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Tuning Solvent Miscibility: A Fundamental Assessment on the Example of Induced Methanol/n-Dodecane Phase Separation

Cite this: J. Phys. Chem. B 2019, 123, 20, 4400–4407
Publication Date (Web):April 29, 2019
https://doi.org/10.1021/acs.jpcb.9b00839

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Abstract

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In this work, we assess the fundamental aspects of mutual miscibility of solvents by studying the mixing of two potential candidates, methanol and n-dodecane, for nonaqueous solvent extraction. To do so, 1H NMR spectroscopy and molecular dynamics simulations are used jointly. The NMR spectra show that good phase separation can be obtained by adding LiCl and that the addition of a popular extractant (tri-n-butyl phosphate) yields the opposite effect. It is also demonstrated that in a specific case the poor phase separation is not due to the migration of n-dodecane into the more polar phase, but due to the transfer of the extractant into it, which is especially relevant when considering industrial applications of solvent extraction. With the aid of molecular dynamics simulations, explanations of this behavior are given. Specifically, an increase of all hydrogen-bond lifetimes is found to be consequent to the addition of LiCl which implies an indirect influence on the methanol liquid structure, by favoring a stronger hydrogen-bond network. Therefore, we found that better phase separation is not directly due to the presence of LiCl, but due to the “hardening” of the hydrogen-bond network.

Introduction

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Nowadays, hydrometallurgical approaches, which involve aqueous chemistry for the recovery of metals, are fundamental for the extractive metallurgy of many elements. The protocols for the extraction of many metal ions are based on a combination of pyrometallurgy, which consists of thermal treatments aimed to produce physical and chemical transformations, and hydrometallurgy. The latter is often used in the first part of a flow sheet. (1−3) Unfortunately, pyrometallurgical methods are usually incapable of treating low-grade ores or residues in an economic way and hydrometallurgy with strong acid leaching is weakly selective. These problems, together with the aim to develop and establish a circular economy, (4) led to the innovative concept of solvometallurgy. (5)
Solvometallurgy implies processes that are similar to those of hydrometallurgy, but not involving aqueous phases. This opens a wide spectrum of choices, including molecular organic solvents, ionic liquids, deep-eutectic solvents, and inorganic solvents. (6−12) It must be clarified that the term “nonaqueous solvents” in the paradigm of solvometallurgy does not necessarily imply anhydrous conditions, but rather a solvent in which the water content is lower than 50 vol %. Solvometallurgy has already been exploited for the recovery of copper from chrysocolla, (13,14) rare earth and other metals from complex silica-rich ores, (15) uranium from carbonate ores and for reprocessing of spent nuclear fuel. (16) In hydrometallurgy, conventional solvent extraction is commonly used. In this case, metals are distributed between an aqueous phase and an immiscible organic phase. As mentioned above, the new paradigm of solvometallurgy replaces the aqueous phase by a nonaqueous solvent. (17,18) Additionally, some conditions have to be fulfilled: (1) the process should involve two mutually immiscible liquid phases, (2) phase separation should be fast, (3) good solubility of the extractant in the less polar phase, (4) good solubility of the extracted metal complex, and (5) poor solubility of the extractant in the more polar phase. The first condition is crucial, especially with regard to industrial applications. Hence, enormous effort is made in the identification of suitable solvent pairs for nonaqueous solvent extraction. The first indication of mutual miscibility is a volume change of two phases that are in contact with each other. This does not hold if the mutual solubility is the same in the two phases. Therefore, the mutual solubility can be quantified for instance by Fourier transform infrared spectroscopy, gas chromatography, or NMR spectroscopy. Furthermore, mutual miscibility is a function of the temperature and the concentration of cosolvents, which can be a dissolved salt or extractant. For example, it is known that two completely miscible solvents can be phase-separated with the addition of salt to the more polar phase. (19,20) The opposite is also possible, that is, the addition of extractants to the less polar phase can worsen the phase separation by causing the transfer of the solvent molecules from one phase to the other.
In miscibility experiments, two candidates are mixed together and the phase separation is evaluated. To improve the phase separation, the addition of salts can be taken into account. When the obtained phase separation is satisfactory, the extractant is added and the evaluation is repeated. The addition of the extractant can be considered as a critical step since for many systems it leads to unsuccessful phase separation.
In this paper, we aim to highlight those fundamental aspects that act as a driving force to the mutual solubility of two solvents in the presence of cosolvents and cosolutes and, therefore, have to be taken into account while screening for new solvent pairs. To do this, 1H NMR spectroscopy and molecular dynamics (MD) simulations are used to obtain a fundamental insight into the mixing and demixing of two potential candidate solvents for solvent extractions, such as methanol (MeOH) and n-dodecane (DD). (21−23) Quantitative analysis experiments were performed and based on these results, several systems were simulated (see Figure 1) to provide molecular-level insight into the polar phase obtained after the mixing and demixing of the two solvents. The system was also tested and simulated in the presence of LiCl and the popular solvating extractant tri-n-butyl phosphate (TBP).

Figure 1

Figure 1. Schematic representation of the procedure described in this work.

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Methods

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Chemicals and Instrumentation

Methanol (>99.9%), n-dodecane (>99%), and acetone-d6 (99.9 atom % D) were purchased from Acros Organics (Geel, Belgium) while TBP (>98%) was purchased from Alfa-Aesar (Karlsruhe, Germany). Chloroform-d (99.8 atom % D) and LiCl (99.9%) were obtained from Sigma-Aldrich (Diegem, Belgium). The mixing was performed using an Eppendorf ThermoMixer C, in T Corning Centrifuge Tubes with CentriSTAR caps. Centrifugation was performed by means of a Heraeus Megafuge 1.0. 1H NMR spectra were recorded on a Bruker Avance 300 spectrometer, operating at 300 MHz. The chemical shifts are noted in parts per million (ppm), referenced to tetramethylsilane. All chemicals were used as received without any further purification.

Experimental Details

Solutions of MeOH with different concentrations (0, 0.5, 1, 2, and 3 mol·L–1) of LiCl were prepared and stirred overnight. 5 mL of each solution was dispensed in centrifuge tubes and mixed with 5 mL of DD or DD + TBP (1 mol·L–1). The centrifuge tubes were shaken for 30 min at 2000 rpm and centrifuged for 15 min at 4000 rpm. The volume changes of the phases were evaluated using the indicated graduation of the centrifuge tubes. The same procedure was then repeated with a fixed concentration of LiCl (2 mol·L–1) and by varying the concentration of TBP (0.25, 0.5, 1, 2 mol·L–1). After each step, solutions were left to rest to reach thermal equilibrium with the environment.
1H NMR spectra of the different phases were collected. The amount of dissolved solvent and in some cases also the amount of extractant were quantified using the ratio of the integrated peak values belonging to different molecules. To do so, two NMR solvents were used: deuterated acetone (acetone-d6) and deuterated chloroform (chloroform-d1). The NMR solvent should not strongly interact with any compound to avoid interference. Deuterated acetone was found to have an effect on the MeOH acidic hydrogen atom peak. Nevertheless, the integration has been performed on the peaks of the methyl group hydrogen atoms, therefore, these spectra provide the qualitative and quantitative information we were looking for. With the aim of testing the effect of a different number of scans and different relaxation times on the final spectra, the data were collected with different number of scans (8–32) and different relaxation times (20–40 s). All spectra are in good agreement with each other, validating the gathered data.

Computational Details

The initial configuration of the simulation boxes was generated using PACKMOL (version 17.039). (24) Classical molecular dynamics simulations were performed using the LAMMPS program package (version 17 Nov 2016). (25) For MeOH, DD, and LiCl, the well-known OPLS-AA force field were used, (26) whereas for TBP we opted for the force field recently developed by Ali et al. (27) since it was proven to perform very well when TBP is mixed with n-dodecane. Nonbonded interactions were described by the 6–12 Lennard-Jones potential. (28) Lorentz–Berthelot mixing rules were used to obtain parameters for pairs of different atoms. A cutoff of 1.6 nm was selected for the calculation of Lennard-Jones and Coulombic interactions and a particle–particle particle–mesh solver, mapping the atom charge to a three-dimensional mesh. (29) Equilibration of the systems was obtained by simulating for 0.5 ns using the NPT ensemble. Constant pressure and temperature were achieved applying the Nosé–Hoover chain thermostat and barostat (T = 297.15 K, τ = 100 fs and P = 1 bar, τ = 1000 fs, respectively). (30,31) The cell vectors, averaged over the last 250 ps were taken to perform the production run within the NVT ensemble. The production run consisted of 10 ns of the simulation time after 0.5 ps of equilibration. The time step was set to 0.5 fs during the whole procedure. The obtained trajectories were analyzed with the TRAVIS code. (32) This tool offers different kinds of functions allowing the analysis of the interaction among the components of the systems. Intra- and intermolecular interactions can be taken into account. In this work radial distribution functions (RDFs), Voronoi analysis, mean square displacements (MSDs), and hydrogen-bond lifetime calculations were exploited. (33−37)

Results

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Volume Analysis

As mentioned in the Introduction, one of the key conditions for two solvents to be suitable for solvent extraction is the formation of two immiscible liquid phases. In this respect, the most immediate test is to check the volume change of the solvents, after mixing and phase separation. Volume changes which are not significant in a laboratory workflow might result in a critical solvent loss on an industrial scale. The optimal situation is when no volume change occurs. The observed volume changes of the phases are reported in Table 1. Numbers are referring to the volume ratio between the top phase (apolar) and the bottom phase (polar). The values reported in Table 1 show that the neat binary system MeOH + DD is not suitable for solvent extraction. Increasing the concentration of LiCl improved phase separation. An optimal phase ratio was already obtained for 0.5 mol·L–1 of this salt. Yet, Table 1 shows that with 1 mol·L–1 of TBP the phase separation worsened and that even with higher concentrations of LiCl (up to 3 mol·L–1) it was not possible to obtain the 1:1 ratio. In Table 2 the volume ratios are reported for a fixed concentration of LiCl (2 mol·L–1), and the TBP concentration ranging from 0 to 2 mol·L–1. Again, in the presence of TBP, the optimal phase separation could not be achieved.
Table 1. Volume Ratios upon Mixing Equal Volumes of MeOH and DD (at Room Temperature) with Respect to Different Concentrations (mol·L–1) of LiCl in MeOH and TBP (always 1 mol·L–1) in DDa
MeOH + LiClDDDD + TBP
neat MeOH1:1.11:2.3
0.51:11:2
11:11:1.9
21:11:1.9
31:11:1.9
a

Data are referred to the volume ratio between the top phase (apolar) and the bottom phase (polar) obtained after mixing and phase separation.

Table 2. Volume Ratios with Respect to Different Concentrations (mol·L–1) of the Extractant for MeOH + LiCl 2 mol·L–1 a
DD + TBPMeOH + LiCl
neat DD1:1
0.251:1.4
0.51:1.8
11:1.9
21:4.5
a

Data reported is referred to the volume ratio between the top phase (apolar) and the bottom phase (polar) after mixing and phase separation.

1H NMR Spectra

We used 1H NMR spectroscopy to evaluate the amount of DD and MeOH being transferred from one phase to the other. Figures S1–S4 (see the Supporting Information (SI)) show the spectra of the polar (bottom) phase which were obtained after shaking and centrifuging of, respectively, MeOH and DD (MD), MD in the presence of TBP (MDT), MD in the presence of LiCl (MLD), and MD in the presence of both TBP and LiCl (MLDT). The evaluation of the ratio of the integrals of specific peaks allowed to estimate the ratio of the number of molecules of MeOH, DD, and TBP in the polar phase. For the MD system, the MeOH/DD ratio was 92. With the addition of up to 3 mol·L–1 LiCl, the situation radically changed, moving the ratio to 1840. In the case of MDT, the ratio was 30, whereas for MLDT it was 300. The following conclusion can be drawn: the addition of LiCl prevents the transfer of the DD into the polar phase, whereas TBP facilitates it. Moreover, the volume change is not only due to the transfer of DD into the bottom phase but also due to the transfer of TBP. In fact, the MeOH/DD ratios are not fully in line with what we expected from the phase volume analysis. Specifically, the “quality” of the phase separation of these systems according to the volume analysis is MLD > MD > MLDT > MDT, whereas according to the MeOH/DD ratios in NMR spectra, we have MLD > MLDT > MD > MDT. The explanation may be found in the MeOH/TBP ratios, which remain stable even in the presence of LiCl. Therefore, MLDT shows a worse phase separation than MD, despite the fact that the amount of DD in the polar phase is much lower. To align the volume ratio analysis with the NMR results, we can count TBP together with DD and recalculate the MeOH/(DD + TBP) ratios for all systems. We finally obtain MLD(1838) > MD(92) > MLDT(23) > MDT(13), which is in line with the volume analysis.

Systems Simulated via Molecular Dynamics

Four systems were simulated in accordance with the evidence gathered from the 1H NMR spectra (see Figure 1), namely, the bottom phases that were obtained after the mixing and phase separation of the MeOH + DD mixture (MD) also in the presence of TBP (MDT), LiCl (MLD), and both (MLDT). Table 3 reports the number of molecules in each system.
Table 3. Compositions of Simulated Bottom Phase Systems Chosen According to 1H NMR Spectraa
systemMDMDTMLDMLDT
MeOH3000300030003000
DD32100210
TBP 120 120
LiCl  361361
a

Data refer to the number of molecules in each system.

Solvent Partitioning

By means of the Voronoi tessellation method, a domain analysis was performed. (38−40) With this analysis, we divided the systems into subsets of different molecules or groups of atoms and calculated the average number of domains of each subset during the simulation. A large value means the subset is dispersed, whereas a small value means the atoms belonging to the subset are all connected. We decided to divide the systems into subsets of molecules of the same kind, and also to pair MeOH + LiCl and DD + TBP. The results of these different analyses are reported in Table 4. A value close to one means high aggregation. Large values indicate small dispersed agglomerates.
Table 4. Bottom Phases Domain Analysisa
systemMeOHMeOH + LiClLiClDDDD + TBPTBP
MD1.0  12.9 (32)  
MLD1.01.0291.8 (361)1.5 (2)  
MDT2.7  9.3 (100)3.96.4 (120)
MLDT1.21.2274.0 (361)7.0 (10)3.43.6 (120)
a

The values refer to the average number of domains of the subsets into the systems (within parentheses is the number of molecules for that component. For MeOH, the number of molecules is always 3000).

