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Investigation of Hydration States of Ionic Liquids by Fourier Transform Infrared Absorption Spectroscopy: Relevance to Stabilization of Protein Molecules
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  • Navin Rajapriya Inbaraj
    Navin Rajapriya Inbaraj
    Department of Materials Science and Engineering, School of Materials Science and Chemical Technology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama-shi, Kanagawa-ken 226-8502, Japan
    More by Navin Rajapriya Inbaraj
    Orcidhttps://orcid.org/0000-0001-5318-5983
  • Subin Song
    Subin Song
    Department of Materials Science and Engineering, School of Materials Science and Chemical Technology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama-shi, Kanagawa-ken 226-8502, Japan
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  • Ryongsok Chang
    Ryongsok Chang
    Department of Materials Science and Engineering, School of Materials Science and Chemical Technology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama-shi, Kanagawa-ken 226-8502, Japan
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  • Kyoko Fujita*
    Kyoko Fujita
    Department of Pathophysiology, Tokyo University of Pharmacy and Life Sciences, 1432-1 Horinouchi, Hachioji, Tokyo 192-0392, Japan
    *Email: [email protected]
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  • Tomohiro Hayashi*
    Tomohiro Hayashi
    Department of Materials Science and Engineering, School of Materials Science and Chemical Technology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama-shi, Kanagawa-ken 226-8502, Japan
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0002-4065-1807
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Langmuir

Cite this: Langmuir 2023, 39, 7, 2558–2568
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https://doi.org/10.1021/acs.langmuir.2c02851
Published February 8, 2023

Copyright © 2023 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

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Among many kinds of ionic liquids, some hydrated ionic liquids (Hy ILs) have shown an exceptional capability to stabilize protein molecules and maintain their structure and functions over a long period. However, the complex IL–water interaction among these protein-stabilizing Hy ILs has yet to be elucidated clearly. In this work, we investigate the origin of the compatibility of ionic liquid with proteins from the viewpoint of hydration structure. We systematically analyzed the hydrogen-bonding state of water molecules around ionic liquid using Fourier transform infrared absorption (FT-IR) spectroscopy. We found that the native hydrogen-bonding network of water remained relatively unperturbed in the protein-stabilizing ILs. We also observed that the protein-stabilizing ILs have a strong electric field interaction with the surrounding water molecules and this water–IL interaction did not disrupt the water–water hydrogen-bonding interaction. On the other hand, protein-denaturing ILs perturb the hydrogen-bonding network of the water molecules to a greater extent. Furthermore, the protein-denaturing ILs were found to have a weak electric field effect on the water molecules. We speculate that the direct hydrogen bonding of the ILs with water molecules and the strong electric field of the ions lasting several hydration shells while maintaining the relatively unperturbed hydrogen-bonding network of the water molecules play an essential role in protein stabilization.

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1. Introduction

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Ionic liquids (ILs) are molten salts composed purely of ions and are generally considered to have a melting temperature below 100 °C. (1) The ions are large and asymmetrical, which prevents the dense packing to achieve a crystalline phase at relatively lower temperatures. Some ILs remain liquid even at ambient temperature and are called room-temperature ILs. (2) Due to the interionic interaction and low melting temperature, ILs have unique physicochemical properties: negligible vapor pressure, (3) thermal stability up to 400 °C, (4) wide liquidus range (>200 °C), (5) and superior ionic conductivity. (6) The physicochemical properties of IL enable a wide range of applications as an electrolyte, (7) lubricant, (8) thermal fluid, (9) separating agent, (10) etc.
Recently, several IL and water mixtures were investigated for their protein-stabilizing capability. For example, choline dihydrogen phosphate [Ch][dHp] mixed with 20 wt % water stabilized lysozyme molecules for up to a month with higher thermal stability and only 20% loss of activity. (11) The same IL–water mixture stabilized cytochrome c for up to 1 year without affecting its native structure. (12) Interestingly, several protein-stabilizing IL–water mixtures showed a unique phase transition behavior, cold crystallization (CC), during the slow heating of the supercooled amorphous phase. (13) This CC behavior of IL–water mixtures could be detected in differential scanning calorimetry (DSC) as an exothermic peak in the thermogram. For example, [Ch][dHp], a protein-stabilizing IL mixed with water at a molar ratio of 7:1, water to IL, showed a CC behavior at −80 °C. However, [Ch][dBp], a protein denaturant, did not show CC behavior, irrespective of the water content. (13) The authors also suggested that CC could be used as a screening method to determine the protein-stabilizing capability of Hy ILs. Similarly, CC behavior was also observed in biocompatible polymer–water systems of poly(ethylene glycol), (14) poly(2-methoxyethyl acrylate), (15,16) poly(tetrahydrofurfuryl acrylate), (17) and poly(2-methacryloyloxyethyl phosphorylcholine). (18) The authors mentioned that the CC behavior arises from the intermediate water, which is weakly hydrogen-bonded to the polymer or the surrounding water molecules. (19−21)
Although the CC behavior of protein-stabilizing ILs was speculated to arise from a similar hydrogen-bonding network of water molecules in biocompatible polymers, the structure of the water molecules around the ILs and the nature of their interaction is yet to be explained clearly. The difficulty in elucidating the IL-water interaction could be owed to the complex and competitive hydrogen bonding between cations, anions, and water molecules. (22) Furthermore, the hydration state of the ions could also differ based on their structure; for instance, the ion’s hydrogen-bonding capacity, (23,24) hydrophobicity, (25,26) and charge locality. (27,28)
The IL–water interaction plays a dominant role in stabilizing proteins. Nevertheless, why do only particular Hy ILs show protein-stabilizing capability? How does this IL–water interaction differ from the protein-denaturing Hy ILs? We hypothesize that protein-stabilizing ILs have a unique intermolecular interaction with water molecules. In this study, we attempt to explain the hydration state of protein-stabilizing ILs by drawing connections between IL–water intermolecular interaction and the water structure around diverse Hy ILs at room temperature.

2. Materials and Methods

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2.1. Preparation of Hy ILs

In this study, we analyzed the hydration state of choline dihydrogen phosphate ([Ch][dHp]), choline dihydrogen citrate ([Ch][dhC]), 1-butyl-3-methylimidazolium dihydrogen phosphate ([C4mim][dHp]), phosphocholine (PC), tetrabutylphosphonium dihydrogen phosphate ([P4444][dHp]), tetrahexylphosphonium dihydrogen phosphate([P6666][dHp]), tributyldodecylphosphonium dihydrogen phosphate ([P44412][dHp]) and tetrabutylammonium dihydrogen phosphate ([N4444][dHp]), choline dibutylphosphate ([Ch][dBp]), choline bromide ([Ch]Br), and choline thiocyanate ([Ch][SCN]). The chemical structure of the ILs used in this study is depicted in Figure 1; their protein-stabilizing capability and CC behavior are summarized in Table 1. The ILs were synthesized according to the literature. (12,29,30) ILs were identified by 1H NMR and electrospray ionization mass spectrometry (ESI-MS) or elemental analysis. [P4444][dHp] ESI-MS ESI+m/z 259.25 (P4444+), ESI– 78.96 (dHp–-H2O),1H NMR (CDCl3, 400 MHz): d 0.80 (s, 12H), d 1.39 (s, 15H), d 2.20 (s, 8H), [N4444][dHp] ESI-MS ESI+m/z 242.28 (N4444+), ESI– 253.80 (3 dHp–-2H2O), 1H NMR (CDCl3, 400 MHz): d 0.84 (s, 12H), d 1.31 (s, 8H), d 1.50 (s, 8H), d 3.16 (s, 8H), [P44412][dHp] ESI-MS ESI+m/z 371.38 (P44412+), ESI– 96.96 (dHp–), 1H NMR (CDCl3, 400 MHz): d 0.89 (m, 12H), d 1.21 (s, 18H), d 1.49 (s, 14H), d 2.27 (s, 8H), [P6666][dHp] ESI-MS ESI+m/z 371.37 (P6666+), ESI– 253.80 (3 dHp–-2H2O) 1H NMR (CDCl3, 400 MHz): d 0.84 (s, 12H), d 1.28 (m, 32H), d 2.46 (s, 8H). The ILs were mixed with water at 3:1, 7:1, and 15:1 molar ratios of water to IL.

Figure 1

Figure 1. Chemical structures of the ILs investigated in this study (a–k).

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Table 1. Protein-Stabilizing Capability and CC Behavior of the Hy ILs Used in This Study
Hy ILCC (water/IL molar ratio)*protein stabilizationrefs
[Ch][dHp]Yes (7:1)Yes (13)
[Ch][dhC]Yes (7:1)Yes (13)
[C4mim][dHp]Yes (12:1)Yes (13)
[N4444][dHp]Yes (7:1)Yes (27)
[P4444][dHp]Yes (7:1)Yes (27)
[P6666][dHp]Yes (7:1)Yes (27)
PCYes (7:1)Yes (13)
[P44412][dHp]Yes (15:1)Yes (27)
[Ch][dBp]NoNo (13)
[Ch][SCN]NoNo (29)
[Ch]BrNoNo (29)
*

The molar ratio at which the CC was observed is represented in brackets.

2.2. ATR-IR Measurement

The IR spectra of all of the Hy ILs were obtained from a single-reflection ATR accessory (“ATR PRO ONE”, JASCO, Japan), assembled in an FT/IR-4600 Fourier transform infrared (FTIR) spectrometer (JASCO, Japan) equipped with a DLATGS detector and a Ge/KBr beam splitter. A tiny droplet of the Hy IL was pipetted on top of the diamond ATR prism and sealed with a lid to mitigate the evaporation of water. All measurements were performed at room temperature with a continuous flow of dry N2 gas. The spectra were averaged over 150 scans with a resolution of 4 cm–1 (1 cm–1 in the data step). The IR absorption spectrum was processed with Spectral Manager (JASCO, Japan) and analyzed with Igor Pro (Wavemetrics, USA). We confirmed that the conditions of the measurements and spectral processing can reveal the detailed structural features (number of peaks and peak positions) of the spectra.

2.3. Curve Fitting Analysis

The IR spectra of the OH stretching band is broad, which results from the intermolecular vibrational dynamics of stretching vibrations of water molecules at various hydrogen-bonding state. In the case of Hy ILs, the broadness arises from the mixing of the CH stretching mode and the Fermi resonance in the OH stretching region of the IR spectra. To deconvolute this broad OH stretching band, we performed Gaussian deconvolution. In this section, we will demonstrate the curve fitting analysis using Hy [Ch][dHp] 7:1 (protein-stabilizing Hy IL) and [Ch]Br 7:1 (protein-denaturing Hy IL). The remaining curve fitting results of the OH stretching band of Hy ILs are included in the Supporting Information (Figures S1–S3)
Second derivative analysis was performed on the OH stretching bands to identify the number of peaks and their positions (Figure 2). The negative peaks of the second derivative spectra in the 3700–3100 cm–1 appear to be broader than the negative peaks in the 3100–2800 cm–1 region.

Figure 2

Figure 2. Number of peaks and their positions identified from the second derivative analysis of the OH stretching bands of Hy [Ch][dHp] 7:1 and Hy [Ch]Br 7:1.

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The negative peaks in the 3700–3100 cm–1 region could be correlated with the OH stretching peaks, and the negative peaks in the 3100–2800 cm–1 region could be correlated with the CH stretching peaks.
Curve fitting of the OH bands of the Hy ILs was performed with a Gaussian function using the peak positions identified from the second derivative spectra (Figure 3). However, the peaks in the CH stretching region were fixed as per the second derivative spectra. Fixing the CH stretching peaks mitigated the fluctuation of fitting results. Together with the error during fitting, the peak areas fluctuated within 4%.

Figure 3

Figure 3. The number of peaks and their positions identified from the second derivative analysis of the OH stretching bands of Hy [Ch][dHp] 7:1 and Hy [Ch]Br 7:1.

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The protein-stabilizing Hy ILs show a relatively broader OH stretching band compared to the protein-denaturing Hy ILs. This broadness of the OH band arises from the Fermi resonance caused by the mixing of the overtone of the out-of-plane bending vibration mode of the P–OH group of the [dHp]− anion with the CH stretching and OH stretching bands of the cation. (31) The assignments of the peaks are represented in Table 2.
Table 2. Assignment of the Gaussian Peaks of Hy ILs in the 3700–2700 cm–1 Region of IR Spectra
IR peak frequencies (cm–1)assignments
3700–3000OH stretching band
3090–3070CH asymmetric stretching mode of (N)CH3 group
3040–3020CH symmetric stretching mode of (N)CH3 group
2980–2960CH asymmetric stretching mode of CH3 group
2940–2920CH symmetric stretching mode of CH3 group
2880–2870CH asymmetric stretching mode of CH2 group
2850–2820CH symmetric stretching mode of CH2 group
2800–2700Fermi resonance from the overtone of P–OH out-of-plane bending peak of the [dHp] anion mixing with the CH and OH stretching bands

3. Results and Discussion

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Although several Hy ILs have been reported to show protein-stabilizing capability, the intermolecular interaction between water and IL has yet to be elucidated clearly. This study aims to probe into the intermolecular interaction between several Hy ILs ionic liquids and analyze its effect on the hydrogen-bonding network of the water molecules. By investigating the intermolecular interaction between IL and water, we try to suggest an explanation for the protein-stabilizing capability of specific Hy ILs.

3.1. OH Stretching Band

3.1.1. Hydrogen-Bonding Network of Water Molecules Is Relatively Less Perturbed in Protein-Stabilizing Hy ILs Than in Protein-Denaturing Hy ILs

The OH stretching bands of the [Ch] cation and [dHp] anion are mixed with the OH stretching band of water. However, the OH stretching band of the pure ionic liquid (IL) (red plot) was weak in absorbance and broad, spanning from ∼3700 to ∼2000 cm–1 (Figure 4). Hence, the dominant contribution to the OH stretching band in hydrated (Hy) [Ch][dHp] comes from water. Therefore, we concluded that the OH stretching band dominantly arises from the stretching vibration of the water molecules.

Figure 4

Figure 4. IR spectra of the stretching band of [Ch][dHp] with various water contents.

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The OH stretching band is considered sensitive to the variation of the hydrogen-bonding network of water molecules. (32−34) The OH stretching band of the Hy ILs gives us an idea about the structure of water molecules in the hydration shell of ions. (35−37) We investigated the OH stretching band of water structure in the most biocompatible IL–water composition, 7:1 molar ratio, as seven water molecules per ion pair was reported to be the threshold hydration state to ensure biological activity. (38)
The ATR-IR spectra of the OH stretching band of Hy ILs at a 7:1 molar ratio are illustrated in Figure 5. To analyze the difference in the hydration structure of protein-stabilizing and protein-denaturing Hy ILs, the OH stretching band was deconvoluted into four Gaussian peaks. The number of peaks and their peak positions were evaluated from the second derivative analysis. Figure 6 demonstrates the Gaussian peak deconvolution of the OH band of Hy [Ch][dHp] 7:1 (a protein-stabilizing Hy IL) and Hy [Ch]Br 7:1 (a protein-denaturing Hy IL). The remaining deconvolution results of the Hy ILs and pure water (neat water) are included in the Supporting Information (Figures S1–S3).

Figure 5

Figure 5. ATR-IR spectra of the OH stretching band of protein-stabilizing Hy ILs (black line) and protein-denaturing Hy ILs (blue line) at a 7:1 molar ratio. The OH stretching bands of the spectra were height-normalized at the OH stretching region for comparison.

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Figure 6

Figure 6. OH stretching band of (a) Hy [Ch][dHp] 7:1 (protein-stabilizing IL) and (b) Hy [Ch]Br 7:1 (protein-denaturing IL) decomposed into four Gaussian peaks. The number of peaks and their positions were identified from the second derivative analysis.

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The OH band has two prominent peaks at ∼3200 cm–1 (peak 3) and ∼3400 cm–1 (peak 2). These two peaks represent the two most abundant local hydrogen-bonding networks present within the water clusters. The peak at around ∼3200 cm–1 was previously associated with strongly hydrogen-bonded water molecules, and the peak at around ∼3400 cm–1 was associated with weakly hydrogen-bonded water molecules. (32,39) We used these two prominent peaks to analyze the difference in the trend of the OH stretching band among the Hy ILs. We calculated the area of the Gaussian peaks 2 and 3 of the OH stretching band of Hy IL 7:1 and pure water. The area ratios (peak 3: peak 2) of the Gaussian components are summarized in Figure 7. The peak area ratio of pure water was 0.89, and the peak area ratio of protein-stabilizing Hy ILs ranged from 0.83 to 1.18.

Figure 7

Figure 7. Summarized peak area ratio of the prominent Gaussian peaks of the OH stretching band of pure water and Hy ILs at a 7:1 molar ratio.

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Moreover, in protein-denaturing Hy ILs, Hy [Ch]Br 7:1 and Hy [Ch][SCN] 7:1 had area ratios of 0.52 and 0.78, while Hy [Ch][dBp] 7:1 showed an area ratio of 1.30. From Figure 7, it is evident that there is a window within which the protein-stabilizing ILs share an area ratio close to that of pure water (0.89). Beyond this window, the area ratios of the protein-denaturing ILs are distant from that of pure water.
Suppose the protein-stabilizing Hy ILs showed an area ratio close to that of pure water. In that case, this means that the proportion of strongly hydrogen-bonded water molecules to weakly hydrogen-bonded water molecules within the Hy IL is relatively similar to that of pure water. In other words, in the protein-stabilizing Hy ILs, to a certain extent, the hydrogen-bonding network of the water molecules was unperturbed. (40) In the case of protein-denaturing Hy ILs, the area ratio is further away from that of pure water. This finding may indicate that the local hydrogen-bonding network of water molecules was severely disrupted in the protein-denaturing IL. For example, Hy [Ch]Br 7:1 and Hy [Ch][SCN] 7:1 had an area ratio of 0.52 and 0.78, respectively, much lower than that of the pure waters, 0.89. A lower area ratio means that the number of water molecules engaged in weak hydrogen bonding with anions is greater, which reduces the number of strongly hydrogen-bonded water molecules. (35,36,41) We also investigated the correlation between the area ratios of other peaks and the stabilization of proteins. However, we did not find a clear correlation; the summary of the peak areas is included in the Supporting Information (Figure S4).
The Br– and [SCN]− are large and aprotic anions; the increased surface area and the lack of a hydrogen-bonding site increase the surface interaction with many water molecules. (42) This surface interaction of water molecules with anions causes the hydrogenic part of the water molecule to face toward the anion, which prevents hydrogen-bonding interaction with neighboring water molecules. (43) On the other hand, Hy [Ch][dBp] 7:1, also a protein-denaturing IL with an area ratio (1.30), is much larger than that of the protein-stabilizing IL. The component associated with the strongly hydrogen-bonded water molecules was more significant than the weakly hydrogen-bonded water molecules. This interaction is unlike the native hydrogen-bonding interaction of pure water, and hence the area ratio of Hy [Ch][dBp] 7:1 falls outside of the window. One possible explanation for this behavior could be that the weak interaction of water molecules with the [dBp]− anion and its long alkyl chain facilitates a favorable configuration of the water cluster. (25,37) The pocket created by the anion’s hydrophobic domains may increase the water–water aggregation. Hence, the component of strongly hydrogen-bonded water is the dominant local hydrogen-bonding state of the Hy [Ch][dBp] system. The area ratios of the two components indicate that in protein-stabilizing ILs, the native hydrogen-bonding state of water molecules is relatively less perturbed than in protein-denaturing ILs.
Similar phenomena were observed in biocompatible zwitterionic polymers such as poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), (44) poly[N,N-dimethyl-N-(3-sulfopropyl)-3′-methacrylamidopropanaminium inner salt] (poly(SPB)), (45) and poly(2-methoxyethyl acrylate) (PMEA). (46) The vibrational spectra of the water molecules in the vicinity of the polymers mentioned above were reported to be similar to the spectrum of pure water. (47) It was also mentioned that the high blood compatibility of the zwitterionic copolymer of MPC and butyl methacrylate (BMA) [poly(MPC-r-BMA)], and PMEA may have arisen from the intact hydrogen-bonding network of water in its vicinity.
This unperturbed hydration shell may mediate the electrostatic interaction between the ions of the ILs and the protein molecules, preserving the structure and function of protein molecules. The absence of the pure water-like hydrogen-bonding network in Hy ILs may result in an imbalanced interaction between the ions, water molecules, and protein molecules. That is, the ions may directly interact with the charged moiety of the protein and denature it.

