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Long-Term Stability of n-Alkane-in-Water Pickering Nanoemulsions: Effect of Aqueous Solubility of Droplet Phase on Ostwald Ripening
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Long-Term Stability of n-Alkane-in-Water Pickering Nanoemulsions: Effect of Aqueous Solubility of Droplet Phase on Ostwald Ripening
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Langmuir

Cite this: Langmuir 2018, 34, 31, 9289–9297
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https://doi.org/10.1021/acs.langmuir.8b01835
Published July 12, 2018

Copyright © 2018 American Chemical Society. This publication is licensed under CC-BY.

Abstract

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High-pressure microfluidization is used to prepare a series of oil-in-water Pickering nanoemulsions using sterically-stabilized diblock copolymer nanoparticles as the Pickering emulsifier. The droplet phase comprised either n-octane, n-decane, n-dodecane, or n-tetradecane. This series of oils enabled the effect of aqueous solubility on Ostwald ripening to be studied, which is the primary instability mechanism for such nanoemulsions. Analytical centrifugation (LUMiSizer instrument) was used to evaluate the long-term stability of these Pickering nanoemulsions over time scales of weeks/months. This technique enables convenient quantification of the fraction of growing oil droplets and confirmed that using n-octane (aqueous solubility = 0.66 mg dm–3 at 20 °C) leads to instability even over relatively short time periods. However, using n-tetradecane (aqueous solubility = 0.386 μg dm–3 at 20 °C) leads to significantly improved long-term stability with respect to Ostwald ripening, with all droplets remaining below 1 μm diameter after 6 weeks storage at 20 °C. In the case of n-dodecane, the long-term stability of these new copolymer-stabilized Pickering nanoemulsions is significantly better than the silica-stabilized Pickering nanoemulsions reported in the literature by Persson et al. (Colloids Surf., A,2014,459, 48–57). This is attributed to a much greater interfacial yield stress for the former system, as recently described in the literature (see P. J. Betramo et al. Proc. Natl. Acad. Sci. U.S.A.,2017,114, 10373–10378).

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Introduction

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Pickering emulsions can be defined as emulsion droplets that are stabilized by solid particles and have been recognized since the beginning of the last century. (1,2) Both water-in-oil (w/o) (3−5) or oil-in-water (o/w) (6−8) Pickering emulsions are widely reported in the literature, with additional recent examples including water-in-water (w/w) (9,10) and oil-in-oil (o/o) Pickering emulsions. (11,12) Examples of suitable Pickering emulsifiers include silica, (6,13) titania, (14) clays, (15−18) carbon black, (19) and latex particles; (5,7,20) in this context, emulsifier efficacy is governed primarily by surface wettability rather than by bulk chemical composition.
Recently, there have been a few literature reports of the preparation of Pickering nanoemulsions. (21−23) A nanoemulsion comprises very fine droplets that are less than approximately 200 nm in diameter. (24) Compared to macroemulsions, nanoemulsions are significantly less susceptible to gravitational creaming or sedimentation. In addition, their much higher surface area is expected to lead to more active formulations for cosmetics, (25) drug delivery, (26) food manufacturing, (27) and agrochemical applications. (28)
To date, most nanoemulsion studies have utilized various surfactants as the emulsifier. (29−32) Significantly, fewer literature examples involve Pickering nanoemulsions, perhaps because their stabilization requires relatively small nanoparticles (typically at least 5–10 times smaller than the mean droplet diameter) which may not be readily available. (21−23,33) Nevertheless, Persson et al. (21) used a high-pressure microfluidizer to prepare o/w Pickering nanoemulsions of between 100 and 200 nm diameter stabilized using a commercial 7 nm silica sol. Unfortunately, Ostwald ripening was observed over time periods of hours (for n-octane) to weeks (for n-tetradecane) when using various n-alkanes, even for relatively water-insoluble n-alkanes such as n-hexadecane. However, this instability mechanism, which is well-known for nanoemulsions owing to their relatively high surface area, (30) could be suppressed by employing a highly water-insoluble oil such as squalene.
Using a similar high-pressure microfluidizer setup as that utilized by Persson et al., we recently reported (34) the preparation of relatively stable n-dodecane-in-water Pickering nanoemulsions stabilized by poly(glycerol monomethacrylate-block-2,2,2-trifluoroethyl methacrylate) (PGMA–PTFEMA) diblock copolymer nanoparticles of 25 nm diameter. Nanoemulsions with mean hydrodynamic droplet diameters as small as 220 nm (35) could be prepared that remained stable for up to 4 months, as judged by dynamic light scattering (DLS) studies. Herein, we extend this preliminary study by comparing the relative long-term stabilities of o/w Pickering nanoemulsions prepared using n-octane, n-decane, n-dodecane, or n-tetradecane. This series of four oils was chosen to examine the extent of Ostwald ripening. These nanoemulsions were characterized using three well-established particle-sizing techniques, namely DLS (Malvern Nanosizer), laser diffraction (Malvern Mastersizer), and analytical centrifugation (LUMiSizer instrument).

Experimental Section

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Materials

Glycerol monomethacrylate (GMA, 99.8% purity) was obtained from GEO Specialty Chemicals (Hythe, UK) and was used as received. 2-Cyano-2-propyl benzodithioate, 2,2,2-trifluoroethyl methacrylate (TFEMA), 4,4′-azobis(4-cyanopentanoic acid) (ACVA), fluorescein O-methacrylate (FluMA), n-dodecane, n-octane, n-tetradecane, dichloromethane, and deuterium oxide were purchased from Aldrich (UK) and were used as received, unless otherwise stated. Ethanol and dimethylformamide (DMF) were purchased from VWR chemicals (UK), while n-decane was purchased from Alfa Aesar (UK).

Synthesis of PGMA48 Macro-CTA via RAFT Solution Polymerization in Ethanol

A PGMA48 macro-CTA (hereafter described as PGMA48) was synthesized via RAFT polymerization of glycerol monomethacrylate in ethanol at 70 °C, as described previously. (21,30)1H NMR studies indicated a mean DP of 48 via end-group analysis (integrated aromatic RAFT end-group signals at 7.1–7.4 ppm were compared to those of the two oxymethylene protons at 3.5–4.4 ppm). DMF gel permeation chromatography (GPC) studies (refractive index detector) indicated an Mn of 12 700 g mol–1 and an Mw/Mn of 1.17, with a series of near-monodisperse poly(methyl methacrylate) standards being used for calibration.

Synthesis of PGMA48–PTFEMA50 Diblock Copolymer Nanoparticles via RAFT Aqueous Emulsion Polymerization

PGMA48–PTFEMA50 diblock copolymer nanoparticles were synthesized as follows: PGMA48 macro-CTA (2.830 g), ACVA (0.020 g, 71.4 μmol; macro-CTA/ACVA molar ratio = 5.0), and water (52.65 g, 10 w/w %) were weighed into a 100 mL round-bottomed flask, sealed with a rubber septum and degassed with nitrogen for 30 min. TFEMA (2.54 mL, 17.8 mmol, target DP = 50), which had been deoxygenated separately with nitrogen for 15 min, was then added to the solution under nitrogen and immersed in an oil bath set at 70 °C. The reaction solution was stirred for 20 h to ensure complete TFEMA monomer conversion, and the polymerization was quenched by exposure to air. 19F NMR spectroscopy analysis of the copolymer dissolved in d6-acetone indicated less than 1% residual TFEMA monomer. DMF GPC studies indicated an Mn of 19 100 g mol–1 and a Mw/Mn of 1.14, with a series of near-monodisperse poly(methyl methacrylate) standards being used for calibration.

Preparation of PGMA48–PTFEMA50-Stabilized Pickering Macroemulsions Using High-Shear Homogenization

A PGMA48–PTFEMA50 aqueous dispersion (8.0 mL; 7.0 w/w % solids) was added to a 14 mL glass vial and homogenized with 2.0 mL of n-dodecane for 2.0 min at 20 °C using an IKA Ultra-Turrax T-18 homogenizer with a 10 mm dispersing tool operating at 15 500 rpm. The resulting milky oil-in-water emulsion was then analyzed by optical microscopy and laser diffraction. The same protocol was used for the other three n-alkanes used in this work.

Preparation of PGMA48–PTFEMA50-Stabilized Pickering Nanoemulsions Using High-Pressure Microfluidization

A Pickering macroemulsion (6.0 mL) was further processed using an LV1 Low Volume Microfluidizer Processor (Microfluidics, USA). The pressure was adjusted to 20 000 psi, and the number of passes through the LV1 was fixed at 10.

Characterization

NMR Spectroscopy

1H and 19F NMR spectra were recorded in d6-acetone, D2O, or CD3OD using a Bruker AVANCE 400 spectrometer operating at 400 MHz.

Gel Permeation Chromatography

Molecular weights and dispersities were assessed using a GPC instrument equipped with a Varian 290-LC pump injection module, a Varian 390-LC refractive index detector, and two Polymer Laboratories PL gel 5 μm mixed-C columns with a DMF mobile phase containing 0.01 M LiBr operating at 60 °C with a constant flow rate of 1.0 mL min–1. Dimethyl sulfoxide was used as a flow-rate marker, and calibration was achieved using a series of near-monodisperse poly(methyl methacrylate) standards.

Dynamic Light Scattering

Z-average hydrodynamic diameters were determined by DLS at 25 °C using a Malvern Zetasizer NanoZS instrument at a fixed scattering angle of 173°. Dilute aqueous dispersions (0.01 w/w %) of PGMA48–PTFEMA50 nanoparticles were analyzed using disposable cuvettes, and the results were averaged over three consecutive runs. The deionized water used to dilute each sample was ultrafiltered through a 0.20 μm membrane to remove extraneous dust.

Laser Diffraction

Each macroemulsion was sized using a Malvern Mastersizer 3000 instrument equipped with a hydro EV wet sample dispersion unit, a red HeNe laser operating at 633 nm and a light-emitting diode blue-light source operating at 470 nm. The stirring rate was adjusted to 1500 rpm to avoid creaming of the emulsion during analysis. After each measurement, the cell was rinsed three times with deionized water; the glass walls of the cell were carefully wiped with a lens cleaning tissue to avoid cross-contamination, and the laser was aligned centrally to the detector prior to data acquisition.

Analytical Centrifugation (LUMiSizer)

Droplet size distributions were assessed using a LUMiSizer analytical photocentrifuge (LUM GmbH, Berlin, Germany) at 20 °C. Measurements were conducted on diluted Pickering nanoemulsions (1.0 v/v % n-alkane) in 2 mm path length polyamide cells at 200 rpm for 200 profiles (allowing 20 s between profiles), and then the rate of centrifugation was increased up to 4000 rpm for a further 800 profiles. The slow initial rate of centrifugation enabled detection of any larger oil droplets that might be present within the nanoemulsion. The LUMiSizer instrument employs space- and time-resolved extinction profiles technology to measure the intensity of transmitted near-infrared light as a function of time and position over the entire cell length simultaneously. The gradual progression of these transmission profiles contains information on the rate of creaming and, in principle, enables assessment of the droplet size distribution. The nanoemulsion droplet density is an essential input parameter for such analytical centrifugation studies. In the present case, the effective droplet density is somewhat higher than that of the pure n-alkane owing to the presence of the adsorbed monolayer of relatively dense PGMA48–PTFEMA50 nanoparticles. Nevertheless, the true droplet density must lie between that of the pure n-alkane and water because visual inspection confirmed that droplet creaming always occurred during the LUMiSizer analysis. The effective density of the nanoparticle-coated n-alkane droplets can be estimated by using eqs 1–3 (see below). One caveat here is that a density distribution is superimposed on the droplet size distribution for each Pickering nanoemulsion. On the basis of our earlier studies, (36,37) this is expected to lead to an artefactual broadening (38) of the LUMiSizer droplet size distribution. Moreover, the superimposed density distribution is expected to change over time as the droplet size distribution broadens as a result of Ostwald ripening. Thus, correcting for this secondary effect is rather complex and is considered to be beyond the scope of the present study, which instead focuses on assessing the relative long-term stabilities of Pickering nanoemulsions prepared using four different n-alkanes. In view of this, we chose to estimate the effective droplet density from the initial z-average DLS diameter recorded for each fresh nanoemulsion, and this value was utilized for all subsequent time-dependent LUMiSizer studies.

Small-Angle X-ray Scattering (SAXS)

SAXS data were acquired using a laboratory-based SAXS instrument (Xeuss 2.0, Xenocs, France) equipped with a liquid gallium MetalJet X-ray source (Excillum, Sweden, wavelength λ = 0.134 nm), with motorized scatterless slits for beam collimation and a Dectris PILATUS 1M pixel detector. The sample-to-detector distance (or camera length) was 2.493 m. SAXS patterns were recorded over a q range of 0.04 nm–1 < q < 3.0 nm–1, where q (=4π sin θ/λ) is the scattering vector and θ is one-half of the scattering angle. Glass capillaries of approximately 2.0 mm diameter were used for all samples. Data were reduced using the Foxtrot software package developed by SOLEIL and further analyzed using Irena SAS macros (39) for Igor Pro.

Calculation of Nanoparticle and Droplet Densities

The PGMA48–PTFEMA50 nanoparticle density (ρparticle) was calculated using eq 1, as described in our earlier study. (36)
(1)
Here, ρPTFEMA and Rs denote the density and radius of the PTFEMA50 core-forming block, and ρPGMA and Rg are the density and radius of gyration of the shell component (PGMA48). 2Rg is equal to the PGMA stabilizer shell thickness. The numerical value of ρPTFEMA was taken to be 1.47 g cm–3, which is the density of nonsolvated PTFEMA, whereas the highly hydrated PGMA shell was estimated to have a density ρPGMA of approximately 1.00 g cm–3. Rs = 6.81 nm and Rg = 1.59 nm were calculated by fitting the SAXS pattern recorded for the PGMA48–PTFEMA50 nanoparticles to a spherical micelle model; (36) this approach afforded a ρparticle of 1.15 g cm–3. The volume-average particle diameter, DSAXS, was calculated to be 20.0 nm using DSAXS = (2Rs + 4Rg).
The effective density of the n-alkane droplets was estimated by first calculating the number of nanoparticles packed around a single droplet (N).
(2)
Here, mparticle is the mass of particles used to prepare the nanoemulsion, NA is Avogadro’s constant, and Ns is the number of PGMA48–PTFEMA50 chains per particle, determined by SAXS to be 139 using , Mn is the number–average molecular mass of the PGMA48–PTFEMA50 chains, Voil is the total volume of oil used to prepare each nanoemulsion, and roil is the radius of the minimum possible droplet diameter. We calculate roil to be the mean DLS droplet radius (rDLS) minus the adsorbed nanoparticle diameter to give the minimum bare oil droplet radius (roil = rDLS – 2rparticle).
The PGMA48–PTFEMA50 nanoparticle diameter d was determined to be 20.0 nm by SAXS (where d = 2Rs + 4Rg). However, we consider the effective adsorbed diameter (2rparticle) at the oil/water interface to be given by 2Rcore + 2Rg (or 16.8 nm). This is because the PGMA48 stabilizer chains that are in direct contact with the oil phase are expected to be fully collapsed and hence should occupy negligible volume at the oil/water interface (Scheme 1). For example, a nanoemulsion droplet with an overall diameter of 220 nm should have a core droplet diameter of 220 – (2 × 16.8) ≈ 186 nm. Several assumptions are made in these calculations: (i) the z-average droplet diameter reported by DLS includes both the oil droplet and the nanoparticle shell and (ii) the nanoparticles adsorb at the o/w interface with an effective contact angle of 0° with respect to the nanoparticle core (see Scheme 1). Clearly, this is not the true nanoparticle contact angle; therefore, the droplet diameter will be slightly overestimated. However, this is likely to be a relatively small systematic error and therefore is not expected to affect the main findings of this study.

Scheme 1

Scheme 1. Schematic Representation of the Adsorption of PGMA48–PTFEMA50 Nanoparticles at the n-Alkane/Water Interfacea
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aIt is assumed that (i) the nanoparticles are adsorbed with an effective contact angle of 0° and (ii) PGMA48 stabilizer chains in direct contact with the n-alkane/water interface are fully collapsed and hence do not contribute to the adsorbed nanoparticle radius. Thus, given that the effective thickness of the adsorbed sterically stabilized nanoparticles is given by 2Rs + 2Rg (rather than 2Rs + 4Rg), the approximate effective sphere radius, rparticle, of the adsorbed nanoparticles is given by rparticle = Rs + Rg or 8.4 nm. Experimental values for Rs and Rg were obtained from SAXS analysis of the PGMA48–PTFEMA50 nanoparticles prior to emulsification, see the main text.