In the MD system, MeOH forms one single domain, most probably due to the large excess of this solvent. The 32 molecules of DD form an average of 12.9 domains, which means that DD molecules are clustering in groups of 2–3 molecules into MeOH.
In MLD system, MeOH again forms one single domain and LiCl is well solvated by MeOH itself (in fact the subset MeOH + LiCl forms one domain, while LiCl is divided into 291.8 domains, corresponding to an average of 2.4 Li/Cl per domain). According to the 1H NMR spectra, in the presence of LiCl, to reproduce the experimental concentration only two molecules of DD were allowed; these molecules form an average of 1.5 domains, which means that they were clustered together for half of the simulation.
In the MDT system, the total number of DD molecules was 100, which can be related to the activity of TBP. In this case, MeOH is divided into 2.7 domains, which is reasonable once we take into account the amount of DD and TBP in the system. DD forms 9.3 domains (meaning clusters of 10–11 molecules) and TBP 6.4 (clusters of 20–21 molecules). Taking into account the number of domains for the DD + TBP subset, and comparing it to the individual DD and TBP, we find a reduction to 3.9 domains. This shows that DD and TBP cluster together, and consequently “stabilize” each other.
This is even more evident in the MLDT system. For this system, the number of domains for the subsets MeOH and MeOH + LiCl is 1.2, due to the smaller amount of DD and TBP. In this case, TBP forms 3.6 domains, showing the negative influence of LiCl in the solvation of TBP by MeOH. On the other hand, the 10 molecules of DD form 7 domains, which is misleading since it might seem that in this system DD can be well solvated. Analysis of the DD + TBP subset is especially handy in this case. In fact, for this subset, we find an average number of domains very close to the one of the TBP subset. Hence, we can draw the conclusion that DD has to be fully solvated by TBP to transfer into the polar phase. This is also shown from the snapshot reported in Figure 2, where DD is completely surrounded by TBP and LiCl is surrounded by MeOH.

Figure 2

Figure 2. Snapshot taken from the MLDT system. All atoms are represented by their van der Waals radii. Color scheme: yellow for LiCl, red for MeOH, green for TBP, and blue for DD.

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Mobility

The dynamics of the systems were studied by calculating the mean square displacement (MSD) and consequently the diffusion constant of the different components (see Figure 3 and Table 5). The MSDs in Figure 3 allow an easy comparison of the mobility of different compounds in the systems.

Figure 3

Figure 3. Mean square displacements of the bottom phase components. Top left: MeOH; top right: DD; bottom left: Li+ and Cl–; bottom right: DD and TBP.

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Table 5. Diffusion Coefficient (×10–9, m2·s–1) in the Bottom Phases
componentMeOHLi+Cl–DDTBP
MD2.53  1.74 
MLD0.910.380.480.47 
MDT2.27  0.710.71
MLDT0.820.360.390.690.39
We observed that MeOH is mainly affected by the presence of LiCl, which lowers its mobility (increasing the total viscosity of the system) to half of the values for pure methanol. TBP also slightly decreases the mobility of MeOH, which is in line with the fact that TBP and MeOH can interact via hydrogen bonds, and since TBP is a bulky molecule (with low mobility), it decreases the mobility of MeOH as a consequence.
DD is more mobile in the MD system, where the lack of strong interactions with TBP and the small amount of DD itself (which prevents a strong clustering with itself) allows DD to move “freely”. The lower mobility of DD in the MDT and MLDT systems can be easily attributed to the clustering of DD with itself or with TBP. Again, the higher viscosities in the MLD and MLDT systems (due to LiCl) strongly influence the results. With respect to the addition of TBP, MSDs of Li+ and Cl– seem to comply with those of MeOH by decreasing accordingly, pointing to a correlation between them. Interestingly, Cl– is slightly faster than Li+.
To allow some further considerations, we plotted the MSDs of TBP and DD in the MDT and MLDT systems. The curves show that the dynamics of these molecules might be related. Yet, it is clear that TBP is more influenced by the addition of LiCl than DD.
Table 5 shows the diffusion coefficients of the components in different systems. The experimental diffusion coefficient of neat MeOH is 2.6 × 10–9 m2·s–1, and in our simulation of the system MD, we obtained 2.5 × 10–9 m2·s–1, which is in very good agreement. We see that in all systems, MeOH molecules are more mobile than DD, the ions and TBP. It is also interesting that the dynamics of the ions are only slightly influenced by the presence of TBP.

Structural Properties

We used radial pair distribution functions (RDFs) to further interpret the data gathered in the previous sections. Figure 4 displays the RDFs of the interplay between MeOH molecules and Li+. With respect to the MeOH–MeOH interactions, the MDT system shows the highest value first peak. The second highest value of the first peak is from the MD system. Interestingly, MLD and MLDT systems show the smallest first peak. This is easy to explain once we take into account the shoulder of both curves at 340 pm. These shoulders are only visible for systems which contain LiCl and therefore, they are due to the proximity of MeOH molecules that are interacting with the same LiCl ion. (41) It is reasonable to conclude that because of the strong interaction between MeOH and LiCl, the ions interpose between MeOH molecules, creating an even stronger network (showing the kosmotropic character of LiCl), as shown by the increase in viscosity in these systems. This is also confirmed by the highest peaks in Figure 4, which represent the interactions between MeOH molecules and Li+. Figure 5 shows a snapshot of the aforementioned situation. It is clear that MeOH interacts with Li+ and Cl– via the oxygen and the hydrogen atoms, respectively. Furthermore, it is possible to notice some hydrogen bonds between MeOH molecules, a topic which we tackle in the next section.

Figure 4

Figure 4. RDFs of the interactions of MeOH molecules in the bottom phases.

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Figure 5

Figure 5. Example of the liquid structure of MeOH in the presence of LiCl taken from the MLDT system. LiCl is represented with van der Waals radii while MeOH is displayed with the ball-and-stick models. Color scheme: brown for Li, green for Cl, orange for C, red for O, and white for H.

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In Figure 6, we present an intramolecular analysis of the RDFs of the two terminal carbons of DD. With the aid of this analysis, we could define when DD is more crumpled or stretched out in different systems. In the picture, we identified three distances for which the molecules seem more stabilized: (1) 400 pm, which implies a crumpled structure, (2) 1050 pm, which implies a stretched out structure, and (3) 850 pm, which is the half-way case. In the MD system, the crumpled structure is almost equally relevant as the stretched one, whereas the addition of TBP in MDT and MLDT systems induces a relaxation of the structure toward the more stretched one. The comparison between MDT and MLDT systems confirmed once more the stabilizing effect of TBP on DD. In fact, in these systems, the amount of TBP was the same while having 100 DD molecules in the MDT system and only 10 DD molecules in the MLDT system. Consequently, it is reasonable that in the MLDT system, DD is completely surrounded by TBP, which allows the full relaxation of DD. In the MDT system, due to this larger amount of DD, TBP may not be able to interpose completely between DD and MeOH due to the larger amount of DD, which leads to a more crumpled structure of DD. For the sake of completeness, we also reported the analysis for the MLD system. However, since this system contains only two molecules of DD, the statistical sampling for this specific analysis is too poor and does not allow to draw any conclusion.

Figure 6

Figure 6. Intramolecular RDFs of the distance between the terminal carbons of DD molecules.

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Hydrogen-Bond (HB) Network

Following the observations of the previous section, we decided to calculate the average hydrogen-bond (HB) lifetimes in our systems (see Table 6), to confirm the effect of LiCl on the solvent liquid structure. This was possible by means of the methodology introduced by Luzar and Chandler. (34−37) The HB donor is always the MeOH oxygen atom and depending on the system, we have different acceptors: MeOH(O), LiCl(Cl–), and TBP(Osp2). By comparing the HB dynamics for MeOH–MeOH in different systems, we can evaluate the influence of other compounds and extend our molecular-level understanding of the systems.
Table 6. Average Lifetime (ps) of the Hydrogen Bond between MeOH (Donor) and the Components of Different Systems
 MeOHCl–TBP
MD13.8  
MLD28.815.5 
MDT16.5 10.2
MLDT32.618.720.1
We see that the addition of LiCl strongly slows down the HB dynamics for MeOH–MeOH. This observation is crucial as it shows that LiCl has a sort of “pinning effect” on MeOH. This pinning effect organizes and stabilizes the MeOH liquid structure by slowing down the HB dynamics without hindering the HB formation (Figure 5). Since the presence of LiCl is clearly related to the lower miscibility of DD in MeOH, we can easily correlate the slower, therefore stronger, HB network to the lower miscibility. In fact, the diffusional motion of the hydrophobic solute molecule can be explained via the diffusion of a void containing the molecule itself. This diffusion process becomes less likely with such a slow HB dynamics, (42) which explains the lower miscibility. Interestingly, TBP slows down the dynamics of MeOH–MeOH HB. On the other hand, since the interaction between TBP and MeOH (shown by the RDFs analysis in the SI) is weaker, we cannot draw the same conclusion we drew for LiCl. An easy explanation for the slower dynamics in the presence of TBP is that the dissolution of any molecule, be it hydrophobic or hydrophilic, tends to reduce solvent entropy in hydrogen-bonding solvents. Therefore, DD and TBP influence on the HB network agrees with previous studies on the effect of hydrophobic molecules on the solvent HBs. (43−46) Another effect to keep in mind, which further justifies our findings, is the effect of secondary alkyl groups on the HB formation energetics. (47)

Conclusions

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By means of a comprehensive approach, which includes experimental data and theoretical simulations, we studied the fundamental aspects of solvent miscibility. We investigated the mixing of two potential candidate solvents (methanol and n-dodecane) for solvent extraction, in the paradigm of solvometallurgy. Experimentally, we found that good separation of the two solvents can be achieved by the addition of a salt (in this case LiCl) and that the addition of a popular extractant appeared to worsen the phase separation. We also highlighted, by means of 1H NMR spectroscopy, in the case of the full MLDT system, that poor phase separation is not due to the migration of n-dodecane into the more polar phase, but rather due to the migration of tri-n-butyl phosphate into the polar phase. This is an interesting result, especially for industrial applications of nonaqueous solvent extraction.
The “polar” phases obtained during the experimental study were investigated by means of classical molecular dynamics simulations. We found that n-dodecane tends to cluster into methanol and tends to dissolve into tri-n-butyl phosphate microphase. LiCl exhibited a greater impact with respect to the overall dynamics. In fact, we observed an increase of the viscosity, which we correlated to an increase of the hydrogen-bond lifetimes. Consequently, the addition of LiCl not only implied an addition of strong interactions between different components of the system, but also an indirect influence on the liquid structure of methanol, by favoring a stronger hydrogen-bond network. This may be the reason why experimentally we have found that the amount of n-dodecane in the bottom phase of the MLDT system is lower than in the MDT system, with the amount of tri-n-butyl phosphate being the same, since tri-n-butyl phosphate can be part of the hydrogen-bond network, and consequently the exclusion of n-dodecane is not due to the direct presence of LiCl, but due to the “hardening” of the hydrogen-bond network. This effect recalls what had been previously observed by Jiang et al. (48) while studying the roles of the hydrophobic effect and hydrogen bonding in systems able to selectively recognize hydrophilic molecules in water. Following the concept that it is not the addition of the salt that directly improves the phase separation, but rather a consequence of the hydrogen-bond network, new ways of improving phase separation of two solvents can be thought and tested. For example, it was proven that the local structure of water reorganizes in the vicinity of polyelectrolyte brushes, leading to an enhancement of the hydrogen-bond network. (49) Furthermore, a similar effect on the methanol liquid structure can be obtained by playing with temperature and pressure. (50) This kind of effect might be exploited similarly to how LiCl has been used in our work.

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.9b00839.

  • 1H NMR spectra; radial distribution function; RDFs of the interactions of MeOH(O) molecules (PDF)

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Authors
  • Authors
    • Roberto Macchieraldo - Mulliken Center for Theoretical Chemistry, University of Bonn, Beringstrasse 4+6, D-53115 Bonn, Germany
    • Sascha Gehrke - Mulliken Center for Theoretical Chemistry, University of Bonn, Beringstrasse 4+6, D-53115 Bonn, Germany;  Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, D-45413 Mülheim an der Ruhr, Germany
    • Nagaphani K. Batchu - Department of Chemistry, KU Leuven, Celestijnenlaan 200F, bus 2404, B-3001 Heverlee, Belgium;  Orcidhttp://orcid.org/0000-0002-1971-6661
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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B.K., K.B., and R.M. acknowledge the MSCA ETN project SOCRATES (http://etn-socrates.eu/), which has received funding from the European Union’s EU Framework Program for Research and Innovation Horizon 2020 under Grant Agreement No. 721385. S.G. was supported by the International Max Planck Research School for Reactive Structure Analyses for Chemical Reactions. K.B. and N.K.B. acknowledge funding from the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation Program: Grant Agreement 694078—Solvometallurgy for critical metals (SOLCRIMET).