3.2. HOH Bending Mode

3.2.1. Protein-Stabilizing ILs Have a Stronger Electric Field Interaction with Water Molecules Than Protein-Denaturing ILs

Just like the IR OH stretching band, the HOH bending peak is also equally indicative of the hydrogen-bonding interaction of the water molecules. Furthermore, the line shape of the bending mode is also subjected to less vibrational coupling effects from the water molecules’ intra- and intermolecular bending vibrations compared to the OH stretching band. (48) The vibrational frequency and the peak shape of the HOH bending mode spectrum could be used to probe into the electrostatic interaction of the water molecules and ions. (49)
Figure 8 illustrates the HOH bending spectra of Hy IL at 3:1, 7:1, and 15:1 molar ratios. The HOH bending mode frequency of pure water was 1635.3 cm–1. To analyze the effect of IL on the water molecules, the maxima of the HOH bending peak were plotted as a function of increasing water content (Figure 9).

Figure 8

Figure 8. Spectra of HOH bending mode of Hy ILs at (a) 3:1, (b) 7:1, and (c)15:1 molar ratios. The spectra were height-normalized for comparison. The dashed line indicates the peak maxima position of pure water’s HOH bending spectrum.

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Figure 9

Figure 9. Change in the peak top position of HOH bending spectra of Hy ILs with respect to increased water content. Black color indicates protein-stabilizing Hy ILs, and blue color represents protein-denaturing Hy ILs. The horizontal solid line indicates the peak top position of pure water’s HOH bending spectrum.

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At a 3:1 molar ratio, the electric field effect of the ions on the water molecules is the strongest due to the relatively low water–water interaction. The HOH bending frequency of all protein-stabilizing Hy ILs at a 3:1 molar ratio is higher than that of pure water (1635.3 cm–1), and with further addition of water content, the HOH bending vibration gradually reduced. In the case of protein-denaturing Ils, except for Hy [Ch]dBp (1644 cm–1), the HOH bending peak at the 3:1 molar ratio of Hy [Ch]Br and Hy [Ch][SCN] has the same bending vibrational frequency of 1634.4 cm–1, lower than that of pure water. But with increasing water content, the frequency shift of the maxima is unlike the protein-stabilizing Ils. For example, the HOH bending peak of Hy [Ch][dBp] 3:1 remains unchanged until the 7:1 molar ratio and reduces to a lower frequency only at the 15:1 molar ratio. An increase in water concentration showed no effect on the vibrational frequency until the 15:1 molar ratio. In Hy Ch Br 3:1, the peak maximum was 1634.4 cm–1, and at a 7:1 molar ratio, the maximum shifts to reach the frequency of pure water (1635.3 cm–1), and this maximum position remains consistent up to 15:1 molar ratio. In the case of the Ch SCN, the maximum position at 3:1 molar ratio was also 1634.4 cm–1, but with increasing water content it reduced to 1633 cm–1 at 7:1 molar ratio, and at 15:1, it returned to its initial position at 1634.4 cm–1. To summarize, the protein-denaturing Hy Ils showed an HOH bending frequency higher than pure water, while the protein-denaturing Ils showed lower bending vibrational frequency than pure water except [Ch][dBp], and their peak shifts are inconsistent with increasing water concentration. In protein-stabilizing Hy Ils, the HOH bending frequency reduces consistently with increasing water concentration.
The Increase in the HOH bending frequency of the water indicates that the water molecules are subjected to the electric field effect of the protein-stabilizing ILs. The multiple proton donor and acceptor sites in [dHp]− and [dhC]− anions of the protein-stabilizing ILs may enable directional hydrogen bonding with the surrounding water molecules. The directional hydrogen bonding results in a strong electrostatic pull of water molecules’ hydrogen end toward the oxygen of the anion hydrogen-bonding acceptor. This attraction may affect the bending vibrational frequency of the water molecule. As the vibrational bending mode of the water molecule involves the change in the O–H bond angle, during the vibration, the O–H bond pulls away from the anion. However, due to the energy penalty of pulling away from a directional hydrogen bond, the O–H bond vibrates with a slight change in bond angle. Hence, the vibrational frequency is much higher than in pure water. A lower bending vibrational frequency indicates that the electrostatic interaction of water molecules is weaker than that of pure water. [Ch]Br and [Ch][SCN] may have had a structure-breaking effect on the water molecules. That is, the O–H bond of the water molecules is not strongly pulled by neighboring water molecules or the ions. Hence, the bond angle change during bending vibration is significant, resulting in a lower vibrational frequency. (49)
With the further addition of water molecules, the water–water hydrogen-bonding interaction dominates the electric field effect of the ions, and the bending frequency approaches that of pure water. Unlike the other protein-denaturing ILs, Hy [Ch][dBp] 3:1 has a higher frequency of HOH bending vibration, 1644 cm–1. With the further addition of water content, the peak maximum does not change until the 7:1 molar ratio. This result could be interpreted such that the water molecules at a 3:1 molar ratio, when interacting with ions, achieve a favorable cluster, and this hydrogen-bonding network remains stable up to a 7:1 molar ratio, after which the water–water hydrogen-bonding interaction dominates the mixtures. As mentioned in the previous discussion for the area ratio of the OH stretching band, this stable water cluster may be enabled by the weak interaction with the [dBp]− anion. The anion weakly attracts the water molecules, and the long alkyl chain may create nonpolar domains with pockets of water clusters, which prevent interaction with other water molecules until sufficient water concentration is reached.
These results also agree with 1H NMR chemical shifts of water proton in IL 3:1 reported previously by Nikawa et al. (24) The 1H NMR chemical shift of pure water was 4.8 ppm. A change in the chemical shift of the water proton indicates a change in its electron density. To illustrate, a gain in electron density of water proton results in shielding of the hydrogen atom, which leads to an upfield shift, and a decrease in the chemical shift (<4.8 ppm). On the other hand, the loss of electron density of the water proton results in the deshielding of the hydrogen atom, which leads to a downfield shift, that is, the chemical shift value increase (>4.8 ppm). The protein-stabilizing ILs such as Hy [Ch][dHp] (6.3 ppm), Hy [Ch][dhC] (6.8 ppm), Hy PC (5.8 ppm), Hy [C4mim][dHp] (6.1 ppm) showed a downfield shift. Moreover, the protein-denaturing ILs, [Ch][SCN] (4.2 ppm), showed a corresponding upfield chemical shift, and [Ch][dBp] (4.85 ppm) showed a negligible chemical shift from that of pure water. The downfield shift of the protein-stabilizing ILs ranging from 1 to 2.8 ppm from that of the pure water indicates that the ILs have a strong electric field effect on the water molecules. The formation of directional hydrogen bonds with the anions electrostatically screens the water molecules. (50) The significant change in the chemical shift may also indicate that the electric field effect of the protein-stabilizing ILs may span over several hydration shells. An upfield chemical shift of the water proton indicates the structure-breaking effect, disrupting the water’s hydrogen-bonding network due to weak interaction with anions. (43) Our HOH bending results agree with these NMR chemical shift results. We hypothesize that the IR HOH bending peak shifts also represent the ion-dipole and hydrogen-bonding interaction between the IL and water molecules.

3.3.2. Electric Field Effect of Protein-Stabilizing ILs Does Not Disrupt Water’s Hydrogen-Bonding Network As Much As Protein-Denaturing ILs

We consider that there are two reasons for the change in the energy of the HOH bending mode: water-ion and water–water interactions. (50) The HOH bending peak of pure water could be assumed as consisting of only water–water interaction. Hence, any change in the broadness of the HOH bending peak of Hy ILs could indicate a change in the proportions of the two aforementioned water components.
To quantify the broadness, we calculated the full width at half-maximum (FWHM) of the HOH bending mode’s peak of Hy ILs 3:1, where the electric field effect of the IL on the water is dominant. Figure 10 demonstrates the calculated FWHM of [Ch][dHp], protein-stabilizing IL, and [Ch]Br, a protein-denaturing IL. The remaining FWHM results of the Hy ILs are included in the Supporting Information (Figures S5 and S6).

Figure 10

Figure 10. FWHM of HOH bending peaks of (a) Hy [Ch][dHp] 3:1 and (b) Hy [Ch]Br 3:1.

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From the Gaussian peak fitting, the FWHM values of all of the Hy IL 3:1 and pure water were calculated and are summarized in Figure 11. The FWHM of the HOH bending mode’s peak of protein-stabilizing Hy ILs is less than that of protein-denaturing ILs. Pure water has an FWHM of 85 cm–1, the protein-stabilizing ILs showed an FWHM ranging between 80 and 98 cm–1, and the protein-denaturing ILs showed an FWHM ranging from 60 to 78 cm–1. In protein-stabilizing Hy ILs, the FWHM values of the HOH bending peaks were in close range to that of pure water. In protein-denaturing Hy ILs, the FWHM values of HOH bending peaks were smaller than those of protein-stabilizing Hy IL and pure water.

Figure 11

Figure 11. FWHM of the HOH bending peaks of Hy ILs at a 3:1 molar ratio and pure water.

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In pure water, the broadness of the HOH bending peak arises from the continuous distribution of hydrogen-bonding strength and vibrational coupling of the water molecules. (51) Hence, a reduction in the FWHM of HOH bending peak in protein-denaturing Hy ILs indicates that the number of water molecules engaging in pure water-like water–water hydrogen bonding within the hydration shell may have been reduced. (52) On the other hand, the protein-stabilizing ILs show a relatively broader FWHM. This broadness could mean that despite the strong electric field effect of ions, the distribution of hydrogen-bonding strength of the surrounding water molecules is still comparable to that of pure water. Even though the maxima of the HOH bending peaks were blue-shifted due to the electric field effect of ions, the peaks still retained components from the water–water hydrogen-bonding interaction. From these results, we can speculate that even though the protein-stabilizing ILs electrostatically screen the water molecules, this electric field effect does not strip the water molecules from its water–water hydrogen-bonding interaction.
Ahmed et al. reported that the FWHM of the HOH bending mode’s peak of the Raman spectra of aqueous NaCO3 was also as large as pure water’s. (52) They performed Raman spectroscopy and multivariate curve resolution (MCR) analysis to deconvolute the HOH bending spectra into two components. One component corresponds to the vibrational response of the water molecules perturbed by the ions, and the other component corresponds to water in a pure water-like state. The FWHM of the ion-perturbed water spectra in aqueous NaCO3 was almost the same as that of the pure water spectra. This broadness of the HOH band was associated with a greater distribution of the hydrogen-bonding strength of the water molecules in the hydration layer. The authors also suggested that in aqueous NaCO3, the water molecules exist in two types of hydrogen bonding. In the first hydration shell, water molecules are strongly hydrogen-bonded to the oxygen atom of the CO32– anion. In the second hydration shell, the water molecules are hydrogen-bonded to other water molecules by slightly weaker hydrogen bonds than in the first hydration shell. But still stronger than pure water’s hydrogen-bonding strength. We speculate that the water molecules in the protein-stabilizing ILs may also exist in such hydrogen-bonding states. The strong electric field effect of the protein-stabilizing ILs on water molecules and the retainment of pure water-like hydrogen-bonding interaction in the hydration shells may play an essential role in stabilizing protein molecules.

4. Conclusions

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This study used ATR-IR spectroscopy to demonstrate the complex intermolecular interaction between protein-stabilizing ionic liquid and water molecules. By investigating the components of the IR OH stretching band of the Hy ILs, we observed that the native hydrogen-bonding network of the water molecules is less perturbed in the hydration shells around the protein-stabilizing ILs compared to protein-denaturing ILs. From the peak maximum and the FWHM of HOH bending pending peak of the Hy ILs, we found that the water molecules are subjected to a strong electric field by the protein-stabilizing ILs. Furthermore, this electric field effect of the ionic liquids does not disrupt the water–water hydrogen-bonding interaction. On the other hand, protein-denaturing ILs have a weak electric field, and this weak interaction destabilizes the water’s hydrogen-bonding network. We believe that the direct hydrogen bonding of the ILs with water molecules and the strong electric field of the ions lasting several hydration shells while maintaining the relatively unperturbed hydrogen-bonding network of the water molecules play an essential role in protein stabilization.
Although the protein stabilization capability of aqueous salt solutions has been discussed in terms of the Hofmeister series of ions, the protein stabilization mechanisms discussed in this work for Hy ILs may not correlate with aqueous salt solutions. The protein stabilization or denaturation experiments by aqueous salt solutions were carried out at low salt concentrations (e.g., 1–30 wt % of NaCl in water). On the other hand, the concentration of the ILs used in this work is very high (e.g., 50–80wt % of [Ch][dHp] in water). Therefore, protein stabilization and denaturation mechanisms can differ between Hy ILs and the Hofmeister series of ions.
Our findings here will contribute to the design of protein-stabilizing ILs in the future. To quantify the electrostatic screening effect and the strength of hydrogen bonds between ions and water molecules, we will combine the NMR chemical shifts and the IR HOH bending peak shifts of Hy ILs. This work will be published elsewhere.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.langmuir.2c02851.

  • Gaussian deconvolution of the OH stretching band of pure water and Hy ILs (Hy [Ch][dhC] 7:1, Hy [C4mim][dHp] 7:1, Hy PC 7:1, Hy [P4444][dHp] 7:1, Hy [P6666][dHp] 7:1, Hy [P44412][dHp] 7:1, Hy [N4444][dHp] 7:1, Hy [Ch][dBp] 7:1, Hy [Ch][SCN] 7:1) (Figures S1–S3); area ratios of the four Gaussian peaks of the OH stretching band of pure water and the aforementioned Hy ILs at a 7:1 molar ratio (Figure S4); and full width at half-maxima of HOH bending peaks of pure water and the Hy ILs at a 3:1 molar ratio (Figures S5 and S6) (PDF)

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Author Information

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  • Corresponding Authors
    • Kyoko Fujita - Department of Pathophysiology, Tokyo University of Pharmacy and Life Sciences, 1432-1 Horinouchi, Hachioji, Tokyo 192-0392, Japan;  Email: [email protected]
    • Tomohiro Hayashi - Department of Materials Science and Engineering, School of Materials Science and Chemical Technology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama-shi, Kanagawa-ken 226-8502, Japan;  Orcidhttps://orcid.org/0000-0002-4065-1807;  Email: [email protected]
  • Authors
    • Navin Rajapriya Inbaraj - Department of Materials Science and Engineering, School of Materials Science and Chemical Technology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama-shi, Kanagawa-ken 226-8502, Japan;  Orcidhttps://orcid.org/0000-0001-5318-5983
    • Subin Song - Department of Materials Science and Engineering, School of Materials Science and Chemical Technology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama-shi, Kanagawa-ken 226-8502, Japan
    • Ryongsok Chang - Department of Materials Science and Engineering, School of Materials Science and Chemical Technology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama-shi, Kanagawa-ken 226-8502, Japan
  • Funding

    This work was supported by the JSPS KAKENHI grant (grant numbers JP22H04561, JP22H04530, JP21H05511, and JP20H05210). This work was performed under the Research Program of “Five-star Alliance” in “NJRC Mater. & Dev.”

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors thank Ms. Kazue Taki for the administration of this project.

References

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This article references 52 other publications.