The effective density of each nanoemulsion droplet, ρnanoemulsion, was then calculated using a core–shell model where the core is composed of the n-alkane (whose density is denoted by ρoil) and the particulate shell comprises a monolayer of adsorbed PGMA48–PTFEMA50 nanoparticles with an effective adsorbed particle radius (rparticle) of 8.4 nm (37) comprising the particulate shell. (37)
(3)

Results and Discussion

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The PGMA48–PTFEMA50 nanoparticles used in this study were prepared as a 10% aqueous dispersion by RAFT aqueous emulsion polymerization using a technique known as polymerization-induced self-assembly (PISA), as reported previously (see Figure 1A). (36) The z-average nanoparticle diameter was determined to be 24 nm by DLS, whereas fitting SAXS data to a spherical micelle model indicated a volume-average diameter of 20 nm; TEM studies confirmed that the nanoparticles possessed a near-monodisperse spherical morphology (see Figure 1B–D). Preliminary scoping experiments suggested that an excess of relatively small nanoparticles were required to maximize the total surface area available during the high-pressure microfluidization processing step and hence ensure the formation of stable Pickering nanoemulsions. For example, utilizing somewhat larger PGMA48–PTFEMA150 nanoparticles (51 nm diameter by DLS) merely produced highly flocculated nanoemulsions. (34)

Figure 1

Figure 1. (A) Synthesis of PGMA48–PTFEMA50 nanoparticles via RAFT aqueous emulsion polymerization of TFEMA at 70 °C using a PGMA48 macro-CTA; (B) TEM image of the resulting sterically stabilized PGMA48–PTFEMA50 nanoparticles; (C) particle size distribution determined by DLS; (D) SAXS pattern recorded for a 1.0 w/w % aqueous dispersion of the PGMA48–PTFEMA50 nanoparticles (solid red line represents the data fit obtained using a spherical micelle model, which reports a volume-average diameter of 20 ± 1 nm).

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Initially, a 7.0 w/v % aqueous dispersion of PGMA48–PTFEMA50 nanoparticles was used to prepare a relatively coarse Pickering macroemulsion with a mean droplet diameter of 40 μm via conventional high-shear homogenization. Figure 2 illustrates how this precursor macroemulsion was then processed using the LV1 microfluidizer to produce a Pickering nanoemulsion. It is essential to have a substantial excess of PGMA48–PTFEMA50 nanoparticles present after formation of the initial macroemulsion, as these non-adsorbed nanoparticles are required to stabilize the new oil/water interface generated during high-pressure microfluidization. In a recent related study, (34) it was found empirically that an applied pressure of 20 000 psi was optimal for the preparation of stable Pickering nanoemulsions. Lower pressures let to larger, more polydisperse droplets, whereas higher pressures lead to the breakup of PGMA48–PTFEMA50 nanoparticles into individual diblock copolymer chains, which can then act as an amphiphilic copolymer surfactant to form (non-Pickering) nanoemulsions. Our previous studies focused on n-dodecane: for this oil, it was shown that at least eight passes through the microfluidizer were required to achieve a stable nanoemulsion with a unimodal droplet size distribution. (34) Essentially the same observations were made for the other three n-alkanes selected for this study (n-octane, n-decane, and n-tetradecane). These four oils were selected for their differing aqueous solubilities (40,41) because this parameter is known to be important in determining the extent of Ostwald ripening for nanoemulsions. (42−44) After 10 passes through the LV1, a mean DLS droplet diameter of around 180 nm was obtained when using n-octane, n-decane, or n-tetradecane. In contrast, using n-dodecane produced a somewhat larger mean droplet diameter of 257 nm under the same conditions. The reason for this minor discrepancy is not known, but such experiments proved to be reproducible. It does not appear to be related to the mean diameter of the precursor Pickering macroemulsions, which was around 40 μm for each of the four oils.

Figure 2

Figure 2. Schematic representation of the two-step preparation of Pickering nanoemulsions. First, a 7.0 w/v % aqueous dispersion of PGMA48–PTFEMA50 nanoparticles were homogenized with one of four n-alkanes to form an oil-in-water Pickering macroemulsion of around 40–50 μm diameter using conventional high-shear homogenization at 15 500 rpm for 2 min at 20 °C. This relatively coarse precursor emulsion was then refined via 10 passes through the LV1 microfluidizer at 20 000 psi to obtain the final Pickering nanoemulsion.

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Transmission electron microscopy (TEM) images were obtained for dried fresh Pickering nanoemulsions, as shown in Figure 3. Although the volatile droplet phase is no longer present under the ultrahigh vacuum conditions required for TEM, some of the original superstructure of the adsorbed PGMA48–PTFEMA50 nanoparticles is preserved. Such post-mortem studies provide convincing evidence that spherical droplets corresponding to nanoemulsion length scales were achieved when using each of the four oils, albeit with a significant degree of polydispersity. It is perhaps worth mentioning that the relatively high electron density of the PTFEMA cores provides relatively good contrast for such TEM studies, which is one advantage of using these particular amphiphilic PGMA48–PTFEMA50 diblock copolymers.

Figure 3

Figure 3. Representative TEM images obtained for freshly prepared Pickering nanoemulsions prepared via high-pressure microfluidization using 7.0 w/w % PGMA48–PTFEMA50 nanoparticles and 20 v/v % n-alkane. Conditions: applied pressure = 20 000 psi for 10 passes using an LV1 microfluidizer.

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Table 1 summarizes the initial droplet size distributions determined for Pickering nanoemulsions prepared using the four different n-alkanes. DLS and laser diffraction studies were conducted on fresh nanoemulsions within 30 min of their preparation. For three of the oils, the volume-average nanoemulsion diameters reported by laser diffraction are lower than the z-average diameters obtained from DLS, as expected. However, the DLS droplet diameter is actually lower than that reported by laser diffraction for the n-octane-based nanoemulsion. A possible explanation for this unexpected finding is that the latter nanoemulsion already exhibits signs of Ostwald ripening even at this rather early stage and that the DLS technique is surprisingly insensitive to the coarsening droplet size distribution (see the following discussion). The remarkably poor stability of this n-octane nanoemulsion is consistent with findings by Persson et al., (21) who observed the appearance of significantly larger droplet diameters for the same oil after ageing a silica-stabilized nanoemulsion for just 30 min.
Table 1. Summary of the Pickering Nanoemulsions Prepared Using Four n-Alkanes (n-Octane, n-Decane, n-Dodecane, and n-Tetradecane)a
oil typeinitial DLS diameter, DDLS (nm)binitial laser diffraction diameter, Dv (nm)bare droplet diameter, Doil (nm)cnumber of spheres per droplet, Npacking efficiency (%)ρn-alkane (g cm–3)ρnanoemulsion (g cm–3)
n-octane179 ± 59233 ± 245145120440.700.77
n-decane182 ± 56117 ± 92148128450.730.80
n-dodecane257 ± 93167 ± 159223438740.750.81
n-tetradecane180 ± 37120 ± 95146123450.760.82
a

Droplet size distributions were recorded within 30 min of the preparation of each nanoemulsion. The SAXS-derived PGMA48–PTFEMA50 nanoparticle diameter, DSAXS, was determined to be 20.0 nm (where DSAXS = 2Rcore + 4Rg), whereas the effective adsorbed diameter at the oil/water interface was taken to be 2Rcore + 2Rg (16.8 nm) because the collapsed stabilizer chains that are in direct contact with the oil/water interface occupying a negligible volume. The effective nanoparticle density was estimated to be 1.15 g cm–3 using previously reported data. (36)

b

Z-average diameter.

c

Calculated using Doil = DDLS – 4rparticle.

Analytical centrifugation was used to further characterize both the initial and ageing nanoemulsions. This particular technique (LUMiSizer instrument) has a much higher resolution compared to DLS or laser diffraction because droplet fractionation occurs during the measurement (prior to detection). Thus, better insight into how the nanoemulsion size distributions change over time for each oil type was anticipated. One drawback of this technique is that the effective particle density is required to obtain an accurate particle size. (36) This critical input parameter was estimated using eqs 1–3 from the initial DLS droplet diameter using the additional data derived from the SAXS analysis of the PGMA48–PTFEMA50 nanoparticles (see the Experimental Section and the Supporting Information for details). The calculated nanoemulsion densities (ρnanoemulsion) are shown in Table 1 for each n-alkane.
Another important consideration when using analytical centrifugation is the appropriate concentration at which the samples are analyzed. These Pickering nanoemulsions were prepared at a relatively high oil volume fraction (20 v/v % n-alkane). However, analyzing droplets at such high concentrations is known to undersize substantially owing to the phenomenon of hindered sedimentation. (45) In the present case, the rate of creaming of droplets within a relatively concentrated nanoemulsion when subjected to a centrifugal field is significantly retarded as a result of the close proximity of neighboring droplets; thus, the slower creaming rate leads to a smaller apparent droplet diameter. This artefact was indeed observed in the case of these Pickering nanoemulsions. Figure 4A shows that, as the droplet concentration used for analytical centrifugation studies is systematically reduced, the apparent nanoemulsion droplet diameter increases and approaches a constant plateau value at approximately 1.0 v/v %. Thus, all further studies were conducted at this oil droplet concentration, with the 20 v/v % stock nanoemulsion being diluted immediately prior to analysis. Using droplet concentrations lower than 1.0 v/v % becomes problematic: such dilute emulsions scatter light only rather weakly and hence fall outside of the optimum transmission range required for the LUMiSizer instrument (i.e., below 30% transmission). Figure 4B illustrates the effect that varying the oil droplet concentration has on the apparent droplet size distributions reported by the instrument. Analyzing relatively concentrated nanoemulsions (diluted to only 10 v/v %) leads to a significantly smaller apparent droplet diameter (and an apparently narrower droplet size distribution).

Figure 4

Figure 4. (A) Effect of droplet concentration on apparent droplet diameter for an n-dodecane-in-water Pickering nanoemulsion as determined by analytical centrifugation (LUMiSizer instrument). This hindrance function indicates that the optimum droplet concentration for such analyses is approximately 1.0 v/v %, with higher concentrations leading to hindered creaming and hence artefactual undersizing. (B) Droplet size distributions determined by analytical centrifugation using an effective nanoemulsion droplet density of 0.81 g cm–3 at either 10 or 1.0 v/v % n-dodecane. As a result of these initial studies, all further LUMiSizer analyses were conducted at a droplet concentration of 1.0 v/v % n-alkane.

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Figure 5 summarizes the data obtained for various droplet characterization techniques for a freshly-made and 1-week-old n-octane-in-water Pickering nanoemulsion. A significant increase in the mean droplet diameter is discernible by laser diffraction (Figure 5A), analytical centrifugation (Figure 5B), and optical microscopy (Figure 5D) after ageing the 20 v/v % nanoemulsion for 1 week at ambient temperature. Given the relatively high solubility of n-octane in water (0.66 mg dm–3 at 20 °C), (40) such droplet growth is attributed to Ostwald ripening. (42−44) Perhaps surprisingly, DLS analysis indicated only a minimal change in particle size: the z-average diameter actually decreased slightly from 179 ± 59 to 168 ± 72 nm, although a rather small population of micrometer-sized coarser droplets was also detected. However, the latter feature was only observed initially (see Figure 5C); it disappeared in subsequent runs (each measurement comprising an average of three runs). DLS is normally highly biased toward larger populations because the scattered light intensity scales according to the sixth power of the particle radius. This unexpected lack of sensitivity is puzzling, but one possible explanation could be the rapid creaming of the relatively large oil droplets (which were clearly detectable by laser diffraction and analytical centrifugation). This is consistent with the detection of large droplets during the first run but not during the two subsequent runs. Laser diffraction and analytical centrifugation studies indicated that a significant fraction of n-octane droplets (>60% according to analytical centrifugation) became micrometer-sized within 1 week of ageing at 20 °C. This was confirmed by optical microscopy studies performed after 1 week (Figure 5D). According to the analytical centrifugation data, approximately 20% of the droplets had already grown to at least 1–2 μm diameter by the time the fresh nanoemulsion was analyzed. This illustrates the high resolution of this technique: laser diffraction and DLS simply do not have the sensitivity to readily discriminate between a large population of submicrometer-sized droplets and a minor fraction of the micrometer-sized droplets. The former population cannot be observed by optical microscopy because such fine droplets are beyond the optical resolution limit. In the literature, nanoemulsions are typically sized using either DLS or laser diffraction, as such instruments are widely available in many research laboratories. In contrast, there are relatively few reports of sizing nanoemulsions via analytical centrifugation, yet this high-resolution technique appears to offer an important advantage when considering the effect of Ostwald ripening. For this reason, the rest of this article focuses on the characterization of Pickering nanoemulsions via analytical centrifugation. The caveat with this sizing technique is the uncertainty in calculating the effective droplet density for this particular nanoemulsion system and the technical problems associated with the superimposed density distribution on the droplet size distribution (see above).

Figure 5

Figure 5. Various particle sizing techniques used to characterize a freshly-made and 1-week-old 20 v/v % n-octane-in-water Pickering nanoemulsion prepared using 7.0 w/w % PGMA48–PTFEMA50 nanoparticles at 20 000 psi with 10 passes. (A) Laser diffraction, (B) DLS, (C) analytical centrifugation (LUMiSizer, ρnanoemulsion = 0.77 g cm–3), and (D) optical microscopy (no oil droplets were observed by optical microscopy for any of the freshly-made Pickering nanoemulsions reported in this study, but micrometer-sized oil droplets were clearly discernible within 1 week of ageing the n-octane-based Pickering nanoemulsion). Perhaps surprisingly, DLS seemed to be relatively insensitive to the larger oil droplets present within the 1-week-old Pickering nanoemulsion, whereas this coarse fraction was readily detected by laser diffraction and analytical centrifugation.

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Table 2 reports the mean volume-average diameter determined by analytical centrifugation for Pickering nanoemulsions prepared using each of the four n-alkanes on ageing at 20 °C for up to 6 weeks. Unfortunately, the relatively high volatilities of n-octane and n-decane meant that significant evaporation of the oil droplet phase occurred within 1 week, even if these nanoemulsions were well-sealed. Hence, it was not possible to accurately size these two nanoemulsions after ageing for 1 week. However, it is nevertheless evident that significant droplet coalescence occurred for these two nanoemulsions over this time frame compared to those prepared under the same conditions using n-dodecane or n-tetradecane. In particular, the width of the droplet size distributions observed for the fresh nanoemulsions became significantly broader from n-tetradecane to n-octane. For the two more volatile oils, these distributions become highly asymmetric, with pronounced tailing toward larger droplet diameters. This suggests that Ostwald ripening commences immediately after preparation of each nanoemulsion, with n-octane droplets coarsening significantly faster than n-decane droplets. After ageing for 1 week, only the n-tetradecane-based nanoemulsion retains its (approximate) original mean droplet diameter. The n-octane and n-decane nanoemulsions exhibited the largest increases in mean diameter, which correlate with the significantly higher aqueous solubilities of these two oils. All four droplet size distributions became broader, with the most pronounced change being observed for n-octane and only a minimal change being detected for n-tetradecane. Figure 6A shows the volume-average cumulative distributions obtained for each of the four Pickering nanoemulsions recorded after ageing for 1 week. The extent of Ostwald ripening is clearly greatest for n-octane: around 60% of the droplets now exceed 500 nm, with approximately 10% being larger than 4 μm. In comparison, the proportion of droplets exceeding 500 nm for the other three oils is 45% for n-decane, 9% for n-dodecane, but less than 0.1% for n-tetradecane. Although substantial Ostwald ripening had occurred within 1 week, significant proportions of the original n-octane (>35%) and n-decane (>50%) formulations can still be classified as nanoemulsions. This represents much better stability compared to the silica-stabilized nanoemulsions reported by Persson and co-workers. (21) In principle, this difference may be simply related to the relatively large 20 nm PGMA48–PTFEMA50 nanoparticles being more strongly adsorbed at the oil/water interface than the 7 nm charge-stabilized “hard sphere” silica nanoparticles utilized by the Swedish group. (4) However, Vermant and co-workers have just reported that using sterically stabilized “soft spheres” leads to a much greater interfacial yield stress, which is believed to account for the enhanced long-term stability observed for many emulsions (and foams). (46)

Figure 6

Figure 6. Volume-weighted cumulative distributions determined by analytical centrifugation (LUMiSizer instrument) for the n-alkane-in-water nanoemulsions: (A) after ageing for 1 week at 20 °C and (B) after ageing for 6 weeks. Note that significant evaporation of the more volatile n-octane and n-decane oils occurred within 1 week; so, no further analysis was possible in these two cases.