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    Binnemans, K.; Jones, P. T. Solvometallurgy: an emerging branch of extractive netallurgy. J. Sustainable Metall. 2017, 3, 570– 600,  DOI: 10.1007/s40831-017-0128-2
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    Abbott, A. P.; Boothby, D.; Capper, G.; Davies, D. L.; Rasheed, R. K. Deep eutectic solvents formed between choline chloride and carboxylic acids: versatile alternatives to ionic liquids. J. Am. Chem. Soc. 2004, 126, 9142– 9147,  DOI: 10.1021/ja048266j
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    Deep Eutectic Solvents Formed between Choline Chloride and Carboxylic Acids: Versatile Alternatives to Ionic Liquids
    Abbott, Andrew P.; Boothby, David; Capper, Glen; Davies, David L.; Rasheed, Raymond K.
    Journal of the American Chemical Society (2004), 126 (29), 9142-9147CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Deep Eutectic Solvents (DES) can be formed between a variety of quaternary ammonium salts and carboxylic acids. The phys. properties are significantly affected by the structure of the carboxylic acid but the phase behavior of the mixts. can be simply modeled by taking account of the mole fraction of carboxylic acid in the mixt. The phys. properties such as viscosity, cond., and surface tension of these DES are similar to ambient temp. ionic liqs. and insight into the cause of these properties is gained using hole-theory. It is shown that the cond. and viscosity of these liqs. is controlled by ion mobility and the availability of voids of suitable dimensions, and this is consistent with the fluidity of other ionic liqs. and molten salts. The DES are also shown to be good solvents for metal oxides, which could have potential application for metal extn.
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    Rogers, R. D.; Seddon, K. R. Ionic liquids-solvents of the future?. Science 2003, 302, 792– 793,  DOI: 10.1126/science.1090313
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    Chemistry. Ionic liquids--solvents of the future?
    Rogers Robin D; Seddon Kenneth R
    Science (New York, N.Y.) (2003), 302 (5646), 792-3 ISSN:.
    There is no expanded citation for this reference.
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    Gutowski, K. E.; Broker, G. A.; Willauer, H. D.; Huddleston, J. G.; Swatloski, R. P.; Holbrey, J. D.; Rogers, R. D. Controlling the aqueous miscibility of ionic liquids: aqueous biphasic systems of Weter-miscible ionic liquids and water-structuring salts for recycle, metathesis, and separations. J. Am. Chem. Soc. 2003, 125, 6632– 6633,  DOI: 10.1021/ja0351802
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    Controlling the Aqueous Miscibility of Ionic Liquids: Aqueous Biphasic Systems of Water-Miscible Ionic Liquids and Water-Structuring Salts for Recycle, Metathesis, and Separations
    Gutowski, Keith E.; Broker, Grant A.; Willauer, Heather D.; Huddleston, Jonathan G.; Swatloski, Richard P.; Holbrey, John D.; Rogers, Robin D.
    Journal of the American Chemical Society (2003), 125 (22), 6632-6633CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Hydrophilic ionic liqs. can be salted-out and concd. from aq. soln. upon addn. of salts forming aq. biphasic systems as illustrated by the phase behavior of mixts. of 1-butyl-3-methylimidazolium chloride and K3PO4.
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  9. 9
    Earle, M. J.; Esperança, J. M.; Gilea, M. A.; Lopes, J. N. C.; Rebelo, L. P.; Magee, J. W.; Seddon, K. R.; Widegren, J. A. The distillation and volatility of ionic liquids. Nature 2006, 439, 831– 834,  DOI: 10.1038/nature04451
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    The distillation and volatility of ionic liquids
    Earle, Martyn J.; Esperanca, Jose M. S. S.; Gilea, Manuela A.; Canongia Lopes, Jose N.; Rebelo, Luis P. N.; Magee, Joseph W.; Seddon, Kenneth R.; Widegren, Jason A.
    Nature (London, United Kingdom) (2006), 439 (7078), 831-834CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    It is widely believed that a defining characteristic of ionic liqs. (or low-temp. molten salts) is that they exert no measurable vapor pressure, and hence cannot be distd. Here we demonstrate that this is unfounded, and that many ionic liqs. can be distd. at low pressure without decompn. Ionic liqs. represent matter solely composed of ions, and so are perceived as non-volatile substances. During the last decade, interest in the field of ionic liqs. has burgeoned, producing a wealth of intellectual and technol. challenges and opportunities for the prodn. of new chem. and extractive processes, fuel cells and batteries, and new composite materials. Much of this potential is underpinned by their presumed involatility. This characteristic, however, can severely restrict the attainability of high purity levels for ionic liqs. (when they contain poorly volatile components) in recycling schemes, as well as excluding their use in gas-phase processes. We anticipate that our demonstration that some selected families of commonly used aprotic ionic liqs. can be distd. at 200-300 °C and low pressure, with concomitant recovery of significant amts. of pure substance, will permit these currently excluded applications to be realized.
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  10. 10
    Fukumoto, K.; Yoshizawa, M.; Ohno, H. Room temperature ionic liquids from 20 Natural amino acids. J. Am. Chem. Soc. 2005, 127, 2398– 2399,  DOI: 10.1021/ja043451i
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    Room Temperature Ionic Liquids from 20 Natural Amino Acids
    Fukumoto, Kenta; Yoshizawa, Masahiro; Ohno, Hiroyuki
    Journal of the American Chemical Society (2005), 127 (8), 2398-2399CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The authors have succeeded in synthesizing room temp. ionic liqs. from 20 natural amino acids (AA). These amino acid ionic liqs. are H-AA-O-·(1-ethyl-3-methylimidazolium)+ salts, which dissolved native amino acids despite water-free conditions. Furthermore, these ionic liqs. are sol. in various org. solvents, such as chloroform. Effects of acidity, hydrogen bonding ability, and steric factors on the properties of these ionic liqs. were analyzed as the function of side groups.
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    Wang, Y.; Voth, G. A. Unique spatial heterogeneity in ionic liquids. J. Am. Chem. Soc. 2005, 127, 12192– 12193,  DOI: 10.1021/ja053796g
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    Unique Spatial Heterogeneity in Ionic Liquids
    Wang, Yanting; Voth, Gregory A.
    Journal of the American Chemical Society (2005), 127 (35), 12192-12193CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A multiscale coarse-graining model for ionic liqs. has been extended to investigate the unique aggregation of cations in ionic liqs. through computer simulation. It has been found that, with sufficiently long side chains, the tail groups of cations aggregate to form spatially heterogeneous domains, while headgroups of the cations and the anions distribute as uniformly as possible. This is understood as the result of competition between the charged electrostatic interactions between headgroups and anions and the collective short-range interactions between the neutral tail groups. This aggregation can help to explain a no. of exptl. obsd. phys. phenomena in ionic liqs.
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    Scurto, A. M.; Aki, S. N. V. K.; Brennecke, J. F. CO2 as a separation switch for ionic liquid/organic mixtures. J. Am. Chem. Soc. 2002, 124, 10276– 10277,  DOI: 10.1021/ja0268682
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    CO2 as a Separation Switch for Ionic Liquid/Organic Mixtures
    Scurto, Aaron M.; Aki, Sudhir N. V. K.; Brennecke, Joan F.
    Journal of the American Chemical Society (2002), 124 (35), 10276-10277CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A novel technique to sep. ionic liqs. from org. compds. is introduced which uses carbon dioxide to induce the formation of an ionic liq.-rich phase and an org.-rich liq. phase in mixts. of methanol and 3-butyl-1-methyl-imidazolium hexafluorophosphate ([C4mim][PF6]). If the temp. is above the crit. temp. of CO2 then the methanol-rich phase can become completely miscible with the CO2-rich phase, and this new phase is completely ionic liq.-free. Since CO2 is nonpolar, it is not equipped to solvate ions. As the CO2 dissolves in the methanol/[C4mim][PF6] mixt., the solvent power of the CO2-expanded liq. is significantly reduced, inducing the formation of the second liq. phase that is rich in ionic liq. This presents a new way to recover products from ionic liq. mixts. and purify org. phases that have been contaminated with ionic liq. Moreover, these results have important implications for reactions done in CO2/ionic liq. biphasic mixts.
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    Sastri, V. Selective leaching of metals from ores with organic solvents and chelating agents. J. Sci. Ind. Res. 1975, 34, 663– 665
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    Selective leaching of metals from ores with organic solvents and chelating agents
    Sastri, V. S.
    Journal of Scientific & Industrial Research (1975), 34 (12), 663-5CODEN: JSIRAC; ISSN:0022-4456.
    A review with 16 refs. is given on the use of org. solvents and chelating agents in the selective leaching of metals from ores.
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    Raghavan, S.; Fuerstenau, D. A Lyometallurgical Process for Leaching Copper from Chrysocolla. In Hydrometallurgy Fundamentals, Technology and Innovations , Proceedings of the Milton E., Wordsworth (IV) Int. Symposium on Hydrometallurgy, 1993; pp 283– 297.
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    Chizhevskaya, S. V.; Chekmarev, A. M.; Klimenko, O. M.; Povetkina, M. V.; Sinegribova, O. A.; Cox, M. In Nontraditional Methods of Treating High-Silicon Ores Containing Rare Elements , Hydrometallurgy ’94: Papers presented at the international symposium ‘Hydrometallurgy ’94’ organized by the Institution of Mining and Metallurgy and the Society of Chemical Industry, and held in Cambridge, England, from 11 to 15 July, 1994; pp 219– 228.
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    Hiroshi, T.; Kazushige, T.; Yoshimi, O. Direct Dissolution of Water-Insoluble Uranium Compounds by Contact with Neutral Organic Solvents Pretreated with Nitric Acid. U.S. Patent US3288568, 1966.
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    Batchu, N. K.; Vander Hoogerstraete, T.; Banerjee, D.; Binnemans, K. Non-aqueous solvent extraction of rare-earth nitrates from ethylene glycol to n-dodecane by Cyanex 923. Sep. Purif. Technol. 2017, 174, 544– 553,  DOI: 10.1016/j.seppur.2016.10.039
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    17
    Non-aqueous solvent extraction of rare-earth nitrates from ethylene glycol to n-dodecane by Cyanex 923
    Batchu, Nagaphani Kumar; Vander Hoogerstraete, Tom; Banerjee, Dipanjan; Binnemans, Koen
    Separation and Purification Technology (2017), 174 (), 544-553CODEN: SPUTFP; ISSN:1383-5866. (Elsevier B.V.)
    A solvent extn. process comprising two immiscible org. phases was developed for the extn. of rare earths. The more polar org. phase was ethylene glycol with dissolved rare-earth nitrate salts and lithium nitrate, while the less polar phase was a soln. of the neutral extractant Cyanex 923 dissolved in n-dodecane. The solvent extn. mechanism was detd. by slope anal. and the main species in the org. phase was identified by Extended x-ray Absorption Fine Structure (EXAFS) studies. The extn. from the ethylene glycol soln. was compared with extn. from an aq. feed soln. When compared to aq. feed solns., the light rare-earth elements (LREEs) are less efficiently extd. and the heavy rare-earth elements (HREEs) more efficiently extd. from an ethylene glycol feed soln., resulting into the easy sepn. of HREEs from LREEs. The sepn. factors between neighboring elements are higher for this non-aq. solvent extn. process than for extn. from an aq. feed soln.
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    Li, Z.; Li, X.; Raiguel, S.; Binnemans, K. Separation of transition metals from rare earths by non-aqueous solvent extraction from ethylene glycol solutions using Aliquat 336. Sep. Purif. Technol. 2018, 201, 318– 326,  DOI: 10.1016/j.seppur.2018.03.022
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    18
    Separation of transition metals from rare earths by nonaqueous solvent extraction from ethylene glycol solutions using Aliquat 336
    Li, Zheng; Li, Xiaohua; Raiguel, Stijn; Binnemans, Koen
    Separation and Purification Technology (2018), 201 (), 318-326CODEN: SPUTFP; ISSN:1383-5866. (Elsevier B.V.)
    Solvent extn. is a widely used sepn. technique in extractive metallurgy. A conventional solvent extn. system consists of an aq. phase and an immiscible org. phase. Replacement of H2O by a polar org. solvent can lead to superior metal sepns. Co(II) and Sm(III) chlorides dissolved in H2O and ethylene glycol (EG), resp., with LiCl as chloride source, were extd. by Aliquat 336 dild. in toluene. Both Co and Sm were extd. from the aq. solns., but Co was extd. more efficiently from the ethylene glycol solns. than from the aq. solns., whereas Sm was not extd. at all from the ethylene glycol solns. As a result, Co and Sm could be sepd. completely in a single extn. step from ethylene glycol solns. The mechanisms of Co extn. by Aliquat 336 from the ethylene glycol and aq. solns. are similar, as validated by slope anal. and UV-visible absorption spectroscopy. Sm was extd. from the aq. soln. through the salting-out effect of LiCl. LiCl has a much lower salting-out effect for Sm in ethylene glycol than in H2O due to the lower dielec. const. of ethylene glycol and the lower soly. of LiCl in ethylene glycol. Consequently, Sm is not salted out from ethylene glycol, leading to a very efficient sepn. of Co and Sm. This sepn. effect can also be applied to the sepn. of other transition metal and rare-earth metal pairs, including Fe/Nd and Zn/Eu.
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    Misawa, M.; Yoshida, K.; Maruyama, K.; Munemura, H.; Hosokawa, Y. Salt-induced phase separation in aqueous solution. J. Phys. Chem. Solids 1999, 60, 1301– 1306,  DOI: 10.1016/S0022-3697(99)00108-0
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    Salt-induced phase separation in aqueous solution
    Misawa, M.; Yoshida, K.; Maruyama, K.; Munemura, H.; Hosokawa, Y.
    Journal of Physics and Chemistry of Solids (1999), 60 (8-9), 1301-1306CODEN: JPCSAW; ISSN:0022-3697. (Elsevier Science Ltd.)
    KCl-induced phase sepn. phenomenon in 1-propanol aq. soln. has been studied by means of small angle (SANS) and quasi-elastic neutron scattering measurements. The significant increase of SANS intensity by the addn. of KCl is attributed to the aggregate of clusters, of which the fractal nature of dimension 1.8-1.9 is found. The motion of water mols. is much suppressed in the 1-propanol-water soln., while it is slightly recovered by the addn. of KCl. The aggregate of clusters and liq.-liq. phase sepn. are discussed in relation to the motion of water mols.
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  20. 20
    Batchu, N. K.; Vander Hoogerstraete, T.; Banerjee, D.; Binnemans, K. Separation of rare-earth ions from ethylene glycol (+LiCl) solutions by non-aqueous solvent extraction with Cyanex 923. RSC Adv. 2017, 7, 45351– 45362,  DOI: 10.1039/C7RA09144C
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    Separation of rare-earth ions from ethylene glycol (+LiCl) solutions by non-aqueous solvent extraction with Cyanex 923
    Batchu, Nagaphani Kumar; Vander Hoogerstraete, Tom; Banerjee, Dipanjan; Binnemans, Koen