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    The structural change of water restrained by poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) was investigated by differential scanning calorimetry (DSC), since the biocompatibility of PMPC and related biopolymers is affected by the structure of water on the polymer surface. The phase transition behavior of PMPC-water systems with a water content (Wc = mass of water/mass of dry sample, g g-1) in the range 0-2.0 was measured in the temp. range -150 to 50 °C. Glass transition, cold crystn. and melting were obsd. Cold crystn., which has been suggested as an index of biocompatibility, was detected for PMPC with a Wc in the range 0.5-0.9. The amts. of two types of bound water, non-freezing water and freezing bound water, were calcd. from the melting enthalpy. The amt. of non-freezing water of PMPC was ∼0.48. It was found that the phase transition behavior and amt. of bound water of PMPC were quite similar to those of water-sol. polysaccharide electrolytes. The results indicate that the bound water, not the free water, is restrained by PMPC.
  19. 19
    Tanaka, M.; Hayashi, T.; Morita, S. The Roles of Water Molecules at the Biointerface of Medical Polymers. Polym. J. 2013, 45, 701– 710,  DOI: 10.1038/pj.2012.229
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    The roles of water molecules at the biointerface of medical polymers
    Tanaka, Masaru; Hayashi, Tomohiro; Morita, Shigeaki
    Polymer Journal (Tokyo, Japan) (2013), 45 (7), 701-710CODEN: POLJB8; ISSN:0032-3896. (NPG Nature Asia-Pacific)
    A review. A no. of materials have been proposed for use as biomaterials, including hydrophilic, phase-sepd. and zwitterionic polymers. The mechanisms responsible for the bio/blood compatibility (bioinertness) of these polymers at the mol. level have not been clearly demonstrated, although many theor. and exptl. efforts have been made to understand these mechanisms. Water interactions have been recognized as fundamental for the biol. response to contact with biomaterials. We have proposed the intermediate water' concept, in which water clearly exhibits defined peaks for cold crystn. in the differential scanning calorimetry chart and presents a strong peak at 3400cm-1 in a time-resolved IR spectrum. We found a localized hydration structure consisting of three hydrated waters in poly(2-methoxyethyl acrylate). We hypothesized that intermediate water, which prevents the proteins and blood cells from directly contacting the polymer surface, or non-freezing water on the polymer surface has an important role in the bio/blood compatibility of polymers. We will provide an overview of the recent exptl. progress and a theor. description of the bio/blood compatibility mechanisms as detd. by thermal, spectroscopic and surface force measurements.
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    Hayashi, T. Water at Interfaces: Its Behavior and Roles in Interfacial Phenomena. Chem. Lett. 2021, 50, 1173– 1180,  DOI: 10.1246/cl.210049
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    Water at Interfaces: Its Behavior and Roles in Interfacial Phenomena
    Hayashi, Tomohiro
    Chemistry Letters (2021), 50 (6), 1173-1180CODEN: CMLTAG; ISSN:0366-7022. (Chemical Society of Japan)
    A review. Water at interfaces plays essential roles in many natural phenomena and engineering applications. However, the mol. behavior of interfacial water is still a matter of intense debate. Thus far, many exptl. and theor. methods have been employed to elucidate the real picture (structure, dynamics, hydrogen bonding states, etc.) of interfacial water and its relevance to interfacial phenomena and material functions. The author reviews these works and discusses the principles and limitations of the anal. methods. The significant representative findings on water-mediated interfacial phenomena are also introduced.
  21. 21
    Chang, R.; Asatyas, S.; Lkhamsuren, G.; Hirohara, M.; Mondarte, E. A. Q.; Suthiwanich, K.; Sekine, T.; Hayashi, T. Water near Bioinert Self-Assembled Monolayers. Polym. J. 2018, 50, 563– 571,  DOI: 10.1038/s41428-018-0075-1
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    Water near bioinert self-assembled monolayers
    Chang, Ryongsok; Asatyas, Syifa; Lkhamsuren, Ganchimeg; Hirohara, Makoto; Mondarte, Evan Angelo Quimada; Suthiwanich, Kasinan; Sekine, Taito; Hayashi, Tomohiro
    Polymer Journal (Tokyo, Japan) (2018), 50 (8), 563-571CODEN: POLJB8; ISSN:0032-3896. (Nature Research)
    A review. For a long time, water has been speculated to play an essential role in the interactions of proteins and cells with artificial biocompatible materials. The current question is how water mols. at the interfaces affect the adsorption of proteins and the adhesion of cells. To answer this question, we introduce recent works that investigated the interfacial behavior of water near self-assembled monolayers (SAMs) by different types of anal. techniques. By combining these findings, we discuss how interfacial water affects the protein and cell resistance of various bioinert SAMs.
  22. 22
    Gupta, A.; Kaur, S.; Kashyap, H. K. How Water Permutes the Structural Organization and Microscopic Dynamics of Cholinium Glycinate Biocompatible Ionic Liquid. J. Phys. Chem. B 2019, 123, 2057– 2069,  DOI: 10.1021/acs.jpcb.8b10235
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    How Water Permutes the Structural Organization and Microscopic Dynamics of Cholinium Glycinate Biocompatible Ionic Liquid
    Gupta, Aditya; Kaur, Supreet; Kashyap, Hemant K.
    Journal of Physical Chemistry B (2019), 123 (9), 2057-2069CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
    The authors study the structural organization and microscopic dynamics of aq. cholinium glycinate ([Ch][Gly]), a biocompatible ionic liq. (IL), by employing all-atom mol. dynamics simulations. Herein, the authors observe the effect of H2O content on the mol.-level arrangement of ions in the IL-H2O mixt. through simulated x-ray scattering structure function, their partial components, and real-space correlation functions. The study reveals a principal peak in the total structure function of the neat [Ch][Gly] IL at around q = 1.4 Å-1. The corresponding correlation tends to decrease and shifts toward shorter length scales with increase in the H2O content. The principal peak mainly originates from the correlations between counterions. Hydrogen bond anal. reveals that H2O mols. compete with the anions to form hydrogen bond with the hydroxyl hydrogen of cation. Concomitantly, strong hydrogen bonding is also obsd. between [Gly]- anion and H2O, which depreciates with the increasing hydration level. Hydrogen-bond autocorrelation function anal. manifests that av. lifetimes of different possible hydrogen bonds decrease with increase in mole fraction of H2O. The mobilities of the ions are also significantly affected by H2O, showing a nonlinear increase with the increasing H2O content. The [Gly]- anion is found to show faster dynamics on the addn. of H2O as compared to [Ch]+ cation.
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    Yaghini, N.; Nordstierna, L.; Martinelli, A. Effect of Water on the Transport Properties of Protic and Aprotic Imidazolium Ionic Liquids – an Analysis of Self-Diffusivity, Conductivity, and Proton Exchange Mechanism. Phys. Chem. Chem. Phys. 2014, 16, 9266– 9275,  DOI: 10.1039/C4CP00527A
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    Effect of water on the transport properties of protic and aprotic imidazolium ionic liquids - an analysis of self-diffusivity, conductivity, and proton exchange mechanism
    Yaghini, N.; Nordstierna, L.; Martinelli, A.
    Physical Chemistry Chemical Physics (2014), 16 (20), 9266-9275CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    In this paper we report on the transport properties of protic and aprotic ionic liqs. of the imidazolium cation (C2C1Im+ or C2HIm+) and the TFSI- or TfO- anion as a function of added water. We observe that the self-diffusion coeff. of the ionic species increases upon addn. of water, and that the cation diffuses faster than the anion in the entire water concn. range investigated. We also observe that the overall increase of anionic and cationic diffusion coeffs. is significant for C2HImTfO while it is rather weak for C2C1ImTFSI, the former being more hydrophilic. Moreover, the difference between cationic and anionic self-diffusivity specifically depends on the structure of the ionic liq.'s ions. The degree of ion-ion assocn. has been investigated by comparing the molar cond. obtained by impedance measurements with the molar cond. calcd. from NMR data using the Nernst-Einstein equation. Our data indicate that the ions are partly dissocd. (Λimp/ΛNMR in the range 0.45-0.75) but also that the degree of assocn. decreases in the order C2HImTfO > C2HImTFSI ≈ C2C1ImTfO > C2C1ImTFSI. From these results, it seems that water finds different sites of interaction in the protic and aprotic ionic liqs., with a strong preference for hydrogen bonding to the -NH group (when available) and a stronger affinity to the TfO anion as compared to the TFSI. For the protic ionic liqs., the anal. of 1H NMR chem. shifts (upon addn. of H2O and D2O, resp.) indicates a water-cation interaction of hydrogen bonding nature. In addn., we could probe proton exchange between the -NH group and deuterated water for the protic cation, which occurs at a significantly faster rate if assocd. with the TfO anion as compared to the TFSI.
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    Nikawa, Y.; Fujita, K.; Ohno, H. Quantitative Assessment of Kosmotropicity of Hydrated Ionic Liquids by Nuclear Magnetic Resonance. Phys. Chem. Chem. Phys. 2017, 19, 8148– 8151,  DOI: 10.1039/C6CP07463D
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    Quantitative assessment of kosmotropicity of hydrated ionic liquids by nuclear magnetic resonance
    Nikawa, Yohsuke; Fujita, Kyoko; Ohno, Hiroyuki
    Physical Chemistry Chemical Physics (2017), 19 (12), 8148-8151CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    NMR studies revealed that the chem. shift of H2O in hydrated ionic liqs. varied with their component ions. The variation reflected the formation of hydrogen bonding networks between ions and water mols. The chem. shift relative to bulk water was used to quant. assess the kosmotropicity.
  25. 25
    Kaneko, K.; Saihara, K.; Masuda, Y.; Yoshimura, Y.; Shimizu, A. Dynamic Properties of Water Molecules in Ionic Liquid/Water Mixture with Various Alkyl Chain Length. J. Mol. Liq. 2018, 264, 337– 342,  DOI: 10.1016/j.molliq.2018.05.043
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    Dynamic properties of water molecules in ionic liquid/water mixture with various alkyl chain length
    Kaneko, Kazuyoshi; Saihara, Koji; Masuda, Yuichi; Yoshimura, Yukihiro; Shimizu, Akio
    Journal of Molecular Liquids (2018), 264 (), 337-342CODEN: JMLIDT; ISSN:0167-7322. (Elsevier B.V.)
    This work investigated the alkyl chain length dependence of dynamic properties of water mols. in imidazolium-based ionic liq. (IL)/water mixts. The microscopic (i.e., self-diffusion coeffs. of water mol. and ILs) properties of these systems suggested that the movement of IL/water mixts. decreases with increasing alkyl chain length of the IL. In contrast, the self-diffusion coeff. ratios (Dwater/Danion or Dwater/Dcation) at a water concn. extrapolated to 0 mol% (i.e., the value of a pure IL) increased as the alkyl chain length of the IL increased. This finding indicates that the water mols. in the IL/water mixts. move more independent of the IL as the alkyl chain length of the IL increases. Based on these results, we propose a plausible model for water mols. in a space within the IL.
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    Han, Q.; Wang, X.; Bynre, N. Utilizing Water Activity as a Simple Measure to Understand Hydrophobicity in Ionic Liquids. Frontiers in Chemistry 2019, 7, 112,  DOI: 10.3389/FCHEM.2019.00112
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    Utilizing water activity as a simple measure to understand hydrophobicity in ionic liquids
    Han, Qi; Wang, Xungai; Bynre, Nolene
    Frontiers in Chemistry (Lausanne, Switzerland) (2019), 7 (), 112CODEN: FCLSAA; ISSN:2296-2646. (Frontiers Media S.A.)
    Ionic liqs. (ILs) are regarded as designable solvents finding use in a variety of applications. One of the challenges of the design and selection process is to understand the ionic liq. properties. In this work, we selected seven ILs contg. three types of hydrophilic anions and examd. several key properties, which are correlated to hydrophobicity. In particular, we measured the hydrogen bond basicity β and water activity aw of IL and IL-water mixts., and suggested that these two properties are linearly correlated particularly in hydrated ILs. We then used NMR to evaluate the chem. shift of H2O in hydrated ILs. Correlating the outcomes of each of these techniques with respect to understanding the hydrophobicity of the ILs is discussed. It is shown that water activity aw is the most facile technique to represent and understand hydrophobicity of ILs.
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    Voss, J. M.; Marsh, B. M.; Zhou, J.; Garand, E. Interaction between Ionic Liquid Cation and Water: Infrared Predissociation Study of [Bmim]+·(H2O)N Clusters. Phys. Chem. Chem. Phys. 2016, 18, 18905– 18913,  DOI: 10.1039/c6cp02730j
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    Interaction between ionic liquid cation and water: infrared predissociation study of [bmim]+·(H2O)n clusters
    Voss, Jonathan M.; Marsh, Brett M.; Zhou, Jia; Garand, Etienne
    Physical Chemistry Chemical Physics (2016), 18 (28), 18905-18913CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The IR predissocn. spectra of [bmim]+·(H2O)n, n = 1-8, in the 2800-3800 cm-1 region are presented and analyzed with the help of electronic structure calcns. The water mols. solvate [bmim]+ by predominately interacting with the imidazolium C2-H moiety for the small n = 1 and 2 clusters. This is characterized by a red shifted and relatively intense C(2)-H stretch. For n ≥ 4 clusters, hydrogen-bond interactions between the water mols. drive the formation of ring isomers which interact on top of the imidazolium ring without any direct interaction with the C2-H. The water arrangement in [bmim]+·(H2O)n is similar to the low energy isomers of neutral water clusters up to the n = 6 cluster. This is not the case for the n = 8 cluster, which has the imidazolium ring disrupting the otherwise preferred cubic water structure. The evolution of the solvation network around [bmim]+ illustrates the competing [bmim]+-water and water-water interactions.
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    Kohno, Y.; Ohno, H. Ionic Liquid/Water Mixtures: From Hostility to Conciliation. Chem. Commun. 2012, 48, 7119– 7130,  DOI: 10.1039/C2CC31638B
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    Ionic liquid/water mixtures: from hostility to conciliation
    Kohno, Yuki; Ohno, Hiroyuki
    Chemical Communications (Cambridge, United Kingdom) (2012), 48 (57), 7119-7130CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    A review. Water was originally inimical to ionic liqs. (ILs) esp. in the anal. of their detailed properties. Various data on the properties of ILs indicate that there are two ways to design functions of ionic liqs. The first is to change the structure of component ions, to provide "task-specific ILs". The second is to mix ILs with other components, such as other ILs, org. solvents or water. Mixing makes it easy to control the properties of the soln. In this strategy, water is now a very important partner. Below, the authors summarize recent results on the properties of IL/water mixts. Stable phase sepn. is an effective method in some sepn. processes. Conversely, a dynamic phase change between a homogeneous mixt. and sepn. of phases is important in many fields. Anal. of the relation between phase behavior and the hydration state of the component ions indicates that the pattern of phase sepn. is governed by the hydrophilicity of the ions. Sufficiently hydrophilic ions yielded ILs that are miscible with water, and hydrophobic ions gave stable phase sepn. with water. ILs composed of hydrophobic but hydrated ions undergo a dynamic phase change between a homogeneous mixt. and sep. phases according to temp. ILs having more than seven water mols. per ion pair undergo this phase transition. These dynamic phase changes are considered, with some examples, and application is made to the sepn. of water-sol. proteins.
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    Fujita, K.; Nakano, R.; Nakaba, R.; Nakamura, N.; Ohno, H. Hydrated Ionic Liquids Enable Both Solubilisation and Refolding of Aggregated Concanavalin A. Chem. Commun. 2019, 55, 3578– 3581,  DOI: 10.1039/c8cc10102g
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    Hydrated ionic liquids enable both solubilisation and refolding of aggregated concanavalin A
    Fujita, Kyoko; Nakano, Roka; Nakaba, Risa; Nakamura, Nobuhumi; Ohno, Hiroyuki
    Chemical Communications (Cambridge, United Kingdom) (2019), 55 (25), 3578-3581CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    The dissoln. and refolding of aggregated Con A have been achieved, in hydrated ionic liqs. contg. a limited no. of water mols. Both ammonium and phosphonium salts were examd. to find a suitable hydrophobicity of ions and water content for refolding. Recovery of sugar recognition was confirmed as a proof of refolding.
  30. 30
    Fujita, K.; MacFarlane, D. R.; Forsyth, M.; Yoshizawa-Fujita, M.; Murata, K.; Nakamura, N.; Ohno, H. Solubility and Stability of Cytochrome c in Hydrated Ionic Liquids: Effect of Oxo Acid Residues and Kosmotropicity. Biomacromolecules 2007, 8, 2080– 2086,  DOI: 10.1021/bm070041o
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    Solubility and Stability of Cytochrome c in Hydrated Ionic Liquids: Effect of Oxo Acid Residues and Kosmotropicity
    Fujita, Kyoko; MacFarlane, Douglas R.; Forsyth, Maria; Yoshizawa-Fujita, Masahiro; Murata, Kenichi; Nakamura, Nobuhumi; Ohno, Hiroyuki
    Biomacromolecules (2007), 8 (7), 2080-2086CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)
    Hydrated ionic liqs. (ILs) were prepd. by adding appropriate amts. of water to hydrophilic ILs. Some hydrated ILs show excellent solubilizing ability for proteins, keeping the basic properties of ILs. The soly. of cytochrome c (cyt c) depended on the structure of the component ions. When component anions have oxo acid residues, the resulting hydrated ILs solubilize cyt c quite well. In such hydrated ILs, the structure and activity of cyt c is influenced by the kosmotropicity of the component ions. We synthesized ILs from various ions having different kosmotropicity, including dihydrogen phosphate (dhp), dibutylphosphate, acetate, lactate, and methanesulfonate as anions. The activity of the dissolved cyt c depends on the permutations of kosmotropicity of the component ions. Cyt c shows no structural change and retains its activity when dissolved in the hydrated choline dhp, which is an excellent combination of chaotropic cation and kosmotropic anion. Furthermore, cyt c dissolved in the hydrated choline dhp remained in a native state and was active after 18 mo of storage at room temp.
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    de Souza, Í. F. T.; Paschoal, V. H.; Bernardino, K.; Lima, T. A.; Daemen, L. L.; Z, Y.; Ribeiro, M. C. C. Vibrational Spectroscopy and Molecular Dynamics Simulation of Choline Oxyanions Salts. J. Mol. Liq. 2021, 340, 117100  DOI: 10.1016/j.molliq.2021.117100
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    Vibrational spectroscopy and molecular dynamics simulation of choline oxyanions salts
    de Souza, Icaro F. T.; Paschoal, Vitor H.; Bernardino, Kalil; Lima, Thamires A.; Daemen, Luke L.; Z, Y.; Ribeiro, Mauro C. C.
    Journal of Molecular Liquids (2021), 340 (), 117100CODEN: JMLIDT; ISSN:0167-7322. (Elsevier B.V.)
    The structure of choline salts contg. the anions acetate, [Chol][Ac], and dihydrogen phosphate, [Chol][DHP], were investigated by IR, Raman, and inelastic neutron scattering (INS). The chosen systems allow for the comparison of structural effects related to the bond acceptor characteristic of [Ac] and the simultaneous acceptor and donor characteristics of [DHP] in forming hydrogen bonds (H-bond) in salts of [Chol], which is itself prone to forming H-bonds. Different computational tools were used for the anal. of different spectral ranges. The calcn. of the low-frequency range of Raman and INS spectra of the cryst. phases at low-temps. by solid state DFPT (d. functional perturbation theory) unveils the coupling between vibrations of the H-bonds and intramol. modes. Changes obsd. in the spectral pattern of lattice and [DHP] modes upon heating cryst. [Chol][DHP] are analogous to the ferroelec.-paraelec. phase transition known in the potassium salt of [DHP]. The fingerprint region of the vibrational spectra provides information concerning the [Chol] conformation in the solid phase (gauche in [Chol][Ac] and anti in [Chol][DHP]) and in aq. soln. DFT calcns. of ionic pairs and ionic clusters unveil the interplay between [Chol] conformation and the [DHP] ability to form H-bonded dimers of anions. The high-frequency spectral range and the structures driven by H-bonds are discussed using classical mol. dynamics (MD) simulations. The MD simulations of aq. solns. highlight the strong anion-cation H-bond in [Chol][Ac], in contrast to the strong anion-anion H-bond in [Chol][DHP] due to occurrence of dimers and larger clusters of [DHP].
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    Sun, Q. The Raman OH Stretching Bands of Liquid Water. Vib. Spectrosc. 2009, 51, 213– 217,  DOI: 10.1016/j.vibspec.2009.05.002
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    The Raman OH stretching bands of liquid water
    Sun, Qiang
    Vibrational Spectroscopy (2009), 51 (2), 213-217CODEN: VISPEK; ISSN:0924-2031. (Elsevier B.V.)
    From the discussion on water structure and clusters, it can be deduced that the OH stretching vibration is closely related to local H-bonded network for a water mol., and different OH vibrations can be assigned to OH groups engaged in various H-bonding. At ambient condition, the main local H-bonding for a mol. can be classified as DDAA (double donor-double acceptor), DDA (double donor-single acceptor), DAA (single donor-double acceptor) and DA (single donor-single acceptor) and free OH vibrations. As for water at 290 K and 0.1 MPa pressure, the OH stretching region of the Raman spectrum can be deconvoluted into 5 sub-bands, which are located at 3014, 3226, 3432, 3572, and 3636 cm-1, and can be assigned to νDAA-OH, νDDAA-OH, νDA-OH, νDDA-OH, and free OH2 sym. stretching vibrations, resp.
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    Schmidt, D. A.; Miki, K. Structural Correlations in Liquid Water: A New Interpretation of IR Spectroscopy. J. Phys. Chem. A 2007, 111, 10119– 10122,  DOI: 10.1021/JP074737N
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    Structural Correlations in Liquid Water: A New Interpretation of IR Spectroscopy
    Schmidt, Diedrich A.; Miki, Kazushi
    Journal of Physical Chemistry A (2007), 111 (40), 10119-10122CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    The authors present a new and alternative interpretation of the structure of the IR vibrational mode (ν(OH) band) of pure H2O. The re-interpretation is based on the influence of the cooperative H bonding arising from a network of H bonds in the liq. The ν(OH) band has six components that are dominated by differences in their O-H bond lengths but deviate from thermodynamically av. values due to interactions with the H bond network. The phys. origin of the structure in the ν(OH) band is directly related to the O-H bond length, and variations in this bond length are caused by the influence of the surrounding H-bonded network of H2O mols.
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    Arunan, E.; Desiraju, G. R.; Klein, R. A.; Sadlej, J.; Scheiner, S.; Alkorta, I.; Clary, D. C.; Crabtree, R. H.; Dannenberg, J. J.; Hobza, P.; Kjaergaard, H. G.; Legon, A. C.; Mennucci, B.; Nesbitt, D. J. Definition of the Hydrogen Bond (IUPAC Recommendations 2011). Pure Appl. Chem. 2011, 83, 1637– 1641,  DOI: 10.1351/PAC-REC-10-01-02
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    Definition of the hydrogen bond (IUPAC Recommendations 2011)
    Arunan, Elangannan; Desiraju, Gautam R.; Klein, Roger A.; Sadlej, Joanna; Scheiner, Steve; Alkorta, Ibon; Clary, David C.; Crabtree, Robert H.; Dannenberg, Joseph J.; Hobza, Pavel; Kjaergaard, Henrik G.; Legon, Anthony C.; Mennucci, Benedetta; Nesbitt, David J.
    Pure and Applied Chemistry (2011), 83 (8), 1637-1641CODEN: PACHAS; ISSN:0033-4545. (International Union of Pure and Applied Chemistry)
    A novel definition for the hydrogen bond is recommended here. It takes into account the theor. and exptl. knowledge acquired over the past century. This definition insists on some evidence. Six criteria are listed that could be used as evidence for the presence of a hydrogen bond.
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    Kitadai, N.; Sawai, T.; Tonoue, R.; Nakashima, S.; Katsura, M.; Fukushi, K. Effects of Ions on the OH Stretching Band of Water as Revealed by ATR-IR Spectroscopy. J. Solution Chem. 2014, 43, 1055– 1077,  DOI: 10.1007/s10953-014-0193-0
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    Effects of Ions on the OH Stretching Band of Water as Revealed by ATR-IR Spectroscopy
    Kitadai, Norio; Sawai, Takashi; Tonoue, Ryota; Nakashima, Satoru; Katsura, Makoto; Fukushi, Keisuke
    Journal of Solution Chemistry (2014), 43 (6), 1055-1077CODEN: JSLCAG; ISSN:0095-9782. (Springer)
    The effects of various cations (Li+, Na+, K+, Rb+, Cs+, Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, and Ni2+) and anions (Cl-, Br-, I-, NO-3, ClO-4, HCO-3, and CO2-3) on the molar absorptivity of water in the OH stretching band region (2,600-3,800 cm-1) were ascertained from attenuated total reflection IR spectra of aq. electrolyte solns. (22 in all). The OH stretching band mainly changes linearly with ion concns. up to 2 mol·L-1, but several specific combinations of cations and anions (Cs2SO4, Li2SO4, and MgSO4) present different trends. That deviation is attributed to ion pair formation and cooperativity in ion hydration, which indicates that the extent of the ion-water interaction reflected by the OH stretching band of water is beyond the first solvation shell of water mols. directly surrounding the ion. The obtained dataset was then correlated with several quant. parameters representing structural and dynamic properties of water mols. around ions: ΔGHB, the structural entropy (Sstr), the viscosity B-coeff. (Bη), and the ionic B-coeff. of NMR relaxation (BNMR). Results show that modification of the OH stretching band of water caused by ions has quasi-linear relations with all of these parameters. Vibrational spectroscopy can be a useful means for evaluating ion-water interaction in aq. solns.