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Table 2. Change in Mean Droplet Diameter with Ageing Time as Determined by Analytical Centrifugation (LUMiSizer) for the Four n-Alkane Pickering Nanoemulsions Reported in This Studya
   mean nanoemulsion diameter by analytical centrifugation (nm)
oil typeaqueous solubility of oil at 20 °C (mg dm–3)ρnanoemulsion (g cm–3)fresh1 week4 weeks6 weeks
n-octane0.660.77132 ± 637234 ± 1850  
n-decane0.03960.80115 ± 219258 ± 1108  
n-dodecane0.00340.81160 ± 92182 ± 156186 ± 227175 ± 335
n-tetradecane0.0003860.82145 ± 67141 ± 87146 ± 146156 ± 180
a

All nanoemulsions were prepared with 20 v/v % n-alkane using 7.0 w/w % PGMA48–PTFEMA50 nanoparticles at 20 000 psi after 10 passes through a LV1 high-pressure microfluidizer. Analytical centrifugation studies of the “fresh” nanoemulsions were conducted within 24 h of microfluidization in all cases. (N.B. significant evaporation of the n-octane- and n-decane-based nanoemulsions occurred within 1 week, which meant that no further analysis could be undertaken in these two cases).

After ageing for 4–6 weeks, the n-dodecane- and n-tetradecane-based nanoemulsions gradually undergo further Ostwald ripening, albeit to a much smaller extent than that observed for the n-octane and n-decane nanoemulsions. Analytical centrifugation studies indicate that the mean droplet diameter remains almost unchanged (Table 2), but the droplet distributions become significantly broader over time, with the n-dodecane-based nanoemulsion being prone to greater coarsening (with 6% of droplets exceeding 1 μm diameter after 6 weeks) than that produced using the less water-soluble n-tetradecane. After 6 weeks ageing at 20 °C, all of the latter oil droplets remained below 1 μm diameter, with around 65% still being classified as a nanoemulsion. In principle, at least some of this droplet coarsening could be attributed to a superimposed density distribution, as discussed earlier. (36−38) However, in practice, this is likely to be a secondary effect. Overall, the Ostwald ripening observed for these Pickering nanoemulsions strongly correlate with the aqueous solubilities of the four n-alkanes used in this study. Given the bimodal size distributions observed for these Pickering nanoemulsions on ageing, it was not possible to examine whether the cube of the mean diameter for the ripening droplets depended linearly on the ageing time, as reported previously by Solans and co-workers. (47)

Conclusions

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In summary, a series of oil-in-water Pickering nanoemulsions have been prepared using four different n-alkanes stabilized by PGMA48–PTFEMA50 nanoparticles. The Pickering nature of these nanoemulsions was confirmed by TEM studies because drying led to close-packed spherical superstructures comprising the original nanoparticles. Mean droplet diameters for the fresh nanoemulsions were determined by DLS, laser diffraction, and analytical centrifugation. Perhaps surprisingly, DLS proved to be inadequate for monitoring the Ostwald ripening of these Pickering nanoemulsions over time. This lack of sensitivity may be the result of rapid creaming of micrometer-sized droplets or it could simply reflect the relatively low upper size limit for this instrument. In contrast, laser diffraction was able to detect the growing population of larger oil droplets, but this technique suffered from relatively low resolution. Analytical centrifugation proved to be the most sensitive technique for studying droplet growth because the oil droplets are fractionated according to their size prior to detection. Time-dependent studies using the latter technique indicated that the n-octane nanoemulsions were rather unstable as expected, given the relatively high aqueous solubility of this oil (0.66 mg dm–3). These droplets coarsened significantly on ageing for 1 week at 20 °C, with larger mean droplet diameters and a much broader droplet size distribution being obtained, as confirmed by optical microscopy studies. Nanoemulsions prepared using n-decane (aqueous solubility = 0.0396 mg dm–3) also proved to be relatively unstable on ageing. In contrast, nanoemulsions prepared using either n-dodecane or n-tetradecane were significantly more stable, as expected given their much lower aqueous solubilities (0.0034 and 0.000386 mg dm–3, respectively). After ageing for 6 weeks at 20 °C, only minimal change in the mean droplet diameter was observed for these latter two oils, but significant broadening of their respective droplet size distributions was discernible. Nevertheless, 94% of n-dodecane droplets and 100% of n-tetradecane droplets remained below 1 μm diameter after this ageing period.

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b01835.

  • Details of the spherical micelle models used for the SAXS analysis (PDF)

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Author Information

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  • Corresponding Authors
    • Kate L. Thompson - Department of Chemistry, University of Sheffield, Dainton Building, Brook Hill, Sheffield, South Yorkshire S3 7HF, U.K.;  Email: [email protected]
    • Steven P. Armes - Department of Chemistry, University of Sheffield, Dainton Building, Brook Hill, Sheffield, South Yorkshire S3 7HF, U.K.;  Orcidhttp://orcid.org/0000-0002-8289-6351;  Email: [email protected]
  • Authors
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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S.P.A. thanks the financial support from EPSRC (Fellowship grant EP/R003009/1) and also acknowledges a five-year Advanced Investigator ERC grant (PISA 320372).