    RSC Advances (2017), 7 (72), 45351-45362CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    The sepn. of a mixt. of rare earths by non-aq. solvent extn. with two immiscible org. phases has been studied. The more polar org. phase was ethylene glycol with dissolved lithium chloride and the less polar org. phase was the extractant dild. in n-dodecane. Cyanex 923 was found to be the most performant extractant amongst the investigated acidic, basic and solvating extractants: Cyanex 272, Cyphos IL 101, Aliquat 336, bis(2-ethylhexyl)amine, trioctylphosphine oxide (TOPO) and Cyanex 923. The replacement of the aq. chloride feed solns. by non-aq. ethylene glycol feed solns. had a profound effect on the distribution ratios and sepn. factors. The sepn. factors for extn. of pairs of rare earths from aq. chloride solns. by Cyanex 923 are too low to be of practical use. On the contrary, a mixt. of rare earths can be sepd. conveniently in four different groups by extn. with Cyanex 923 from ethylene glycol (+LiCl) solns. The influence of several parameters such as the chloride concn., the type of chloride salt, the addn. of other polar solvents to the ethylene glycol phase, the addn. of second extractant to the less polar org. phase, and the addn. of complexing agents to the ethylene glycol phase has been studied. The extn. mechanism for extn. of ytterbium(III) was studied by slope anal. expts. The ytterbium(III) species in the ethylene glycol phase and the extd. species in the n-dodecane phase were detd. by EXAFS. Furthermore, a conceptual flow sheet for the fractionation of rare earths from an ethylene glycol (+LiCl) feed soln. into different groups by extn. with Cyanex 923 has been proposed. The new extn. system is useful for extn. of scandium and for sepn. of scandium from the other REEs.
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    Casás, L. M.; Touriño, A.; Orge, B.; Marino, G.; Iglesias, M.; Tojo, J. Thermophysical properties of acetone or methanol + n–alkane (C9 to C12) mixtures. J. Chem. Eng. Data 2002, 47, 887– 893,  DOI: 10.1021/je0103059
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    Thermophysical Properties of Acetone or Methanol + n-Alkane (C9 to C12) Mixtures
    Casas, Lidia M.; Tourino, Antonio; Orge, Beatriz; Marino, Gonzalo; Iglesias, Miguel; Tojo, Jose
    Journal of Chemical and Engineering Data (2002), 47 (4), 887-893CODEN: JCEAAX; ISSN:0021-9568. (American Chemical Society)
    Liq.-liq. equil. for methanol + n-alkanes (C9 to C12) were detd. in the temp. range from 278.15 to 308.15 K, using a visual static method. Densities, refractive indexes, and speeds of sound were also measured at 288.15, 298.15, and 308.15 K for acetone + n-alkane mixts. and at 298.15 and 308.15 K for methanol + n-alkane mixts. to study the temp. influence on mixing phenomena. The results were used to model liq.-liq. coexistence and derived properties trend. The calcd. values based on the UNIQUAC equation were found to be similar to those based on the NRTL model. The Prigogine-Flory-Paterson theory was applied to est. the excess molar volumes and the UNIFAC group contribution model for phase equil.
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    Casás, L. M.; Orge, B.; Díaz, C.; Tojo, J. Liquid-liquid equilibria for mixtures of methyl acetate+methanol+n-alkane (C10–C12) at several temperatures and 1 atm. J. Chem. Thermodyn. 2004, 36, 237– 243,  DOI: 10.1016/j.jct.2003.11.009
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    Liquid-liquid equilibria for mixtures of {methyl acetate + methanol + n-alkane (C10-C12)} at several temperatures and 1 atm
    Casas, Lidia M.; Orge, Beatriz; Diaz, Concepcion; Tojo, Jose
    Journal of Chemical Thermodynamics (2004), 36 (3), 237-243CODEN: JCTDAF; ISSN:0021-9614. (Elsevier Science Ltd.)
    The liq.-liq. equil. of the mixts. {methyl acetate + methanol + n-alkane (C10-C12)} at atm. pressure in the temp. range (278.15 to 308.15) K are presented. The equil. compns. were measured by gas chromatog. from samples splitted isothermally into a glass-stirred device and phase diagrams are reported at each temp. The exptl. detd. liq.-liq. equil. were satisfactorily correlated by the NRTL and UNIQUAC equations. A poor description has been obtained using the UNIFAC method and their modifications.
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    Touriño, A.; Casás, L.; Marino, G.; Iglesias, M.; Orge, B.; Tojo, J. Liquid phase behaviour and thermodynamics of acetone+methanol+n-alkane (C9–C12) mixtures. Fluid Phase Equilib. 2003, 206, 61– 85,  DOI: 10.1016/S0378-3812(02)00306-0
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    Liquid phase behaviour and thermodynamics of acetone+methanol+n-alkane (C9-C12) mixtures
    Tourino, A.; Casas, L. M.; Marino, G.; Iglesias, M.; Orge, B.; Tojo, J.
    Fluid Phase Equilibria (2003), 206 (1-2), 61-85CODEN: FPEQDT; ISSN:0378-3812. (Elsevier Science B.V.)
    This paper reports the results of a new exptl. study of two-liq. phases equil. as a function of temp. and one-liq. phase thermodn. magnitudes (densities, refractive indexes and speeds of sound), covering the compn. diagrams for the mixts. acetone + methanol + n-alkane (C9-C12). The obtained coexistence curves were very asym. with respect to equimolar alc. + n-alkane compn., such effect increasing with the length of the aliph. chain and temp. A comparative anal. was performed by application of different methods to predict exptl. liq.-liq. equil. (LLE) behavior and thermodn. of these ternary mixts. The obtained exptl. results let us know the potential role of the linear aliph. alkanes as sepn. agents for heterogeneous modified distn. of the azeotrope acetone + methanol.
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    Martínez, L.; Andrade, R.; Birgin, E. G.; Martínez, J. M. A package for building initial configurations for molecular dynamics simulations. J. Comput. Chem. 2009, 30, 2157– 2164,  DOI: 10.1002/jcc.21224
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    PACKMOL: A package for building initial configurations for molecular dynamics simulations
    Martinez, L.; Andrade, R.; Birgin, E. G.; Martinez, J. M.
    Journal of Computational Chemistry (2009), 30 (13), 2157-2164CODEN: JCCHDD; ISSN:0192-8651. (John Wiley & Sons, Inc.)
    Adequate initial configurations for mol. dynamics simulations consist of arrangements of mols. distributed in space in such a way to approx. represent the system's overall structure. In order that the simulations are not disrupted by large van der Waals repulsive interactions, atoms from different mols. must keep safe pairwise distances. Obtaining such a mol. arrangement can be considered a packing problem: Each type mol. must satisfy spatial constraints related to the geometry of the system, and the distance between atoms of different mols. must be greater than some specified tolerance. We have developed a code able to pack millions of atoms, grouped in arbitrarily complex mols., inside a variety of three-dimensional regions. The regions may be intersections of spheres, ellipses, cylinders, planes, or boxes. The user must provide only the structure of one mol. of each type and the geometrical constraints that each type of mol. must satisfy. Building complex mixts., interfaces, solvating biomols. in water, other solvents, or mixts. of solvents, is straightforward. In addn., different atoms belonging to the same mol. may also be restricted to different spatial regions, in such a way that more ordered mol. arrangements can be built, as micelles, lipid double-layers, etc. The packing time for state-of-the-art mol. dynamics systems varies from a few seconds to a few minutes in a personal computer. The input files are simple and currently compatible with PDB, Tinker, Molden, or Moldy coordinate files. The package is distributed as free software and can be downloaded from . © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009.
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    Fast parallel algorithms for short-range molecular dynamics
    Plimpton, Steve
    Journal of Computational Physics (1995), 117 (1), 1-19CODEN: JCTPAH; ISSN:0021-9991.
    Three parallel algorithms for classical mol. dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-at. forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for mol. dynamics models which can be difficult to parallelize efficiently - those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a std. Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers - the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C90 processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex mol. dynamics simulations are also discussed.
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    Jorgensen, W. L.; Maxwell, D. S.; Tirado-Rives, J. Development and testing of the OPLS all-atom force field on conformational energetics and properties of organic liquids. J. Am. Chem. Soc. 1996, 118, 11225– 11236,  DOI: 10.1021/ja9621760
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    Development and Testing of the OPLS All-Atom Force Field on Conformational Energetics and Properties of Organic Liquids
    Jorgensen, William L.; Maxwell, David S.; Tirado-Rives, Julian
    Journal of the American Chemical Society (1996), 118 (45), 11225-11236CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The parametrization and testing of the OPLS all-atom force field for org. mols. and peptides are described. Parameters for both torsional and nonbonded energetics have been derived, while the bond stretching and angle bending parameters have been adopted mostly from the AMBER all-atom force field. The torsional parameters were detd. by fitting to rotational energy profiles obtained from ab initio MO calcns. at the RHF/6-31G*//RHF/6-31G* level for more than 50 org. mols. and ions. The quality of the fits was high with av. errors for conformational energies of less than 0.2 kcal/mol. The force-field results for mol. structures are also demonstrated to closely match the ab initio predictions. The nonbonded parameters were developed in conjunction with Monte Carlo statistical mechanics simulations by computing thermodn. and structural properties for 34 pure org. liqs. including alkanes, alkenes, alcs., ethers, acetals, thiols, sulfides, disulfides, aldehydes, ketones, and amides. Av. errors in comparison with exptl. data are 2% for heats of vaporization and densities. The Monte Carlo simulations included sampling all internal and intermol. degrees of freedom. It is found that such non-polar and monofunctional systems do not show significant condensed-phase effects on internal energies in going from the gas phase to the pure liqs.
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    Das, A.; Sahu, P.; Ali, S. M. Molecular dynamics simulation for the calibration of the OPLS force field using DFT derived partial charges for the screening of alkyl phosphate ligands by studying structure, dynamics, and thermodynamics. J. Chem. Eng. Data 2017, 62, 2280– 2295,  DOI: 10.1021/acs.jced.7b00096
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    Molecular Dynamics Simulation for the Calibration of the OPLS Force Field Using DFT Derived Partial Charges for the Screening of Alkyl Phosphate Ligands by Studying Structure, Dynamics, and Thermodynamics
    Das, Arya; Sahu, Pooja; Ali, Sk. Musharaf
    Journal of Chemical & Engineering Data (2017), 62 (8), 2280-2295CODEN: JCEAAX; ISSN:0021-9568. (American Chemical Society)
    Mol. dynamics (MD) simulations were performed to calibrate the all-atom optimized potential for liq. simulations (OPLS-AA) force field using partial quantum charges calcd. from four different population anal. methods: Mulliken, Lowdin, NPA, and ChelpG for predicting the thermophys. properties of pure liqs. like tri-n-butylphosphate (TBP), tri-isoamylphosphate (TiAP), triethylphosphate (TEP), and dodecane to det. a potential solvent for the nuclear fuel cycle. The structural, dynamic, and thermodn. properties were calcd. in NVT ensembles by introducing the partial charges on each atom calcd. from d. functional theory (DFT). The calcd. structural and dynamic properties were affected by the different partial charges on TBP, TiAP, and TEP. The estd. liq. d. employing partial charges obtained from Mulliken population anal. with OPLS force field leads to an excellent agreement with the exptl. data (within 0.36-1.41%). The diffusivity and the pair correlation function (PCF) for all of the ligands have been calcd. and validated wherever literature data is available. The free energies of hydration and solvation for all of the ligands were evaluated using thermodn. integration technique and the hydration free energy for TEP is within 8.3% of the exptl. value, and for other properties they are not available in the literature for comparison. Furthermore, the partition coeff. of the ligands calcd. using MD derived free energy difference between the water-dodecane system resembles the trend predicted by DFT/COSMO-RS calcns. which is in qual. agreement with the exptl. results. Among the four-charge model, the computed dipole moment of TBP and TEP using the Mulliken charge is found to be in good agreement with the exptl. results. Finally, the superiority of TiAP over TBP as an extg. agent for the UO22+ ion has been demonstrated by a higher calcd. free energy of extn., ΔGext, over TBP using DFT. Overall the Mulliken charge embedded calibrated OPLS-AA force field is perhaps the most reliable one as it does not incorporate any arbitrary scaling in the force field or Lennard-Jones parameters and thus can be used indubitably to evaluate the liq. state properties of alkyl phosphates and n-alkanes which eventually assist in the invent of future generation extractants.
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    Proceedings of the Physical Society, London (1931), 43 (), 461-82CODEN: PPSOAU; ISSN:0370-1328.
    A review. The various types of cohesive forces, such as homeopolar, ionic and metallic, are considered from the viewpoint of wave mechanics; each type is discussed. The nature of the van der Waals fields is treated and previous work, including the author's, is discussed.
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    Brehm, M.; Kirchner, B. TRAVIS - A free analyzer and visualizer for monte carlo and molecular dynamics trajectories. J. Chem. Inf. Model. 2011, 51, 2007– 2023,  DOI: 10.1021/ci200217w
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    TRAVIS - A Free Analyzer and Visualizer for Monte Carlo and Molecular Dynamics Trajectories
    Brehm, Martin; Kirchner, Barbara
    Journal of Chemical Information and Modeling (2011), 51 (8), 2007-2023CODEN: JCISD8; ISSN:1549-9596. (American Chemical Society)
    We present TRAVIS ("TRajectory Analyzer and VISualizer"), a free program package for analyzing and visualizing Monte Carlo and mol. dynamics trajectories. The aim of TRAVIS is to collect as many analyses as possible in one program, creating a powerful tool and making it unnecessary to use many different programs for evaluating simulations. This should greatly rationalize and simplify the work-flow of analyzing trajectories. TRAVIS is written in C++, open-source free-ware and licensed under the terms of the GNU General Public License (GPLv3). It is easy to install (platform independent, no external libraries) and easy to use. In this article, we present some of the algorithms that are implemented in TRAVIS - many of them widely known for a long time, but some of them also to appear in literature for the first time. All shown analyses only require a std. MD trajectory as input data.
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    Brehm, M.; Weber, H.; Thomas, M.; Hollòczki, O.; Kirchner, B. Domain analysis in nanostructured liquids: A post-molecular dynamics study at the example of ionic liquids. ChemPhysChem 2015, 16, 3271– 3277,  DOI: 10.1002/cphc.201500471
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    Domain Analysis in Nanostructured Liquids: A Post-Molecular Dynamics Study at the Example of Ionic Liquids
    Brehm, Martin; Weber, Henry; Thomas, Martin; Holloczki, Oldamur; Kirchner, Barbara
    ChemPhysChem (2015), 16 (15), 3271-3277CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
    In the present computational work, we develop a new tool for our trajectory anal. program TRAVIS to analyze the well-known behavior of liqs. to sep. into microphases. The dissection of the liq. into domains of different subsets, for example, in the case of fluorinated ionic liqs. with anionic and cationic head groups (forming together the polar domain), fluorous, and alkyl subsets is followed by radical Voronoi tessellation. This leads to useful av. quantities of the subset neighbor count, i.e., the domain count that gives the amt. of particular domains in the liq., the domain vol. and surface, as well as the isoperimetric quotient, which provides a measure of the deviation of the domains from a spherical shape. Thus, the newly implemented method allows anal. of the domains in terms of their nos. and shapes on a qual. and also quant. basis. It is a simple, direct, and automated anal. that does not need evaluation of the structure beforehand in terms of, for example, first solvent shell min. In the microheterogeneous ionic liqs. that we used as examples, the polar subsets always form a single domain in all investigated liqs. Although the fluorous side chains are also more or less connected in one or, maximally, two domains, the alkyl phases are dispersed.