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    Schmidt, D. A.; Miki, K. Defective Continuous Hydrogen-Bond Networks: An Alternative Interpretation of IR Spectroscopy. Chemphyschem 2008, 9, 1914– 1919,  DOI: 10.1002/CPHC.200800236
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    Defective continuous hydrogen-bond networks: an alternative interpretation of IR spectroscopy
    Schmidt, Diedrich A.; Miki, Kazushi
    ChemPhysChem (2008), 9 (13), 1914-1919CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The authors apply previously developed deconvolution method and interpretation to analyze changes in the OH stretching band [ν(OH) band] of low-concn. (≤0.2 m) aq. solns. of NaCl and KCl. The authors treat these simple, monovalent ions as defects in the H-bond network of pure H2O and quantify the changes in the spectra at low defect concn. with an order parameter. Order-parameter anal. of difference spectra of the 2 solns. leads to hydration nos. of 7.0 ± 1.0 and 5.9 ± 0.3 for K+ and Na+, resp. Addnl., changes in the ν(OH) band due to low concns. of ions result from changes in the topol. of the H-bond network.
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    Cammarata, L.; Kazarian, S. G.; Salter, P. A.; Welton, T. Molecular States of Water in Room Temperature Ionic Liquids. Phys. Chem. Chem. Phys. 2001, 3, 5192– 5200,  DOI: 10.1039/b106900d
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    Molecular states of water in room temperature ionic liquids
    Cammarata, L.; Kazarian, S. G.; Salter, P. A.; Welton, T.
    Physical Chemistry Chemical Physics (2001), 3 (23), 5192-5200CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    ATR and transmission IR spectroscopy have been used to investigate the state of water in room temp. ionic liqs. (RTILs) based on the 1-alkyl-3-methylimidazolium cation with the anions: [PF6]-, [SbF6]-, [BF4]-, [ClO4]-, [CF3SO3]-, [(CF3SO2)2N]-, [NO3]- and [CF3CO2]-. It has been shown that in these RTILs water mols. absorbed from the air are present mostly in the "free" (not self-assocd.) state, bound via H-bonding with [PF6]-, [BF4]-, [SbF6]-, [ClO4]-, [CF3SO3]-, [(CF3SO2)2N]- with the concns. of dissolved water in the range 0.2-1.0 mol dm-3. It has been concluded that most of the water mols. at these concns. exist in sym. 1 : 2 type H-bonded complexes: anion...HOH...anion. Addnl. evidence that the preferred sites of interaction with water mols. are the anions has been obtained from the expts. with RTILs of the 1-butyl-2,3-dimethylimidazolium and 1-butyl-2,3,4,5-tetramethylimidazolium cations. Water mols. can also form assocd. liq.-like formations in RTILs with anions of stronger basicity such as [NO3]- and [CF3CO2]-. When these RTILs are exposed to air the water concns. exceed 1.0 mol dm-3. The strength of H-bonding between water mols. and anions increases in the order [PF6]- < [SbF6]- < [BF4]- < [(CF3SO2)2N]- < [ClO4]- < [CF3SO3]- < [NO3]- < [CF3CO2]-. The energies of this H-bonding were estd. from spectral shifts, with the resulting enthalpies being in the range 8-13 kJ mol-1. ATR-IR spectroscopy has also been used to study H-bonding between methanol and RTILs.
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    Ohno, Hiroyuki.; Fujita, Kyoko.; Kohno, Yuki. Is Seven the Minimum Number of Water Molecules per Ion Pair for Assured Biological Activity in Ionic Liquid–Water Mixtures?. Phys. Chem. Chem. Phys. 2015, 17, 14454– 14460,  DOI: 10.1039/C5CP00768B
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    Is seven the minimum number of water molecules per ion pair for assured biological activity in ionic liquid-water mixtures?
    Ohno, Hiroyuki; Fujita, Kyoko; Kohno, Yuki
    Physical Chemistry Chemical Physics (2015), 17 (22), 14454-14460CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    A review. Ionic liqs. (ILs) contg. small amts. of water are called hydrated ILs and they show diverse physico-chem. properties that are strongly dependent on their water content. Some properties of hydrated ILs, such as biol. activity and phase transition behavior, were found to change non-linearly, with an inflection at a water mol. to ion pair ratio of around 7:1. This crit. hydration no. of ILs has been discussed in this paper with respect to the state of solvated water mols.
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    Woutersen, S.; Emmerichs, U.; Bakker, H. J. Femtosecond Mid-IR Pump-Probe Spectroscopy of Liquid Water: Evidence for a Two-Component Structure. Science 1997, 278, 658– 660,  DOI: 10.1126/science.278.5338.658
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    A femtosecond mid-IR pump-probe study of the vibrational and orientational dynamics of the OH-stretching mode of HDO dissolved in D2O is presented. The orientational relaxation of the HDO mols. occurs on either a very slow or a very fast time scale, with assocd. time consts. of τR = 13 ps and τR = 0.7 ps. Strongly H-bonded H2O mols. only relax through the slow orientational relaxation process, whereas the fast process dominates for weakly H-bonded mols. Probably with respect to orientational dynamics, two distinct mol. species exist in liq. H2O.
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    Bharmoria, P.; Gupta, H.; Mohandas, V. P.; Ghosh, P. K.; Kumar, A. Temperature Invariance of NaCl Solubility in Water: Inferences from Salt-Water Cluster Behavior of NaCl, KCl, and NH4Cl. J. Phys. Chem. B 2012, 116, 11712– 11719,  DOI: 10.1021/jp307261g
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    Temperature Invariance of NaCl Solubility in Water: Inferences from Salt-Water Cluster Behavior of NaCl, KCl, and NH4Cl
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    Journal of Physical Chemistry B (2012), 116 (38), 11712-11719CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
    The growth and stability of salt-water clusters were exptl. studied in aq. solns. of NaCl, KCl, and NH4Cl from dil. to near-satn. conditions employing dynamic light scattering and zeta potential measurements. In order to examine cluster stability, the changes in the cluster sizes were monitored as a function of temp. Compared to the other cases, the av. size of NaCl-water clusters remained almost const. over the studied temp. range of 20-70°. Information obtained from the temp.-dependent soln. compressibility (detd. from speed of sound and d. measurements), multinuclear NMR (1H, 17O, 35Cl NMR), and FTIR were utilized to explain the cluster behavior. Comparison of NMR chem. shifts of satd. salt solns. with solid-state NMR data of pure salts, and evaluation of spectral modifications in the OH stretch region of satd. salt solns. as compared to that of pure water, provided important clues on ion pair-water interactions and water structure in the clusters. The high stability and temp. independence of the cluster sizes in aq. NaCl shed light on the temp. invariance of its soly.
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    Riemenschneider, J.; Holzmann, J.; Ludwig, R. Salt Effects on the Structure of Water Probed by Attenuated Total Reflection Infrared Spectroscopy and Molecular Dynamics Simulations. Chemphyschem 2008, 9, 2731– 2736,  DOI: 10.1002/cphc.200800571
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    We study aq. solns. of alk. chlorides (NaCl, KCl, RbCl and CsCl) with a combination of attenuated total reflection IR (ATR-IR) spectroscopy and mol. dynamics (MD) simulations using the TlP4P-Ew water model, covering concn. ranges between 0.1 and 6 M. Spectral modifications in the OH stretch region are evaluated and correlated to the various salts and salt concns. By taking the difference spectra between the spectral line shapes of aq. salt solns. and those of pure water, we specifically focus on the small quasi-"free OH" band appearing at the highest wavenumbers in the spectra. This free-OH feature is found constantly at 3650 cm-1 for all salts and salt concns., but it shows a characteristic intensity depending on the chosen cation. In the order from Na+ to Cs+, the free OH intensity decreases compared to that of pure water. To interpret the exptl. results, we performed MD simulations for similar salt solns. The exptl. obsd. effects can be correlated with structural alterations indicated by differences between the site-site pair correlation functions of water in aq. salt solns. and those of pure water.
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    Scipioni, R.; Schmidt, D. A.; Boero, M. A First Principles Investigation of Water Dipole Moment in a Defective Continuous Hydrogen Bond Network. J. Chem. Phys. 2009, 130, 024502  DOI: 10.1063/1.3054197
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    First principles mol. dynamics simulations of an aq. soln. salt system at finite concn. contg. both Na+ and Cl- ions show that a change in the distribution of the mol. dipole moment of H2O monomers appears when ions are present in soln. Simulations suggest a lowering of the dipole moments of the water mols. in the solvation shells of Na+ and Cl- as compared to the pure water case, while the dipoles of the rest of the mols. are hardly affected. However, finer anal. in terms of the Wannier centers distribution suggests a change in the electronic structure of the water mols. even in the bulk. Also a change of the H-bond network arrangement was found and correlation between dipole and MOH parameter evidences such subtle effects, suggesting a lowering of tetrahedral order in salty solns. All these changes can be related to observable quantities such as the IR spectra thus allowing for a rationalization of the exptl. outcome on neutral aq. solns. (c) 2009 American Institute of Physics.
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    Akitt, J. W. Proton Chemical Shifts of Water in Cationic Hydration Complexes and Their Contribution to Water Shifts in Electrolyte Solutions. J. Chem. Soc. Dalton Trans. 1973, 42– 49,  DOI: 10.1039/DT9730000042
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    Proton chemical shifts of water in cationic hydration complexes and their contribution to water shifts in electrolyte solutions
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    The proton chem. shifts of H2O in hydration complexes of Sn4+, Al3+, Ga3+, In3+, Mg2+, and Be2+ were measured at low temps. The shifts were strongly downfield and correlated with shift increments calcd. to arise from the elec. field of the ion. The total cationic H2O shift contained 2 contributions; a downfield one from the elec. field and a smaller upfield one from a structural effect. The correlation was used to est. the proton shift by hydration H2O of the larger Group II and Group I cations where proton exchange is very fast. The contribution of cationic hydration to the molal shifts of a large variety of salt solns. was thus obtained, and comparison with measured molal shifts allowed estn. of the upfield anionic contribution which arises from solvent structure-breaking. A 3-site model was developed to explain the soln. shifts and previous measurements were shown to be consistent with a hydration no. of 6 for Mg2+, Ca2+, Sr2+, and Ba2+, 4 for the alkali metal ions, and 0 for Me4N+. SO42- is also probably tetrahydrated.
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    Kitano, H.; Sudo, K.; Ichikawa, K.; Ide, M.; Ishihara, K. Raman Spectroscopic Study on the Structure of Water in Aqueous Polyelectrolyte Solutions. J. Phys. Chem. B 2000, 104, 11425– 11429,  DOI: 10.1021/jp000429c
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    Raman spectroscopic study on the structure of water in aqueous polyelectrolyte solutions
    Kitano, Hiromi; Sudo, Kurao; Ichikawa, Ken; Ide, Makoto; Ishihara, Kazuhiko
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    The structure and hydrogen bonding of water in various kinds of aq. polyelectrolyte solns. were analyzed with contours of O-H stretching of polarized Raman spectra. Effects of chem. properties of the polymers and water domains surrounded by the polymer chains on the relative intensity of collective band (C value) corresponding to a long-range coupling of O-H stretchings were discussed. The C values for various polymer solns. were almost const. in a relatively low mol. wt. (Mw) region, and decreased with an increase in Mw value. When the size of the space surrounded by the pseudo-network was sufficiently small, the structure of water in the space was altered to have a relatively lower av. no. of hydrogen bonds between water mols. than that of bulk water. The no. of hydrogen bonds collapsed by the presence of one monomer residue (N value) of polyelectrolyte (sodium polyethylenesulfonate, poly-L-lysine hydrobromide, etc.) with a small Mw was much larger than those for neutral polymers such as poly(ethylene glycol) and poly(N-vinylpyrrolidone). This result indicates that the monomer residues of water-sol. neutral polymers do not disturb the structure of water significantly, whereas electrostriction effect by the polyelectrolyte is quite effective on the structure of water. In contrast, the N value for poly(2-methacryloyloxyethyl phosphorylcholine) with a small Mw was nearly zero, suggesting that the zwitterionic-type monomer residues do not disturb the hydrogen bonding between water mols.
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    Kitano, H.; Imai, M.; Sudo, K.; Ide, M. Hydrogen-Bonded Network Structure of Water in Aqueous Solution of Sulfobetaine Polymers. J. Phys. Chem. B 2002, 106, 11391– 11396,  DOI: 10.1021/jp020185r
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    Hydrogen-Bonded Network Structure of Water in Aqueous Solution of Sulfobetaine Polymers
    Kitano, Hiromi; Imai, Makoto; Sudo, Kurao; Ide, Makoto
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    Poly(N,N-dimethylaminopropyl methacrylamide) (poly(DMAPMA)) was incubated with 1,3-propanesultone and 1,4-butanesultone to afford polymers with various contents of N,N-dimethyl-N-(3-sulfopropyl)-3'-methacrylamidopropanaminium inner salt residues and N,N-dimethyl-N-(4-sulfobutyl)-3'-methacrylamidopropanaminium inner salt residues, resp. The structure and hydrogen bonding of water in an aq. soln. of the sulfobetaine polymers were analyzed using the contours of the O-H stretching in the polarized Raman spectra. With an increase in the content of the sulfobetaine residue, the relative intensity of the collective band (C value) corresponding to a long-range coupling of the O-H stretching in the aq. polymer solns. became larger and approached the C value of pure water. The no. of hydrogen bonds disrupted because of the presence of one monomer residue (N value) for the polymers with a large sulfobetaine content was a small pos. value and comparable to those for neutral polymers such as poly(ethylene glycol) and poly(N-vinylpyrrolidone). This is in significant contrast with the largely pos. N values for the precursor polymer (poly(DMAPMA)), and ordinary polyelectrolytes such as sodium polyethylenesulfonate, poly-L-lysine hydrobromide, sodium polyacrylate, and poly(acrylic acid). The N value for a small mol. wt. zwitterionic compd., 3-aminopropanesulfonic acid, was also slightly pos., which is consistent with the tendency obsd. for the sulfobetaine polymers. The present results clearly indicate that the zwitterionic polymers do not significantly disturb the hydrogen-bonded network structure of water, probably because of the counteraction of the electrostriction effect by the proximity between the anionic and cationic groups.
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    Kitano, H.; Imai, M.; Mori, T.; Gemmei-Ide, M.; Yokoyama, Y.; Ishihara, K. Structure of Water in the Vicinity of Phospholipid Analogue Copolymers As Studied by Vibrational Spectroscopy. Langmuir 2003, 19, 10260– 10266,  DOI: 10.1021/la0349673
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    Structure of Water in the Vicinity of Phospholipid Analogue Copolymers As Studied by Vibrational Spectroscopy
    Kitano, Hiromi; Imai, Makoto; Mori, Takayuki; Gemmei-Ide, Makoto; Yokoyama, Yoshiyuki; Ishihara, Kazuhiko
    Langmuir (2003), 19 (24), 10260-10266CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    The structure and H bonding of H2O in the vicinity of phospholipid analog random copolymers [poly(2-methacryloyloxyethyl phosphorylcholine-r-Bu methacrylate), poly(MPC-r-BMA)] with various mol. wts. were analyzed in their aq. solns. and thin films with contours of O-H stretching of Raman and attenuated total reflection IR (ATR-IR) spectra, resp. The relative intensity of the collective band (C value) corresponding to a long-range coupling of O-H stretchings of the Raman spectra for the aq. soln. of poly(MPC-r-BMA) was close to that for pure H2O, which is in contrast with the smaller C value in the aq. soln. of ordinary polyelectrolytes. The no. of H bonds collapsed by the presence of 1 monomer residue (Ncorr value) of poly(MPC-r-BMA) (Mw 1.3 × 104, 3.0 × 104, and 9.3 × 104) was much smaller than those for ordinary polyelectrolytes and close to those for neutral polymers such as poly(ethylene glycol) and poly(N-vinylpyrrolidone). H2O-insol. poly(MPC-r-BMA) with a large mol. wt. (4.2 × 105) could be cast as a thin film (thickness, ∼10 μm) on a ZnSe crystal for the ATR-IR spectra. At an early stage of sorption of H2O into the poly(MPC-r-BMA) film, the O-H stretching band of the IR spectra for the H2O incorporated in the film was similar to that for free H2O, which is in contrast with the drastic change in the O-H stretching band of H2O incorporated in polymer films such as PHEMA, PMMA, and PBMA. Probably the phospholipid analog monomer residues with a zwitterionic structure do not significantly disturb the H bonding between H2O mols. in either the aq. soln. or the thin film systems.
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    Kitano, H.; Mori, T.; Takeuchi, Y.; Tada, S.; Gemmei-Ide, M.; Yokoyama, Y.; Tanaka, M. Structure of Water Incorporated in Sulfobetaine Polymer Films as Studied by ATR-FTIR. Macromol. Biosci. 2005, 5, 314– 321,  DOI: 10.1002/mabi.200400212
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    Structure of water incorporated in sulfobetaine polymer films as studied by ATR-FTIR
    Kitano, Hiromi; Mori, Takayuki; Takeuchi, Yuki; Tada, Susumu; Gemmei-Ide, Makoto; Yokoyama, Yoshiyuki; Tanaka, Masaru
    Macromolecular Bioscience (2005), 5 (4), 314-321CODEN: MBAIBU; ISSN:1616-5187. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The structure and hydrogen bonding of water in the vicinity of a thin film of a sulfobetaine copolymer (poly[(N,N-dimethyl-N-(3-sulfoproyl)-3'-methacrylamido-propanaminium inner salt)-ran-(Bu methacrylate)], poly(SPB-r-BMA)), were analyzed with band shapes of O-H stretching of attenuated total reflection IR (ATR-IR) spectra. The copolymer could be cast as a thin film, of approx. thickness 10 μm, on a ZnSe crystal for the ATR-IR spectroscopy. At an early stage of sorption of water into the polymer film, the O-H stretching band of the IR spectra for the water incorporated in the film was similar to that for free water. This is consistent with the tendency for another zwitterionic polymeric material, poly[(2-methacryloyloxyethylphosphorylcholine)-ran-(Bu methacrylate)] (poly-(MPC-r-BMA)). It is, however, contradictory to the drastic change in the O-H stretching band for water incorporated into films of polymers such as poly(2-hydroxyethyl methacrylate), poly(Me methacrylate) and poly(Bu methacrylate). These results suggest that polymers with a zwitterionic structure do not significantly disturb the hydrogen bonding between water mols. incorporated in the thin films. The investigation into the blood-compatibility of both the poly(SPB-r-BMA) and the poly(MPC-r-BMA) films indicate a definite correlation between the blood-compatibility of the polymers and the lack of effect of the polymeric materials on the structure of the incorporated water.
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    Seki, T.; Chiang, K.-Y.; Yu, C.-C.; Yu, X.; Okuno, M.; Hunger, J.; Nagata, Y.; Bonn, M. The Bending Mode of Water: A Powerful Probe for Hydrogen Bond Structure of Aqueous Systems. J. Phys. Chem. Lett. 2020, 11, 8459– 8469,  DOI: 10.1021/acs.jpclett.0c01259
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    The Bending Mode of Water: A Powerful Probe for Hydrogen Bond Structure of Aqueous Systems
    Seki, Takakazu; Chiang, Kuo-Yang; Yu, Chun-Chieh; Yu, Xiaoqing; Okuno, Masanari; Hunger, Johannes; Nagata, Yuki; Bonn, Mischa
    Journal of Physical Chemistry Letters (2020), 11 (19), 8459-8469CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    A review. Insights into the microscopic structure and dynamics of the water's hydrogen-bonded network are crucial to understand the role of water in biol., atm. and geochem. processes, and chem. reactions in aq. systems. Vibrational spectroscopy of water has provided many such insights, in particular using the O-H stretch mode. In this Perspective, we summarize our recent studies that have revealed that the H-O-H bending mode can be an equally powerful reporter for the microscopic structure of water and provides more direct access to the hydrogen-bonded network than the conventionally studied O-H stretch mode. We discuss the fundamental vibrational properties of the water bending mode, such as the intermol. vibrational coupling, and its effects on the spectral lineshapes and vibrational dynamics. Several examples of static and ultrafast bending mode spectroscopy illustrate how the water bending mode provides an excellent window on the microscopic structure of both bulk and interfacial water.
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    Piatkowski, L.; Bakker, H. J. Vibrational Dynamics of the Bending Mode of Water Interacting with Ions. J. Chem. Phys. 2011, 135, 214509  DOI: 10.1063/1.3664866
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    Vibrational dynamics of the bending mode of water interacting with ions
    Piatkowski, L.; Bakker, H. J.
    Journal of Chemical Physics (2011), 135 (21), 214509/1-214509/8CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    The authors studied the vibrational relaxation dynamics of the bending mode (ν2) of the water H2O mols. in the presence of different salts (LiCl, LiBr, LiI, NaI, CsI, NaClO4, and NaBF4). The linear and nonlinear spectra of the bending mode show distinct responses of H2O mols. hydrating the anions. The bending mode of H2O mols. that are H-bonded to an anion exhibits much slower relaxation rates (T1 ∼ 1 ps) than H2O mols. that are H-bonded to other H2O mols. (T1 = 400 fs). The effect of the anion on the absorption spectrum and relaxation time const. of the H2O bending mode is not only detd. by the strength of the H-bond interaction but also by the shape of the anion. (c) 2011 American Institute of Physics.
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    Zahn, S.; Wendler, K.; Delle Site, L.; Kirchner, B. Depolarization of Water in Protic Ionic Liquids. Phys. Chem. Chem. Phys. 2011, 13, 15083– 15093,  DOI: 10.1039/c1cp20288j
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    Depolarization of water in protic ionic liquids
    Zahn, Stefan; Wendler, Katharina; Delle Site, Luigi; Kirchner, Barbara
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    A mixt. of the protic ionic liq. mono-methylammonium nitrate with 1.6 wt% water was investigated from Car-Parrinello mol. dynamics simulations. In contrast to imidazolium-based ionic liqs., the cation possesses strong directional hydrogen bonds to water and all hydrogen bonds in the mixt. have a comparable strength. This results in a good incorporation of water into the hydrogen bond network of mono-methylammonium nitrate and a tetrahedral hydrogen bond coordination of water. Hence, one might expect a larger dipole moment of water in the investigated mixt. compared to neat water due to the good hydrogen bond network incorporation and the charged vicinity of water in the protic ionic liq. However, the opposite is obsd. pointing to strong electrostatic screening in protic ionic liqs. The influence of water on the properties of the protic ionic liq. is discussed.
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    Intermolecular coupling of bending vibrations in liquid water
    Kabisch, G.
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    Raman spectra of liq. H2O, D2O and their isotopic mixts. were investigated in the region of the deformation modes. Comparison with ν2(HDO) shows that differences in band shape and peak frequencies in isotropic scattering for H2O and D2O are caused by intermol. coupling, and not by the presence of discrete mol. species in liq. water. The changes in ν2 band parameters, produced by addn. of electrolytes and nonelectrolytes, are discussed.
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    Ahmed, M.; Namboodiri, V.; Singh, A. K.; Mondal, J. A. On the Intermolecular Vibrational Coupling, Hydrogen Bonding, and Librational Freedom of Water in the Hydration Shell of Mono- and Bivalent Anions. J. Chem. Phys. 2014, 141, 164708  DOI: 10.1063/1.4899070
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    On the intermolecular vibrational coupling, hydrogen bonding, and librational freedom of water in the hydration shell of mono- and bivalent anions
    Ahmed, Mohammed; Namboodiri, V.; Singh, Ajay K.; Mondal, Jahur A.
    Journal of Chemical Physics (2014), 141 (16), 164708/1-164708/8CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    The hydration energy of an ion largely resides within the first few layers of water mols. in its hydration shell. Hence, it is important to understand the transformation of water properties, such as hydrogen-bonding, intermol. vibrational coupling, and librational freedom in the hydration shell of ions. We investigated these properties in the hydration shell of mono- (Cl- and I-) and bivalent (SO42- and CO32-) anions by using Raman multivariate curve resoln. (Raman-MCR) spectroscopy in the OH stretch, HOH bend, and [bend+librational] combination bands of water. Raman-MCR of aq. Na-salt (NaCl, NaI, Na2SO4, and Na2CO3) solns. provides ion-correlated spectra (IC-spectrum) which predominantly bear the vibrational characteristics of water in the hydration shell of resp. anions. Comparison of these IC-spectra with the Raman spectrum of bulk water in different spectral regions reveals that the water is vibrationally decoupled with its neighbors in the hydration shell. Hydrogen-bond strength and librational freedom also vary with the nature of anion: hydrogen-bond strength, for example, decreases as CO32- > SO42- > bulk water ≈ Cl- > I-; and the librational freedom increases as CO32- ≈ SO42- < bulk water < Cl- < I-. It is believed that these structural perturbations influence the dynamics of coherent energy transfer and librational reorientation of water in the hydration shell of anions. (c) 2014 American Institute of Physics.