References

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    Langmuir (2010), 26 (11), 8836-8841CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Particle-stabilized high internal phase emulsions have been used to synthesize tough and very high porosity macroporous polymers with a closed-cell pore structure. In this study, we show that Pickering water-in-oil emulsion templates with up to an 85 vol. % internal phase can be stabilized by only 1 wt. % of titania particles with their surfaces suitably modified by the adsorption of 3.5 ± 0.5 wt. % oleic acid. The pore structure and mech. properties of the resulting macroporous polymers were tailored by altering the internal phase vol. ratio of the emulsion template and the titania particle concn. used to stabilize the emulsion templates. The pore size and pore size distributions increase with increasing internal phase vol. of the emulsion template as well as decreasing titania particle concn. used to stabilize the emulsion template. The mech. properties, namely, Young's modulus and the crush strength of the macroporous polymers, increased with decreasing porosity and increasing foam d. The toughest macroporous polymer had the lowest porosity but also the smallest pore size and narrowest pore size distribution.
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  15. 15
    Cui, Y.; Threlfall, M.; van Duijneveldt, J. S. Optimizing organoclay stabilized Pickering emulsions. J. Colloid Interface Sci. 2011, 356, 665– 671,  DOI: 10.1016/j.jcis.2011.01.046
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    Cui, Y.; van Duijneveldt, J. S. Adsorption of polyetheramines on montmorillonite at high pH. Langmuir 2010, 26, 17210– 17217,  DOI: 10.1021/la103278v
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    Cui, Y.; van Duijneveldt, J. S. Microcapsules composed of cross-linked organoclay. Langmuir 2012, 28, 1753– 1757,  DOI: 10.1021/la2040856
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    Microcapsules Composed of Cross-Linked Organoclay
    Cui, Yannan; van Duijneveldt, Jeroen S.
    Langmuir (2012), 28 (3), 1753-1757CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Polyelectrolyte-modified montmorillonite particles were used to stabilize oil-in-water Pickering emulsions, which were then bound together by an oil-sol. crosslinker to obtain microcapsules. It was detd. how the morphol. and rigidity of the microcapsules changed as polyelectrolyte and crosslinker concns. were varied. Well-defined microcapsules could be formed by using a moderate concn. of polyelectrolyte, and the higher the crosslinker concn., the more rigid the microcapsules. Dried microcapsules were obsd. using SEM, and it was shown that the clay platelets lie flat next to each other on the microcapsule surface, forming an armor-like structure.
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    Williams, M.; Armes, S. P.; York, D. W. Clay-based colloidosomes. Langmuir 2012, 28, 1142– 1148,  DOI: 10.1021/la2046405
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    Clay-Based Colloidosomes
    Williams, Mark; Armes, S. P.; York, David W.
    Langmuir (2012), 28 (2), 1142-1148CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Poly(ethylene imine) (PEI) was adsorbed onto the surface of Laponite clay nanoparticles from aq. soln. at pH 9 to produce an efficient hybrid Pickering emulsifier. This facile protocol gave stable sunflower oil-in-water Pickering emulsions via homogenization at 12,000 rpm for 2 min at 20°. The effect of varying the extent of PEI adsorption on the Pickering emulsifier performance of the surface-modified Laponite was studied for five oils of varying polarity using aq. electrophoresis, TGA, and laser diffraction studies. A min. vol.-av. emulsion droplet diam. of ∼60 μm was achieved at a Laponite concn. of 0.50% by mass when using a PEI/Laponite mass ratio of 0.50. Such emulsions proved to be very stable toward droplet coalescence over time scales of months, although creaming is obsd. on standing within days due to the relatively large droplet size. These conditions correspond to submonolayer coverage of the Laponite particles by the PEI, which ensures that there is little or no excess PEI remaining in the aq. continuous phase. This situation is confirmed by visual inspection of the underlying aq. phase of the creamed emulsion when using fluorescently labeled PEI. These Pickering emulsions are readily converted into novel clay-based colloidosomes via reaction of the primary and/or secondary amine groups on the PEI chains adsorbed at the Laponite surface with either oil-sol. poly(propylene glycol) diglycidyl ether or water-sol. poly(ethylene glycol) diglycidyl ether cross-linkers. These colloidosomes were sufficiently robust to survive the removal of the internal oil phase after washing with excess alc., as judged by both optical and fluorescence microscopy. However, dye release studies conducted with clay-based colloidosomes suggest that these microcapsules are highly permeable and hence do not provide an effective barrier for retarding the release of small mols.
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    Katepalli, H.; John, V. T.; Bose, A. The response of carbon black stabilized oil-in-water emulsions to the addition of surfactant solutions. Langmuir 2013, 29, 6790– 6797,  DOI: 10.1021/la400037c
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    The Response of Carbon Black Stabilized Oil-in-Water Emulsions to the Addition of Surfactant Solutions
    Katepalli, Hari; John, Vijay T.; Bose, Arijit
    Langmuir (2013), 29 (23), 6790-6797CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    The authors use carboxyl-terminated, neg. charged, carbon black (CB) particles suspended in water to create CB-stabilized octane-in-water emulsions, and examine the consequences of adding aq. anionic (SOS, SDS), cationic (OTAB, DTAB), and nonionic (Triton X-100) surfactant solns. to these emulsions. Depending upon the amphiphile's interaction with particles, interfacial activity, and bulk concn., some CB particles get displaced from the octane-water interfaces and are replaced by surfactants. The emulsions remain stable through this exchange. Particles leave the octane-water interfaces by two distinct modes that depend on the nature of particle-surfactant interactions. Both happen over time scales of the order of seconds. For anionic and nonionic surfactants that bind to the CB through hydrophobic interactions, individual particles or small agglomerates stream away steadily from the interface. Cationic surfactants bind strongly to the carboxylate groups, reduce the magnitude of the surface potential, and cause the CB particles to agglomerate into easily visible chunks at the droplet interfaces. These chunks then leave the interfaces at discrete intervals, rather than in a steady stream. For the longer chain cationic surfactant, DTAB, the particle ejection mode reverts back to a steady stream as the concn. is increased beyond a threshold. This change from chunks of particles leaving intermittently to steady streaming is because of the formation of a surfactant bilayer on the particles that reverses the particle surface charge and makes them highly hydrophilic. The charge reversal also suppresses agglomeration. Zeta potentials of CB particles measured after exposure to surfactant solns. support this hypothesis. These results are the 1st systematic observations of different particle release modes from oil-water interfaces produced by variations in interactions between surfactants and particles. They can be generalized to other particle-surfactant systems and exploited for materials synthesis.
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  20. 20
    Gautier, F.; Destribats, M.; Perrier-Cornet, R.; Dechézelles, J.-F.; Giermanska, J.; Héroguez, V.; Ravaine, S.; Leal-Calderon, F.; Schmitt, V. Pickering emulsions with stimulable particles: from highly- to weakly-covered interfaces. Phys. Chem. Chem. Phys. 2007, 9, 6455– 6462,  DOI: 10.1039/b710226g
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    Pickering emulsions with stimulable particles: from highly- to weakly-covered interfaces
    Gautier, Florent; Destribats, Mathieu; Perrier-Cornet, Romain; Dechezelles, Jean-Francois; Giermanska, Joanna; Heroguez, Valerie; Ravaine, Serge; Leal-Calderon, Fernando; Schmitt, Veronique
    Physical Chemistry Chemical Physics (2007), 9 (48), 6455-6462CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    We study oil-in-water emulsions stabilized by pH-sensitive colloidal silica or latex particles. Depending on the compn. of the continuous phase, the same type of particles and the same emulsification process lead to emulsions characterized either by large drops densely covered by the particles, or to small droplets which are weakly covered. The two kinetically stable states can be tuned reversibly by using pH or salinity as compositional stimuli. We examine the emulsions' behavior in these two limiting cases and we discuss the possible mechanisms allowing stabilization, esp. in the case of low surface coverage.
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    Persson, K. H.; Blute, I. A.; Mira, I. C.; Gustafsson, J. Creation of well-defined particle stabilized oil-in-water nanoemulsions. Colloids Surf., A 2014, 459, 48– 57,  DOI: 10.1016/j.colsurfa.2014.06.034
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    Creation of well-defined particle stabilized oil-in-water nanoemulsions
    Persson, Karin H.; Blute, Irena A.; Mira, Isabel C.; Gustafsson, Jonas
    Colloids and Surfaces, A: Physicochemical and Engineering Aspects (2014), 459 (), 48-57CODEN: CPEAEH; ISSN:0927-7757. (Elsevier B.V.)
    The prepn. of oil-in-water (o/w) nanoemulsions stabilized with silica nanoparticle sols has been investigated. The emulsification was performed using a high shear homogenizer (Microfluidizer TM processor, Microfluidics, USA). The effect of different processing conditions on the droplet size distribution and stability was investigated in emulsions prepd. using different types of oils, oil concn. and particle/oil ratios. It was the ability of the particles to attach to, and stabilize the newly created interface, rather than their ability to lower the interfacial tension, what proved important for the drop size of the resulting emulsions. Changes in drop size distribution with time, attributed to Ostwald ripening effects, were obsd. for the more sol. oils, while stable nanoemulsions with droplet size of ∼100-200 nm could be produced using a virtually water-insol. oil such as squalene.
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    Sihler, S.; Schrade, A.; Cao, Z.; Ziener, U. Inverse Pickering emulsions with droplet sizes below 500 nm. Langmuir 2015, 31, 10392– 10401,  DOI: 10.1021/acs.langmuir.5b02735
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    Inverse Pickering Emulsions with Droplet Sizes below 500 nm
    Sihler, Susanne; Schrade, Anika; Cao, Zhihai; Ziener, Ulrich
    Langmuir (2015), 31 (38), 10392-10401CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Inverse Pickering emulsions with droplet diams. between 180 and 450 nm, a narrow droplet size distribution, and an outstanding stability were prepd. using a miniemulsion technique. Com. available hydrophilic silica nanoparticles were used to stabilize the emulsions. They were hydrophobized in situ by the adsorption of various neutral polymeric surfactants. The influence of different parameters, such as kind and amt. of surfactant as hydrophobizing agent, size and charge of the silica particles, and amt. of water in the dispersed phase, as well as the kind of osmotic agent (sodium chloride and phosphate-buffered saline), on the emulsion characteristics was investigated. The systems were characterized by dynamic light scattering, transmission electron microscopy, cryo-SEM (cryo-SEM), thermogravimetric anal., and semiquant. attenuated total reflection IR spectroscopy. Cryo-SEM shows that some silica particles are obviously rendered hydrophilic and form a three-dimensional network inside the droplets.
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    Sacanna, S.; Kegel, W. K.; Philipse, A. P. Thermodynamically stable Pickering emulsions. Phys. Rev. Lett. 2007, 98, 158301,  DOI: 10.1103/physrevlett.98.158301
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    Thermodynamically Stable Pickering Emulsions
    Sacanna, S.; Kegel, W. K.; Philipse, A. P.
    Physical Review Letters (2007), 98 (15), 158301/1-158301/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    We show that under appropriate conditions, mixts. of oil, water, and nanoparticles form thermodynamically stable oil-in-water emulsions with monodisperse droplet diams. in the range of 30-150 nm. This observation challenges current wisdom that so-called Pickering emulsions are at most metastable and points to a new class of mesoscopic equil. structures. Thermodn. stability is demonstrated by the spontaneous evolution of binary droplet mixts. towards one intermediate size distribution. Equil. interfacial curvature due to an asym. charge distribution induced by adsorbed colloids explains the growth of emulsion droplets upon salt addn. Moreover, the existence of a minimal radius of curvature with a concomitant expulsion of excess oil is in close analogy with microemulsions.
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    Solans, C.; Izquierdo, P.; Nolla, J.; Azemar, N.; Garcia-Celma, M. J. Nano-emulsions. Curr. Opin. Colloid Interface Sci. 2005, 10, 102– 110,  DOI: 10.1016/j.cocis.2005.06.004
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    Nano-emulsions
    Solans, C.; Izquierdo, P.; Nolla, J.; Azemar, N.; Garcia-Celma, M. J.
    Current Opinion in Colloid & Interface Science (2005), 10 (3,4), 102-110CODEN: COCSFL; ISSN:1359-0294. (Elsevier B.V.)
    A review. The formation, properties and applications of nano-emulsions (also referred to as miniemulsions, ultrafine emulsions, submicron emulsions) are reviewed and summarized. Nano-emulsion droplet sizes fall typically in the range of 20-200 nm and show narrow size distributions. Although most of the publications on either oil-in-water (O/W) or water-in-oil (W/O) nano-emulsions report their formation by dispersion or high-energy emulsification methods, an increased interest is obsd. in the study of nano-emulsion formation by condensation or low-energy emulsification methods (based on the phase transitions that take place during the emulsification process). Phase behavior studies have shown that the size of the droplets is governed by the surfactant phase structure (bicontinuous microemulsion or lamellar) at the inversion point induced by either temp. or compn. Studies on nano-emulsion formation by the phase inversion temp. (PIT) method have shown a relation between min. droplet size and complete solubilization of the oil in a microemulsion bicontinuous phase independently of whether the initial phase equil. is single or multiphase. Due to their small droplet size nano-emulsions possess stability against sedimentation or creaming with Ostwald ripening forming the main mechanism of nano-emulsion breakdown. The main application of nano-emulsions is the prepn. of nanoparticles using a polymerizable monomer as the disperse phase (the so-called miniemulsion polymn. method) where nano-emulsion droplets act as nanoreactors. Novel complex polymeric materials as well as hybrid org./inorg. materials, such as magnetic polymeric nanospheres, are among the new applications developed. Another interesting application which is experiencing an active development is the use of nano-emulsions as formulations, namely, for controlled drug delivery and targeting.
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    Sonneville-Aubrun, O.; Simonnet, J.-T.; L’Alloret, F. Nanoemulsions: a new vehicle for skincare products. Adv. Colloid Interface Sci. 2004, 108, 145– 149,  DOI: 10.1016/j.cis.2003.10.026
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    Rapoport, N. Y.; Kennedy, A. M.; Shea, J. E.; Scaife, C. L.; Nam, K.-H. Controlled and targeted tumor chemotherapy by ultrasound-activated nanoemulsions/microbubbles. J. Controlled Release 2009, 138, 268– 276,  DOI: 10.1016/j.jconrel.2009.05.026
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    Controlled and targeted tumor chemotherapy by ultrasound-activated nanoemulsions/microbubbles
    Rapoport, Natalya Y.; Kennedy, Anne M.; Shea, Jill E.; Scaife, Courtney L.; Nam, Kweon-Ho
    Journal of Controlled Release (2009), 138 (3), 268-276CODEN: JCREEC; ISSN:0168-3659. (Elsevier B.V.)
    The paper reports the results of nanotherapy of ovarian, breast, and pancreatic cancerous tumors by paclitaxel-loaded nanoemulsions that convert into microbubbles locally in tumor tissue under the action of tumor-directed therapeutic ultrasound. Tumor accumulation of nanoemulsions was confirmed by ultrasound imaging. Dramatic regression of ovarian, breast, and orthotopic pancreatic tumors was obsd. in tumor therapy through systemic injections of drug-loaded nanoemulsions combined with therapeutic ultrasound, signifying efficient ultrasound-triggered drug release from tumor-accumulated nanodroplets. The mechanism of drug release in the process of droplet-to-bubble conversion is discussed. No therapeutic effect from the nanodroplet/ultrasound combination was obsd. without the drug, indicating that therapeutic effect was caused by the ultrasound-enhanced chemotherapeutic action of the tumor-targeted drug, rather than the mech. or thermal action of ultrasound itself. Tumor recurrence was obsd. after the completion of the first treatment round; a second treatment round with the same regimen proved less effective, suggesting that drug-resistant cells were either developed or selected during the first treatment round.
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    McClements, D. J.; Rao, J. Food-grade nanoemulsions: Formulation, fabrication, properties, performance, biological fate, and potential toxicity. Crit. Rev. Food Sci. Nutr. 2011, 51, 285– 330,  DOI: 10.1080/10408398.2011.559558
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    Food-Grade Nanoemulsions: Formulation, Fabrication, Properties, Performance, Biological Fate, and Potential Toxicity
    McClements, David Julian; Rao, Jiajia
    Critical Reviews in Food Science and Nutrition (2011), 51 (4), 285-330CODEN: CRFND6; ISSN:1040-8398. (Taylor & Francis, Inc.)
    A review. Nanoemulsions fabricated from food-grade ingredients are being increasingly utilized in the food industry to encapsulate, protect, and deliver lipophilic functional components, such as biol.-active lipids (e.g., ω-3 fatty acids, conjugated linoleic acid) and oil-sol. flavors, vitamins, preservatives, and nutraceuticals. The small size of the particles in nanoemulsions (r < 100 nm) means that they have a no. of potential advantages over conventional emulsions-higher stability to droplet aggregation and gravitational sepn., high optical clarity, ability to modulate product texture, and, increased bioavailability of lipophilic components. On the other hand, there may also be some risks assocd. with the oral ingestion of nanoemulsions, such as their ability to change the biol. fate of bioactive components within the gastrointestinal tract and the potential toxicity of some of the components used in their fabrication. This review article provides an overview of the current status of nanoemulsion formulation, fabrication, properties, applications, biol. fate, and potential toxicity with emphasis on systems suitable for utilization within the food and beverage industry.
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    Du, Z.; Wang, C.; Tai, X.; Wang, G.; Liu, X. Optimization and characterization of biocompatible oil-in-water nanoemulsion for pesticide delivery. ACS Sustain. Chem. Eng. 2016, 4, 983– 991,  DOI: 10.1021/acssuschemeng.5b01058
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    Optimization and Characterization of Biocompatible Oil-in-Water Nanoemulsion for Pesticide Delivery
    Du, Zhiping; Wang, Chuanxin; Tai, Xiumei; Wang, Guoyong; Liu, Xiaoying
    ACS Sustainable Chemistry & Engineering (2016), 4 (3), 983-991CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
    The formation of promising oil-in-water nanoemulsion suitable for pesticide delivery has been achieved by using Me laurate as oil phase, and alkyl polyglycoside (APG) and polyoxyethylene 3-lauryl ether (C12E3) as mixed surfactant. Effects of APG and C12E3 mixing ratios, oil wt. fraction, and total surfactant concn. on droplet size and distribution of the nanoemulsion were systematically investigated. Long-term stabilities of the nanoemulsions prepd. with various surfactant mixing ratios were assessed by measuring droplet size at different time intervals; the results indicated that the main driving force for droplet size increase over time was Ostwald ripening. On this basis, a practical water-insol. pesticide β-cypermethrin (β-CP) was incorporated into two optimized nanoemulsion systems to demonstrate potential applications. The results of dynamic light scattering (DLS) and transmission electron microscopy (TEM) measurements showed that the nanoemulsions had a nearly monodisperse droplet size distribution (PDI < 0.2) and incorporation of β-CP had no notable effect on the size and stability of the nanoemulsions. For consideration of practical application, diln. stability and spreading properties of the pesticide-loaded nanoemulsion were studied by DLS, contact angle, and dynamic surface tension, resp. The nanoemulsion was still homogeneous after diln., although destabilization in droplet size was obsd. by DLS. The results of contact angle and dynamic surface tension demonstrated the excellent spreading performance of the optimized nanoemulsion.
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    Teo, A.; Goh, K. K. T.; Wen, J.; Oey, I.; Ko, S.; Kwak, H.-S.; Lee, S. J. Physicochemical properties of whey protein, lactoferrin and Tween 20 stabilised nanoemulsions: Effect of temperature, pH and salt. Food Chem. 2016, 197, 297– 306,  DOI: 10.1016/j.foodchem.2015.10.086
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    Physicochemical properties of whey protein, lactoferrin and Tween 20 stabilised nanoemulsions: Effect of temperature, pH and salt
    Teo, Anges; Goh, Kelvin K. T.; Wen, Jingyuan; Oey, Indrawati; Ko, Sanghoon; Kwak, Hae-Soo; Lee, Sung Je
    Food Chemistry (2016), 197 (Part_A), 297-306CODEN: FOCHDJ; ISSN:0308-8146. (Elsevier Ltd.)
    Oil-in-water nanoemulsions were prepd. by emulsification and solvent evapn. using whey protein isolate (WPI), lactoferrin and Tween 20 as emulsifiers. Protein-stabilized nanoemulsions showed a decrease in particle size with increasing protein concn. from 0.25% to 1% (wt./wt.) level with Z-av. diam. between 70 and 90 nm. However, larger droplets were produced by Tween 20 (120-450 nm) esp. at concn. above 0.75% (wt./wt.). The stability of nanoemulsions to temp. (30-90 °C), pH (2-10) and ionic strength (0-500 mM NaCl or 0-90 mM CaCl2) was also tested. Tween 20 nanoemulsions were unstable to heat treatment at 90 °C for 15 min. WPI-stabilized nanoemulsions exhibited droplet aggregation near the isoelec. point at pH 4.5 and 5 and they were also unstable at salt concn. above 30 mM CaCl2. These results indicated that stable nanoemulsions can be prepd. by careful selection of emulsifiers.
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    Wooster, T. J.; Golding, M.; Sanguansri, P. Impact of oil type on nanoemulsion formation and ostwald ripening stability. Langmuir 2008, 24, 12758– 12765,  DOI: 10.1021/la801685v
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    Impact of Oil Type on Nanoemulsion Formation and Ostwald Ripening Stability
    Wooster, Tim J.; Golding, Matt; Sanguansri, Peerasak
    Langmuir (2008), 24 (22), 12758-12765CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    The formation of stable transparent nanoemulsions poses 2 challenges: the ability to initially create an emulsion where the entire droplet size distribution is <80 nm, and the subsequent stabilization of this emulsion against Ostwald ripening. The phys. properties of the oil phase and the nature of the surfactant layer have a considerable impact on nanoemulsion formation and stabilization. Nanoemulsions made with high viscosity oils, such as long chain triglycerides (LCT), were considerably larger (D = 120 nm) than nanoemulsions prepd. with low viscosity oils such as hexadecane (D = 80 nm). The optimization of surfactant architecture, and differential viscosity ηD/ηC, gave remarkably small nanoemulsions. With av. sizes <40 nm they are some of the smallest homogenized emulsions ever reported. What is more remarkable is that LCT nanoemulsions do not undergo Ostwald ripening and are phys. stable for over 3 mo. Ostwald ripening is prevented by the large molar volume of long chain triglyceride oils, which makes them insol. in H2O thus providing a kinetic barrier to Ostwald ripening. Examn. of the Ostwald ripening of mixed oil nanoemulsions found that the entropy gain assocd. with oil demixing provided a thermodn. barrier to Ostwald ripening. Not only are the nanoemulsions created in this work some of the smallest reported, but they are also thermodynamically stable to Ostwald ripening when at least 50% of the oil phase is an insol. triglyceride.
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    Gutiérrez, J. M.; González, C.; Maestro, A.; Solé, I.; Pey, C. M.; Nolla, J. Nano-emulsions: New applications and optimization of their preparation. Curr. Opin. Colloid Interface Sci. 2008, 13, 245– 251,  DOI: 10.1016/j.cocis.2008.01.005
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    Nano-emulsions: New applications and optimization of their preparation
    Gutierrez, J. M.; Gonzalez, C.; Maestro, A.; Sole, I.; Pey, C. M.; Nolla, J.
    Current Opinion in Colloid & Interface Science (2008), 13 (4), 245-251CODEN: COCSFL; ISSN:1359-0294. (Elsevier B.V.)
    A review. Nano-emulsions, as nonequil. systems, present characteristics and properties which depend not only on compn. but also on the prepn. method. Although interest in nano-emulsions was developed since ∼20 years ago, mainly for nanoparticle prepn., it is in the last years that direct applications of nano-emulsions in consumer products are being developed, mainly in pharmacy and cosmetics. These recent applications have made that studies on optimization methods for nano-emulsion prepn. be a requirement. This review is focused on the most recent literature on developments of nano-emulsions as final application products and on the optimization of their prepn.
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    The use of ultrasonics for nanoemulsion preparation
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    Jiménez Saelices, C.; Capron, I. Design of Pickering micro and nanoemulsions based on the structural characteristics of nanocelluloses. Biomacromolecules 2018, 19, 460,  DOI: 10.1021/acs.biomac.7b01564
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    Thompson, K. L.; Cinotti, N.; Jones, E. R.; Mable, C. J.; Fowler, P. W.; Armes, S. P. Bespoke diblock copolymer nanoparticles enable the production of relatively stable oil-in-water Pickering nanoemulsions. Langmuir 2017, 33, 12616– 12623,  DOI: 10.1021/acs.langmuir.7b02267
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    Bespoke diblock copolymer nanoparticles enable production of relatively stable oil-in-water Pickering nanoemulsions
    Thompson, Kate L.; Cinotti, Natacha; Jones, Elizabeth R.; Mable, Charlotte J.; Fowler, Patrick W.; Armes, Steven P.
    Langmuir (2017), 33 (44), 12616-12623CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Sterically stabilized diblock copolymer nanoparticles with an intensity-av. diam. of 25 nm are prepd. in the form of a concd. aq. dispersion using polymn.-induced self-assembly (PISA). The addn. of n-dodecane followed by high-shear homogenization produces n-dodecane-in-water Pickering macroemulsions of 22-46 μm diam. If the nanoparticles are present in sufficient excess, then subsequent processing using a high-pressure microfluidizer leads to the formation of Pickering nanoemulsions with a mean oil droplet diam. below 200 nm. The size of these Pickering nanoemulsions can be tuned by systematically varying the nanoparticle concn., applied pressure, no. of passes, and oil vol. fraction. High-internal-phase emulsions can also be achieved by increasing the n-dodecane vol. fraction up to 0.80. TEM studies of (dried) n-dodecane droplets confirm the presence of intact nanoparticles and suggest a relatively high surface coverage, which is consistent with model packing calcns. based on radius ratios. Such Pickering nanoemulsions proved to be surprisingly stable with respect to Ostwald ripening, with no significant change in the mean DLS droplet diam. after storage for approx. 4 mo at 20 °C.
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  35. 35

    This overall diameter of 220 nm necessarily includes the thickness of the adsorbed PGMA48–PTFEMA50 nanoparticle layer. If this contribution is subtracted from the overall droplet diameter, the mean diameter for the underlying “naked” oil droplet is less than 200 nm, which meets the criterion for a nanoemulsion according to the literature (see ref (24)).

    There is no corresponding record for this reference.
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    Akpinar, B.; Fielding, L. A.; Cunningham, V. J.; Ning, Y.; Mykhaylyk, O. O.; Fowler, P. W.; Armes, S. P. Determining the effective density and stabilizer layer thickness of sterically stabilized nanoparticles. Macromolecules 2016, 49, 5160– 5171,  DOI: 10.1021/acs.macromol.6b00987
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    Determining the Effective Density and Stabilizer Layer Thickness of Sterically Stabilized Nanoparticles
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    Macromolecules (Washington, DC, United States) (2016), 49 (14), 5160-5171CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
    A series of model sterically stabilized diblock copolymer nanoparticles has been designed to aid the development of anal. protocols in order to det. two key parameters: the effective particle d. and the steric stabilizer layer thickness. The former parameter is essential for high resoln. particle size anal. based on anal. (ultra)centrifugation techniques (e.g., disk centrifuge photosedimentometry, DCP), whereas the latter parameter is of fundamental importance in detg. the effectiveness of steric stabilization as a colloid stability mechanism. The diblock copolymer nanoparticles were prepd. via polymn.-induced self-assembly (PISA) using RAFT aq. emulsion polymn.: this approach affords relatively narrow particle size distributions and enables the mean particle diam. and the stabilizer layer thickness to be adjusted independently via systematic variation of the mean d.p. of the hydrophobic and hydrophilic blocks, resp. The hydrophobic core-forming block was poly(2,2,2-trifluoroethyl methacrylate) [PTFEMA], which was selected for its relatively high d. The hydrophilic stabilizer block was poly(glycerol monomethacrylate) [PGMA], which is a well-known non-ionic polymer that remains water-sol. over a wide range of temps. Four series of PGMAx-PTFEMAy nanoparticles were prepd. (x = 28, 43, 63, and 98, y = 100-1400) and characterized via transmission electron microscopy (TEM), dynamic light scattering (DLS), and small-angle X-ray scattering (SAXS). It was found that the d.p. of both the PGMA stabilizer and core-forming PTFEMA had a strong influence on the mean particle diam., which ranged from 20 to 250 nm. Furthermore, SAXS was used to det. radii of gyration of 1.46 to 2.69 nm for the solvated PGMA stabilizer blocks. Thus, the mean effective d. of these sterically stabilized particles was calcd. and detd. to lie between 1.19 g cm-3 for the smaller particles and 1.41 g cm-3 for the larger particles; these values are significantly lower than the solid-state d. of PTFEMA (1.47 g cm-3). Since anal. centrifugation requires the d. difference between the particles and the aq. phase, detg. the effective particle d. is clearly vital for obtaining reliable particle size distributions. Furthermore, selected DCP data were recalcd. by taking into account the inherent d. distribution superimposed on the particle size distribution. Consequently, the true particle size distributions were found to be somewhat narrower than those calcd. using an erroneous single d. value, with smaller particles being particularly sensitive to this artifact.
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    Fielding, L. A.; Mykhaylyk, O. O.; Armes, S. P.; Fowler, P. W.; Mittal, V.; Fitzpatrick, S. Correcting for a density distribution: Particle size analysis of core–shell nanocomposite particles using disk centrifuge photosedimentometry. Langmuir 2012, 28, 2536– 2544,  DOI: 10.1021/la204841n
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    Correcting for a Density Distribution: Particle Size Analysis of Core-Shell Nanocomposite Particles Using Disk Centrifuge Photosedimentometry
    Fielding, Lee A.; Mykhaylyk, Oleksandr O.; Armes, Steven P.; Fowler, Patrick W.; Mittal, Vikas; Fitzpatrick, Stephen
    Langmuir (2012), 28 (5), 2536-2544CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Many types of colloidal particles possess a core-shell morphol. In this Article, if the core and shell densities differ, this morphol. leads to an inherent d. distribution for particles of finite polydispersity. If the shell is denser than the core, this d. distribution implies an artificial narrowing of the particle size distribution as detd. by disk centrifuge photosedimentometry (DCP). In the specific case of polystyrene/silica nanocomposite particles, which consist of a polystyrene core coated with a monolayer shell of silica nanoparticles, the particle d. distribution can be detd. by anal. ultracentrifugation and introduce a math. method to account for this d. distribution by reanalyzing the raw DCP data. Using the mean silica packing d. calcd. from small-angle x-ray scattering, the real particle d. can be calcd. for each data point. The cor. DCP particle size distribution is both broader and more consistent with particle size distributions reported for the same polystyrene/silica nanocomposite sample using other sizing techniques, such as electron microscopy, laser light diffraction, and dynamic light scattering. Artifactual narrowing of the size distribution also probably occurs for many other polymer/inorg. nanocomposite particles comprising a low-d. core of variable dimensions coated with a high-d. shell of const. thickness, or for core-shell latexes where the shell is continuous rather than particulate.
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  38. 38

    For the present nanoemulsion system, the core comprises a low-density oil and the shell is composed of high-density nanoparticles. However, in this particular case the overall droplet density is less than that of the continuous phase, so centrifugation leads to creaming rather than sedimentation. Thus artefactual broadening of the droplet size distribution is anticipated, rather than artefactual narrowing.