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    Luzar, A.; Chandler, D. Effect of environment on hydrogen bond dynamics in liquid water. Phys. Rev. Lett. 1996, 76, 928– 931,  DOI: 10.1103/PhysRevLett.76.928
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    Effect of environment on hydrogen bond dynamics in liquid water
    Luzar, Alenka; Chandler, David
    Physical Review Letters (1996), 76 (6), 928-31CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    In a series of mol.-dynamics calcns., we simulate the dynamics of forming and breaking a hydrogen bond in liq. water at room temp. We show that this dynamics is clearly nonexponential, yet virtually uncorrelated with the fluctuations of neighboring bonds.
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    Luzar, A.; Chandler, D. Hydrogen-bond kinetics in liquid water. Nature 1996, 379, 55– 57,  DOI: 10.1038/379055a0
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    Hydrogen-bond kinetics in liquid water
    Luzar, Alenka; Chandler, David
    Nature (London) (1996), 379 (6560), 55-7CODEN: NATUAS; ISSN:0028-0836. (Macmillan Magazines)
    Hydrogen bonds play a crucial role in the behavior of water; their spatial patterns and fluctuations characterize the structure and dynamics of the liq. The processes of breaking and making hydrogen bonds in the condensed phase can be probed indirectly by a variety of exptl. techniques, and more quant. information can be obtained from computer simulations. In particular, simulations have revealed that on long timescales the relaxation behavior of hydrogen bonds in liq. water exhibit non-exponential kinetics, suggesting that bond making and breaking are not simple processes characterized by well defined rate consts. Here we show that these kinetics can be understood in terms of an interplay between diffusion and hydrogen-bond dynamics. In our model, which can be extended to other hydrogen-bonded liqs., diffusion governs whether a specific pair of water mols. are near neighbors, and hydrogen bonds between such pairs form and persist at random with av. lifetimes detd. by rate consts. for bond making and breaking.
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    Luzar, A. Resolving the hydrogen bond dynamics conundrum. J. Chem. Phys. 2000, 113, 10663– 10675,  DOI: 10.1063/1.1320826
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    Resolving the hydrogen bond dynamics conundrum
    Luzar, Alenka
    Journal of Chemical Physics (2000), 113 (23), 10663-10675CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    This paper analyzes dynamic properties of hydrogen bonds in liq. water. We use mol. dynamics simulation to calc. different probability densities that govern the time evolution of the formation and rupture of hydrogen bonds. We provide anal. connections between these functions. Excellent agreement with our simulation results is obsd. We prove transition state theory rate const. to be identical to the inverse of the assocd. mean first passage time (hydrogen bond lifetime). Hence, the anal. establishes its Arrhenius temp. dependence. We give the explicit relation between reactive flux correlation function for the relaxation dynamics of hydrogen bonds, and their first passage time probability densities. All the different observations in the existing literature, assocd. with various ests. of hydrogen bonding times in liq. water that are affected (or not affected) by particular bond criteria, as well as by different definitions of hydrogen bond lifetimes applied in simulation, can be easily reconciled within the framework of reactive flux correlation function approach.
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    Gehrke, S.; von Domaros, M.; Clark, R.; Hollòczki, O.; Brehm, M.; Welton, T.; Luzar, A.; Kirchner, B. Structure and lifetimes in ionic liquids and their mixtures. Faraday Discuss. 2018, 206, 219– 245,  DOI: 10.1039/C7FD00166E
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    Structure and lifetimes in ionic liquids and their mixtures
    Gehrke, Sascha; von Domaros, Michael; Clark, Ryan; Holloczki, Oldamur; Brehm, Martin; Welton, Tom; Luzar, Alenka; Kirchner, Barbara
    Faraday Discussions (2018), 206 (Ionic Liquids: From Fundamental Properties to Practical Applications), 219-245CODEN: FDISE6; ISSN:1359-6640. (Royal Society of Chemistry)
    With the aid of mol. dynamics simulations, we study the structure and dynamics of different ionic liq. systems, with focus on hydrogen bond, ion pair and ion cage formation. To do so, we report radial distribution functions, their no. integrals, and various time-correlation functions, from which we ext. well-defined lifetimes by means of the reactive flux formalism. We explore the influence of polarizable force fields vs. non-polarizable ones with downscaled charges (±0.8) for the example of 1-butyl-3-methylimidazolium bromide. Furthermore, we use 1-butyl-3-methylimidazolium trifluoromethanesulfonate to investigate the impact of temp. and mixing with water as well as with the chloride ionic liq. Smaller coordination nos., larger distances, and tremendously accelerated dynamics are obsd. when the polarizable force field is applied. The same trends are found with increasing temp. Adding water decreases the ion-ion coordination nos. whereas the water-ion and water-water coordination is enhanced. A domain anal. reveals that the nonpolar parts of the ions are dispersed and when more water is added the water clusters increase in size. The dynamics accelerate in general upon addn. of water. In the ionic liq. mixt., the coordination no. around the cation changes between the two anions, but the no. integrals of the cation around the anions remain const. and the dynamics slow down with increasing content of the chloride ionic liq.
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  38. 38
    Brehm, M.; Weber, H.; Thomas, M.; Hollòczki, O.; Kirchner, B. Domain analysis in nanostructured liquids: a post-molecular dynamics study at the example of ionic liquids. ChemPhysChem 2015, 16, 3271– 3277,  DOI: 10.1002/cphc.201500471
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    38
    Domain Analysis in Nanostructured Liquids: A Post-Molecular Dynamics Study at the Example of Ionic Liquids
    Brehm, Martin; Weber, Henry; Thomas, Martin; Holloczki, Oldamur; Kirchner, Barbara
    ChemPhysChem (2015), 16 (15), 3271-3277CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
    In the present computational work, we develop a new tool for our trajectory anal. program TRAVIS to analyze the well-known behavior of liqs. to sep. into microphases. The dissection of the liq. into domains of different subsets, for example, in the case of fluorinated ionic liqs. with anionic and cationic head groups (forming together the polar domain), fluorous, and alkyl subsets is followed by radical Voronoi tessellation. This leads to useful av. quantities of the subset neighbor count, i.e., the domain count that gives the amt. of particular domains in the liq., the domain vol. and surface, as well as the isoperimetric quotient, which provides a measure of the deviation of the domains from a spherical shape. Thus, the newly implemented method allows anal. of the domains in terms of their nos. and shapes on a qual. and also quant. basis. It is a simple, direct, and automated anal. that does not need evaluation of the structure beforehand in terms of, for example, first solvent shell min. In the microheterogeneous ionic liqs. that we used as examples, the polar subsets always form a single domain in all investigated liqs. Although the fluorous side chains are also more or less connected in one or, maximally, two domains, the alkyl phases are dispersed.
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    Elfgen, R.; Hollòczki, O.; Kirchner, B. A molecular level understanding of template effects in ionic liquids. Acc. Chem. Res. 2017, 50, 2949– 2957,  DOI: 10.1021/acs.accounts.7b00436
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    A Molecular Level Understanding of Template Effects in Ionic Liquids
    Elfgen, Roman; Holloczki, Oldamur; Kirchner, Barbara
    Accounts of Chemical Research (2017), 50 (12), 2949-2957CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    The structure-directing or template effect has been invoked several times for ionic liqs. to explain the different outcome in material synthesis, namely, different scaffolds or geometrical arrangements with varying ionic liqs. It is obvious to assume that such an effect can originate from the most likely complex microstructure, being present within the ionic liq. itself. In that regard, ionic liqs. have already been shown to undergo a nanosegregation into polar and nonpolar phases, which is commonly known and denoted as microheterogeneity. In order to provide detailed insight on the mol. level and to understand the effects rising from this structuring, the authors performed mol. dynamics simulations on selected very simple model systems composed of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, considering Et, Bu, hexyl, and octyl side chains attached to the cations, mixed with either n-dodecanol or n-butanol. By analyzing snapshots of the simulation boxes and calcg. spatial distribution functions, one can visualize that with increasing side chains, the systems show considerable nanosegregation into polar and nonpolar domains. Combined angular and distance distribution functions show that in case of the nanosegregating systems the side chains of the cations are preferentially arranged in a parallel fashion, which indicates a micelle-like structure for the ionic liqs. The alc. mols. participate in and are, therefore, influenced by this microheterogeneity. It can be shown that in the case of the short IL alkyl side chains, the self-aggregation of the nonpolar units of the alcs. is much stronger, while for the long chain cations, the nonpolar entities of the alcs. are most often connected to the nonpolar units of the ionic liqs. Using our domain anal. tool, we can quantify these observations by tracking the no., size, and shape of the polar and nonpolar entities present in the different investigated systems. The aforementioned combined angular-distance distribution functions reveal a structure-directing effect of the ionic liqs. on the alc. mols. within our simple model systems. The ionic liqs. act as template and order the alc. mols. according to their own structure, resulting in a parallel alignment of the alkyl side chains of the alcs. and ionic liq. cations, with both polar groups being at the same side. These observations show that the microheterogeneous structure of ionic liqs. can indeed be applied to order substrates with respect to each other or, for example, to catalysts in a predetd. fashion, opening new possibilities for explaining or enhancing selectivities of chem. reactions in ionic liqs.
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    Macchieraldo, R.; Esser, L.; Elfgen, R.; Voepel, P.; Zahn, S.; Smarsly, B. M.; Kirchner, B. Hydrophilic ionic liquid mixtures of weakly and strongly coordinating anions with and without water. ACS Omega 2018, 3, 8567– 8582,  DOI: 10.1021/acsomega.8b00995
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    Hydrophilic Ionic Liquid Mixtures of Weakly and Strongly Coordinating Anions with and without Water
    Macchieraldo, Roberto; Esser, Lars; Elfgen, Roman; Voepel, Pascal; Zahn, Stefan; Smarsly, Bernd M.; Kirchner, Barbara
    ACS Omega (2018), 3 (8), 8567-8582CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)
    With the aid of ab initio mol. dynamics simulations, we investigate an ionic liq. mixt. composed of three components 1-butyl-3-methylimidazolium [C4C1Im]+, tetrafluoroborate [BF4]- and chloride [Cl]- without and with water. In the pure IL mixt., we observe an already complex network of interactions between cations and anions, adding water to the system even extends the complexity. Obsd. no. integrals show that the coordination no. between cations and anions is reduced in the system with water compared to the pure system. Further studies show, that the Coulombic network of the strongly coordinating anion [Cl]- is disturbed by water, while the weakly coordinating anion [BF4]- is not. These observations can also be confirmed by the Voronoi polyhedra anal., which shows that the polar network of microheterogeneous IL collapses by the introduction of water. Hydrogen-acceptor interactions revealed, that the [Cl]- anions are transferred from being situated in the IL to the water continuum while [BF4]-is almost unperturbed, these effects mainly influence the interplay of the ionic liq. network.
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    Ray, P.; Vogl, T.; Balducci, A.; Kirchner, B. Structural investigations on lithium-doped protic and aprotic ionic liquids. J. Phys. Chem. B 2017, 121, 5279– 5292,  DOI: 10.1021/acs.jpcb.7b02636
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    Structural Investigations on Lithium-Doped Protic and Aprotic Ionic Liquids
    Ray, Promit; Vogl, Thomas; Balducci, Andrea; Kirchner, Barbara
    Journal of Physical Chemistry B (2017), 121 (20), 5279-5292CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
    Solns. of lithium bis(trifluoromethanesulfonyl)imide (LiNTf2), in four different [NTf2]--based ionic liqs., are extensively investigated as potential electrolytes for lithium-ion batteries. Solvation of the [Li]+ ions in the ionic liqs. and its impact on their physicochem. properties are studied herein with the aid of mol. dynamics simulations. The cationic components of the investigated liqs. were systematically varied so as to individually evaluate effects of specific structural changes; increase in ring size, the addn. of an alkyl chain and absence of an acidic proton, on the solvation and mobility of the [Li]+ cations. The studied cations also allow for a direct comparison between solns. of [Li]+ salt in protic and aprotic ionic liqs. Emphasis is laid on elucidating the interactions between the [Li]+ and [NTf2]- ions revealing slightly higher coordination nos. for the aprotic solvent, benchmarked against exptl. measurements. The study suggests that the ionic liqs. largely retain their structure upon salt addn., with interactions within the liqs. only slightly perturbed. The rattling motion of the [Li]+ cations within cages formed by the surrounding [NTf2]- anions is examd. by the anal. of [Li]+ autocorrelation functions. Overall, the solvation mechanism of [Li]+ salt, within the hydrogen-bonded network of the ionic liqs., is detailed from classical and ab initio mol. dynamics simulations.
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    Kirchner, B.; Hutter, J.; Kuo, I.-F. W.; Mundy, C. J. Hydrophobic hydratation from Car-Parrinello simulations. Int. J. Mod. Phys. B 2004, 18, 1951– 1962,  DOI: 10.1142/S0217979204007241
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    Hydrophobic hydration from Car-Parrinello simulations
    Kirchner, Barbara; Hutter, Juerg; Kuo, I.-Feng W.; Mundy, Christopher J.
    International Journal of Modern Physics B: Condensed Matter Physics, Statistical Physics, Applied Physics (2004), 18 (14), 1951-1962CODEN: IJPBEV; ISSN:0217-9792. (World Scientific Publishing Co. Pte. Ltd.)
    In this work we investigate the fast anomalous diffusion of hydrogen mols. in water using Car-Parrinello mol. dynamics simulations. We employ Voronoi polyhedra anal. to distinguish between void diffusion and void hopping. Our results indicate that a combination of both mechanism is sufficient to explain anomalous diffusion. Furthermore, we investigate the geometry and the electronic structure of the first solvation shell.
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    The H bond structure of liq. MeOH was studied as a function of pressure and temp. up to 2.8 kbar and from 297 to 413 K. Chem. shifts of the CH3 and OH groups were monitored throughout this pressure and temp. regime, and the chem. shift difference between these two groups was used to describe changes of the H bond network in MeOH. The H bond equil. was studied using mol. dynamics simulations and a phenomenol. model describing clustering in liq. MeOH. Results are presented concerning the size and distribution of H-bonded clusters in MeOH as a function of pressure and temp. The extent of H bonding decreases upon an increase in temp. The results for pressure are equivocal, the phenomenol. model suggests that H bonding decreases with increasing pressure, which supports earlier interpretations regarding the measured self-diffusion coeffs. in deuterated MeOH as a function of pressure. The mol. dynamics simulations, however, show an increase in H bonding with increasing pressure.
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  • Abstract