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  • Abstract

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    Figure 1

    Figure 1. Chemical structures of the ILs investigated in this study (a–k).

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    Figure 2

    Figure 2. Number of peaks and their positions identified from the second derivative analysis of the OH stretching bands of Hy [Ch][dHp] 7:1 and Hy [Ch]Br 7:1.

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    Figure 3

    Figure 3. The number of peaks and their positions identified from the second derivative analysis of the OH stretching bands of Hy [Ch][dHp] 7:1 and Hy [Ch]Br 7:1.

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    Figure 4

    Figure 4. IR spectra of the stretching band of [Ch][dHp] with various water contents.

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    Figure 5

    Figure 5. ATR-IR spectra of the OH stretching band of protein-stabilizing Hy ILs (black line) and protein-denaturing Hy ILs (blue line) at a 7:1 molar ratio. The OH stretching bands of the spectra were height-normalized at the OH stretching region for comparison.

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    Figure 6

    Figure 6. OH stretching band of (a) Hy [Ch][dHp] 7:1 (protein-stabilizing IL) and (b) Hy [Ch]Br 7:1 (protein-denaturing IL) decomposed into four Gaussian peaks. The number of peaks and their positions were identified from the second derivative analysis.

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    Figure 7

    Figure 7. Summarized peak area ratio of the prominent Gaussian peaks of the OH stretching band of pure water and Hy ILs at a 7:1 molar ratio.

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    Figure 8

    Figure 8. Spectra of HOH bending mode of Hy ILs at (a) 3:1, (b) 7:1, and (c)15:1 molar ratios. The spectra were height-normalized for comparison. The dashed line indicates the peak maxima position of pure water’s HOH bending spectrum.

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    Figure 9

    Figure 9. Change in the peak top position of HOH bending spectra of Hy ILs with respect to increased water content. Black color indicates protein-stabilizing Hy ILs, and blue color represents protein-denaturing Hy ILs. The horizontal solid line indicates the peak top position of pure water’s HOH bending spectrum.

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    Figure 10

    Figure 10. FWHM of HOH bending peaks of (a) Hy [Ch][dHp] 3:1 and (b) Hy [Ch]Br 3:1.

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    Figure 11

    Figure 11. FWHM of the HOH bending peaks of Hy ILs at a 3:1 molar ratio and pure water.

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  • References

    This article references 52 other publications.