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    Ilavsky, J.; Jemian, P. R. Irena: tool suite for modeling and analysis of small-angle scattering. J. Appl. Crystallogr. 2009, 42, 347– 353,  DOI: 10.1107/s0021889809002222
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    Irena, a tool suite for anal. of both x-ray and neutron small-angle scattering (SAS) data within the com. Igor Pro application, brings together a comprehensive suite of tools useful for studies in materials science, physics, chem., polymer science and other fields. In addn. to Guinier and Porod fits, the suite combines a variety of advanced SAS data evaluation tools for the modeling of size distribution in the dil. limit using max. entropy and other methods, dil. limit small-angle scattering from multiple noninteracting populations of scatterers, the pair-distance distribution function, a unified fit, the Debye-Bueche model, the reflectivity (x-ray and neutron) using Parratt's formalism, and small-angle diffraction. There are also a no. of support tools, such as a data import/export tool supporting a broad sampling of common data formats, a data modification tool, a presentation-quality graphics tool optimized for small-angle scattering data, and a neutron and x-ray scattering contrast calculator. These tools are brought together into one suite with consistent interfaces and functionality. The suite allows robust automated note recording and saving of parameters during export.
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    The solubilities in H2O at 25 ± 1.5° of methane, ethane, propane, butane, isobutane, pentane, isopentane, hexane, 2-methylpentane, 2,2-dimethylbutane, n-heptane, 2,4-dimethylpentane, n-octane, 2,2,4-trimethylpentane, cyclopentane, cyclohexane, methylcyclopentane, methylcyclohexane, benzene, toluene, o-xylene, ethylbenzene, and isopropylbenzene were detd.
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    Taylor, P. Ostwald ripening in emulsions: estimation of solution thermodynamics of the disperse phase. Adv. Colloid Interface Sci. 2003, 106, 261– 285,  DOI: 10.1016/s0001-8686(03)00113-1
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    The rate of Ostwald ripening was measured for small emulsions prepd. by the diln. of O/W microemulsions contg. a range of n-alkanes. The rates were detd. as a function of temp. for decane, undecane, dodecane, and tridecane using photon correlation spectroscopy. The aq. solubilities of these oils were estd. from the rates of ripening using the Lifshitz, Slyozov and Wagner theory of Ostwald ripening. These solubilities were used to calc. the free energy, enthalpy and entropy of soln. for the 4 alkanes. Values obtained were in good agreement with literature values. The free energy of soln. of hexadecane and octadecane were also detd. from the rate of ripening of mixed oil phase emulsions with dodecane using the treatment suggested by Kabalnov. The values obtained agreed well with extrapolated values of the free energy from lower alkanes, suggesting that coiling or aggregation of the longer chain mols. in soln. was not affecting the soly.
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    Binks, B. P.; Cho, W.-G.; Fletcher, P. D. I.; Petsev, D. N. Stability of oil-in-water emulsions in a low interfacial tension system. Langmuir 2000, 16, 1025– 1034,  DOI: 10.1021/la990952m
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    Stability of Oil-in-Water Emulsions in a Low Interfacial Tension System
    Binks, B. P.; Cho, W-G.; Fletcher, P. D. I.; Petsev, D. N.
    Langmuir (2000), 16 (3), 1025-1034CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    The stability of oil-in-water emulsions to both creaming and coalescence was measured as a function of salt concn. in heptane + water mixts. stabilized by sodium bis(2-ethylhexylsulfosuccinate) (AOT). Emulsions were prepd. from pre-equilibrated phases in Winsor I systems. Up to 0.035 M NaCl, the creaming rate decreases with salt concn., with no visible sign of coalescence. Above 0.035 M and approaching the phase inversion salt concn. of 0.055 M, the creaming rate increases quite markedly and coalescence becomes appreciable. The creaming at low salt concns. is due mainly to the buoyancy motion of single drops. A simple model for the time evolution of resolved water is developed which successfully describes the behavior. The drop size changes obsd. are due to Ostwald ripening, the rate of which decreases with salt concn. Exptl. ripening rates are consistent with a mechanism by which oil is transported between emulsion drops via microemulsion droplets present in the continuous phase. We calc. the energy of interdrop interaction allowing for drop deformation using exptl. detd. parameters of interfacial tension, drop radius, and zeta potential. At high [NaCl], due mainly to the low interfacial tension, the drops can deform and the attraction between them becomes significant. As a result, flocculation occurs which leads to coalescence instability.
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    Kinetics of Swelling of Oil-in-Water Emulsions
    Binks, B. P.; Clint, J. H.; Fletcher, P. D. I.; Rippon, S.; Lubetkin, S. D.; Mulqueen, P. J.
    Langmuir (1998), 14 (19), 5402-5411CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    The kinetics of swelling of squalane-in-water emulsion drops are investigated by the addn. of decane-in-water emulsion drops. Squalane is sufficiently insol. in the aq. continuous phase that it cannot transfer between oil drops. Decane is able to transfer between drops and swells the squalane drops. The mixed emulsions were stabilized by the nonionic surfactant n-dodecyl octaoxyethylene glycol ether (C12E8) and were stable with respect to drop coalescence. A systematic series of expts. were made in which the swelling rates were detd. as functions of the initial drop radii, vol. fractions, and oil compns. of both types of emulsions. Using a theor. model originally developed by Ugelstad et al., the entire data set was successfully fitted with a single adjustable parameter equal to the product of the soly. of decane (C∞) and its diffusion coeff. in the aq. continuous phase (D). The measured value of C∞D was consistent with a mechanism of decane transport in which micelles of C12E8 act as carriers facilitating decane transport between emulsion drops. Also in agreement with this mechanism, it was obsd. that increasing the aq. phase concn. of C12E8 increased the swelling rate. Ostwald ripening rates of the decane-in-water emulsions gave values of C∞D consistent with those derived from swelling expts.
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    Binks, B. P.; Clint, J. H.; Fletcher, P. D. I.; Rippon, S.; Lubetkin, S. D.; Mulqueen, P. J. Kinetics of swelling of oil-in-water emulsions stabilized by different surfactants. Langmuir 1999, 15, 4495– 4501,  DOI: 10.1021/la990054q
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    Kinetics of Swelling of Oil-in-Water Emulsions Stabilized by Different Surfactants
    Binks, B. P.; Clint, J. H.; Fletcher, P. D. I.; Rippon, S.; Lubetkin, S. D.; Mulqueen, P. J.
    Langmuir (1999), 15 (13), 4495-4501CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    The effects of different concns. of a range of surfactants on the kinetics of swelling of squalane-in-water emulsion drops by the addn. of decane-in-water emulsion drops were obsd. Using a model described in detail in an earlier paper (Langmuir 1998, 14, 5402), kinetic measurements were used to det. the product of the decane soly. C∞ (expressed as a dimensionless vol. fraction) and effective diffusion coeff. D for the transport of decane between drops across the aq. continuous phase. The issue is addressed as to whether the decane transfer occurs by "mol." transport or whether the surfactant micelles present in the continuous aq. phase act as "carriers" for the decane. Swelling rates were investigated for different concns. of two nonionic surfactants, dodecyl hexaoxyethylene glycol ether (C12E6) and dodecyl octaoxyethylene glycol ether (C12E8), the cationic surfactant tetradecyl trimethylammonium bromide (TTAB) and the anionic species sodium dodecyl sulfate (SDS). The solubilization of decane was detd. for aq. solns. of all the surfactants. For mol. transport, the product C∞D is predicted to be of the order of 10-17 m2 s-1 and independent of the surfactant concn. in the continuous aq. phase. From the swelling expts., the product C∞D is found to be on the order of 10-14-10-13 m2 s-1 and to increase with surfactant concn. for all species except SDS. The measured magnitudes of C∞D for all the surfactants are consistent with oil transfer occurring by the "micelle carrier" mechanism with negligible energy barrier to the transport process.
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    Walter, J.; Thajudeen, T.; Süβ, S.; Segets, D.; Peukert, W. New possibilities of accurate particle characterisation by applying direct boundary models to analytical centrifugation. Nanoscale 2015, 7, 6574– 6587,  DOI: 10.1039/c5nr00995b
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    New possibilities of accurate particle characterisation by applying direct boundary models to analytical centrifugation
    Walter, Johannes; Thajudeen, Thaseem; Sueβ, Sebastian; Segets, Doris; Peukert, Wolfgang
    Nanoscale (2015), 7 (15), 6574-6587CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Anal. centrifugation (AC) is a powerful technique for the characterization of nanoparticles in colloidal systems. As a direct and abs. technique it requires no calibration or measurements of stds. Moreover, it offers simple exptl. design and handling, high sample throughput as well as moderate investment costs. However, the full potential of AC for nanoparticle size anal. requires the development of powerful data anal. techniques. In this study we show how the application of direct boundary models to AC data opens up new possibilities in particle characterization. An accurate anal. method, successfully applied to sedimentation data obtained by anal. ultracentrifugation (AUC) in the past, was used for the first time in analyzing AC data. Unlike traditional data evaluation routines for AC using a designated no. of radial positions or scans, direct boundary models consider the complete sedimentation boundary, which results in significantly better statistics. We demonstrate that meniscus fitting, as well as the correction of radius and time invariant noise significantly improves the signal-to-noise ratio and prevents the occurrence of false positives due to optical artifacts. Moreover, hydrodynamic non-ideality can be assessed by the residuals obtained from the anal. The sedimentation coeff. distributions obtained by AC are in excellent agreement with the results from AUC. Brownian dynamics simulations were used to generate numerical sedimentation data to study the influence of diffusion on the obtained distributions. Our approach is further validated using polystyrene and silica nanoparticles. In particular, we demonstrate the strength of AC for analyzing multimodal distributions by means of gold nanoparticles.
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    Beltramo, P. J.; Gupta, M.; Alicke, A.; Liascukiene, I.; Gunes, D. Z.; Baroud, C. N.; Vermant, J. Arresting dissolution by interfacial rheology design. Proc. Natl. Acad. Sci. U.S.A. 2017, 114, 10373,  DOI: 10.1073/pnas.1705181114
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    Arresting dissolution by interfacial rheology design
    Beltramo, Peter J.; Gupta, Manish; Alicke, Alexandra; Liascukiene, Irma; Gunes, Deniz Z.; Baroud, Charles N.; Vermant, Jan
    Proceedings of the National Academy of Sciences of the United States of America (2017), 114 (39), 10373-10378CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    A strategy to halt dissoln. of particle-coated air bubbles in water based on interfacial rheol. design is presented. Previously a dense monolayer was believed to be required for such an armored bubble to resist dissoln.; in fact, engineering a two-dimensional yield stress interface achieves such performance at sub-monolayer particle coverage. A suite of interfacial rheol. techniques characterized spherical and ellipsoidal particles at an air/water interface as a function of surface coverage. Bubbles with varying particle coverage were made and their resistance to dissoln. evaluated using a microfluidic technique. A bare bubble only had a single pressure at which a given radius was stable; the authors detd. a range of pressures over which armored bubble dissoln. was arrested. The link between interfacial rheol. and macroscopic dissoln. of ∼100 μm bubbles coated with ∼1 um particles is presented and discussed. Generic design rationale was confirmed using non-spherical particles, which develop significant yield stress at even lower surface coverages. Thus, it can be used to successfully inhibit Ostwald ripening in a multitude of foam and emulsion applications.
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    Izquierdo, P.; Esquena, J.; Tadros, T. F.; Dederen, C.; Garcia, M. J.; Azemar, N.; Solans, C. Formation and stability of nano-emulsions prepared using the phase inversion temperature method. Langmuir 2002, 18, 26– 30,  DOI: 10.1021/la010808c
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    Formation and Stability of Nano-Emulsions Prepared Using the Phase Inversion Temperature Method
    Izquierdo, P.; Esquena, J.; Tadros, Th. F.; Dederen, C.; Garcia, M. J.; Azemar, N.; Solans, C.
    Langmuir (2002), 18 (1), 26-30CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Formation of O/W nano-emulsions was studied in H2O/‾C12‾E4/oil systems by the phase inversion temp. emulsification method. Emulsification was carried out at the corresponding HLB (hydrophilic-lipophilic balance) temp., and then the emulsions were cooled fast to 25°. The influence of surfactant concn. and oil soly. on HLB temp., nano-emulsion droplet size, and stability also was studied. Droplet size was detd. by dynamic light scattering, and nano-emulsion stability was assessed, measuring the variation of droplet size as a function of time. The results obtained showed that the breakdown process of nano-emulsions studied could be attributed to Ostwald ripening. An increase of nano-emulsion instability with the increase in surfactant concn. and oil soly. was also found.
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  1. Saul J. Hunter, Priyanka Chohan, Spyridon Varlas, Steven P. Armes. Effect of Temperature, Oil Type, and Copolymer Concentration on the Long-Term Stability of Oil-in-Water Pickering Nanoemulsions Prepared Using Diblock Copolymer Nanoparticles. Langmuir 2024, 40 (7) , 3702-3714. https://doi.org/10.1021/acs.langmuir.3c03423
  2. Yanlin Guo, Xinpeng Zhang, Xiaohan Wang, Li Zhang, Zhenghe Xu, Dejun Sun. Nanoemulsions Stable against Ostwald Ripening. Langmuir 2024, 40 (2) , 1364-1372. https://doi.org/10.1021/acs.langmuir.3c03019
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Cite this: Langmuir 2018, 34, 31, 9289–9297
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https://doi.org/10.1021/acs.langmuir.8b01835
Published July 12, 2018

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  • Abstract

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    Scheme 1

    Scheme 1. Schematic Representation of the Adsorption of PGMA48–PTFEMA50 Nanoparticles at the n-Alkane/Water Interfacea
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    aIt is assumed that (i) the nanoparticles are adsorbed with an effective contact angle of 0° and (ii) PGMA48 stabilizer chains in direct contact with the n-alkane/water interface are fully collapsed and hence do not contribute to the adsorbed nanoparticle radius. Thus, given that the effective thickness of the adsorbed sterically stabilized nanoparticles is given by 2Rs + 2Rg (rather than 2Rs + 4Rg), the approximate effective sphere radius, rparticle, of the adsorbed nanoparticles is given by rparticle = Rs + Rg or 8.4 nm. Experimental values for Rs and Rg were obtained from SAXS analysis of the PGMA48–PTFEMA50 nanoparticles prior to emulsification, see the main text.