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    Figure 1

    Figure 1. Schematic representation of the procedure described in this work.

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    Figure 2

    Figure 2. Snapshot taken from the MLDT system. All atoms are represented by their van der Waals radii. Color scheme: yellow for LiCl, red for MeOH, green for TBP, and blue for DD.

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    Figure 3

    Figure 3. Mean square displacements of the bottom phase components. Top left: MeOH; top right: DD; bottom left: Li+ and Cl–; bottom right: DD and TBP.

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    Figure 4

    Figure 4. RDFs of the interactions of MeOH molecules in the bottom phases.

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    Figure 5

    Figure 5. Example of the liquid structure of MeOH in the presence of LiCl taken from the MLDT system. LiCl is represented with van der Waals radii while MeOH is displayed with the ball-and-stick models. Color scheme: brown for Li, green for Cl, orange for C, red for O, and white for H.

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    Figure 6

    Figure 6. Intramolecular RDFs of the distance between the terminal carbons of DD molecules.

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      A novel technique to sep. ionic liqs. from org. compds. is introduced which uses carbon dioxide to induce the formation of an ionic liq.-rich phase and an org.-rich liq. phase in mixts. of methanol and 3-butyl-1-methyl-imidazolium hexafluorophosphate ([C4mim][PF6]). If the temp. is above the crit. temp. of CO2 then the methanol-rich phase can become completely miscible with the CO2-rich phase, and this new phase is completely ionic liq.-free. Since CO2 is nonpolar, it is not equipped to solvate ions. As the CO2 dissolves in the methanol/[C4mim][PF6] mixt., the solvent power of the CO2-expanded liq. is significantly reduced, inducing the formation of the second liq. phase that is rich in ionic liq. This presents a new way to recover products from ionic liq. mixts. and purify org. phases that have been contaminated with ionic liq. Moreover, these results have important implications for reactions done in CO2/ionic liq. biphasic mixts.
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      Selective leaching of metals from ores with organic solvents and chelating agents
      Sastri, V. S.
      Journal of Scientific & Industrial Research (1975), 34 (12), 663-5CODEN: JSIRAC; ISSN:0022-4456.
      A review with 16 refs. is given on the use of org. solvents and chelating agents in the selective leaching of metals from ores.
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      Raghavan, S.; Fuerstenau, D. A Lyometallurgical Process for Leaching Copper from Chrysocolla. In Hydrometallurgy Fundamentals, Technology and Innovations , Proceedings of the Milton E., Wordsworth (IV) Int. Symposium on Hydrometallurgy, 1993; pp 283– 297.
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      Chizhevskaya, S. V.; Chekmarev, A. M.; Klimenko, O. M.; Povetkina, M. V.; Sinegribova, O. A.; Cox, M. In Nontraditional Methods of Treating High-Silicon Ores Containing Rare Elements , Hydrometallurgy ’94: Papers presented at the international symposium ‘Hydrometallurgy ’94’ organized by the Institution of Mining and Metallurgy and the Society of Chemical Industry, and held in Cambridge, England, from 11 to 15 July, 1994; pp 219– 228.
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      Batchu, N. K.; Vander Hoogerstraete, T.; Banerjee, D.; Binnemans, K. Non-aqueous solvent extraction of rare-earth nitrates from ethylene glycol to n-dodecane by Cyanex 923. Sep. Purif. Technol. 2017, 174, 544– 553,  DOI: 10.1016/j.seppur.2016.10.039
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      Non-aqueous solvent extraction of rare-earth nitrates from ethylene glycol to n-dodecane by Cyanex 923
      Batchu, Nagaphani Kumar; Vander Hoogerstraete, Tom; Banerjee, Dipanjan; Binnemans, Koen
      Separation and Purification Technology (2017), 174 (), 544-553CODEN: SPUTFP; ISSN:1383-5866. (Elsevier B.V.)
      A solvent extn. process comprising two immiscible org. phases was developed for the extn. of rare earths. The more polar org. phase was ethylene glycol with dissolved rare-earth nitrate salts and lithium nitrate, while the less polar phase was a soln. of the neutral extractant Cyanex 923 dissolved in n-dodecane. The solvent extn. mechanism was detd. by slope anal. and the main species in the org. phase was identified by Extended x-ray Absorption Fine Structure (EXAFS) studies. The extn. from the ethylene glycol soln. was compared with extn. from an aq. feed soln. When compared to aq. feed solns., the light rare-earth elements (LREEs) are less efficiently extd. and the heavy rare-earth elements (HREEs) more efficiently extd. from an ethylene glycol feed soln., resulting into the easy sepn. of HREEs from LREEs. The sepn. factors between neighboring elements are higher for this non-aq. solvent extn. process than for extn. from an aq. feed soln.
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      Li, Z.; Li, X.; Raiguel, S.; Binnemans, K. Separation of transition metals from rare earths by non-aqueous solvent extraction from ethylene glycol solutions using Aliquat 336. Sep. Purif. Technol. 2018, 201, 318– 326,  DOI: 10.1016/j.seppur.2018.03.022
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      Separation of transition metals from rare earths by nonaqueous solvent extraction from ethylene glycol solutions using Aliquat 336
      Li, Zheng; Li, Xiaohua; Raiguel, Stijn; Binnemans, Koen
      Separation and Purification Technology (2018), 201 (), 318-326CODEN: SPUTFP; ISSN:1383-5866. (Elsevier B.V.)
      Solvent extn. is a widely used sepn. technique in extractive metallurgy. A conventional solvent extn. system consists of an aq. phase and an immiscible org. phase. Replacement of H2O by a polar org. solvent can lead to superior metal sepns. Co(II) and Sm(III) chlorides dissolved in H2O and ethylene glycol (EG), resp., with LiCl as chloride source, were extd. by Aliquat 336 dild. in toluene. Both Co and Sm were extd. from the aq. solns., but Co was extd. more efficiently from the ethylene glycol solns. than from the aq. solns., whereas Sm was not extd. at all from the ethylene glycol solns. As a result, Co and Sm could be sepd. completely in a single extn. step from ethylene glycol solns. The mechanisms of Co extn. by Aliquat 336 from the ethylene glycol and aq. solns. are similar, as validated by slope anal. and UV-visible absorption spectroscopy. Sm was extd. from the aq. soln. through the salting-out effect of LiCl. LiCl has a much lower salting-out effect for Sm in ethylene glycol than in H2O due to the lower dielec. const. of ethylene glycol and the lower soly. of LiCl in ethylene glycol. Consequently, Sm is not salted out from ethylene glycol, leading to a very efficient sepn. of Co and Sm. This sepn. effect can also be applied to the sepn. of other transition metal and rare-earth metal pairs, including Fe/Nd and Zn/Eu.
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      Misawa, M.; Yoshida, K.; Maruyama, K.; Munemura, H.; Hosokawa, Y. Salt-induced phase separation in aqueous solution. J. Phys. Chem. Solids 1999, 60, 1301– 1306,  DOI: 10.1016/S0022-3697(99)00108-0
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      Salt-induced phase separation in aqueous solution
      Misawa, M.; Yoshida, K.; Maruyama, K.; Munemura, H.; Hosokawa, Y.
      Journal of Physics and Chemistry of Solids (1999), 60 (8-9), 1301-1306CODEN: JPCSAW; ISSN:0022-3697. (Elsevier Science Ltd.)
      KCl-induced phase sepn. phenomenon in 1-propanol aq. soln. has been studied by means of small angle (SANS) and quasi-elastic neutron scattering measurements. The significant increase of SANS intensity by the addn. of KCl is attributed to the aggregate of clusters, of which the fractal nature of dimension 1.8-1.9 is found. The motion of water mols. is much suppressed in the 1-propanol-water soln., while it is slightly recovered by the addn. of KCl. The aggregate of clusters and liq.-liq. phase sepn. are discussed in relation to the motion of water mols.
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      Batchu, N. K.; Vander Hoogerstraete, T.; Banerjee, D.; Binnemans, K. Separation of rare-earth ions from ethylene glycol (+LiCl) solutions by non-aqueous solvent extraction with Cyanex 923. RSC Adv. 2017, 7, 45351– 45362,  DOI: 10.1039/C7RA09144C
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      Separation of rare-earth ions from ethylene glycol (+LiCl) solutions by non-aqueous solvent extraction with Cyanex 923
      Batchu, Nagaphani Kumar; Vander Hoogerstraete, Tom; Banerjee, Dipanjan; Binnemans, Koen
      RSC Advances (2017), 7 (72), 45351-45362CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      The sepn. of a mixt. of rare earths by non-aq. solvent extn. with two immiscible org. phases has been studied. The more polar org. phase was ethylene glycol with dissolved lithium chloride and the less polar org. phase was the extractant dild. in n-dodecane. Cyanex 923 was found to be the most performant extractant amongst the investigated acidic, basic and solvating extractants: Cyanex 272, Cyphos IL 101, Aliquat 336, bis(2-ethylhexyl)amine, trioctylphosphine oxide (TOPO) and Cyanex 923. The replacement of the aq. chloride feed solns. by non-aq. ethylene glycol feed solns. had a profound effect on the distribution ratios and sepn. factors. The sepn. factors for extn. of pairs of rare earths from aq. chloride solns. by Cyanex 923 are too low to be of practical use. On the contrary, a mixt. of rare earths can be sepd. conveniently in four different groups by extn. with Cyanex 923 from ethylene glycol (+LiCl) solns. The influence of several parameters such as the chloride concn., the type of chloride salt, the addn. of other polar solvents to the ethylene glycol phase, the addn. of second extractant to the less polar org. phase, and the addn. of complexing agents to the ethylene glycol phase has been studied. The extn. mechanism for extn. of ytterbium(III) was studied by slope anal. expts. The ytterbium(III) species in the ethylene glycol phase and the extd. species in the n-dodecane phase were detd. by EXAFS. Furthermore, a conceptual flow sheet for the fractionation of rare earths from an ethylene glycol (+LiCl) feed soln. into different groups by extn. with Cyanex 923 has been proposed. The new extn. system is useful for extn. of scandium and for sepn. of scandium from the other REEs.
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      Casás, L. M.; Touriño, A.; Orge, B.; Marino, G.; Iglesias, M.; Tojo, J. Thermophysical properties of acetone or methanol + n–alkane (C9 to C12) mixtures. J. Chem. Eng. Data 2002, 47, 887– 893,  DOI: 10.1021/je0103059
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      Thermophysical Properties of Acetone or Methanol + n-Alkane (C9 to C12) Mixtures
      Casas, Lidia M.; Tourino, Antonio; Orge, Beatriz; Marino, Gonzalo; Iglesias, Miguel; Tojo, Jose
      Journal of Chemical and Engineering Data (2002), 47 (4), 887-893CODEN: JCEAAX; ISSN:0021-9568. (American Chemical Society)
      Liq.-liq. equil. for methanol + n-alkanes (C9 to C12) were detd. in the temp. range from 278.15 to 308.15 K, using a visual static method. Densities, refractive indexes, and speeds of sound were also measured at 288.15, 298.15, and 308.15 K for acetone + n-alkane mixts. and at 298.15 and 308.15 K for methanol + n-alkane mixts. to study the temp. influence on mixing phenomena. The results were used to model liq.-liq. coexistence and derived properties trend. The calcd. values based on the UNIQUAC equation were found to be similar to those based on the NRTL model. The Prigogine-Flory-Paterson theory was applied to est. the excess molar volumes and the UNIFAC group contribution model for phase equil.
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      Casás, L. M.; Orge, B.; Díaz, C.; Tojo, J. Liquid-liquid equilibria for mixtures of methyl acetate+methanol+n-alkane (C10–C12) at several temperatures and 1 atm. J. Chem. Thermodyn. 2004, 36, 237– 243,  DOI: 10.1016/j.jct.2003.11.009
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      Liquid-liquid equilibria for mixtures of {methyl acetate + methanol + n-alkane (C10-C12)} at several temperatures and 1 atm
      Casas, Lidia M.; Orge, Beatriz; Diaz, Concepcion; Tojo, Jose
      Journal of Chemical Thermodynamics (2004), 36 (3), 237-243CODEN: JCTDAF; ISSN:0021-9614. (Elsevier Science Ltd.)
      The liq.-liq. equil. of the mixts. {methyl acetate + methanol + n-alkane (C10-C12)} at atm. pressure in the temp. range (278.15 to 308.15) K are presented. The equil. compns. were measured by gas chromatog. from samples splitted isothermally into a glass-stirred device and phase diagrams are reported at each temp. The exptl. detd. liq.-liq. equil. were satisfactorily correlated by the NRTL and UNIQUAC equations. A poor description has been obtained using the UNIFAC method and their modifications.
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    23. 23
      Touriño, A.; Casás, L.; Marino, G.; Iglesias, M.; Orge, B.; Tojo, J. Liquid phase behaviour and thermodynamics of acetone+methanol+n-alkane (C9–C12) mixtures. Fluid Phase Equilib. 2003, 206, 61– 85,  DOI: 10.1016/S0378-3812(02)00306-0
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      Liquid phase behaviour and thermodynamics of acetone+methanol+n-alkane (C9-C12) mixtures
      Tourino, A.; Casas, L. M.; Marino, G.; Iglesias, M.; Orge, B.; Tojo, J.
      Fluid Phase Equilibria (2003), 206 (1-2), 61-85CODEN: FPEQDT; ISSN:0378-3812. (Elsevier Science B.V.)
      This paper reports the results of a new exptl. study of two-liq. phases equil. as a function of temp. and one-liq. phase thermodn. magnitudes (densities, refractive indexes and speeds of sound), covering the compn. diagrams for the mixts. acetone + methanol + n-alkane (C9-C12). The obtained coexistence curves were very asym. with respect to equimolar alc. + n-alkane compn., such effect increasing with the length of the aliph. chain and temp. A comparative anal. was performed by application of different methods to predict exptl. liq.-liq. equil. (LLE) behavior and thermodn. of these ternary mixts. The obtained exptl. results let us know the potential role of the linear aliph. alkanes as sepn. agents for heterogeneous modified distn. of the azeotrope acetone + methanol.
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      Martínez, L.; Andrade, R.; Birgin, E. G.; Martínez, J. M. A package for building initial configurations for molecular dynamics simulations. J. Comput. Chem. 2009, 30, 2157– 2164,  DOI: 10.1002/jcc.21224
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      PACKMOL: A package for building initial configurations for molecular dynamics simulations
      Martinez, L.; Andrade, R.; Birgin, E. G.; Martinez, J. M.
      Journal of Computational Chemistry (2009), 30 (13), 2157-2164CODEN: JCCHDD; ISSN:0192-8651. (John Wiley & Sons, Inc.)
      Adequate initial configurations for mol. dynamics simulations consist of arrangements of mols. distributed in space in such a way to approx. represent the system's overall structure. In order that the simulations are not disrupted by large van der Waals repulsive interactions, atoms from different mols. must keep safe pairwise distances. Obtaining such a mol. arrangement can be considered a packing problem: Each type mol. must satisfy spatial constraints related to the geometry of the system, and the distance between atoms of different mols. must be greater than some specified tolerance. We have developed a code able to pack millions of atoms, grouped in arbitrarily complex mols., inside a variety of three-dimensional regions. The regions may be intersections of spheres, ellipses, cylinders, planes, or boxes. The user must provide only the structure of one mol. of each type and the geometrical constraints that each type of mol. must satisfy. Building complex mixts., interfaces, solvating biomols. in water, other solvents, or mixts. of solvents, is straightforward. In addn., different atoms belonging to the same mol. may also be restricted to different spatial regions, in such a way that more ordered mol. arrangements can be built, as micelles, lipid double-layers, etc. The packing time for state-of-the-art mol. dynamics systems varies from a few seconds to a few minutes in a personal computer. The input files are simple and currently compatible with PDB, Tinker, Molden, or Moldy coordinate files. The package is distributed as free software and can be downloaded from . © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009.
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      Plimpton, S. Fast parallel algorithms for short-range molecular dynamics. J. Comput. Phys. 1995, 117, 1– 19,  DOI: 10.1006/jcph.1995.1039
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      Fast parallel algorithms for short-range molecular dynamics
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      Journal of Computational Physics (1995), 117 (1), 1-19CODEN: JCTPAH; ISSN:0021-9991.
      Three parallel algorithms for classical mol. dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-at. forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for mol. dynamics models which can be difficult to parallelize efficiently - those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a std. Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers - the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C90 processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex mol. dynamics simulations are also discussed.