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      A review. Room temp. ionic liqs. are salts that are liq. at room temp. and their use as catalysts and catalytic support has been studied extensively. They are also being considered as "green solvents" for various sepn. processes. Recent measurements reported on the properties of pure ionic liqs. and their mixts., including gas and liq. soly. in common org. solvents are reviewed. While some property values are in good agreement, some show large differences. These values are compared and reasons for the discrepancies are conjectured. Since traditional approaches to predicting the properties of fluid liqs. require extensive LLE and VLE measurements, alternative predictive methods need to be explored. The predictions of the properties of mixts. of ionic liqs. using COSMOtherm, an approach based on unimol. quantum chem. calcns. of the individual mols., are presented.
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      Choline 2-hexenoate is an ionic compd. that is a liq. at room temp., and is just one of a class of compds. that have huge potential in biomedical research and clin. applications, explains Eden E. L. Tanner.
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      Ngo, H. L.; LeCompte, K.; Hargens, L.; McEwen, A. B. Thermal Properties of Imidazolium Ionic Liquids. Thermochim. Acta 2000, 357–358, 97– 102,  DOI: 10.1016/S0040-6031(00)00373-7
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      We investigated the thermal properties of several imidazolium salts using DSC and TGA/SDTA data. Many of these salts are liqs. at sub-ambient temps. These ionic liqs. form glasses at low temps. and have minimal vapor pressure up to their thermal decompn. temp. (>400°C). Thermal decompn. is endothermic with the inorg. anions and exothermic with the org. anions investigated. Halide anions drastically reduce the thermal stability of these salts (<300°C). We have obsd. that aluminum catalyzes the decompn. of the salts contg. the inorg. fluoride anions. The imidazolium cations are thermally more stable than the tetraalkyl ammonium cations.
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      Brennecke, J. F.; Maginn, E. J. Ionic Liquids: Innovative Fluids for Chemical Processing. AIChE J. 2001, 47, 2384– 2389,  DOI: 10.1002/aic.690471102
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      Ionic liquids: innovative fluids for chemical processing
      Brennecke, Joan F.; Maginn, Edward J.
      AIChE Journal (2001), 47 (11), 2384-2389CODEN: AICEAC; ISSN:0001-1541. (American Institute of Chemical Engineers)
      A review of the use of ionic liqs. as efficient and environmentally friendly solvents for chem. processing. Ionic liqs. are org. salts whose cations, substituents, and anions can be varied virtually at will to change their chem. and phys. properties. The various challenges and opportunities offered by the use of ionic liqs. are discussed.
    6. 6
      Shin, J.-H.; Henderson, W. A.; Passerini, S. Ionic Liquids to the Rescue? Overcoming the Ionic Conductivity Limitations of Polymer Electrolytes. Electrochem. Commun. 2003, 5, 1016– 1020,  DOI: 10.1016/j.elecom.2003.09.017
      6
      Ionic liquids to the rescue? Overcoming the ionic conductivity limitations of polymer electrolytes
      Shin, Joon-Ho; Henderson, Wesley A.; Passerini, Stefano
      Electrochemistry Communications (2003), 5 (12), 1016-1020CODEN: ECCMF9; ISSN:1388-2481. (Elsevier Science B.V.)
      Polymer electrolytes - solid polymeric membranes with dissolved salts - are being studied for use in all-solid-state Li-metal-polymer batteries to power consumer electronic devices. The low ionic cond. at room temp. of existing polymer electrolytes has hindered the development of such batteries. The incorporation of salts, which are liq. at room temp. (room temp. ionic liqs. or RTILs) into polymer electrolytes may be the soln. to overcoming the inherent ionic cond. limitations of dry polymer electrolytes.
    7. 7
      Galiński, M.; Lewandowski, A.; Stępniak, I. Ionic Liquids as Electrolytes. Electrochim. Acta 2006, 51, 5567– 5580,  DOI: 10.1016/j.electacta.2006.03.016
      7
      Ionic liquids as electrolytes
      Galinski, Maciej; Lewandowski, Andrzej; Stepniak, Izabela
      Electrochimica Acta (2006), 51 (26), 5567-5580CODEN: ELCAAV; ISSN:0013-4686. (Elsevier B.V.)
      A review. Salts having a low m.p. are liq. at room temp., or even below, and form a new class of liqs. usually called room temp. ionic liqs. (RTIL). Information about RTILs can be found in the literature with such key words as: room temp. molten salt, low-temp. molten salt, ambient-temp. molten salt, liq. org. salt or simply ionic liq. Their physicochem. properties are the same as high temp. ionic liqs., but the practical aspects of their maintenance or handling are different enough to merit a distinction. The class of ionic liqs., based on tetraalkylammonium cation and chloroaluminate anion, was extensively studied since late 1970s of the XX century, following the works of Osteryoung. Systematic research on the application of chloroaluminate ionic liqs. as solvents was performed in 1980s. However, ionic liqs. based on Al halides are moisture sensitive. During the last decade an increasing no. of new ionic liqs. were prepd. and used as solvents. The general aim of this paper was to review the phys. and chem. properties of RTILs from the point of view of their possible application as electrolytes in electrochem. processes and devices. The following points are discussed: melting and freezing, cond., viscosity, temp. dependence of cond., transport and transference nos., electrochem. stability, possible application in Al electroplating, Li batteries and in electrochem. capacitors.
    8. 8
      Bermúdez, M.-D.; Jiménez, A.-E.; Sanes, J.; Carrión, F.-J. Ionic Liquids as Advanced Lubricant Fluids. Molecules 2009, 14, 2888– 2908,  DOI: 10.3390/molecules14082888
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      Ionic liquids as advanced lubricant fluids
      Bermudez, Maria-Dolores; Jimenez, Ana-Eva; Sanes, Jose; Carrion, Francisco-Jose
      Molecules (2009), 14 (8), 2888-2908CODEN: MOLEFW; ISSN:1420-3049. (Molecular Diversity Preservation International)
      A review. Ionic liqs. (ILs) are finding technol. applications as chem. reaction media and engineering fluids. Some emerging fields are those of lubrication, surface engineering and nanotechnol. ILs are thermally stable, nonflammable highly polar fluids with negligible volatility, these characteristics make them ideal candidates for new lubricants under severe conditions, were conventional oils and greases or solid lubricants fail. Such conditions include ultra-high vacuum and extreme temps. Other very promising areas which depend on the interaction between IL mols. and material surfaces are the use of ILs in the lubrication of microelectromechanic and nanoelectromechanic systems (MEMS and NEMS), the friction and wear redn. of reactive light alloys and the modification of nanophases.
    9. 9
      Holbrey, J. D.; Reichert, W. M.; Reddy, R. G.; Rogers, R. D. Heat Capacities of Ionic Liquids and Their Applications as Thermal Fluids. In Ionic Liquids as Green Solvents, ACS Symposium Series; American Chemical Society, 2003; Vol. 856, pp 121– 133.
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      Baba, Y.; Kubota, F.; Kamiya, N.; Goto, M. Recent Advances in Extraction and Separation of Rare Earth Metals Using Ionic Liquids. J. Chem. Eng. Jpn. 2010, 44, 679– 685,  DOI: 10.1252/jcej.10we279
      There is no corresponding record for this reference.
    11. 11
      Vrikkis, R. M.; Fraser, K. J.; Fujita, K.; MacFarlane, D. R.; Elliott, G. D. Biocompatible Ionic Liquids: A New Approach for Stabilizing Proteins in Liquid Formulation. J. Biomech. Eng. 2009, 131, 074514,  DOI: 10.1115/1.3156810
      11
      Biocompatible ionic liquids: a new approach for stabilizing proteins in liquid formulation
      Vrikkis Regina M; Fraser Kevin J; Fujita Kyoko; Macfarlane Douglas R; Elliott Gloria D
      Journal of biomechanical engineering (2009), 131 (7), 074514 ISSN:0148-0731.
      Ionic liquids (ILs) have shown excellent promise as both solutes and solvents for stabilizing proteins at room temperature. Because many modern drugs are protein-based, these stabilizing characteristics have great potential to provide advances in the field of liquid formulation of therapeutic proteins. However, before these developments can be translated into clinical solutions it is essential to establish data related to the biocompatibility of these ILs. The current work investigates the cytotoxicity of several ILs that were rationally synthesized from natural biomolecules and compounds that have already been approved as excipients for drug formulations. The effect of choline dihydrogen phosphate (choline dhp), choline saccharinate, and 1-butyl 3-methyl imidazolium lactate (bmim lactate) on the metabolic activity of a mouse macrophage cell line (J774) was assessed using the reduction in resazurin as an indicator of activity and, by extension, viability. Two formulations of lysozyme (10 mg/ml and 100 mg/ml) in 80 wt % choline dhp (aq) were prepared and the proteins were evaluated for structural stability immediately following formulation and again at 1 month. Equivalent formulations in 0.1 M Na acetate aqueous buffer were evaluated as controls. A differential scanning microcalorimeter (DSC) was used to evaluate the structural stability on the basis of the unfolding temperature and the enthalpy of unfolding, and a micrococcus lysodiekticus activity test was used to evaluate functional activity. All compounds were found to be relatively benign, with toxicity increasing in the order choline dhp<choline saccharinate<bmim lactate. At 1 month lysozyme that had been stored in choline dhp had a higher activity and folded fraction than lysozyme that had been stored in aqueous buffer. These results suggest that biocompatibility and protein stabilization characteristics can be rationally designed into ionic liquids.
    12. 12
      Fujita, K. Solubility and Stability of Cytochrome c in Hydrated Ionic Liquids: Effect of Oxo Acid Residues and Kosmotropicity. Biomacromolecules 2007, 8, 2080– 2086,  DOI: 10.1021/BM070041O
      12
      Solubility and Stability of Cytochrome c in Hydrated Ionic Liquids: Effect of Oxo Acid Residues and Kosmotropicity
      Fujita, Kyoko; MacFarlane, Douglas R.; Forsyth, Maria; Yoshizawa-Fujita, Masahiro; Murata, Kenichi; Nakamura, Nobuhumi; Ohno, Hiroyuki
      Biomacromolecules (2007), 8 (7), 2080-2086CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)
      Hydrated ionic liqs. (ILs) were prepd. by adding appropriate amts. of water to hydrophilic ILs. Some hydrated ILs show excellent solubilizing ability for proteins, keeping the basic properties of ILs. The soly. of cytochrome c (cyt c) depended on the structure of the component ions. When component anions have oxo acid residues, the resulting hydrated ILs solubilize cyt c quite well. In such hydrated ILs, the structure and activity of cyt c is influenced by the kosmotropicity of the component ions. We synthesized ILs from various ions having different kosmotropicity, including dihydrogen phosphate (dhp), dibutylphosphate, acetate, lactate, and methanesulfonate as anions. The activity of the dissolved cyt c depends on the permutations of kosmotropicity of the component ions. Cyt c shows no structural change and retains its activity when dissolved in the hydrated choline dhp, which is an excellent combination of chaotropic cation and kosmotropic anion. Furthermore, cyt c dissolved in the hydrated choline dhp remained in a native state and was active after 18 mo of storage at room temp.
    13. 13
      Fujita, K.; Nikawa, Y.; Ohno, H. Cold Crystallisation Behaviour of Water Molecules in Ionic Liquids as a Screening Method to Evaluate Biocompatibility of the Hydrated Ionic Liquids. Chem. Commun. 2013, 49, 3257– 3259,  DOI: 10.1039/c3cc39033k
      13
      Cold crystallization behavior of water molecules in ionic liquids as a screening method to evaluate biocompatibility of the hydrated ionic liquids
      Fujita, Kyoko; Nikawa, Yohsuke; Ohno, Hiroyuki
      Chemical Communications (Cambridge, United Kingdom) (2013), 49 (31), 3257-3259CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      Hydrated ionic liqs., exhibiting cold crystn. behavior of water mols. in a certain range of water contents, successfully dissolved cytochrome c maintaining the original spin state of heme.
    14. 14
      Tanaka, M.; Motomura, T.; Ishii, N.; Shimura, K.; Onishi, M.; Mochizuki, A.; Hatakeyama, T. Cold Crystallization of Water in Hydrated Poly(2-Methoxyethyl Acrylate) (PMEA). Polym. Int. 2000, 49, 1709– 1713,  DOI: 10.1002/1097-0126(200012)49:12<1709::aid-pi601>3.0.co;2-l
      14
      Cold crystallization of water in hydrated poly(2-methoxyethyl acrylate) (PMEA)
      Tanaka, Masaru; Motomura, Tadahiro; Ishii, Naoki; Shimura, Kenichi; Onishi, Makoto; Mochizuki, Akira; Hatakeyama, Tatsuko
      Polymer International (2000), 49 (12), 1709-1713CODEN: PLYIEI; ISSN:0959-8103. (John Wiley & Sons Ltd.)
      The structure of water assocd. with poly(2-methoxyethyl acrylate) (PMEA), which is known to exhibit excellent blood compatibility, has been investigated using DSC. The total equil. water content (EWC) of PMEA was 9.0wt%. Water in the PMEA could be classified into three types: non-freezing, freezing-bound and free water. Cold crystn. of water was clearly obsd. at about -42°C on heating when the water content was more than 3.0wt%. Cold crystn. is attributed to the phase transition from the amorphous ice to the crystal ice in PMEA. The relative proportions of freezing-bound water at the EWC is 48% of all the water in hydrated PMEA.
    15. 15
      Hatakeyma, T.; Kasuga, H.; Tanaka, M.; Hatakeyama, H. Cold Crystallization of Poly(Ethylene Glycol)–Water Systems. Thermochim. Acta 2007, 465, 59– 66,  DOI: 10.1016/j.tca.2007.09.005
      15
      Cold crystallization of poly(ethylene glycol)-water systems
      Hatakeyma, Tatsuko; Kasuga, Hazuki; Tanaka, Masaru; Hatakeyama, Hyoe
      Thermochimica Acta (2007), 465 (1-2), 59-66CODEN: THACAS; ISSN:0040-6031. (Elsevier B.V.)
      Phase transition behavior of poly(ethylene glycol) (PEG)-water systems was investigated by differential scanning calorimetry (DSC) in a temp. range from 150 to 350 K and water content (mass of water/mass of PEG) range from 0 to 10 g g-1. In DSC heating curves, glass transition, cold crystn., melting of eutectic crystal, water and PEG crystal were obsd. depending on water content. The cold crystn. of the system, which is thought to be used as an index of biocompatibility of polymer-water interaction, received particular attention. It was found that cold crystn. and glass transition were obsd. in a wide water content range from 0.05 to 10 g g-1. From the enthalpy balance of transitions in both heating and cooling DSC curves, it was confirmed that cold crystn. is attributable to the mol. rearrangement of PEG mols. assocd. with amorphous ice. When four water mols. are attached to one repeating unit of PEG, the heat capacity difference at glass transition temp. attains the largest value and the enthalpy of cold crystn. shows the max. value.
    16. 16
      Tanaka, M.; Morita, S.; Hayashi, T. Role of Interfacial Water in Determining the Interactions of Proteins and Cells with Hydrated Materials. Colloids Surf. B Biointerfaces 2021, 198, 111449  DOI: 10.1016/j.colsurfb.2020.111449
      16
      Role of interfacial water in determining the interactions of proteins and cells with hydrated materials
      Tanaka, Masaru; Morita, Shigeaki; Hayashi, Tomohiro
      Colloids and Surfaces, B: Biointerfaces (2021), 198 (), 111449CODEN: CSBBEQ; ISSN:0927-7765. (Elsevier B.V.)
      A review. Water mols. play a crucial role in biointerfacial interactions, including protein adsorption and desorption. To understand the role of water in the interaction of proteins and cells at biol. interfaces, it is important to compare particular states of hydration water with various physicochem. properties of hydrated biomaterials. In this review, we discuss the fundamental concepts for detg. the interactions of proteins and cells with hydrated materials along with selected examples corresponding to our recent studies, including poly(2-methoxyethyl acrylate) (PMEA), PMEA derivs., and other biomaterials. The states of water were analyzed by differential scanning calorimetry, in situ attenuated total reflection IR spectroscopy, and surface force measurements. We found that intermediate water which is loosely bound to a biomaterial, is a useful indicator of the bioinertness of material surfaces. This finding on intermediate water provides novel insights and helps develop novel exptl. models for understanding protein adsorption in a wide range of materials, such as those used in biomedical applications.
    17. 17
      Mochizuki, A.; Hatakeyama, T.; Tomono, Y.; Tanaka, M. Water Structure and Blood Compatibility of Poly(Tetrahydrofurfuryl Acrylate). J. Biomater. Sci. Polym. Ed. 2009, 20, 591– 603,  DOI: 10.1163/156856209X426411
      17
      Water structure and blood compatibility of poly(tetrahydrofurfuryl acrylate)
      Mochizuki, Akira; Hatakeyama, Tatsuko; Tomono, Yuka; Tanaka, Masaru
      Journal of Biomaterials Science, Polymer Edition (2009), 20 (5-6), 591-603CODEN: JBSEEA; ISSN:0920-5063. (VSP)
      We previously reported that poly(2-methoxyethyl acrylate) (PMEA), which has excellent blood compatibility, contains a large amt. of freezing bound water. In order to confirm the role of freezing bound water in detg. blood compatibility, poly(tetrahydrofurfuryl acrylate) (PTHFA), was newly synthesized and the thermal properties of water in PTHFA were investigated by differential scanning calorimetry (DSC), as freezing bound water was obsd. as cold crystn. in DSC heating curves. In addn., the blood compatibility of PTHFA, including activations of platelets, the coagulation system and the complement system, was investigated. The temp. of cold crystn. of water in PTHFA was higher than that of water in PMEA; moreover, the amt. of freezing bound water in PTHFA was smaller than that in PMEA. The effect of freezing bound water on blood compatibility was investigated by comparing PTHFA, PMEA, poly(2-hydroxyethyl methacrylate) (PHEMA) and poly(2-methoxyethyl methacrylate) (PMEMA). The latter two samples showed no cold crystn. Activations of platelets, the coagulation system and the complement system were enhanced in the following order: PMEA < PHEMA < PTHFA < PMEMA, PMEA < PMEMA < PTHFA < PHEMA and PMEA < PTHFA < PMEMA < PHEMA, resp. The above results were reasonably explained by the amt. and/or the stability of freezing bound water.
    18. 18
      Hatakeyama, T.; Tanaka, M.; Hatakeyama, H. Studies on Bound Water Restrained by Poly(2-Methacryloyloxyethyl Phosphorylcholine): Comparison with Polysaccharide-Water Systems. Acta Biomater. 2010, 6, 2077– 2082,  DOI: 10.1016/j.actbio.2009.12.018
      18
      Studies on bound water restrained by poly(2-methacryloyloxyethyl phosphorylcholine): comparison with polysaccharide-water systems
      Hatakeyama, T.; Tanaka, M.; Hatakeyama, H.
      Acta Biomaterialia (2010), 6 (6), 2077-2082CODEN: ABCICB; ISSN:1742-7061. (Elsevier Ltd.)
      The structural change of water restrained by poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) was investigated by differential scanning calorimetry (DSC), since the biocompatibility of PMPC and related biopolymers is affected by the structure of water on the polymer surface. The phase transition behavior of PMPC-water systems with a water content (Wc = mass of water/mass of dry sample, g g-1) in the range 0-2.0 was measured in the temp. range -150 to 50 °C. Glass transition, cold crystn. and melting were obsd. Cold crystn., which has been suggested as an index of biocompatibility, was detected for PMPC with a Wc in the range 0.5-0.9. The amts. of two types of bound water, non-freezing water and freezing bound water, were calcd. from the melting enthalpy. The amt. of non-freezing water of PMPC was ∼0.48. It was found that the phase transition behavior and amt. of bound water of PMPC were quite similar to those of water-sol. polysaccharide electrolytes. The results indicate that the bound water, not the free water, is restrained by PMPC.
    19. 19
      Tanaka, M.; Hayashi, T.; Morita, S. The Roles of Water Molecules at the Biointerface of Medical Polymers. Polym. J. 2013, 45, 701– 710,  DOI: 10.1038/pj.2012.229
      19
      The roles of water molecules at the biointerface of medical polymers
      Tanaka, Masaru; Hayashi, Tomohiro; Morita, Shigeaki
      Polymer Journal (Tokyo, Japan) (2013), 45 (7), 701-710CODEN: POLJB8; ISSN:0032-3896. (NPG Nature Asia-Pacific)
      A review. A no. of materials have been proposed for use as biomaterials, including hydrophilic, phase-sepd. and zwitterionic polymers. The mechanisms responsible for the bio/blood compatibility (bioinertness) of these polymers at the mol. level have not been clearly demonstrated, although many theor. and exptl. efforts have been made to understand these mechanisms. Water interactions have been recognized as fundamental for the biol. response to contact with biomaterials. We have proposed the intermediate water' concept, in which water clearly exhibits defined peaks for cold crystn. in the differential scanning calorimetry chart and presents a strong peak at 3400cm-1 in a time-resolved IR spectrum. We found a localized hydration structure consisting of three hydrated waters in poly(2-methoxyethyl acrylate). We hypothesized that intermediate water, which prevents the proteins and blood cells from directly contacting the polymer surface, or non-freezing water on the polymer surface has an important role in the bio/blood compatibility of polymers. We will provide an overview of the recent exptl. progress and a theor. description of the bio/blood compatibility mechanisms as detd. by thermal, spectroscopic and surface force measurements.
    20. 20
      Hayashi, T. Water at Interfaces: Its Behavior and Roles in Interfacial Phenomena. Chem. Lett. 2021, 50, 1173– 1180,  DOI: 10.1246/cl.210049
      20
      Water at Interfaces: Its Behavior and Roles in Interfacial Phenomena
      Hayashi, Tomohiro
      Chemistry Letters (2021), 50 (6), 1173-1180CODEN: CMLTAG; ISSN:0366-7022. (Chemical Society of Japan)
      A review. Water at interfaces plays essential roles in many natural phenomena and engineering applications. However, the mol. behavior of interfacial water is still a matter of intense debate. Thus far, many exptl. and theor. methods have been employed to elucidate the real picture (structure, dynamics, hydrogen bonding states, etc.) of interfacial water and its relevance to interfacial phenomena and material functions. The author reviews these works and discusses the principles and limitations of the anal. methods. The significant representative findings on water-mediated interfacial phenomena are also introduced.
    21. 21
      Chang, R.; Asatyas, S.; Lkhamsuren, G.; Hirohara, M.; Mondarte, E. A. Q.; Suthiwanich, K.; Sekine, T.; Hayashi, T. Water near Bioinert Self-Assembled Monolayers. Polym. J. 2018, 50, 563– 571,  DOI: 10.1038/s41428-018-0075-1
      21
      Water near bioinert self-assembled monolayers
      Chang, Ryongsok; Asatyas, Syifa; Lkhamsuren, Ganchimeg; Hirohara, Makoto; Mondarte, Evan Angelo Quimada; Suthiwanich, Kasinan; Sekine, Taito; Hayashi, Tomohiro
      Polymer Journal (Tokyo, Japan) (2018), 50 (8), 563-571CODEN: POLJB8; ISSN:0032-3896. (Nature Research)
      A review. For a long time, water has been speculated to play an essential role in the interactions of proteins and cells with artificial biocompatible materials. The current question is how water mols. at the interfaces affect the adsorption of proteins and the adhesion of cells. To answer this question, we introduce recent works that investigated the interfacial behavior of water near self-assembled monolayers (SAMs) by different types of anal. techniques. By combining these findings, we discuss how interfacial water affects the protein and cell resistance of various bioinert SAMs.
    22. 22
      Gupta, A.; Kaur, S.; Kashyap, H. K. How Water Permutes the Structural Organization and Microscopic Dynamics of Cholinium Glycinate Biocompatible Ionic Liquid. J. Phys. Chem. B 2019, 123, 2057– 2069,  DOI: 10.1021/acs.jpcb.8b10235