    Figure 1

    Figure 1. (A) Synthesis of PGMA48–PTFEMA50 nanoparticles via RAFT aqueous emulsion polymerization of TFEMA at 70 °C using a PGMA48 macro-CTA; (B) TEM image of the resulting sterically stabilized PGMA48–PTFEMA50 nanoparticles; (C) particle size distribution determined by DLS; (D) SAXS pattern recorded for a 1.0 w/w % aqueous dispersion of the PGMA48–PTFEMA50 nanoparticles (solid red line represents the data fit obtained using a spherical micelle model, which reports a volume-average diameter of 20 ± 1 nm).

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    Figure 2

    Figure 2. Schematic representation of the two-step preparation of Pickering nanoemulsions. First, a 7.0 w/v % aqueous dispersion of PGMA48–PTFEMA50 nanoparticles were homogenized with one of four n-alkanes to form an oil-in-water Pickering macroemulsion of around 40–50 μm diameter using conventional high-shear homogenization at 15 500 rpm for 2 min at 20 °C. This relatively coarse precursor emulsion was then refined via 10 passes through the LV1 microfluidizer at 20 000 psi to obtain the final Pickering nanoemulsion.

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    Figure 3

    Figure 3. Representative TEM images obtained for freshly prepared Pickering nanoemulsions prepared via high-pressure microfluidization using 7.0 w/w % PGMA48–PTFEMA50 nanoparticles and 20 v/v % n-alkane. Conditions: applied pressure = 20 000 psi for 10 passes using an LV1 microfluidizer.

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    Figure 4

    Figure 4. (A) Effect of droplet concentration on apparent droplet diameter for an n-dodecane-in-water Pickering nanoemulsion as determined by analytical centrifugation (LUMiSizer instrument). This hindrance function indicates that the optimum droplet concentration for such analyses is approximately 1.0 v/v %, with higher concentrations leading to hindered creaming and hence artefactual undersizing. (B) Droplet size distributions determined by analytical centrifugation using an effective nanoemulsion droplet density of 0.81 g cm–3 at either 10 or 1.0 v/v % n-dodecane. As a result of these initial studies, all further LUMiSizer analyses were conducted at a droplet concentration of 1.0 v/v % n-alkane.

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    Figure 5

    Figure 5. Various particle sizing techniques used to characterize a freshly-made and 1-week-old 20 v/v % n-octane-in-water Pickering nanoemulsion prepared using 7.0 w/w % PGMA48–PTFEMA50 nanoparticles at 20 000 psi with 10 passes. (A) Laser diffraction, (B) DLS, (C) analytical centrifugation (LUMiSizer, ρnanoemulsion = 0.77 g cm–3), and (D) optical microscopy (no oil droplets were observed by optical microscopy for any of the freshly-made Pickering nanoemulsions reported in this study, but micrometer-sized oil droplets were clearly discernible within 1 week of ageing the n-octane-based Pickering nanoemulsion). Perhaps surprisingly, DLS seemed to be relatively insensitive to the larger oil droplets present within the 1-week-old Pickering nanoemulsion, whereas this coarse fraction was readily detected by laser diffraction and analytical centrifugation.

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    Figure 6

    Figure 6. Volume-weighted cumulative distributions determined by analytical centrifugation (LUMiSizer instrument) for the n-alkane-in-water nanoemulsions: (A) after ageing for 1 week at 20 °C and (B) after ageing for 6 weeks. Note that significant evaporation of the more volatile n-octane and n-decane oils occurred within 1 week; so, no further analysis was possible in these two cases.