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      Jorgensen, W. L.; Maxwell, D. S.; Tirado-Rives, J. Development and testing of the OPLS all-atom force field on conformational energetics and properties of organic liquids. J. Am. Chem. Soc. 1996, 118, 11225– 11236,  DOI: 10.1021/ja9621760
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      Development and Testing of the OPLS All-Atom Force Field on Conformational Energetics and Properties of Organic Liquids
      Jorgensen, William L.; Maxwell, David S.; Tirado-Rives, Julian
      Journal of the American Chemical Society (1996), 118 (45), 11225-11236CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The parametrization and testing of the OPLS all-atom force field for org. mols. and peptides are described. Parameters for both torsional and nonbonded energetics have been derived, while the bond stretching and angle bending parameters have been adopted mostly from the AMBER all-atom force field. The torsional parameters were detd. by fitting to rotational energy profiles obtained from ab initio MO calcns. at the RHF/6-31G*//RHF/6-31G* level for more than 50 org. mols. and ions. The quality of the fits was high with av. errors for conformational energies of less than 0.2 kcal/mol. The force-field results for mol. structures are also demonstrated to closely match the ab initio predictions. The nonbonded parameters were developed in conjunction with Monte Carlo statistical mechanics simulations by computing thermodn. and structural properties for 34 pure org. liqs. including alkanes, alkenes, alcs., ethers, acetals, thiols, sulfides, disulfides, aldehydes, ketones, and amides. Av. errors in comparison with exptl. data are 2% for heats of vaporization and densities. The Monte Carlo simulations included sampling all internal and intermol. degrees of freedom. It is found that such non-polar and monofunctional systems do not show significant condensed-phase effects on internal energies in going from the gas phase to the pure liqs.
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      Das, A.; Sahu, P.; Ali, S. M. Molecular dynamics simulation for the calibration of the OPLS force field using DFT derived partial charges for the screening of alkyl phosphate ligands by studying structure, dynamics, and thermodynamics. J. Chem. Eng. Data 2017, 62, 2280– 2295,  DOI: 10.1021/acs.jced.7b00096
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      Molecular Dynamics Simulation for the Calibration of the OPLS Force Field Using DFT Derived Partial Charges for the Screening of Alkyl Phosphate Ligands by Studying Structure, Dynamics, and Thermodynamics
      Das, Arya; Sahu, Pooja; Ali, Sk. Musharaf
      Journal of Chemical & Engineering Data (2017), 62 (8), 2280-2295CODEN: JCEAAX; ISSN:0021-9568. (American Chemical Society)
      Mol. dynamics (MD) simulations were performed to calibrate the all-atom optimized potential for liq. simulations (OPLS-AA) force field using partial quantum charges calcd. from four different population anal. methods: Mulliken, Lowdin, NPA, and ChelpG for predicting the thermophys. properties of pure liqs. like tri-n-butylphosphate (TBP), tri-isoamylphosphate (TiAP), triethylphosphate (TEP), and dodecane to det. a potential solvent for the nuclear fuel cycle. The structural, dynamic, and thermodn. properties were calcd. in NVT ensembles by introducing the partial charges on each atom calcd. from d. functional theory (DFT). The calcd. structural and dynamic properties were affected by the different partial charges on TBP, TiAP, and TEP. The estd. liq. d. employing partial charges obtained from Mulliken population anal. with OPLS force field leads to an excellent agreement with the exptl. data (within 0.36-1.41%). The diffusivity and the pair correlation function (PCF) for all of the ligands have been calcd. and validated wherever literature data is available. The free energies of hydration and solvation for all of the ligands were evaluated using thermodn. integration technique and the hydration free energy for TEP is within 8.3% of the exptl. value, and for other properties they are not available in the literature for comparison. Furthermore, the partition coeff. of the ligands calcd. using MD derived free energy difference between the water-dodecane system resembles the trend predicted by DFT/COSMO-RS calcns. which is in qual. agreement with the exptl. results. Among the four-charge model, the computed dipole moment of TBP and TEP using the Mulliken charge is found to be in good agreement with the exptl. results. Finally, the superiority of TiAP over TBP as an extg. agent for the UO22+ ion has been demonstrated by a higher calcd. free energy of extn., ΔGext, over TBP using DFT. Overall the Mulliken charge embedded calibrated OPLS-AA force field is perhaps the most reliable one as it does not incorporate any arbitrary scaling in the force field or Lennard-Jones parameters and thus can be used indubitably to evaluate the liq. state properties of alkyl phosphates and n-alkanes which eventually assist in the invent of future generation extractants.
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      Brehm, Martin; Kirchner, Barbara
      Journal of Chemical Information and Modeling (2011), 51 (8), 2007-2023CODEN: JCISD8; ISSN:1549-9596. (American Chemical Society)
      We present TRAVIS ("TRajectory Analyzer and VISualizer"), a free program package for analyzing and visualizing Monte Carlo and mol. dynamics trajectories. The aim of TRAVIS is to collect as many analyses as possible in one program, creating a powerful tool and making it unnecessary to use many different programs for evaluating simulations. This should greatly rationalize and simplify the work-flow of analyzing trajectories. TRAVIS is written in C++, open-source free-ware and licensed under the terms of the GNU General Public License (GPLv3). It is easy to install (platform independent, no external libraries) and easy to use. In this article, we present some of the algorithms that are implemented in TRAVIS - many of them widely known for a long time, but some of them also to appear in literature for the first time. All shown analyses only require a std. MD trajectory as input data.
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    33. 33
      Brehm, M.; Weber, H.; Thomas, M.; Hollòczki, O.; Kirchner, B. Domain analysis in nanostructured liquids: A post-molecular dynamics study at the example of ionic liquids. ChemPhysChem 2015, 16, 3271– 3277,  DOI: 10.1002/cphc.201500471
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      Domain Analysis in Nanostructured Liquids: A Post-Molecular Dynamics Study at the Example of Ionic Liquids
      Brehm, Martin; Weber, Henry; Thomas, Martin; Holloczki, Oldamur; Kirchner, Barbara
      ChemPhysChem (2015), 16 (15), 3271-3277CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
      In the present computational work, we develop a new tool for our trajectory anal. program TRAVIS to analyze the well-known behavior of liqs. to sep. into microphases. The dissection of the liq. into domains of different subsets, for example, in the case of fluorinated ionic liqs. with anionic and cationic head groups (forming together the polar domain), fluorous, and alkyl subsets is followed by radical Voronoi tessellation. This leads to useful av. quantities of the subset neighbor count, i.e., the domain count that gives the amt. of particular domains in the liq., the domain vol. and surface, as well as the isoperimetric quotient, which provides a measure of the deviation of the domains from a spherical shape. Thus, the newly implemented method allows anal. of the domains in terms of their nos. and shapes on a qual. and also quant. basis. It is a simple, direct, and automated anal. that does not need evaluation of the structure beforehand in terms of, for example, first solvent shell min. In the microheterogeneous ionic liqs. that we used as examples, the polar subsets always form a single domain in all investigated liqs. Although the fluorous side chains are also more or less connected in one or, maximally, two domains, the alkyl phases are dispersed.
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      Luzar, A.; Chandler, D. Effect of environment on hydrogen bond dynamics in liquid water. Phys. Rev. Lett. 1996, 76, 928– 931,  DOI: 10.1103/PhysRevLett.76.928
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      Effect of environment on hydrogen bond dynamics in liquid water
      Luzar, Alenka; Chandler, David
      Physical Review Letters (1996), 76 (6), 928-31CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      In a series of mol.-dynamics calcns., we simulate the dynamics of forming and breaking a hydrogen bond in liq. water at room temp. We show that this dynamics is clearly nonexponential, yet virtually uncorrelated with the fluctuations of neighboring bonds.
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      Luzar, A.; Chandler, D. Hydrogen-bond kinetics in liquid water. Nature 1996, 379, 55– 57,  DOI: 10.1038/379055a0
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      Hydrogen-bond kinetics in liquid water
      Luzar, Alenka; Chandler, David
      Nature (London) (1996), 379 (6560), 55-7CODEN: NATUAS; ISSN:0028-0836. (Macmillan Magazines)
      Hydrogen bonds play a crucial role in the behavior of water; their spatial patterns and fluctuations characterize the structure and dynamics of the liq. The processes of breaking and making hydrogen bonds in the condensed phase can be probed indirectly by a variety of exptl. techniques, and more quant. information can be obtained from computer simulations. In particular, simulations have revealed that on long timescales the relaxation behavior of hydrogen bonds in liq. water exhibit non-exponential kinetics, suggesting that bond making and breaking are not simple processes characterized by well defined rate consts. Here we show that these kinetics can be understood in terms of an interplay between diffusion and hydrogen-bond dynamics. In our model, which can be extended to other hydrogen-bonded liqs., diffusion governs whether a specific pair of water mols. are near neighbors, and hydrogen bonds between such pairs form and persist at random with av. lifetimes detd. by rate consts. for bond making and breaking.
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      Luzar, A. Resolving the hydrogen bond dynamics conundrum. J. Chem. Phys. 2000, 113, 10663– 10675,  DOI: 10.1063/1.1320826
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      Resolving the hydrogen bond dynamics conundrum
      Luzar, Alenka
      Journal of Chemical Physics (2000), 113 (23), 10663-10675CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      This paper analyzes dynamic properties of hydrogen bonds in liq. water. We use mol. dynamics simulation to calc. different probability densities that govern the time evolution of the formation and rupture of hydrogen bonds. We provide anal. connections between these functions. Excellent agreement with our simulation results is obsd. We prove transition state theory rate const. to be identical to the inverse of the assocd. mean first passage time (hydrogen bond lifetime). Hence, the anal. establishes its Arrhenius temp. dependence. We give the explicit relation between reactive flux correlation function for the relaxation dynamics of hydrogen bonds, and their first passage time probability densities. All the different observations in the existing literature, assocd. with various ests. of hydrogen bonding times in liq. water that are affected (or not affected) by particular bond criteria, as well as by different definitions of hydrogen bond lifetimes applied in simulation, can be easily reconciled within the framework of reactive flux correlation function approach.
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      Gehrke, S.; von Domaros, M.; Clark, R.; Hollòczki, O.; Brehm, M.; Welton, T.; Luzar, A.; Kirchner, B. Structure and lifetimes in ionic liquids and their mixtures. Faraday Discuss. 2018, 206, 219– 245,  DOI: 10.1039/C7FD00166E
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      Structure and lifetimes in ionic liquids and their mixtures
      Gehrke, Sascha; von Domaros, Michael; Clark, Ryan; Holloczki, Oldamur; Brehm, Martin; Welton, Tom; Luzar, Alenka; Kirchner, Barbara
      Faraday Discussions (2018), 206 (Ionic Liquids: From Fundamental Properties to Practical Applications), 219-245CODEN: FDISE6; ISSN:1359-6640. (Royal Society of Chemistry)
      With the aid of mol. dynamics simulations, we study the structure and dynamics of different ionic liq. systems, with focus on hydrogen bond, ion pair and ion cage formation. To do so, we report radial distribution functions, their no. integrals, and various time-correlation functions, from which we ext. well-defined lifetimes by means of the reactive flux formalism. We explore the influence of polarizable force fields vs. non-polarizable ones with downscaled charges (±0.8) for the example of 1-butyl-3-methylimidazolium bromide. Furthermore, we use 1-butyl-3-methylimidazolium trifluoromethanesulfonate to investigate the impact of temp. and mixing with water as well as with the chloride ionic liq. Smaller coordination nos., larger distances, and tremendously accelerated dynamics are obsd. when the polarizable force field is applied. The same trends are found with increasing temp. Adding water decreases the ion-ion coordination nos. whereas the water-ion and water-water coordination is enhanced. A domain anal. reveals that the nonpolar parts of the ions are dispersed and when more water is added the water clusters increase in size. The dynamics accelerate in general upon addn. of water. In the ionic liq. mixt., the coordination no. around the cation changes between the two anions, but the no. integrals of the cation around the anions remain const. and the dynamics slow down with increasing content of the chloride ionic liq.
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    38. 38
      Brehm, M.; Weber, H.; Thomas, M.; Hollòczki, O.; Kirchner, B. Domain analysis in nanostructured liquids: a post-molecular dynamics study at the example of ionic liquids. ChemPhysChem 2015, 16, 3271– 3277,  DOI: 10.1002/cphc.201500471
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      38
      Domain Analysis in Nanostructured Liquids: A Post-Molecular Dynamics Study at the Example of Ionic Liquids
      Brehm, Martin; Weber, Henry; Thomas, Martin; Holloczki, Oldamur; Kirchner, Barbara
      ChemPhysChem (2015), 16 (15), 3271-3277CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
      In the present computational work, we develop a new tool for our trajectory anal. program TRAVIS to analyze the well-known behavior of liqs. to sep. into microphases. The dissection of the liq. into domains of different subsets, for example, in the case of fluorinated ionic liqs. with anionic and cationic head groups (forming together the polar domain), fluorous, and alkyl subsets is followed by radical Voronoi tessellation. This leads to useful av. quantities of the subset neighbor count, i.e., the domain count that gives the amt. of particular domains in the liq., the domain vol. and surface, as well as the isoperimetric quotient, which provides a measure of the deviation of the domains from a spherical shape. Thus, the newly implemented method allows anal. of the domains in terms of their nos. and shapes on a qual. and also quant. basis. It is a simple, direct, and automated anal. that does not need evaluation of the structure beforehand in terms of, for example, first solvent shell min. In the microheterogeneous ionic liqs. that we used as examples, the polar subsets always form a single domain in all investigated liqs. Although the fluorous side chains are also more or less connected in one or, maximally, two domains, the alkyl phases are dispersed.
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    39. 39
      Elfgen, R.; Hollòczki, O.; Kirchner, B. A molecular level understanding of template effects in ionic liquids. Acc. Chem. Res. 2017, 50, 2949– 2957,  DOI: 10.1021/acs.accounts.7b00436
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      A Molecular Level Understanding of Template Effects in Ionic Liquids
      Elfgen, Roman; Holloczki, Oldamur; Kirchner, Barbara
      Accounts of Chemical Research (2017), 50 (12), 2949-2957CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      The structure-directing or template effect has been invoked several times for ionic liqs. to explain the different outcome in material synthesis, namely, different scaffolds or geometrical arrangements with varying ionic liqs. It is obvious to assume that such an effect can originate from the most likely complex microstructure, being present within the ionic liq. itself. In that regard, ionic liqs. have already been shown to undergo a nanosegregation into polar and nonpolar phases, which is commonly known and denoted as microheterogeneity. In order to provide detailed insight on the mol. level and to understand the effects rising from this structuring, the authors performed mol. dynamics simulations on selected very simple model systems composed of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, considering Et, Bu, hexyl, and octyl side chains attached to the cations, mixed with either n-dodecanol or n-butanol. By analyzing snapshots of the simulation boxes and calcg. spatial distribution functions, one can visualize that with increasing side chains, the systems show considerable nanosegregation into polar and nonpolar domains. Combined angular and distance distribution functions show that in case of the nanosegregating systems the side chains of the cations are preferentially arranged in a parallel fashion, which indicates a micelle-like structure for the ionic liqs. The alc. mols. participate in and are, therefore, influenced by this microheterogeneity. It can be shown that in the case of the short IL alkyl side chains, the self-aggregation of the nonpolar units of the alcs. is much stronger, while for the long chain cations, the nonpolar entities of the alcs. are most often connected to the nonpolar units of the ionic liqs. Using our domain anal. tool, we can quantify these observations by tracking the no., size, and shape of the polar and nonpolar entities present in the different investigated systems. The aforementioned combined angular-distance distribution functions reveal a structure-directing effect of the ionic liqs. on the alc. mols. within our simple model systems. The ionic liqs. act as template and order the alc. mols. according to their own structure, resulting in a parallel alignment of the alkyl side chains of the alcs. and ionic liq. cations, with both polar groups being at the same side. These observations show that the microheterogeneous structure of ionic liqs. can indeed be applied to order substrates with respect to each other or, for example, to catalysts in a predetd. fashion, opening new possibilities for explaining or enhancing selectivities of chem. reactions in ionic liqs.