      22
      How Water Permutes the Structural Organization and Microscopic Dynamics of Cholinium Glycinate Biocompatible Ionic Liquid
      Gupta, Aditya; Kaur, Supreet; Kashyap, Hemant K.
      Journal of Physical Chemistry B (2019), 123 (9), 2057-2069CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
      The authors study the structural organization and microscopic dynamics of aq. cholinium glycinate ([Ch][Gly]), a biocompatible ionic liq. (IL), by employing all-atom mol. dynamics simulations. Herein, the authors observe the effect of H2O content on the mol.-level arrangement of ions in the IL-H2O mixt. through simulated x-ray scattering structure function, their partial components, and real-space correlation functions. The study reveals a principal peak in the total structure function of the neat [Ch][Gly] IL at around q = 1.4 Å-1. The corresponding correlation tends to decrease and shifts toward shorter length scales with increase in the H2O content. The principal peak mainly originates from the correlations between counterions. Hydrogen bond anal. reveals that H2O mols. compete with the anions to form hydrogen bond with the hydroxyl hydrogen of cation. Concomitantly, strong hydrogen bonding is also obsd. between [Gly]- anion and H2O, which depreciates with the increasing hydration level. Hydrogen-bond autocorrelation function anal. manifests that av. lifetimes of different possible hydrogen bonds decrease with increase in mole fraction of H2O. The mobilities of the ions are also significantly affected by H2O, showing a nonlinear increase with the increasing H2O content. The [Gly]- anion is found to show faster dynamics on the addn. of H2O as compared to [Ch]+ cation.
    23. 23
      Yaghini, N.; Nordstierna, L.; Martinelli, A. Effect of Water on the Transport Properties of Protic and Aprotic Imidazolium Ionic Liquids – an Analysis of Self-Diffusivity, Conductivity, and Proton Exchange Mechanism. Phys. Chem. Chem. Phys. 2014, 16, 9266– 9275,  DOI: 10.1039/C4CP00527A
      23
      Effect of water on the transport properties of protic and aprotic imidazolium ionic liquids - an analysis of self-diffusivity, conductivity, and proton exchange mechanism
      Yaghini, N.; Nordstierna, L.; Martinelli, A.
      Physical Chemistry Chemical Physics (2014), 16 (20), 9266-9275CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      In this paper we report on the transport properties of protic and aprotic ionic liqs. of the imidazolium cation (C2C1Im+ or C2HIm+) and the TFSI- or TfO- anion as a function of added water. We observe that the self-diffusion coeff. of the ionic species increases upon addn. of water, and that the cation diffuses faster than the anion in the entire water concn. range investigated. We also observe that the overall increase of anionic and cationic diffusion coeffs. is significant for C2HImTfO while it is rather weak for C2C1ImTFSI, the former being more hydrophilic. Moreover, the difference between cationic and anionic self-diffusivity specifically depends on the structure of the ionic liq.'s ions. The degree of ion-ion assocn. has been investigated by comparing the molar cond. obtained by impedance measurements with the molar cond. calcd. from NMR data using the Nernst-Einstein equation. Our data indicate that the ions are partly dissocd. (Λimp/ΛNMR in the range 0.45-0.75) but also that the degree of assocn. decreases in the order C2HImTfO > C2HImTFSI ≈ C2C1ImTfO > C2C1ImTFSI. From these results, it seems that water finds different sites of interaction in the protic and aprotic ionic liqs., with a strong preference for hydrogen bonding to the -NH group (when available) and a stronger affinity to the TfO anion as compared to the TFSI. For the protic ionic liqs., the anal. of 1H NMR chem. shifts (upon addn. of H2O and D2O, resp.) indicates a water-cation interaction of hydrogen bonding nature. In addn., we could probe proton exchange between the -NH group and deuterated water for the protic cation, which occurs at a significantly faster rate if assocd. with the TfO anion as compared to the TFSI.
    24. 24
      Nikawa, Y.; Fujita, K.; Ohno, H. Quantitative Assessment of Kosmotropicity of Hydrated Ionic Liquids by Nuclear Magnetic Resonance. Phys. Chem. Chem. Phys. 2017, 19, 8148– 8151,  DOI: 10.1039/C6CP07463D
      24
      Quantitative assessment of kosmotropicity of hydrated ionic liquids by nuclear magnetic resonance
      Nikawa, Yohsuke; Fujita, Kyoko; Ohno, Hiroyuki
      Physical Chemistry Chemical Physics (2017), 19 (12), 8148-8151CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      NMR studies revealed that the chem. shift of H2O in hydrated ionic liqs. varied with their component ions. The variation reflected the formation of hydrogen bonding networks between ions and water mols. The chem. shift relative to bulk water was used to quant. assess the kosmotropicity.
    25. 25
      Kaneko, K.; Saihara, K.; Masuda, Y.; Yoshimura, Y.; Shimizu, A. Dynamic Properties of Water Molecules in Ionic Liquid/Water Mixture with Various Alkyl Chain Length. J. Mol. Liq. 2018, 264, 337– 342,  DOI: 10.1016/j.molliq.2018.05.043
      25
      Dynamic properties of water molecules in ionic liquid/water mixture with various alkyl chain length
      Kaneko, Kazuyoshi; Saihara, Koji; Masuda, Yuichi; Yoshimura, Yukihiro; Shimizu, Akio
      Journal of Molecular Liquids (2018), 264 (), 337-342CODEN: JMLIDT; ISSN:0167-7322. (Elsevier B.V.)
      This work investigated the alkyl chain length dependence of dynamic properties of water mols. in imidazolium-based ionic liq. (IL)/water mixts. The microscopic (i.e., self-diffusion coeffs. of water mol. and ILs) properties of these systems suggested that the movement of IL/water mixts. decreases with increasing alkyl chain length of the IL. In contrast, the self-diffusion coeff. ratios (Dwater/Danion or Dwater/Dcation) at a water concn. extrapolated to 0 mol% (i.e., the value of a pure IL) increased as the alkyl chain length of the IL increased. This finding indicates that the water mols. in the IL/water mixts. move more independent of the IL as the alkyl chain length of the IL increases. Based on these results, we propose a plausible model for water mols. in a space within the IL.
    26. 26
      Han, Q.; Wang, X.; Bynre, N. Utilizing Water Activity as a Simple Measure to Understand Hydrophobicity in Ionic Liquids. Frontiers in Chemistry 2019, 7, 112,  DOI: 10.3389/FCHEM.2019.00112
      26
      Utilizing water activity as a simple measure to understand hydrophobicity in ionic liquids
      Han, Qi; Wang, Xungai; Bynre, Nolene
      Frontiers in Chemistry (Lausanne, Switzerland) (2019), 7 (), 112CODEN: FCLSAA; ISSN:2296-2646. (Frontiers Media S.A.)
      Ionic liqs. (ILs) are regarded as designable solvents finding use in a variety of applications. One of the challenges of the design and selection process is to understand the ionic liq. properties. In this work, we selected seven ILs contg. three types of hydrophilic anions and examd. several key properties, which are correlated to hydrophobicity. In particular, we measured the hydrogen bond basicity β and water activity aw of IL and IL-water mixts., and suggested that these two properties are linearly correlated particularly in hydrated ILs. We then used NMR to evaluate the chem. shift of H2O in hydrated ILs. Correlating the outcomes of each of these techniques with respect to understanding the hydrophobicity of the ILs is discussed. It is shown that water activity aw is the most facile technique to represent and understand hydrophobicity of ILs.
    27. 27
      Voss, J. M.; Marsh, B. M.; Zhou, J.; Garand, E. Interaction between Ionic Liquid Cation and Water: Infrared Predissociation Study of [Bmim]+·(H2O)N Clusters. Phys. Chem. Chem. Phys. 2016, 18, 18905– 18913,  DOI: 10.1039/c6cp02730j
      27
      Interaction between ionic liquid cation and water: infrared predissociation study of [bmim]+·(H2O)n clusters
      Voss, Jonathan M.; Marsh, Brett M.; Zhou, Jia; Garand, Etienne
      Physical Chemistry Chemical Physics (2016), 18 (28), 18905-18913CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The IR predissocn. spectra of [bmim]+·(H2O)n, n = 1-8, in the 2800-3800 cm-1 region are presented and analyzed with the help of electronic structure calcns. The water mols. solvate [bmim]+ by predominately interacting with the imidazolium C2-H moiety for the small n = 1 and 2 clusters. This is characterized by a red shifted and relatively intense C(2)-H stretch. For n ≥ 4 clusters, hydrogen-bond interactions between the water mols. drive the formation of ring isomers which interact on top of the imidazolium ring without any direct interaction with the C2-H. The water arrangement in [bmim]+·(H2O)n is similar to the low energy isomers of neutral water clusters up to the n = 6 cluster. This is not the case for the n = 8 cluster, which has the imidazolium ring disrupting the otherwise preferred cubic water structure. The evolution of the solvation network around [bmim]+ illustrates the competing [bmim]+-water and water-water interactions.
    28. 28
      Kohno, Y.; Ohno, H. Ionic Liquid/Water Mixtures: From Hostility to Conciliation. Chem. Commun. 2012, 48, 7119– 7130,  DOI: 10.1039/C2CC31638B
      28
      Ionic liquid/water mixtures: from hostility to conciliation
      Kohno, Yuki; Ohno, Hiroyuki
      Chemical Communications (Cambridge, United Kingdom) (2012), 48 (57), 7119-7130CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      A review. Water was originally inimical to ionic liqs. (ILs) esp. in the anal. of their detailed properties. Various data on the properties of ILs indicate that there are two ways to design functions of ionic liqs. The first is to change the structure of component ions, to provide "task-specific ILs". The second is to mix ILs with other components, such as other ILs, org. solvents or water. Mixing makes it easy to control the properties of the soln. In this strategy, water is now a very important partner. Below, the authors summarize recent results on the properties of IL/water mixts. Stable phase sepn. is an effective method in some sepn. processes. Conversely, a dynamic phase change between a homogeneous mixt. and sepn. of phases is important in many fields. Anal. of the relation between phase behavior and the hydration state of the component ions indicates that the pattern of phase sepn. is governed by the hydrophilicity of the ions. Sufficiently hydrophilic ions yielded ILs that are miscible with water, and hydrophobic ions gave stable phase sepn. with water. ILs composed of hydrophobic but hydrated ions undergo a dynamic phase change between a homogeneous mixt. and sep. phases according to temp. ILs having more than seven water mols. per ion pair undergo this phase transition. These dynamic phase changes are considered, with some examples, and application is made to the sepn. of water-sol. proteins.
    29. 29
      Fujita, K.; Nakano, R.; Nakaba, R.; Nakamura, N.; Ohno, H. Hydrated Ionic Liquids Enable Both Solubilisation and Refolding of Aggregated Concanavalin A. Chem. Commun. 2019, 55, 3578– 3581,  DOI: 10.1039/c8cc10102g
      29
      Hydrated ionic liquids enable both solubilisation and refolding of aggregated concanavalin A
      Fujita, Kyoko; Nakano, Roka; Nakaba, Risa; Nakamura, Nobuhumi; Ohno, Hiroyuki
      Chemical Communications (Cambridge, United Kingdom) (2019), 55 (25), 3578-3581CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      The dissoln. and refolding of aggregated Con A have been achieved, in hydrated ionic liqs. contg. a limited no. of water mols. Both ammonium and phosphonium salts were examd. to find a suitable hydrophobicity of ions and water content for refolding. Recovery of sugar recognition was confirmed as a proof of refolding.
    30. 30
      Fujita, K.; MacFarlane, D. R.; Forsyth, M.; Yoshizawa-Fujita, M.; Murata, K.; Nakamura, N.; Ohno, H. Solubility and Stability of Cytochrome c in Hydrated Ionic Liquids: Effect of Oxo Acid Residues and Kosmotropicity. Biomacromolecules 2007, 8, 2080– 2086,  DOI: 10.1021/bm070041o
      30
      Solubility and Stability of Cytochrome c in Hydrated Ionic Liquids: Effect of Oxo Acid Residues and Kosmotropicity
      Fujita, Kyoko; MacFarlane, Douglas R.; Forsyth, Maria; Yoshizawa-Fujita, Masahiro; Murata, Kenichi; Nakamura, Nobuhumi; Ohno, Hiroyuki
      Biomacromolecules (2007), 8 (7), 2080-2086CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)
      Hydrated ionic liqs. (ILs) were prepd. by adding appropriate amts. of water to hydrophilic ILs. Some hydrated ILs show excellent solubilizing ability for proteins, keeping the basic properties of ILs. The soly. of cytochrome c (cyt c) depended on the structure of the component ions. When component anions have oxo acid residues, the resulting hydrated ILs solubilize cyt c quite well. In such hydrated ILs, the structure and activity of cyt c is influenced by the kosmotropicity of the component ions. We synthesized ILs from various ions having different kosmotropicity, including dihydrogen phosphate (dhp), dibutylphosphate, acetate, lactate, and methanesulfonate as anions. The activity of the dissolved cyt c depends on the permutations of kosmotropicity of the component ions. Cyt c shows no structural change and retains its activity when dissolved in the hydrated choline dhp, which is an excellent combination of chaotropic cation and kosmotropic anion. Furthermore, cyt c dissolved in the hydrated choline dhp remained in a native state and was active after 18 mo of storage at room temp.
    31. 31
      de Souza, Í. F. T.; Paschoal, V. H.; Bernardino, K.; Lima, T. A.; Daemen, L. L.; Z, Y.; Ribeiro, M. C. C. Vibrational Spectroscopy and Molecular Dynamics Simulation of Choline Oxyanions Salts. J. Mol. Liq. 2021, 340, 117100  DOI: 10.1016/j.molliq.2021.117100
      31
      Vibrational spectroscopy and molecular dynamics simulation of choline oxyanions salts
      de Souza, Icaro F. T.; Paschoal, Vitor H.; Bernardino, Kalil; Lima, Thamires A.; Daemen, Luke L.; Z, Y.; Ribeiro, Mauro C. C.
      Journal of Molecular Liquids (2021), 340 (), 117100CODEN: JMLIDT; ISSN:0167-7322. (Elsevier B.V.)
      The structure of choline salts contg. the anions acetate, [Chol][Ac], and dihydrogen phosphate, [Chol][DHP], were investigated by IR, Raman, and inelastic neutron scattering (INS). The chosen systems allow for the comparison of structural effects related to the bond acceptor characteristic of [Ac] and the simultaneous acceptor and donor characteristics of [DHP] in forming hydrogen bonds (H-bond) in salts of [Chol], which is itself prone to forming H-bonds. Different computational tools were used for the anal. of different spectral ranges. The calcn. of the low-frequency range of Raman and INS spectra of the cryst. phases at low-temps. by solid state DFPT (d. functional perturbation theory) unveils the coupling between vibrations of the H-bonds and intramol. modes. Changes obsd. in the spectral pattern of lattice and [DHP] modes upon heating cryst. [Chol][DHP] are analogous to the ferroelec.-paraelec. phase transition known in the potassium salt of [DHP]. The fingerprint region of the vibrational spectra provides information concerning the [Chol] conformation in the solid phase (gauche in [Chol][Ac] and anti in [Chol][DHP]) and in aq. soln. DFT calcns. of ionic pairs and ionic clusters unveil the interplay between [Chol] conformation and the [DHP] ability to form H-bonded dimers of anions. The high-frequency spectral range and the structures driven by H-bonds are discussed using classical mol. dynamics (MD) simulations. The MD simulations of aq. solns. highlight the strong anion-cation H-bond in [Chol][Ac], in contrast to the strong anion-anion H-bond in [Chol][DHP] due to occurrence of dimers and larger clusters of [DHP].
    32. 32
      Sun, Q. The Raman OH Stretching Bands of Liquid Water. Vib. Spectrosc. 2009, 51, 213– 217,  DOI: 10.1016/j.vibspec.2009.05.002
      32
      The Raman OH stretching bands of liquid water
      Sun, Qiang
      Vibrational Spectroscopy (2009), 51 (2), 213-217CODEN: VISPEK; ISSN:0924-2031. (Elsevier B.V.)
      From the discussion on water structure and clusters, it can be deduced that the OH stretching vibration is closely related to local H-bonded network for a water mol., and different OH vibrations can be assigned to OH groups engaged in various H-bonding. At ambient condition, the main local H-bonding for a mol. can be classified as DDAA (double donor-double acceptor), DDA (double donor-single acceptor), DAA (single donor-double acceptor) and DA (single donor-single acceptor) and free OH vibrations. As for water at 290 K and 0.1 MPa pressure, the OH stretching region of the Raman spectrum can be deconvoluted into 5 sub-bands, which are located at 3014, 3226, 3432, 3572, and 3636 cm-1, and can be assigned to νDAA-OH, νDDAA-OH, νDA-OH, νDDA-OH, and free OH2 sym. stretching vibrations, resp.
    33. 33
      Schmidt, D. A.; Miki, K. Structural Correlations in Liquid Water: A New Interpretation of IR Spectroscopy. J. Phys. Chem. A 2007, 111, 10119– 10122,  DOI: 10.1021/JP074737N
      33
      Structural Correlations in Liquid Water: A New Interpretation of IR Spectroscopy
      Schmidt, Diedrich A.; Miki, Kazushi
      Journal of Physical Chemistry A (2007), 111 (40), 10119-10122CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      The authors present a new and alternative interpretation of the structure of the IR vibrational mode (ν(OH) band) of pure H2O. The re-interpretation is based on the influence of the cooperative H bonding arising from a network of H bonds in the liq. The ν(OH) band has six components that are dominated by differences in their O-H bond lengths but deviate from thermodynamically av. values due to interactions with the H bond network. The phys. origin of the structure in the ν(OH) band is directly related to the O-H bond length, and variations in this bond length are caused by the influence of the surrounding H-bonded network of H2O mols.
    34. 34
      Arunan, E.; Desiraju, G. R.; Klein, R. A.; Sadlej, J.; Scheiner, S.; Alkorta, I.; Clary, D. C.; Crabtree, R. H.; Dannenberg, J. J.; Hobza, P.; Kjaergaard, H. G.; Legon, A. C.; Mennucci, B.; Nesbitt, D. J. Definition of the Hydrogen Bond (IUPAC Recommendations 2011). Pure Appl. Chem. 2011, 83, 1637– 1641,  DOI: 10.1351/PAC-REC-10-01-02
      34
      Definition of the hydrogen bond (IUPAC Recommendations 2011)
      Arunan, Elangannan; Desiraju, Gautam R.; Klein, Roger A.; Sadlej, Joanna; Scheiner, Steve; Alkorta, Ibon; Clary, David C.; Crabtree, Robert H.; Dannenberg, Joseph J.; Hobza, Pavel; Kjaergaard, Henrik G.; Legon, Anthony C.; Mennucci, Benedetta; Nesbitt, David J.
      Pure and Applied Chemistry (2011), 83 (8), 1637-1641CODEN: PACHAS; ISSN:0033-4545. (International Union of Pure and Applied Chemistry)
      A novel definition for the hydrogen bond is recommended here. It takes into account the theor. and exptl. knowledge acquired over the past century. This definition insists on some evidence. Six criteria are listed that could be used as evidence for the presence of a hydrogen bond.
    35. 35
      Kitadai, N.; Sawai, T.; Tonoue, R.; Nakashima, S.; Katsura, M.; Fukushi, K. Effects of Ions on the OH Stretching Band of Water as Revealed by ATR-IR Spectroscopy. J. Solution Chem. 2014, 43, 1055– 1077,  DOI: 10.1007/s10953-014-0193-0
      35
      Effects of Ions on the OH Stretching Band of Water as Revealed by ATR-IR Spectroscopy
      Kitadai, Norio; Sawai, Takashi; Tonoue, Ryota; Nakashima, Satoru; Katsura, Makoto; Fukushi, Keisuke
      Journal of Solution Chemistry (2014), 43 (6), 1055-1077CODEN: JSLCAG; ISSN:0095-9782. (Springer)
      The effects of various cations (Li+, Na+, K+, Rb+, Cs+, Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, and Ni2+) and anions (Cl-, Br-, I-, NO-3, ClO-4, HCO-3, and CO2-3) on the molar absorptivity of water in the OH stretching band region (2,600-3,800 cm-1) were ascertained from attenuated total reflection IR spectra of aq. electrolyte solns. (22 in all). The OH stretching band mainly changes linearly with ion concns. up to 2 mol·L-1, but several specific combinations of cations and anions (Cs2SO4, Li2SO4, and MgSO4) present different trends. That deviation is attributed to ion pair formation and cooperativity in ion hydration, which indicates that the extent of the ion-water interaction reflected by the OH stretching band of water is beyond the first solvation shell of water mols. directly surrounding the ion. The obtained dataset was then correlated with several quant. parameters representing structural and dynamic properties of water mols. around ions: ΔGHB, the structural entropy (Sstr), the viscosity B-coeff. (Bη), and the ionic B-coeff. of NMR relaxation (BNMR). Results show that modification of the OH stretching band of water caused by ions has quasi-linear relations with all of these parameters. Vibrational spectroscopy can be a useful means for evaluating ion-water interaction in aq. solns.
    36. 36
      Schmidt, D. A.; Miki, K. Defective Continuous Hydrogen-Bond Networks: An Alternative Interpretation of IR Spectroscopy. Chemphyschem 2008, 9, 1914– 1919,  DOI: 10.1002/CPHC.200800236
      36
      Defective continuous hydrogen-bond networks: an alternative interpretation of IR spectroscopy
      Schmidt, Diedrich A.; Miki, Kazushi
      ChemPhysChem (2008), 9 (13), 1914-1919CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The authors apply previously developed deconvolution method and interpretation to analyze changes in the OH stretching band [ν(OH) band] of low-concn. (≤0.2 m) aq. solns. of NaCl and KCl. The authors treat these simple, monovalent ions as defects in the H-bond network of pure H2O and quantify the changes in the spectra at low defect concn. with an order parameter. Order-parameter anal. of difference spectra of the 2 solns. leads to hydration nos. of 7.0 ± 1.0 and 5.9 ± 0.3 for K+ and Na+, resp. Addnl., changes in the ν(OH) band due to low concns. of ions result from changes in the topol. of the H-bond network.
    37. 37
      Cammarata, L.; Kazarian, S. G.; Salter, P. A.; Welton, T. Molecular States of Water in Room Temperature Ionic Liquids. Phys. Chem. Chem. Phys. 2001, 3, 5192– 5200,  DOI: 10.1039/b106900d
      37
      Molecular states of water in room temperature ionic liquids
      Cammarata, L.; Kazarian, S. G.; Salter, P. A.; Welton, T.
      Physical Chemistry Chemical Physics (2001), 3 (23), 5192-5200CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      ATR and transmission IR spectroscopy have been used to investigate the state of water in room temp. ionic liqs. (RTILs) based on the 1-alkyl-3-methylimidazolium cation with the anions: [PF6]-, [SbF6]-, [BF4]-, [ClO4]-, [CF3SO3]-, [(CF3SO2)2N]-, [NO3]- and [CF3CO2]-. It has been shown that in these RTILs water mols. absorbed from the air are present mostly in the "free" (not self-assocd.) state, bound via H-bonding with [PF6]-, [BF4]-, [SbF6]-, [ClO4]-, [CF3SO3]-, [(CF3SO2)2N]- with the concns. of dissolved water in the range 0.2-1.0 mol dm-3. It has been concluded that most of the water mols. at these concns. exist in sym. 1 : 2 type H-bonded complexes: anion...HOH...anion. Addnl. evidence that the preferred sites of interaction with water mols. are the anions has been obtained from the expts. with RTILs of the 1-butyl-2,3-dimethylimidazolium and 1-butyl-2,3,4,5-tetramethylimidazolium cations. Water mols. can also form assocd. liq.-like formations in RTILs with anions of stronger basicity such as [NO3]- and [CF3CO2]-. When these RTILs are exposed to air the water concns. exceed 1.0 mol dm-3. The strength of H-bonding between water mols. and anions increases in the order [PF6]- < [SbF6]- < [BF4]- < [(CF3SO2)2N]- < [ClO4]- < [CF3SO3]- < [NO3]- < [CF3CO2]-. The energies of this H-bonding were estd. from spectral shifts, with the resulting enthalpies being in the range 8-13 kJ mol-1. ATR-IR spectroscopy has also been used to study H-bonding between methanol and RTILs.
    38. 38
      Ohno, Hiroyuki.; Fujita, Kyoko.; Kohno, Yuki. Is Seven the Minimum Number of Water Molecules per Ion Pair for Assured Biological Activity in Ionic Liquid–Water Mixtures?. Phys. Chem. Chem. Phys. 2015, 17, 14454– 14460,  DOI: 10.1039/C5CP00768B
      38