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      A review. Colloidal particles act in many ways like surfactant mols., particularly if adsorbed to a fluid-fluid interface. Just as the water or oil-liking tendency of a surfactant is quantified in terms of the hydrophile-lipophile balance (HLB) no., so can that of a spherical particle be described in terms of its wettability via contact angle. Important differences exist, however, between the two types of surface-active material, due in part to the fact that particles are strongly held at interfaces. This review attempts to correlate the behavior obsd. in systems contg. either particles or surfactant mols. in the areas of adsorption to interfaces, partitioning between phases and solid-stabilized emulsions and foams.
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    5. 5
      Binks, B. P.; Lumsdon, S. O. Pickering emulsions stabilized by monodisperse latex particles: Effects of particle size. Langmuir 2001, 17, 4540– 4547,  DOI: 10.1021/la0103822
      5
      Pickering Emulsions Stabilized by Monodisperse Latex Particles: Effects of Particle Size
      Binks, B. P.; Lumsdon, S. O.
      Langmuir (2001), 17 (15), 4540-4547CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      The prepn., type, and stability of emulsions of oil and H2O stabilized solely by spherical, monodisperse polystyrene latex particles of different size is described. Two types of behavior occur depending on whether particles remain intact (in the case of cyclohexane) or dissolve to give free polymer chains (in the case of toluene). Emulsions formed with cyclohexane and either hydrophilic aldehyde/sulfate particles or hydrophobic sulfate particles are H2O-in-oil (w/o) over a wide range of salt concns. and H2O vol. fractions. Av. emulsion drop diams. initially increase from 35 to 75 μm with increasing particle diam. and then remain const. Although such emulsions sediment, there is no sign of coalescence for over 6 mo. The authors show evidence of the transition from nonflocculated to flocculated emulsions upon increasing the H2O vol. fraction, as predicted theor. for charged drops in oil. By use of toluene and hydrophilic particles however, emulsions can be inverted from oil-in-H2O (o/w) to w/o with increasing salt concn. The concn. of salt required to screen the repulsions between neg. charged adsorbed polymers increases with initial particle size as the av. mol. wt. also increases. H2O-in-oil emulsions, of ∼1 μm diam., are stable to coalescence for long periods.
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    6. 6
      Binks, B. P.; Lumsdon, S. O. Stability of oil-in-water emulsions stabilised by silica particles. Phys. Chem. Chem. Phys. 1999, 1, 3007– 3016,  DOI: 10.1039/a902209k
      6
      Stability of oil-in-water emulsions stabilised by silica particles
      Binks, B. P.; Lumsdon, S. O.
      Physical Chemistry Chemical Physics (1999), 1 (12), 3007-3016CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      We describe the prepn. and properties of oil-in-water emulsions stabilized by colloidal silica particles alone. The charge on the particles and their extent of flocculation, assessed via turbidity measurements, can be modified by pH control and addn. of simple electrolytes. The stability of emulsions to both creaming and coalescence is low in the absence of electrolyte, and the effects of adding salt are dependent on the type of salt. In systems contg. NaCl, emulsions are less stable once the particles are flocculated. In the presence of either LaCl3 or tetraethylammonium bromide (TEAB), emulsion stability increases dramatically for conditions where the silica particles are weakly flocculated; extensive flocculation of the particles however leads to destabilization of the emulsions. For TEAB, relatively large emulsions of diam. around 40 μm remain very stable for up to 3 mo at salt concns. corresponding to the onset of coagulation of the colloid. Such emulsions are themselves strongly flocculated.
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    7. 7
      Fujii, S.; Randall, D. P.; Armes, S. P. Synthesis of polystyrene/poly2-(dimethylamino) ethyl methacrylate-stat-ethylene glycol dimethacrylatel core-shell latex particles by seeded emulsion polymerization and their application as stimulus-responsive particulate emulsifiers for oil-in-water emulsions. Langmuir 2004, 20, 11329– 11335,  DOI: 10.1021/la048473x
      There is no corresponding record for this reference.
    8. 8
      Kalashnikova, I.; Bizot, H.; Cathala, B.; Capron, I. New Pickering emulsions stabilized by bacterial cellulose nanocrystals. Langmuir 2011, 27, 7471– 7479,  DOI: 10.1021/la200971f
      8
      New Pickering Emulsions Stabilized by Bacterial Cellulose Nanocrystals
      Kalashnikova, Irina; Bizot, Herve; Cathala, Bernard; Capron, Isabelle
      Langmuir (2011), 27 (12), 7471-7479CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      The authors studied oil in water Pickering emulsions stabilized by cellulose nanocrystals obtained by hydrochloric acid hydrolysis of bacterial cellulose. The resulting solid particles, called bacterial cellulose nanocrystals (BCNs), present an elongated shape and low surface charge d., forming a colloidal suspension in water. The BCNs produced proved to stabilize the hexadecane/water interface, promoting monodispersed oil in water droplets around 4 μm in diam. stable for several months. The authors characterized the emulsion and visualized the particles at the surface of the droplets by SEM and calcd. the droplet coverage by varying the BCN concn. in the aq. phase. A 60% coverage limit was defined, above which very stable, deformable droplets are obtained. The high stability of the more covered droplets was attributed to the particle irreversible adsorption assocd. with the formation of a 2-dimensional network. Due to the sustainability and low environmental impact of cellulose, the BCN based emulsions open opportunities for the development of environmentally friendly new materials.
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    9. 9
      Nguyen, B. T.; Wang, W.; Saunders, B. R.; Benyahia, L.; Nicolai, T. pH-Responsive water-in-water Pickering emulsions. Langmuir 2015, 31, 3605– 3611,  DOI: 10.1021/la5049024
      9
      pH-Responsive Water-in-Water Pickering Emulsions
      Nguyen, Bach T.; Wang, Wenkai; Saunders, Brian R.; Benyahia, Lazhar; Nicolai, Taco
      Langmuir (2015), 31 (12), 3605-3611CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      The structure and stability of H2O-in-H2O emulsions was studied in the presence of spherical, pH-sensitive microgels. The emulsions were formed by mixing aq. solns. of dextran and PEO. The microgels consisted of cross-linked, synthetic polymers with a radius that steeply increased from 60 to 220 nm with increasing pH within a narrow range around 7.0. At all pH values between 5.0 and 7.5, the microgels were preferentially situated at the interface, but only in a narrow range between pH 7.0 and 7.5, the emulsions were stable for ≥1 wk. The droplet size was visualized with confocal laser scanning microscopy and is smallest in the stable pH range. Emulsions could be stabilized or destabilized by small changes of the pH. Addn. of small amts. of salt led to a shift of the pH range where the emulsions were stable. The effects of varying the microgel concn. and the polymer compn. were studied.
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    10. 10
      Nicolai, T.; Murray, B. Particle stabilized water in water emulsions. Food Hydrocolloids 2017, 68, 157– 163,  DOI: 10.1016/j.foodhyd.2016.08.036
      10
      Particle stabilized water in water emulsions
      Nicolai, Taco; Murray, Brent
      Food Hydrocolloids (2017), 68 (), 157-163CODEN: FOHYES; ISSN:0268-005X. (Elsevier Ltd.)
      Food products often contain mixts. of incompatible water sol. macromols. such as proteins and polysaccharides. When two aq. solns. of incompatible macromols. are mixed they sep. into two phases each enriched in one of the two macromols. Contrary to oil-water (O/W) emulsions, water/water (W/W) emulsions cannot be stabilized by addn. of surfactants and in food applications macroscopic phase sepn. is avoided by gelling one or both phases. However, recently it was shown that W/W emulsions can be stabilized to varying extents by addn. of particles. Such particle stabilized emulsions are also known as Pickering emulsions and have been studied extensively for O/W emulsions. Here the literature on particle stabilization of W/W emulsions is reviewed. The behavior of particle stabilized W/W emulsions is found to be quite different from that of O/W emulsions due to the much smaller interfacial tension and the much larger length scale at which the interface expresses itself. Besides the particle size, interaction of the particles with the macromols. in the mixt. and with each other at the interface appears to play a decisive role for stabilization.
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    11. 11
      Rizzelli, S. L.; Jones, E. R.; Thompson, K. L.; Armes, S. P. Preparation of non-aqueous Pickering emulsions using anisotropic block copolymer nanoparticles. Colloid Polym. Sci. 2016, 294, 1– 12,  DOI: 10.1007/s00396-015-3785-3
      11
      Preparation of non-aqueous Pickering emulsions using anisotropic block copolymer nanoparticles
      Rizzelli, S. L.; Jones, E. R.; Thompson, K. L.; Armes, S. P.
      Colloid and Polymer Science (2016), 294 (1), 1-12CODEN: CPMSB6; ISSN:0303-402X. (Springer)
      Amphiphilic diblock copolymer worms prepd. via alc. RAFT dispersion polymn. can be used to stabilize nonaq. Pickering emulsions. A previously reported synthesis protocol based on polymn.-induced self-assembly (PISA) was modified to enable the prepn. of poly(2-(dimethylamino)ethyl methacrylate)-poly(benzyl methacrylate) (PDMA-PBzMA) worm-like particles directly in methanol at relatively high solids. A dil. dispersion of these highly anisotropic nanoparticles was then homogenized with sunflower oil to produce sunflower oil-in-methanol emulsions. The mean droplet diam. ranged from 9 to 104 μm, depending on the nanoparticle concn. and the stirring rate used for homogenization. The sunflower oil content was increased systematically, with stable emulsions being obtained up to a vol. fraction of 0.60. In all cases, the sunflower oil droplets gradually increase in size on ageing for up to 4 days. However, stable emulsions were obtained after this time period, with no further change in the mean droplet diam. for at least 2 mo on standing at ambient temp. Turbidimetry studies of the continuous phase after sedimentation of the relatively dense emulsion droplets indicated that the initial adsorption efficiency of the PDMA-PBzMA worms is very high, but this is reduced significantly as the droplet diam. gradually increases during ageing. There is a concomitant increase in fractional surface coverage over the same time period, suggesting that the increase in droplet diam. is the result of limited coalescence, rather than an Ostwald ripening mechanism.
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    12. 12
      Thompson, K. L.; Lane, J. A.; Derry, M. J.; Armes, S. P. Non-aqueous Isorefractive Pickering Emulsions. Langmuir 2015, 31, 4373– 4376,  DOI: 10.1021/acs.langmuir.5b00630
      12
      Non-aqueous Isorefractive Pickering Emulsions
      Thompson, Kate L.; Lane, Jacob A.; Derry, Matthew J.; Armes, Steven P.
      Langmuir (2015), 31 (15), 4373-4376CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Non-aq. Pickering emulsions of 16-240 μm diam. have been prepd. using diblock copolymer worms with ethylene glycol as the droplet phase and an n-alkane as the continuous phase. Initial studies using n-dodecane resulted in stable emulsions that were significantly less turbid than conventional water-in-oil emulsions. This is attributed to the rather similar refractive indexes of the latter two phases. By utilizing n-tetradecane as an alternative oil that almost precisely matches the refractive index of ethylene glycol, almost isorefractive ethylene glycol-in-n-tetradecane Pickering emulsions can be prepd. The droplet diam. and transparency of such emulsions can be systematically varied by adjusting the worm copolymer concn.
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    13. 13
      Fielding, L. A.; Armes, S. P. Preparation of Pickering emulsions and colloidosomes using either a glycerol-functionalised silica sol or core-shell polymer/silica nanocomposite particles. J. Mater. Chem. 2012, 22, 11235– 11244,  DOI: 10.1039/c2jm31433a
      13
      Preparation of Pickering emulsions and colloidosomes using either a glycerol-functionalized silica sol or core-shell polymer/silica nanocomposite particles
      Fielding, Lee A.; Armes, Steven P.
      Journal of Materials Chemistry (2012), 22 (22), 11235-11244CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
      A com. glycerol-modified 19 nm silica sol was homogenized with sunflower oil to form stable Pickering emulsions and also covalently cross-linked colloidosomes. Colloidal core-shell polymer/silica nanocomposite particles produced using this glycerol-functionalized silica were also used to produce both Pickering emulsions and colloidosomes contg. hybrid shells comprising both inorg. and org. components. The formation of stable oil-in-water Pickering emulsions required either low pH or the addn. of electrolyte: this is rationalized in terms of the highly anionic surface character of the silica particles. Colloidosomes are readily obtained on addn. of a polymeric diisocyanate, which reacts with the surface glycerol groups on the silica particles. This oil-sol. cross-linker is confined to the interior of the emulsion droplets, thus avoiding inter-colloidosome aggregation. The oil phase can be removed from the colloidosomes by washing with excess alc., resulting in microcapsules comprising either a 19 nm particulate silica shell or a 240 nm polymer/silica shell. These microcapsules can be imaged by optical microscopy in soln. and by SEM in the dry state. The permeability of these colloidosomes with respect to small mol. release was also examd. by incorporating an oil-sol. fluorescent dye during homogenization that becomes water-sol. on raising the soln. pH of the aq. continuous phase. Finally, control expts. performed with a non-functionalized silica sol confirmed that Pickering emulsions cannot be converted into colloidosomes due to the absence of surface glycerol groups.
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    14. 14
      Ikem, V. O.; Menner, A.; Bismarck, A. High-porosity macroporous polymers sythesized from titania-particle-stabilized medium and high internal phase emulsions. Langmuir 2010, 26, 8836– 8841,  DOI: 10.1021/la9046066
      14
      High-Porosity Macroporous Polymers Synthesized from Titania-Particle-Stabilized Medium and High Internal Phase Emulsions
      Ikem, Vivian O.; Menner, Angelika; Bismarck, Alexander
      Langmuir (2010), 26 (11), 8836-8841CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Particle-stabilized high internal phase emulsions have been used to synthesize tough and very high porosity macroporous polymers with a closed-cell pore structure. In this study, we show that Pickering water-in-oil emulsion templates with up to an 85 vol. % internal phase can be stabilized by only 1 wt. % of titania particles with their surfaces suitably modified by the adsorption of 3.5 ± 0.5 wt. % oleic acid. The pore structure and mech. properties of the resulting macroporous polymers were tailored by altering the internal phase vol. ratio of the emulsion template and the titania particle concn. used to stabilize the emulsion templates. The pore size and pore size distributions increase with increasing internal phase vol. of the emulsion template as well as decreasing titania particle concn. used to stabilize the emulsion template. The mech. properties, namely, Young's modulus and the crush strength of the macroporous polymers, increased with decreasing porosity and increasing foam d. The toughest macroporous polymer had the lowest porosity but also the smallest pore size and narrowest pore size distribution.
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    15. 15
      Cui, Y.; Threlfall, M.; van Duijneveldt, J. S. Optimizing organoclay stabilized Pickering emulsions. J. Colloid Interface Sci. 2011, 356, 665– 671,  DOI: 10.1016/j.jcis.2011.01.046
      There is no corresponding record for this reference.
    16. 16
      Cui, Y.; van Duijneveldt, J. S. Adsorption of polyetheramines on montmorillonite at high pH. Langmuir 2010, 26, 17210– 17217,  DOI: 10.1021/la103278v
      There is no corresponding record for this reference.
    17. 17
      Cui, Y.; van Duijneveldt, J. S. Microcapsules composed of cross-linked organoclay. Langmuir 2012, 28, 1753– 1757,  DOI: 10.1021/la2040856
      17
      Microcapsules Composed of Cross-Linked Organoclay
      Cui, Yannan; van Duijneveldt, Jeroen S.
      Langmuir (2012), 28 (3), 1753-1757CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Polyelectrolyte-modified montmorillonite particles were used to stabilize oil-in-water Pickering emulsions, which were then bound together by an oil-sol. crosslinker to obtain microcapsules. It was detd. how the morphol. and rigidity of the microcapsules changed as polyelectrolyte and crosslinker concns. were varied. Well-defined microcapsules could be formed by using a moderate concn. of polyelectrolyte, and the higher the crosslinker concn., the more rigid the microcapsules. Dried microcapsules were obsd. using SEM, and it was shown that the clay platelets lie flat next to each other on the microcapsule surface, forming an armor-like structure.
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    18. 18
      Williams, M.; Armes, S. P.; York, D. W. Clay-based colloidosomes. Langmuir 2012, 28, 1142– 1148,  DOI: 10.1021/la2046405
      18
      Clay-Based Colloidosomes
      Williams, Mark; Armes, S. P.; York, David W.
      Langmuir (2012), 28 (2), 1142-1148CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Poly(ethylene imine) (PEI) was adsorbed onto the surface of Laponite clay nanoparticles from aq. soln. at pH 9 to produce an efficient hybrid Pickering emulsifier. This facile protocol gave stable sunflower oil-in-water Pickering emulsions via homogenization at 12,000 rpm for 2 min at 20°. The effect of varying the extent of PEI adsorption on the Pickering emulsifier performance of the surface-modified Laponite was studied for five oils of varying polarity using aq. electrophoresis, TGA, and laser diffraction studies. A min. vol.-av. emulsion droplet diam. of ∼60 μm was achieved at a Laponite concn. of 0.50% by mass when using a PEI/Laponite mass ratio of 0.50. Such emulsions proved to be very stable toward droplet coalescence over time scales of months, although creaming is obsd. on standing within days due to the relatively large droplet size. These conditions correspond to submonolayer coverage of the Laponite particles by the PEI, which ensures that there is little or no excess PEI remaining in the aq. continuous phase. This situation is confirmed by visual inspection of the underlying aq. phase of the creamed emulsion when using fluorescently labeled PEI. These Pickering emulsions are readily converted into novel clay-based colloidosomes via reaction of the primary and/or secondary amine groups on the PEI chains adsorbed at the Laponite surface with either oil-sol. poly(propylene glycol) diglycidyl ether or water-sol. poly(ethylene glycol) diglycidyl ether cross-linkers. These colloidosomes were sufficiently robust to survive the removal of the internal oil phase after washing with excess alc., as judged by both optical and fluorescence microscopy. However, dye release studies conducted with clay-based colloidosomes suggest that these microcapsules are highly permeable and hence do not provide an effective barrier for retarding the release of small mols.
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    19. 19
      Katepalli, H.; John, V. T.; Bose, A. The response of carbon black stabilized oil-in-water emulsions to the addition of surfactant solutions. Langmuir 2013, 29, 6790– 6797,  DOI: 10.1021/la400037c
      19
      The Response of Carbon Black Stabilized Oil-in-Water Emulsions to the Addition of Surfactant Solutions
      Katepalli, Hari; John, Vijay T.; Bose, Arijit
      Langmuir (2013), 29 (23), 6790-6797CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      The authors use carboxyl-terminated, neg. charged, carbon black (CB) particles suspended in water to create CB-stabilized octane-in-water emulsions, and examine the consequences of adding aq. anionic (SOS, SDS), cationic (OTAB, DTAB), and nonionic (Triton X-100) surfactant solns. to these emulsions. Depending upon the amphiphile's interaction with particles, interfacial activity, and bulk concn., some CB particles get displaced from the octane-water interfaces and are replaced by surfactants. The emulsions remain stable through this exchange. Particles leave the octane-water interfaces by two distinct modes that depend on the nature of particle-surfactant interactions. Both happen over time scales of the order of seconds. For anionic and nonionic surfactants that bind to the CB through hydrophobic interactions, individual particles or small agglomerates stream away steadily from the interface. Cationic surfactants bind strongly to the carboxylate groups, reduce the magnitude of the surface potential, and cause the CB particles to agglomerate into easily visible chunks at the droplet interfaces. These chunks then leave the interfaces at discrete intervals, rather than in a steady stream. For the longer chain cationic surfactant, DTAB, the particle ejection mode reverts back to a steady stream as the concn. is increased beyond a threshold. This change from chunks of particles leaving intermittently to steady streaming is because of the formation of a surfactant bilayer on the particles that reverses the particle surface charge and makes them highly hydrophilic. The charge reversal also suppresses agglomeration. Zeta potentials of CB particles measured after exposure to surfactant solns. support this hypothesis. These results are the 1st systematic observations of different particle release modes from oil-water interfaces produced by variations in interactions between surfactants and particles. They can be generalized to other particle-surfactant systems and exploited for materials synthesis.
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    20. 20
      Gautier, F.; Destribats, M.; Perrier-Cornet, R.; Dechézelles, J.-F.; Giermanska, J.; Héroguez, V.; Ravaine, S.; Leal-Calderon, F.; Schmitt, V. Pickering emulsions with stimulable particles: from highly- to weakly-covered interfaces. Phys. Chem. Chem. Phys. 2007, 9, 6455– 6462,  DOI: 10.1039/b710226g
      20
      Pickering emulsions with stimulable particles: from highly- to weakly-covered interfaces
      Gautier, Florent; Destribats, Mathieu; Perrier-Cornet, Romain; Dechezelles, Jean-Francois; Giermanska, Joanna; Heroguez, Valerie; Ravaine, Serge; Leal-Calderon, Fernando; Schmitt, Veronique
      Physical Chemistry Chemical Physics (2007), 9 (48), 6455-6462CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      We study oil-in-water emulsions stabilized by pH-sensitive colloidal silica or latex particles. Depending on the compn. of the continuous phase, the same type of particles and the same emulsification process lead to emulsions characterized either by large drops densely covered by the particles, or to small droplets which are weakly covered. The two kinetically stable states can be tuned reversibly by using pH or salinity as compositional stimuli. We examine the emulsions' behavior in these two limiting cases and we discuss the possible mechanisms allowing stabilization, esp. in the case of low surface coverage.
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    21. 21
      Persson, K. H.; Blute, I. A.; Mira, I. C.; Gustafsson, J. Creation of well-defined particle stabilized oil-in-water nanoemulsions. Colloids Surf., A 2014, 459, 48– 57,  DOI: 10.1016/j.colsurfa.2014.06.034
      21
      Creation of well-defined particle stabilized oil-in-water nanoemulsions
      Persson, Karin H.; Blute, Irena A.; Mira, Isabel C.; Gustafsson, Jonas
      Colloids and Surfaces, A: Physicochemical and Engineering Aspects (2014), 459 (), 48-57CODEN: CPEAEH; ISSN:0927-7757. (Elsevier B.V.)
      The prepn. of oil-in-water (o/w) nanoemulsions stabilized with silica nanoparticle sols has been investigated. The emulsification was performed using a high shear homogenizer (Microfluidizer TM processor, Microfluidics, USA). The effect of different processing conditions on the droplet size distribution and stability was investigated in emulsions prepd. using different types of oils, oil concn. and particle/oil ratios. It was the ability of the particles to attach to, and stabilize the newly created interface, rather than their ability to lower the interfacial tension, what proved important for the drop size of the resulting emulsions. Changes in drop size distribution with time, attributed to Ostwald ripening effects, were obsd. for the more sol. oils, while stable nanoemulsions with droplet size of ∼100-200 nm could be produced using a virtually water-insol. oil such as squalene.
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    22. 22
      Sihler, S.; Schrade, A.; Cao, Z.; Ziener, U. Inverse Pickering emulsions with droplet sizes below 500 nm. Langmuir 2015, 31, 10392– 10401,  DOI: 10.1021/acs.langmuir.5b02735
      22
      Inverse Pickering Emulsions with Droplet Sizes below 500 nm
      Sihler, Susanne; Schrade, Anika; Cao, Zhihai; Ziener, Ulrich
      Langmuir (2015), 31 (38), 10392-10401CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Inverse Pickering emulsions with droplet diams. between 180 and 450 nm, a narrow droplet size distribution, and an outstanding stability were prepd. using a miniemulsion technique. Com. available hydrophilic silica nanoparticles were used to stabilize the emulsions. They were hydrophobized in situ by the adsorption of various neutral polymeric surfactants. The influence of different parameters, such as kind and amt. of surfactant as hydrophobizing agent, size and charge of the silica particles, and amt. of water in the dispersed phase, as well as the kind of osmotic agent (sodium chloride and phosphate-buffered saline), on the emulsion characteristics was investigated. The systems were characterized by dynamic light scattering, transmission electron microscopy, cryo-SEM (cryo-SEM), thermogravimetric anal., and semiquant. attenuated total reflection IR spectroscopy. Cryo-SEM shows that some silica particles are obviously rendered hydrophilic and form a three-dimensional network inside the droplets.
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    23. 23
      Sacanna, S.; Kegel, W. K.; Philipse, A. P. Thermodynamically stable Pickering emulsions. Phys. Rev. Lett. 2007, 98, 158301,  DOI: 10.1103/physrevlett.98.158301
      23
      Thermodynamically Stable Pickering Emulsions
      Sacanna, S.; Kegel, W. K.; Philipse, A. P.
      Physical Review Letters (2007), 98 (15), 158301/1-158301/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      We show that under appropriate conditions, mixts. of oil, water, and nanoparticles form thermodynamically stable oil-in-water emulsions with monodisperse droplet diams. in the range of 30-150 nm. This observation challenges current wisdom that so-called Pickering emulsions are at most metastable and points to a new class of mesoscopic equil. structures. Thermodn. stability is demonstrated by the spontaneous evolution of binary droplet mixts. towards one intermediate size distribution. Equil. interfacial curvature due to an asym. charge distribution induced by adsorbed colloids explains the growth of emulsion droplets upon salt addn. Moreover, the existence of a minimal radius of curvature with a concomitant expulsion of excess oil is in close analogy with microemulsions.
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    24. 24
      Solans, C.; Izquierdo, P.; Nolla, J.; Azemar, N.; Garcia-Celma, M. J. Nano-emulsions. Curr. Opin. Colloid Interface Sci. 2005, 10, 102– 110,  DOI: 10.1016/j.cocis.2005.06.004
      24
      Nano-emulsions
      Solans, C.; Izquierdo, P.; Nolla, J.; Azemar, N.; Garcia-Celma, M. J.
      Current Opinion in Colloid & Interface Science (2005), 10 (3,4), 102-110CODEN: COCSFL; ISSN:1359-0294. (Elsevier B.V.)
      A review. The formation, properties and applications of nano-emulsions (also referred to as miniemulsions, ultrafine emulsions, submicron emulsions) are reviewed and summarized. Nano-emulsion droplet sizes fall typically in the range of 20-200 nm and show narrow size distributions. Although most of the publications on either oil-in-water (O/W) or water-in-oil (W/O) nano-emulsions report their formation by dispersion or high-energy emulsification methods, an increased interest is obsd. in the study of nano-emulsion formation by condensation or low-energy emulsification methods (based on the phase transitions that take place during the emulsification process). Phase behavior studies have shown that the size of the droplets is governed by the surfactant phase structure (bicontinuous microemulsion or lamellar) at the inversion point induced by either temp. or compn. Studies on nano-emulsion formation by the phase inversion temp. (PIT) method have shown a relation between min. droplet size and complete solubilization of the oil in a microemulsion bicontinuous phase independently of whether the initial phase equil. is single or multiphase. Due to their small droplet size nano-emulsions possess stability against sedimentation or creaming with Ostwald ripening forming the main mechanism of nano-emulsion breakdown. The main application of nano-emulsions is the prepn. of nanoparticles using a polymerizable monomer as the disperse phase (the so-called miniemulsion polymn. method) where nano-emulsion droplets act as nanoreactors. Novel complex polymeric materials as well as hybrid org./inorg. materials, such as magnetic polymeric nanospheres, are among the new applications developed. Another interesting application which is experiencing an active development is the use of nano-emulsions as formulations, namely, for controlled drug delivery and targeting.
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    25. 25
      Sonneville-Aubrun, O.; Simonnet, J.-T.; L’Alloret, F. Nanoemulsions: a new vehicle for skincare products. Adv. Colloid Interface Sci. 2004, 108, 145– 149,  DOI: 10.1016/j.cis.2003.10.026
      There is no corresponding record for this reference.
    26. 26
      Rapoport, N. Y.; Kennedy, A. M.; Shea, J. E.; Scaife, C. L.; Nam, K.-H. Controlled and targeted tumor chemotherapy by ultrasound-activated nanoemulsions/microbubbles. J. Controlled Release 2009, 138, 268– 276,  DOI: 10.1016/j.jconrel.2009.05.026
      26
      Controlled and targeted tumor chemotherapy by ultrasound-activated nanoemulsions/microbubbles
      Rapoport, Natalya Y.; Kennedy, Anne M.; Shea, Jill E.; Scaife, Courtney L.; Nam, Kweon-Ho
      Journal of Controlled Release (2009), 138 (3), 268-276CODEN: JCREEC; ISSN:0168-3659. (Elsevier B.V.)
      The paper reports the results of nanotherapy of ovarian, breast, and pancreatic cancerous tumors by paclitaxel-loaded nanoemulsions that convert into microbubbles locally in tumor tissue under the action of tumor-directed therapeutic ultrasound. Tumor accumulation of nanoemulsions was confirmed by ultrasound imaging. Dramatic regression of ovarian, breast, and orthotopic pancreatic tumors was obsd. in tumor therapy through systemic injections of drug-loaded nanoemulsions combined with therapeutic ultrasound, signifying efficient ultrasound-triggered drug release from tumor-accumulated nanodroplets. The mechanism of drug release in the process of droplet-to-bubble conversion is discussed. No therapeutic effect from the nanodroplet/ultrasound combination was obsd. without the drug, indicating that therapeutic effect was caused by the ultrasound-enhanced chemotherapeutic action of the tumor-targeted drug, rather than the mech. or thermal action of ultrasound itself. Tumor recurrence was obsd. after the completion of the first treatment round; a second treatment round with the same regimen proved less effective, suggesting that drug-resistant cells were either developed or selected during the first treatment round.
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    27. 27
      McClements, D. J.; Rao, J. Food-grade nanoemulsions: Formulation, fabrication, properties, performance, biological fate, and potential toxicity. Crit. Rev. Food Sci. Nutr. 2011, 51, 285– 330,  DOI: 10.1080/10408398.2011.559558
      27
      Food-Grade Nanoemulsions: Formulation, Fabrication, Properties, Performance, Biological Fate, and Potential Toxicity
      McClements, David Julian; Rao, Jiajia
      Critical Reviews in Food Science and Nutrition (2011), 51 (4), 285-330CODEN: CRFND6; ISSN:1040-8398. (Taylor & Francis, Inc.)
      A review. Nanoemulsions fabricated from food-grade ingredients are being increasingly utilized in the food industry to encapsulate, protect, and deliver lipophilic functional components, such as biol.-active lipids (e.g., ω-3 fatty acids, conjugated linoleic acid) and oil-sol. flavors, vitamins, preservatives, and nutraceuticals. The small size of the particles in nanoemulsions (r < 100 nm) means that they have a no. of potential advantages over conventional emulsions-higher stability to droplet aggregation and gravitational sepn., high optical clarity, ability to modulate product texture, and, increased bioavailability of lipophilic components. On the other hand, there may also be some risks assocd. with the oral ingestion of nanoemulsions, such as their ability to change the biol. fate of bioactive components within the gastrointestinal tract and the potential toxicity of some of the components used in their fabrication. This review article provides an overview of the current status of nanoemulsion formulation, fabrication, properties, applications, biol. fate, and potential toxicity with emphasis on systems suitable for utilization within the food and beverage industry.
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    28. 28
      Du, Z.; Wang, C.; Tai, X.; Wang, G.; Liu, X. Optimization and characterization of biocompatible oil-in-water nanoemulsion for pesticide delivery. ACS Sustain. Chem. Eng. 2016, 4, 983– 991,  DOI: 10.1021/acssuschemeng.5b01058
      28
      Optimization and Characterization of Biocompatible Oil-in-Water Nanoemulsion for Pesticide Delivery
      Du, Zhiping; Wang, Chuanxin; Tai, Xiumei; Wang, Guoyong; Liu, Xiaoying
      ACS Sustainable Chemistry & Engineering (2016), 4 (3), 983-991CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
      The formation of promising oil-in-water nanoemulsion suitable for pesticide delivery has been achieved by using Me laurate as oil phase, and alkyl polyglycoside (APG) and polyoxyethylene 3-lauryl ether (C12E3) as mixed surfactant. Effects of APG and C12E3 mixing ratios, oil wt. fraction, and total surfactant concn. on droplet size and distribution of the nanoemulsion were systematically investigated. Long-term stabilities of the nanoemulsions prepd. with various surfactant mixing ratios were assessed by measuring droplet size at different time intervals; the results indicated that the main driving force for droplet size increase over time was Ostwald ripening. On this basis, a practical water-insol. pesticide β-cypermethrin (β-CP) was incorporated into two optimized nanoemulsion systems to demonstrate potential applications. The results of dynamic light scattering (DLS) and transmission electron microscopy (TEM) measurements showed that the nanoemulsions had a nearly monodisperse droplet size distribution (PDI < 0.2) and incorporation of β-CP had no notable effect on the size and stability of the nanoemulsions. For consideration of practical application, diln. stability and spreading properties of the pesticide-loaded nanoemulsion were studied by DLS, contact angle, and dynamic surface tension, resp. The nanoemulsion was still homogeneous after diln., although destabilization in droplet size was obsd. by DLS. The results of contact angle and dynamic surface tension demonstrated the excellent spreading performance of the optimized nanoemulsion.
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    29. 29
      Teo, A.; Goh, K. K. T.; Wen, J.; Oey, I.; Ko, S.; Kwak, H.-S.; Lee, S. J. Physicochemical properties of whey protein, lactoferrin and Tween 20 stabilised nanoemulsions: Effect of temperature, pH and salt. Food Chem. 2016, 197, 297– 306,  DOI: 10.1016/j.foodchem.2015.10.086
      29
      Physicochemical properties of whey protein, lactoferrin and Tween 20 stabilised nanoemulsions: Effect of temperature, pH and salt
      Teo, Anges; Goh, Kelvin K. T.; Wen, Jingyuan; Oey, Indrawati; Ko, Sanghoon; Kwak, Hae-Soo; Lee, Sung Je
      Food Chemistry (2016), 197 (Part_A), 297-306CODEN: FOCHDJ; ISSN:0308-8146. (Elsevier Ltd.)
      Oil-in-water nanoemulsions were prepd. by emulsification and solvent evapn. using whey protein isolate (WPI), lactoferrin and Tween 20 as emulsifiers. Protein-stabilized nanoemulsions showed a decrease in particle size with increasing protein concn. from 0.25% to 1% (wt./wt.) level with Z-av. diam. between 70 and 90 nm. However, larger droplets were produced by Tween 20 (120-450 nm) esp. at concn. above 0.75% (wt./wt.). The stability of nanoemulsions to temp. (30-90 °C), pH (2-10) and ionic strength (0-500 mM NaCl or 0-90 mM CaCl2) was also tested. Tween 20 nanoemulsions were unstable to heat treatment at 90 °C for 15 min. WPI-stabilized nanoemulsions exhibited droplet aggregation near the isoelec. point at pH 4.5 and 5 and they were also unstable at salt concn. above 30 mM CaCl2. These results indicated that stable nanoemulsions can be prepd. by careful selection of emulsifiers.
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    30. 30
      Wooster, T. J.; Golding, M.; Sanguansri, P. Impact of oil type on nanoemulsion formation and ostwald ripening stability. Langmuir 2008, 24, 12758– 12765,  DOI: 10.1021/la801685v
      30
      Impact of Oil Type on Nanoemulsion Formation and Ostwald Ripening Stability
      Wooster, Tim J.; Golding, Matt; Sanguansri, Peerasak
      Langmuir (2008), 24 (22), 12758-12765CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      The formation of stable transparent nanoemulsions poses 2 challenges: the ability to initially create an emulsion where the entire droplet size distribution is <80 nm, and the subsequent stabilization of this emulsion against Ostwald ripening. The phys. properties of the oil phase and the nature of the surfactant layer have a considerable impact on nanoemulsion formation and stabilization. Nanoemulsions made with high viscosity oils, such as long chain triglycerides (LCT), were considerably larger (D = 120 nm) than nanoemulsions prepd. with low viscosity oils such as hexadecane (D = 80 nm). The optimization of surfactant architecture, and differential viscosity ηD/ηC, gave remarkably small nanoemulsions. With av. sizes <40 nm they are some of the smallest homogenized emulsions ever reported. What is more remarkable is that LCT nanoemulsions do not undergo Ostwald ripening and are phys. stable for over 3 mo. Ostwald ripening is prevented by the large molar volume of long chain triglyceride oils, which makes them insol. in H2O thus providing a kinetic barrier to Ostwald ripening. Examn. of the Ostwald ripening of mixed oil nanoemulsions found that the entropy gain assocd. with oil demixing provided a thermodn. barrier to Ostwald ripening. Not only are the nanoemulsions created in this work some of the smallest reported, but they are also thermodynamically stable to Ostwald ripening when at least 50% of the oil phase is an insol. triglyceride.
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    31. 31
      Gutiérrez, J. M.; González, C.; Maestro, A.; Solé, I.; Pey, C. M.; Nolla, J. Nano-emulsions: New applications and optimization of their preparation. Curr. Opin. Colloid Interface Sci. 2008, 13, 245– 251,  DOI: 10.1016/j.cocis.2008.01.005
      31
      Nano-emulsions: New applications and optimization of their preparation
      Gutierrez, J. M.; Gonzalez, C.; Maestro, A.; Sole, I.; Pey, C. M.; Nolla, J.
      Current Opinion in Colloid & Interface Science (2008), 13 (4), 245-251CODEN: COCSFL; ISSN:1359-0294. (Elsevier B.V.)
      A review. Nano-emulsions, as nonequil. systems, present characteristics and properties which depend not only on compn. but also on the prepn. method. Although interest in nano-emulsions was developed since ∼20 years ago, mainly for nanoparticle prepn., it is in the last years that direct applications of nano-emulsions in consumer products are being developed, mainly in pharmacy and cosmetics. These recent applications have made that studies on optimization methods for nano-emulsion prepn. be a requirement. This review is focused on the most recent literature on developments of nano-emulsions as final application products and on the optimization of their prepn.
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    32. 32
      Kentish, S.; Wooster, T. J.; Ashokkumar, A.; Balachandran, S.; Mawson, R.; Simons, L. The use of ultrasonics for nanoemulsion preparation. Innovat. Food Sci. Emerg. Technol. 2008, 9, 170– 175,  DOI: 10.1016/j.ifset.2007.07.005
      32
      The use of ultrasonics for nanoemulsion preparation
      Kentish, S.; Wooster, T. J.; Ashokkumar, M.; Balachandran, S.; Mawson, R.; Simons, L.
      Innovative Food Science & Emerging Technologies (2008), 9 (2), 170-175CODEN: IFSEBO; ISSN:1466-8564. (Elsevier Ltd.)
      Oil-in-water emulsions are an important vehicles for the delivery of hydrophobic bioactive compds. into a range of food products. The prepn. of very fine emulsions is of increasing interest to the beverage industry, as novel ingredients can be added with negligible impact to soln. clarity. In the present study, both a batch and focused flow-through ultrasonic cell were utilized for emulsification with ultrasonic power generation at 20-24 kHz. Emulsions with a mean droplet size as low as 135 ± 5 nm were achieved using a mixt. of flaxseed oil and water in the presence of Tween 40 surfactant. Results are comparable to those for emulsions prepd. with a microfluidizer operated at 100 MPa. The key to efficient ultrasonic emulsification is to det. an optimum ultrasonic energy intensity input for these systems, as excess energy input may lead to an increase in droplet size.
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    33. 33
      Jiménez Saelices, C.; Capron, I. Design of Pickering micro and nanoemulsions based on the structural characteristics of nanocelluloses. Biomacromolecules 2018, 19, 460,  DOI: 10.1021/acs.biomac.7b01564
      There is no corresponding record for this reference.
    34. 34
      Thompson, K. L.; Cinotti, N.; Jones, E. R.; Mable, C. J.; Fowler, P. W.; Armes, S. P. Bespoke diblock copolymer nanoparticles enable the production of relatively stable oil-in-water Pickering nanoemulsions. Langmuir 2017, 33, 12616– 12623,  DOI: 10.1021/acs.langmuir.7b02267
      34
      Bespoke diblock copolymer nanoparticles enable production of relatively stable oil-in-water Pickering nanoemulsions
      Thompson, Kate L.; Cinotti, Natacha; Jones, Elizabeth R.; Mable, Charlotte J.; Fowler, Patrick W.; Armes, Steven P.
      Langmuir (2017), 33 (44), 12616-12623CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Sterically stabilized diblock copolymer nanoparticles with an intensity-av. diam. of 25 nm are prepd. in the form of a concd. aq. dispersion using polymn.-induced self-assembly (PISA). The addn. of n-dodecane followed by high-shear homogenization produces n-dodecane-in-water Pickering macroemulsions of 22-46 μm diam. If the nanoparticles are present in sufficient excess, then subsequent processing using a high-pressure microfluidizer leads to the formation of Pickering nanoemulsions with a mean oil droplet diam. below 200 nm. The size of these Pickering nanoemulsions can be tuned by systematically varying the nanoparticle concn., applied pressure, no. of passes, and oil vol. fraction. High-internal-phase emulsions can also be achieved by increasing the n-dodecane vol. fraction up to 0.80. TEM studies of (dried) n-dodecane droplets confirm the presence of intact nanoparticles and suggest a relatively high surface coverage, which is consistent with model packing calcns. based on radius ratios. Such Pickering nanoemulsions proved to be surprisingly stable with respect to Ostwald ripening, with no significant change in the mean DLS droplet diam. after storage for approx. 4 mo at 20 °C.
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    35. 35