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      Macchieraldo, R.; Esser, L.; Elfgen, R.; Voepel, P.; Zahn, S.; Smarsly, B. M.; Kirchner, B. Hydrophilic ionic liquid mixtures of weakly and strongly coordinating anions with and without water. ACS Omega 2018, 3, 8567– 8582,  DOI: 10.1021/acsomega.8b00995
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      Hydrophilic Ionic Liquid Mixtures of Weakly and Strongly Coordinating Anions with and without Water
      Macchieraldo, Roberto; Esser, Lars; Elfgen, Roman; Voepel, Pascal; Zahn, Stefan; Smarsly, Bernd M.; Kirchner, Barbara
      ACS Omega (2018), 3 (8), 8567-8582CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)
      With the aid of ab initio mol. dynamics simulations, we investigate an ionic liq. mixt. composed of three components 1-butyl-3-methylimidazolium [C4C1Im]+, tetrafluoroborate [BF4]- and chloride [Cl]- without and with water. In the pure IL mixt., we observe an already complex network of interactions between cations and anions, adding water to the system even extends the complexity. Obsd. no. integrals show that the coordination no. between cations and anions is reduced in the system with water compared to the pure system. Further studies show, that the Coulombic network of the strongly coordinating anion [Cl]- is disturbed by water, while the weakly coordinating anion [BF4]- is not. These observations can also be confirmed by the Voronoi polyhedra anal., which shows that the polar network of microheterogeneous IL collapses by the introduction of water. Hydrogen-acceptor interactions revealed, that the [Cl]- anions are transferred from being situated in the IL to the water continuum while [BF4]-is almost unperturbed, these effects mainly influence the interplay of the ionic liq. network.
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      Ray, P.; Vogl, T.; Balducci, A.; Kirchner, B. Structural investigations on lithium-doped protic and aprotic ionic liquids. J. Phys. Chem. B 2017, 121, 5279– 5292,  DOI: 10.1021/acs.jpcb.7b02636
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      Structural Investigations on Lithium-Doped Protic and Aprotic Ionic Liquids
      Ray, Promit; Vogl, Thomas; Balducci, Andrea; Kirchner, Barbara
      Journal of Physical Chemistry B (2017), 121 (20), 5279-5292CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
      Solns. of lithium bis(trifluoromethanesulfonyl)imide (LiNTf2), in four different [NTf2]--based ionic liqs., are extensively investigated as potential electrolytes for lithium-ion batteries. Solvation of the [Li]+ ions in the ionic liqs. and its impact on their physicochem. properties are studied herein with the aid of mol. dynamics simulations. The cationic components of the investigated liqs. were systematically varied so as to individually evaluate effects of specific structural changes; increase in ring size, the addn. of an alkyl chain and absence of an acidic proton, on the solvation and mobility of the [Li]+ cations. The studied cations also allow for a direct comparison between solns. of [Li]+ salt in protic and aprotic ionic liqs. Emphasis is laid on elucidating the interactions between the [Li]+ and [NTf2]- ions revealing slightly higher coordination nos. for the aprotic solvent, benchmarked against exptl. measurements. The study suggests that the ionic liqs. largely retain their structure upon salt addn., with interactions within the liqs. only slightly perturbed. The rattling motion of the [Li]+ cations within cages formed by the surrounding [NTf2]- anions is examd. by the anal. of [Li]+ autocorrelation functions. Overall, the solvation mechanism of [Li]+ salt, within the hydrogen-bonded network of the ionic liqs., is detailed from classical and ab initio mol. dynamics simulations.
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      Kirchner, B.; Hutter, J.; Kuo, I.-F. W.; Mundy, C. J. Hydrophobic hydratation from Car-Parrinello simulations. Int. J. Mod. Phys. B 2004, 18, 1951– 1962,  DOI: 10.1142/S0217979204007241
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      Hydrophobic hydration from Car-Parrinello simulations
      Kirchner, Barbara; Hutter, Juerg; Kuo, I.-Feng W.; Mundy, Christopher J.
      International Journal of Modern Physics B: Condensed Matter Physics, Statistical Physics, Applied Physics (2004), 18 (14), 1951-1962CODEN: IJPBEV; ISSN:0217-9792. (World Scientific Publishing Co. Pte. Ltd.)
      In this work we investigate the fast anomalous diffusion of hydrogen mols. in water using Car-Parrinello mol. dynamics simulations. We employ Voronoi polyhedra anal. to distinguish between void diffusion and void hopping. Our results indicate that a combination of both mechanism is sufficient to explain anomalous diffusion. Furthermore, we investigate the geometry and the electronic structure of the first solvation shell.
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      Ludwig, R. Water: From Clusters to the Bulk. Angew. Chem., Int. Ed. 2001, 40, 1808– 1827,  DOI: 10.1002/1521-3773(20010518)40:10<1808::AID-ANIE1808>3.0.CO;2-1
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      Water: from clusters to the bulk
      Ludwig, Ralf
      Angewandte Chemie, International Edition (2001), 40 (10), 1808-1827CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH)
      A review with 186 refs. Water is of fundamental importance for human life and plays an important role in many biol. and chem. systems. Although water is the most abundant compd. on earth, it is definitely not a simple liq. It possesses strongly polar hydrogen bonds which are responsible for a striking set of anomalous phys. and chem. properties. For more than a century the combined importance and peculiarity of water inspired scientists to construct conceptual models, which in themselves reproduce the obsd. behavior of the liq. The exploration of structural and binding properties of small water complexes provides a key for understanding bulk water in its liq. and solid phase and for understanding solvation phenomena. Modern ab initio quantum chem. methods and high-resoln. spectroscopy methods have been extremely successful in describing such structures. Cluster models for liq. water try to mimic the transition from these clusters to bulk water. The important question is: What cluster properties are required to describe liq.-phase behavior.
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      Huber, H.; Kirchner, B.; Searles, D. J. Is there an iceberg effect in the water/DMSO mixture? Some information from computational chemistry. J. Mol. Liq. 2002, 98-99, 71– 77,  DOI: 10.1016/S0167-7322(01)00310-5
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      Is There an Iceberg Effect in the Water/DMSO Mixture? Some Information from Computational Chemistry.
      Huber, Hanspeter; Kirchner, Barbara; Searles, Debra J.
      Journal of Molecular Liquids (2002), 98-99 (), 71-77CODEN: JMLIDT; ISSN:0167-7322. (Elsevier Science S.A.)
      The deuteron quadrupole coupling const. of heavy water in a mixt. with DMSO measured by B. C. Gordalla and M. D. Zeidler (1986)shows an unexpected behavior. A completely different evaluation from computational methods with independent approxns. does not reproduce this finding. A crit. discussion of all approxns. and uncertainties of the computational method is given using the limited results available for this system and results from studies on pure water.
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      Galamba, N. Water’s structure around hydrophobic solutes and the iceberg model. J. Phys. Chem. B 2013, 117, 2153– 2159,  DOI: 10.1021/jp310649n
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      Water's Structure around Hydrophobic Solutes and the Iceberg Model
      Galamba, N.
      Journal of Physical Chemistry B (2013), 117 (7), 2153-2159CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
      The structure of water in the hydration shells of small hydrophobic solutes was investigated through mol. dynamics. The results show that a subset of water mols. in the first hydration shell of a nonpolar solute have a significantly enhanced tetrahedrality and a slightly larger no. of hydrogen bonds, relative to the mols. in water at room temp., consistent with the exptl. obsd. neg. excess entropy and increased heat capacity of hydrophobic solns. at room temp. This ordering results from the rearrangement of a small no. of water mols. near the nonpolar solutes that occupy one to two vertices of the enhanced water tetrahedra. Although this structuring is not nearly like that often assocd. with a literal interpretation of the term "iceberg" in the Frank and Evans iceberg model, it does support a moderate interpretation of this model. Thus, the tetrahedral orientational order of this ensemble of water mols. is comparable to that of liq. water at ∼10°C, although not accompanied by the small contraction of the O-O distance obsd. in cold water. The structural changes of water in the vicinity of small nonpolar solutes cannot be inferred from the water radial distribution functions, explaining why this increased ordering is not obsd. through neutron diffraction expts. The present results restore a mol. view where the slower translational and reorientational dynamics of water near hydrophobic groups has a structural equiv. resembling water at low temps.
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      Chowdhary, J.; Ladanyi, B. M. Hydrogen bond dynamics at the water/hydrocarbon interface. J. Phys. Chem. B 2009, 113, 4045– 4053,  DOI: 10.1021/jp8061509
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      Hydrogen Bond Dynamics at the Water/Hydrocarbon Interface
      Chowdhary, Janamejaya; Ladanyi, Branka M.
      Journal of Physical Chemistry B (2009), 113 (13), 4045-4053CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
      The dynamics of hydrogen bond formation and breakage for water in the vicinity of water/hydrocarbon liq. interfaces is studied using mol. dynamics simulations. Several liq. alkanes are considered as the hydrocarbon phase to det. the effects of their chain length and extent of branching on the properties of the adjacent water phase. In addn. to defining the interface location in terms of the lab.-frame d. profiles, the effects of interfacial fluctuations are considered by locating the interface in terms of the proximity of the mols. of the other phase. The hydrogen bond dynamics of interfacial water is weakly influenced by the identity of the hydrocarbon phase and by capillary waves. In addn. to calcg. hydrogen bond time correlations, the authors examine how the hydrogen bond dynamics depend on local coordination and det. the extent of cooperativity in the population relaxation of the hydrogen bonds that a given mol. participates in. The contributions of translational diffusion and reorientation of mol. O-H bonds to the mechanism of hydrogen bond breakage and reformation are studied. In previous work, rotation of the principal axes of water is anisotropic at the interface and depends on the initial orientation of the mol. relative to the interface. Here, the authors extend this anal. to the reorientation of the O-H vector and to hydrogen bond time correlation. Hydrogen bond dynamics are also sensitive to the initial orientation of the mols. participating in the hydrogen bond.
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      Li, Q.; Wu, G.; Yu, Z. The role of methyl groups in the formation of hydrogen bond in DMSO-methanol mixtures. J. Am. Chem. Soc. 2006, 128, 1438– 1439,  DOI: 10.1021/ja0569149
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      The Role of Methyl Groups in the Formation of Hydrogen Bond in DMSO-Methanol Mixtures
      Li, Qingzhong; Wu, Guoshi; Yu, Zhiwu
      Journal of the American Chemical Society (2006), 128 (5), 1438-1439CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      When examg. the formation energetics of a hydrogen-bonded complex R-X-H···Y-R', focus has been almost always on the atoms directly involved, namely the atoms X, Y, and H. Little attention has been paid to the effects of the secondary alkyl groups R and R'. Taking DMSO-methanol binary system as an example, we have studied the roles of the alkyl groups in stabilizing the hydrogen bonds by employing FTIR and NMR techniques and quantum chem. calcns. We found that Me groups play different roles in response to the hydrogen-bonding interactions. The Me groups of DMSO are electron-donating, whereas that of methanol is electron-withdrawing, both making pos. contributions. The findings reveal non-negligible effects of secondary alkyl groups in hydrogen bonding interaction and may shed light on the understanding of other more complicated hydrogen-bonded systems in chem. and biol. systems.
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      Yao, H.; Ke, H.; Zhang, X.; Pan, S.-J.; Li, M.-S.; Yang, L.-P.; Schreckenbach, G.; Jiang, W. Molecular recognition of hydrophilic molecules in water by combining the hydrophobic effect with hydrogen bonding. J. Am. Chem. Soc. 2018, 140, 13466– 13477,  DOI: 10.1021/jacs.8b09157
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      Molecular Recognition of Hydrophilic Molecules in Water by Combining the Hydrophobic Effect with Hydrogen Bonding
      Yao, Huan; Ke, Hua; Zhang, Xiaobin; Pan, San-Jiang; Li, Ming-Shuang; Yang, Liu-Pan; Schreckenbach, Georg; Jiang, Wei
      Journal of the American Chemical Society (2018), 140 (41), 13466-13477CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      During the last half a century, great achievements have been made in mol. recognition in parallel with the invention of numerous synthetic receptors. However, the selective recognition of hydrophilic mols. in water remains a generally accepted challenge in supramol. chem. but is commonplace in nature. In an earlier Communication [Huang et al. J. Am. Chem. Soc. 2016, 138, 14550], we reported a pair of endo-functionalized mol. tubes that surprisingly prefer highly hydrophilic mols. over hydrophobic mols. of a similar size and shape. The hydrophobic effect and hydrogen bonding were proposed to be responsible, but their exact roles were not fully elucidated. In this Article, we present a thorough study on the binding behavior of these mol. tubes toward 44 hydrophilic mols. in water. Principal component anal. reveals that the binding strength is weakly correlated to the hydrophobicity, vol., surface area, and dipole moment of guests. Furthermore, mol. dynamics simulations show the hydrophobic effect through releasing the poorly hydrogen-bonded cavity water contributes to the binding of all the hydrophilic mols., while hydrogen bonding differentiates these mols. and is thus the key to achieve a high selectivity toward certain hydrophilic mols. over other mols. with a similar size and shape. Therefore, a good guest for these mol. tubes should meet the following criteria: the hydrogen-bonding sites should be complementary, and the mol. vol. should be large enough to expel all the cavity water but not too large to cause steric hindrance. This rule of thumb may also be used to design a selective receptor for certain hydrophilic mols. Following these guidelines, a "best-fit" guest was found for the syn-configured mol. tube with a binding const. as high as 106 M-1.
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      The H bond structure of liq. MeOH was studied as a function of pressure and temp. up to 2.8 kbar and from 297 to 413 K. Chem. shifts of the CH3 and OH groups were monitored throughout this pressure and temp. regime, and the chem. shift difference between these two groups was used to describe changes of the H bond network in MeOH. The H bond equil. was studied using mol. dynamics simulations and a phenomenol. model describing clustering in liq. MeOH. Results are presented concerning the size and distribution of H-bonded clusters in MeOH as a function of pressure and temp. The extent of H bonding decreases upon an increase in temp. The results for pressure are equivocal, the phenomenol. model suggests that H bonding decreases with increasing pressure, which supports earlier interpretations regarding the measured self-diffusion coeffs. in deuterated MeOH as a function of pressure. The mol. dynamics simulations, however, show an increase in H bonding with increasing pressure.
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    The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.9b00839.

    • 1H NMR spectra; radial distribution function; RDFs of the interactions of MeOH(O) molecules (PDF)


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