      Is seven the minimum number of water molecules per ion pair for assured biological activity in ionic liquid-water mixtures?
      Ohno, Hiroyuki; Fujita, Kyoko; Kohno, Yuki
      Physical Chemistry Chemical Physics (2015), 17 (22), 14454-14460CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      A review. Ionic liqs. (ILs) contg. small amts. of water are called hydrated ILs and they show diverse physico-chem. properties that are strongly dependent on their water content. Some properties of hydrated ILs, such as biol. activity and phase transition behavior, were found to change non-linearly, with an inflection at a water mol. to ion pair ratio of around 7:1. This crit. hydration no. of ILs has been discussed in this paper with respect to the state of solvated water mols.
    39. 39
      Woutersen, S.; Emmerichs, U.; Bakker, H. J. Femtosecond Mid-IR Pump-Probe Spectroscopy of Liquid Water: Evidence for a Two-Component Structure. Science 1997, 278, 658– 660,  DOI: 10.1126/science.278.5338.658
      39
      Femtosecond mid-IR pump-probe spectroscopy of liquid water: evidence for a two-component structure
      Woutersen, S.; Emmerichs, U.; Bakker, H. J.
      Science (Washington, D. C.) (1997), 278 (5338), 658-660CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      A femtosecond mid-IR pump-probe study of the vibrational and orientational dynamics of the OH-stretching mode of HDO dissolved in D2O is presented. The orientational relaxation of the HDO mols. occurs on either a very slow or a very fast time scale, with assocd. time consts. of τR = 13 ps and τR = 0.7 ps. Strongly H-bonded H2O mols. only relax through the slow orientational relaxation process, whereas the fast process dominates for weakly H-bonded mols. Probably with respect to orientational dynamics, two distinct mol. species exist in liq. H2O.
    40. 40
      Bharmoria, P.; Gupta, H.; Mohandas, V. P.; Ghosh, P. K.; Kumar, A. Temperature Invariance of NaCl Solubility in Water: Inferences from Salt-Water Cluster Behavior of NaCl, KCl, and NH4Cl. J. Phys. Chem. B 2012, 116, 11712– 11719,  DOI: 10.1021/jp307261g
      40
      Temperature Invariance of NaCl Solubility in Water: Inferences from Salt-Water Cluster Behavior of NaCl, KCl, and NH4Cl
      Bharmoria, Pankaj; Gupta, Hariom; Mohandas, V. P.; Ghosh, Pushpito K.; Kumar, Arvind
      Journal of Physical Chemistry B (2012), 116 (38), 11712-11719CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
      The growth and stability of salt-water clusters were exptl. studied in aq. solns. of NaCl, KCl, and NH4Cl from dil. to near-satn. conditions employing dynamic light scattering and zeta potential measurements. In order to examine cluster stability, the changes in the cluster sizes were monitored as a function of temp. Compared to the other cases, the av. size of NaCl-water clusters remained almost const. over the studied temp. range of 20-70°. Information obtained from the temp.-dependent soln. compressibility (detd. from speed of sound and d. measurements), multinuclear NMR (1H, 17O, 35Cl NMR), and FTIR were utilized to explain the cluster behavior. Comparison of NMR chem. shifts of satd. salt solns. with solid-state NMR data of pure salts, and evaluation of spectral modifications in the OH stretch region of satd. salt solns. as compared to that of pure water, provided important clues on ion pair-water interactions and water structure in the clusters. The high stability and temp. independence of the cluster sizes in aq. NaCl shed light on the temp. invariance of its soly.
    41. 41
      Riemenschneider, J.; Holzmann, J.; Ludwig, R. Salt Effects on the Structure of Water Probed by Attenuated Total Reflection Infrared Spectroscopy and Molecular Dynamics Simulations. Chemphyschem 2008, 9, 2731– 2736,  DOI: 10.1002/cphc.200800571
      41
      Salt effects on the structure of water probed by attenuated total reflection infrared spectroscopy and molecular dynamics simulations
      Riemenschneider, Julian; Holzmann, Joerg; Ludwig, Ralf
      ChemPhysChem (2008), 9 (18), 2731-2736CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
      We study aq. solns. of alk. chlorides (NaCl, KCl, RbCl and CsCl) with a combination of attenuated total reflection IR (ATR-IR) spectroscopy and mol. dynamics (MD) simulations using the TlP4P-Ew water model, covering concn. ranges between 0.1 and 6 M. Spectral modifications in the OH stretch region are evaluated and correlated to the various salts and salt concns. By taking the difference spectra between the spectral line shapes of aq. salt solns. and those of pure water, we specifically focus on the small quasi-"free OH" band appearing at the highest wavenumbers in the spectra. This free-OH feature is found constantly at 3650 cm-1 for all salts and salt concns., but it shows a characteristic intensity depending on the chosen cation. In the order from Na+ to Cs+, the free OH intensity decreases compared to that of pure water. To interpret the exptl. results, we performed MD simulations for similar salt solns. The exptl. obsd. effects can be correlated with structural alterations indicated by differences between the site-site pair correlation functions of water in aq. salt solns. and those of pure water.
    42. 42
      Scipioni, R.; Schmidt, D. A.; Boero, M. A First Principles Investigation of Water Dipole Moment in a Defective Continuous Hydrogen Bond Network. J. Chem. Phys. 2009, 130, 024502  DOI: 10.1063/1.3054197
      42
      A first principles investigation of water dipole moment in a defective continuous hydrogen bond network
      Scipioni, Roberto; Schmidt, Diedrich A.; Boero, Mauro
      Journal of Chemical Physics (2009), 130 (2), 024502/1-024502/7CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      First principles mol. dynamics simulations of an aq. soln. salt system at finite concn. contg. both Na+ and Cl- ions show that a change in the distribution of the mol. dipole moment of H2O monomers appears when ions are present in soln. Simulations suggest a lowering of the dipole moments of the water mols. in the solvation shells of Na+ and Cl- as compared to the pure water case, while the dipoles of the rest of the mols. are hardly affected. However, finer anal. in terms of the Wannier centers distribution suggests a change in the electronic structure of the water mols. even in the bulk. Also a change of the H-bond network arrangement was found and correlation between dipole and MOH parameter evidences such subtle effects, suggesting a lowering of tetrahedral order in salty solns. All these changes can be related to observable quantities such as the IR spectra thus allowing for a rationalization of the exptl. outcome on neutral aq. solns. (c) 2009 American Institute of Physics.
    43. 43
      Akitt, J. W. Proton Chemical Shifts of Water in Cationic Hydration Complexes and Their Contribution to Water Shifts in Electrolyte Solutions. J. Chem. Soc. Dalton Trans. 1973, 42– 49,  DOI: 10.1039/DT9730000042
      43
      Proton chemical shifts of water in cationic hydration complexes and their contribution to water shifts in electrolyte solutions
      Akitt, J. W.
      Journal of the Chemical Society, Dalton Transactions: Inorganic Chemistry (1972-1999) (1973), (1), 42-9CODEN: JCDTBI; ISSN:0300-9246.
      The proton chem. shifts of H2O in hydration complexes of Sn4+, Al3+, Ga3+, In3+, Mg2+, and Be2+ were measured at low temps. The shifts were strongly downfield and correlated with shift increments calcd. to arise from the elec. field of the ion. The total cationic H2O shift contained 2 contributions; a downfield one from the elec. field and a smaller upfield one from a structural effect. The correlation was used to est. the proton shift by hydration H2O of the larger Group II and Group I cations where proton exchange is very fast. The contribution of cationic hydration to the molal shifts of a large variety of salt solns. was thus obtained, and comparison with measured molal shifts allowed estn. of the upfield anionic contribution which arises from solvent structure-breaking. A 3-site model was developed to explain the soln. shifts and previous measurements were shown to be consistent with a hydration no. of 6 for Mg2+, Ca2+, Sr2+, and Ba2+, 4 for the alkali metal ions, and 0 for Me4N+. SO42- is also probably tetrahydrated.
    44. 44
      Kitano, H.; Sudo, K.; Ichikawa, K.; Ide, M.; Ishihara, K. Raman Spectroscopic Study on the Structure of Water in Aqueous Polyelectrolyte Solutions. J. Phys. Chem. B 2000, 104, 11425– 11429,  DOI: 10.1021/jp000429c
      44
      Raman spectroscopic study on the structure of water in aqueous polyelectrolyte solutions
      Kitano, Hiromi; Sudo, Kurao; Ichikawa, Ken; Ide, Makoto; Ishihara, Kazuhiko
      Journal of Physical Chemistry B (2000), 104 (47), 11425-11429CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
      The structure and hydrogen bonding of water in various kinds of aq. polyelectrolyte solns. were analyzed with contours of O-H stretching of polarized Raman spectra. Effects of chem. properties of the polymers and water domains surrounded by the polymer chains on the relative intensity of collective band (C value) corresponding to a long-range coupling of O-H stretchings were discussed. The C values for various polymer solns. were almost const. in a relatively low mol. wt. (Mw) region, and decreased with an increase in Mw value. When the size of the space surrounded by the pseudo-network was sufficiently small, the structure of water in the space was altered to have a relatively lower av. no. of hydrogen bonds between water mols. than that of bulk water. The no. of hydrogen bonds collapsed by the presence of one monomer residue (N value) of polyelectrolyte (sodium polyethylenesulfonate, poly-L-lysine hydrobromide, etc.) with a small Mw was much larger than those for neutral polymers such as poly(ethylene glycol) and poly(N-vinylpyrrolidone). This result indicates that the monomer residues of water-sol. neutral polymers do not disturb the structure of water significantly, whereas electrostriction effect by the polyelectrolyte is quite effective on the structure of water. In contrast, the N value for poly(2-methacryloyloxyethyl phosphorylcholine) with a small Mw was nearly zero, suggesting that the zwitterionic-type monomer residues do not disturb the hydrogen bonding between water mols.
    45. 45
      Kitano, H.; Imai, M.; Sudo, K.; Ide, M. Hydrogen-Bonded Network Structure of Water in Aqueous Solution of Sulfobetaine Polymers. J. Phys. Chem. B 2002, 106, 11391– 11396,  DOI: 10.1021/jp020185r
      45
      Hydrogen-Bonded Network Structure of Water in Aqueous Solution of Sulfobetaine Polymers
      Kitano, Hiromi; Imai, Makoto; Sudo, Kurao; Ide, Makoto
      Journal of Physical Chemistry B (2002), 106 (43), 11391-11396CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
      Poly(N,N-dimethylaminopropyl methacrylamide) (poly(DMAPMA)) was incubated with 1,3-propanesultone and 1,4-butanesultone to afford polymers with various contents of N,N-dimethyl-N-(3-sulfopropyl)-3'-methacrylamidopropanaminium inner salt residues and N,N-dimethyl-N-(4-sulfobutyl)-3'-methacrylamidopropanaminium inner salt residues, resp. The structure and hydrogen bonding of water in an aq. soln. of the sulfobetaine polymers were analyzed using the contours of the O-H stretching in the polarized Raman spectra. With an increase in the content of the sulfobetaine residue, the relative intensity of the collective band (C value) corresponding to a long-range coupling of the O-H stretching in the aq. polymer solns. became larger and approached the C value of pure water. The no. of hydrogen bonds disrupted because of the presence of one monomer residue (N value) for the polymers with a large sulfobetaine content was a small pos. value and comparable to those for neutral polymers such as poly(ethylene glycol) and poly(N-vinylpyrrolidone). This is in significant contrast with the largely pos. N values for the precursor polymer (poly(DMAPMA)), and ordinary polyelectrolytes such as sodium polyethylenesulfonate, poly-L-lysine hydrobromide, sodium polyacrylate, and poly(acrylic acid). The N value for a small mol. wt. zwitterionic compd., 3-aminopropanesulfonic acid, was also slightly pos., which is consistent with the tendency obsd. for the sulfobetaine polymers. The present results clearly indicate that the zwitterionic polymers do not significantly disturb the hydrogen-bonded network structure of water, probably because of the counteraction of the electrostriction effect by the proximity between the anionic and cationic groups.
    46. 46
      Kitano, H.; Imai, M.; Mori, T.; Gemmei-Ide, M.; Yokoyama, Y.; Ishihara, K. Structure of Water in the Vicinity of Phospholipid Analogue Copolymers As Studied by Vibrational Spectroscopy. Langmuir 2003, 19, 10260– 10266,  DOI: 10.1021/la0349673
      46
      Structure of Water in the Vicinity of Phospholipid Analogue Copolymers As Studied by Vibrational Spectroscopy
      Kitano, Hiromi; Imai, Makoto; Mori, Takayuki; Gemmei-Ide, Makoto; Yokoyama, Yoshiyuki; Ishihara, Kazuhiko
      Langmuir (2003), 19 (24), 10260-10266CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      The structure and H bonding of H2O in the vicinity of phospholipid analog random copolymers [poly(2-methacryloyloxyethyl phosphorylcholine-r-Bu methacrylate), poly(MPC-r-BMA)] with various mol. wts. were analyzed in their aq. solns. and thin films with contours of O-H stretching of Raman and attenuated total reflection IR (ATR-IR) spectra, resp. The relative intensity of the collective band (C value) corresponding to a long-range coupling of O-H stretchings of the Raman spectra for the aq. soln. of poly(MPC-r-BMA) was close to that for pure H2O, which is in contrast with the smaller C value in the aq. soln. of ordinary polyelectrolytes. The no. of H bonds collapsed by the presence of 1 monomer residue (Ncorr value) of poly(MPC-r-BMA) (Mw 1.3 × 104, 3.0 × 104, and 9.3 × 104) was much smaller than those for ordinary polyelectrolytes and close to those for neutral polymers such as poly(ethylene glycol) and poly(N-vinylpyrrolidone). H2O-insol. poly(MPC-r-BMA) with a large mol. wt. (4.2 × 105) could be cast as a thin film (thickness, ∼10 μm) on a ZnSe crystal for the ATR-IR spectra. At an early stage of sorption of H2O into the poly(MPC-r-BMA) film, the O-H stretching band of the IR spectra for the H2O incorporated in the film was similar to that for free H2O, which is in contrast with the drastic change in the O-H stretching band of H2O incorporated in polymer films such as PHEMA, PMMA, and PBMA. Probably the phospholipid analog monomer residues with a zwitterionic structure do not significantly disturb the H bonding between H2O mols. in either the aq. soln. or the thin film systems.
    47. 47
      Kitano, H.; Mori, T.; Takeuchi, Y.; Tada, S.; Gemmei-Ide, M.; Yokoyama, Y.; Tanaka, M. Structure of Water Incorporated in Sulfobetaine Polymer Films as Studied by ATR-FTIR. Macromol. Biosci. 2005, 5, 314– 321,  DOI: 10.1002/mabi.200400212
      47
      Structure of water incorporated in sulfobetaine polymer films as studied by ATR-FTIR
      Kitano, Hiromi; Mori, Takayuki; Takeuchi, Yuki; Tada, Susumu; Gemmei-Ide, Makoto; Yokoyama, Yoshiyuki; Tanaka, Masaru
      Macromolecular Bioscience (2005), 5 (4), 314-321CODEN: MBAIBU; ISSN:1616-5187. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The structure and hydrogen bonding of water in the vicinity of a thin film of a sulfobetaine copolymer (poly[(N,N-dimethyl-N-(3-sulfoproyl)-3'-methacrylamido-propanaminium inner salt)-ran-(Bu methacrylate)], poly(SPB-r-BMA)), were analyzed with band shapes of O-H stretching of attenuated total reflection IR (ATR-IR) spectra. The copolymer could be cast as a thin film, of approx. thickness 10 μm, on a ZnSe crystal for the ATR-IR spectroscopy. At an early stage of sorption of water into the polymer film, the O-H stretching band of the IR spectra for the water incorporated in the film was similar to that for free water. This is consistent with the tendency for another zwitterionic polymeric material, poly[(2-methacryloyloxyethylphosphorylcholine)-ran-(Bu methacrylate)] (poly-(MPC-r-BMA)). It is, however, contradictory to the drastic change in the O-H stretching band for water incorporated into films of polymers such as poly(2-hydroxyethyl methacrylate), poly(Me methacrylate) and poly(Bu methacrylate). These results suggest that polymers with a zwitterionic structure do not significantly disturb the hydrogen bonding between water mols. incorporated in the thin films. The investigation into the blood-compatibility of both the poly(SPB-r-BMA) and the poly(MPC-r-BMA) films indicate a definite correlation between the blood-compatibility of the polymers and the lack of effect of the polymeric materials on the structure of the incorporated water.
    48. 48
      Seki, T.; Chiang, K.-Y.; Yu, C.-C.; Yu, X.; Okuno, M.; Hunger, J.; Nagata, Y.; Bonn, M. The Bending Mode of Water: A Powerful Probe for Hydrogen Bond Structure of Aqueous Systems. J. Phys. Chem. Lett. 2020, 11, 8459– 8469,  DOI: 10.1021/acs.jpclett.0c01259
      48
      The Bending Mode of Water: A Powerful Probe for Hydrogen Bond Structure of Aqueous Systems
      Seki, Takakazu; Chiang, Kuo-Yang; Yu, Chun-Chieh; Yu, Xiaoqing; Okuno, Masanari; Hunger, Johannes; Nagata, Yuki; Bonn, Mischa
      Journal of Physical Chemistry Letters (2020), 11 (19), 8459-8469CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      A review. Insights into the microscopic structure and dynamics of the water's hydrogen-bonded network are crucial to understand the role of water in biol., atm. and geochem. processes, and chem. reactions in aq. systems. Vibrational spectroscopy of water has provided many such insights, in particular using the O-H stretch mode. In this Perspective, we summarize our recent studies that have revealed that the H-O-H bending mode can be an equally powerful reporter for the microscopic structure of water and provides more direct access to the hydrogen-bonded network than the conventionally studied O-H stretch mode. We discuss the fundamental vibrational properties of the water bending mode, such as the intermol. vibrational coupling, and its effects on the spectral lineshapes and vibrational dynamics. Several examples of static and ultrafast bending mode spectroscopy illustrate how the water bending mode provides an excellent window on the microscopic structure of both bulk and interfacial water.
    49. 49
      Piatkowski, L.; Bakker, H. J. Vibrational Dynamics of the Bending Mode of Water Interacting with Ions. J. Chem. Phys. 2011, 135, 214509  DOI: 10.1063/1.3664866
      49
      Vibrational dynamics of the bending mode of water interacting with ions
      Piatkowski, L.; Bakker, H. J.
      Journal of Chemical Physics (2011), 135 (21), 214509/1-214509/8CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      The authors studied the vibrational relaxation dynamics of the bending mode (ν2) of the water H2O mols. in the presence of different salts (LiCl, LiBr, LiI, NaI, CsI, NaClO4, and NaBF4). The linear and nonlinear spectra of the bending mode show distinct responses of H2O mols. hydrating the anions. The bending mode of H2O mols. that are H-bonded to an anion exhibits much slower relaxation rates (T1 ∼ 1 ps) than H2O mols. that are H-bonded to other H2O mols. (T1 = 400 fs). The effect of the anion on the absorption spectrum and relaxation time const. of the H2O bending mode is not only detd. by the strength of the H-bond interaction but also by the shape of the anion. (c) 2011 American Institute of Physics.
    50. 50
      Zahn, S.; Wendler, K.; Delle Site, L.; Kirchner, B. Depolarization of Water in Protic Ionic Liquids. Phys. Chem. Chem. Phys. 2011, 13, 15083– 15093,  DOI: 10.1039/c1cp20288j
      50
      Depolarization of water in protic ionic liquids
      Zahn, Stefan; Wendler, Katharina; Delle Site, Luigi; Kirchner, Barbara
      Physical Chemistry Chemical Physics (2011), 13 (33), 15083-15093CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      A mixt. of the protic ionic liq. mono-methylammonium nitrate with 1.6 wt% water was investigated from Car-Parrinello mol. dynamics simulations. In contrast to imidazolium-based ionic liqs., the cation possesses strong directional hydrogen bonds to water and all hydrogen bonds in the mixt. have a comparable strength. This results in a good incorporation of water into the hydrogen bond network of mono-methylammonium nitrate and a tetrahedral hydrogen bond coordination of water. Hence, one might expect a larger dipole moment of water in the investigated mixt. compared to neat water due to the good hydrogen bond network incorporation and the charged vicinity of water in the protic ionic liq. However, the opposite is obsd. pointing to strong electrostatic screening in protic ionic liqs. The influence of water on the properties of the protic ionic liq. is discussed.
    51. 51
      Kabisch, G. Intermolecular Coupling of Bending Vibrations in Liquid Water. J. Mol. Struct. 1981, 77, 219– 226,  DOI: 10.1016/0022-2860(81)80066-X
      51
      Intermolecular coupling of bending vibrations in liquid water
      Kabisch, G.
      Journal of Molecular Structure (1981), 77 (3-4), 219-26CODEN: JMOSB4; ISSN:0022-2860.
      Raman spectra of liq. H2O, D2O and their isotopic mixts. were investigated in the region of the deformation modes. Comparison with ν2(HDO) shows that differences in band shape and peak frequencies in isotropic scattering for H2O and D2O are caused by intermol. coupling, and not by the presence of discrete mol. species in liq. water. The changes in ν2 band parameters, produced by addn. of electrolytes and nonelectrolytes, are discussed.
    52. 52
      Ahmed, M.; Namboodiri, V.; Singh, A. K.; Mondal, J. A. On the Intermolecular Vibrational Coupling, Hydrogen Bonding, and Librational Freedom of Water in the Hydration Shell of Mono- and Bivalent Anions. J. Chem. Phys. 2014, 141, 164708  DOI: 10.1063/1.4899070
      52
      On the intermolecular vibrational coupling, hydrogen bonding, and librational freedom of water in the hydration shell of mono- and bivalent anions
      Ahmed, Mohammed; Namboodiri, V.; Singh, Ajay K.; Mondal, Jahur A.
      Journal of Chemical Physics (2014), 141 (16), 164708/1-164708/8CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      The hydration energy of an ion largely resides within the first few layers of water mols. in its hydration shell. Hence, it is important to understand the transformation of water properties, such as hydrogen-bonding, intermol. vibrational coupling, and librational freedom in the hydration shell of ions. We investigated these properties in the hydration shell of mono- (Cl- and I-) and bivalent (SO42- and CO32-) anions by using Raman multivariate curve resoln. (Raman-MCR) spectroscopy in the OH stretch, HOH bend, and [bend+librational] combination bands of water. Raman-MCR of aq. Na-salt (NaCl, NaI, Na2SO4, and Na2CO3) solns. provides ion-correlated spectra (IC-spectrum) which predominantly bear the vibrational characteristics of water in the hydration shell of resp. anions. Comparison of these IC-spectra with the Raman spectrum of bulk water in different spectral regions reveals that the water is vibrationally decoupled with its neighbors in the hydration shell. Hydrogen-bond strength and librational freedom also vary with the nature of anion: hydrogen-bond strength, for example, decreases as CO32- > SO42- > bulk water ≈ Cl- > I-; and the librational freedom increases as CO32- ≈ SO42- < bulk water < Cl- < I-. It is believed that these structural perturbations influence the dynamics of coherent energy transfer and librational reorientation of water in the hydration shell of anions. (c) 2014 American Institute of Physics.

  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.langmuir.2c02851.

    • Gaussian deconvolution of the OH stretching band of pure water and Hy ILs (Hy [Ch][dhC] 7:1, Hy [C4mim][dHp] 7:1, Hy PC 7:1, Hy [P4444][dHp] 7:1, Hy [P6666][dHp] 7:1, Hy [P44412][dHp] 7:1, Hy [N4444][dHp] 7:1, Hy [Ch][dBp] 7:1, Hy [Ch][SCN] 7:1) (Figures S1–S3); area ratios of the four Gaussian peaks of the OH stretching band of pure water and the aforementioned Hy ILs at a 7:1 molar ratio (Figure S4); and full width at half-maxima of HOH bending peaks of pure water and the Hy ILs at a 3:1 molar ratio (Figures S5 and S6) (PDF)


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