      This overall diameter of 220 nm necessarily includes the thickness of the adsorbed PGMA48–PTFEMA50 nanoparticle layer. If this contribution is subtracted from the overall droplet diameter, the mean diameter for the underlying “naked” oil droplet is less than 200 nm, which meets the criterion for a nanoemulsion according to the literature (see ref (24)).

      There is no corresponding record for this reference.
    36. 36
      Akpinar, B.; Fielding, L. A.; Cunningham, V. J.; Ning, Y.; Mykhaylyk, O. O.; Fowler, P. W.; Armes, S. P. Determining the effective density and stabilizer layer thickness of sterically stabilized nanoparticles. Macromolecules 2016, 49, 5160– 5171,  DOI: 10.1021/acs.macromol.6b00987
      36
      Determining the Effective Density and Stabilizer Layer Thickness of Sterically Stabilized Nanoparticles
      Akpinar, Bernice; Fielding, Lee A.; Cunningham, Victoria J.; Ning, Yin; Mykhaylyk, Oleksandr O.; Fowler, Patrick W.; Armes, Steven P.
      Macromolecules (Washington, DC, United States) (2016), 49 (14), 5160-5171CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
      A series of model sterically stabilized diblock copolymer nanoparticles has been designed to aid the development of anal. protocols in order to det. two key parameters: the effective particle d. and the steric stabilizer layer thickness. The former parameter is essential for high resoln. particle size anal. based on anal. (ultra)centrifugation techniques (e.g., disk centrifuge photosedimentometry, DCP), whereas the latter parameter is of fundamental importance in detg. the effectiveness of steric stabilization as a colloid stability mechanism. The diblock copolymer nanoparticles were prepd. via polymn.-induced self-assembly (PISA) using RAFT aq. emulsion polymn.: this approach affords relatively narrow particle size distributions and enables the mean particle diam. and the stabilizer layer thickness to be adjusted independently via systematic variation of the mean d.p. of the hydrophobic and hydrophilic blocks, resp. The hydrophobic core-forming block was poly(2,2,2-trifluoroethyl methacrylate) [PTFEMA], which was selected for its relatively high d. The hydrophilic stabilizer block was poly(glycerol monomethacrylate) [PGMA], which is a well-known non-ionic polymer that remains water-sol. over a wide range of temps. Four series of PGMAx-PTFEMAy nanoparticles were prepd. (x = 28, 43, 63, and 98, y = 100-1400) and characterized via transmission electron microscopy (TEM), dynamic light scattering (DLS), and small-angle X-ray scattering (SAXS). It was found that the d.p. of both the PGMA stabilizer and core-forming PTFEMA had a strong influence on the mean particle diam., which ranged from 20 to 250 nm. Furthermore, SAXS was used to det. radii of gyration of 1.46 to 2.69 nm for the solvated PGMA stabilizer blocks. Thus, the mean effective d. of these sterically stabilized particles was calcd. and detd. to lie between 1.19 g cm-3 for the smaller particles and 1.41 g cm-3 for the larger particles; these values are significantly lower than the solid-state d. of PTFEMA (1.47 g cm-3). Since anal. centrifugation requires the d. difference between the particles and the aq. phase, detg. the effective particle d. is clearly vital for obtaining reliable particle size distributions. Furthermore, selected DCP data were recalcd. by taking into account the inherent d. distribution superimposed on the particle size distribution. Consequently, the true particle size distributions were found to be somewhat narrower than those calcd. using an erroneous single d. value, with smaller particles being particularly sensitive to this artifact.
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    37. 37
      Fielding, L. A.; Mykhaylyk, O. O.; Armes, S. P.; Fowler, P. W.; Mittal, V.; Fitzpatrick, S. Correcting for a density distribution: Particle size analysis of core–shell nanocomposite particles using disk centrifuge photosedimentometry. Langmuir 2012, 28, 2536– 2544,  DOI: 10.1021/la204841n
      37
      Correcting for a Density Distribution: Particle Size Analysis of Core-Shell Nanocomposite Particles Using Disk Centrifuge Photosedimentometry
      Fielding, Lee A.; Mykhaylyk, Oleksandr O.; Armes, Steven P.; Fowler, Patrick W.; Mittal, Vikas; Fitzpatrick, Stephen
      Langmuir (2012), 28 (5), 2536-2544CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Many types of colloidal particles possess a core-shell morphol. In this Article, if the core and shell densities differ, this morphol. leads to an inherent d. distribution for particles of finite polydispersity. If the shell is denser than the core, this d. distribution implies an artificial narrowing of the particle size distribution as detd. by disk centrifuge photosedimentometry (DCP). In the specific case of polystyrene/silica nanocomposite particles, which consist of a polystyrene core coated with a monolayer shell of silica nanoparticles, the particle d. distribution can be detd. by anal. ultracentrifugation and introduce a math. method to account for this d. distribution by reanalyzing the raw DCP data. Using the mean silica packing d. calcd. from small-angle x-ray scattering, the real particle d. can be calcd. for each data point. The cor. DCP particle size distribution is both broader and more consistent with particle size distributions reported for the same polystyrene/silica nanocomposite sample using other sizing techniques, such as electron microscopy, laser light diffraction, and dynamic light scattering. Artifactual narrowing of the size distribution also probably occurs for many other polymer/inorg. nanocomposite particles comprising a low-d. core of variable dimensions coated with a high-d. shell of const. thickness, or for core-shell latexes where the shell is continuous rather than particulate.
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    38. 38

      For the present nanoemulsion system, the core comprises a low-density oil and the shell is composed of high-density nanoparticles. However, in this particular case the overall droplet density is less than that of the continuous phase, so centrifugation leads to creaming rather than sedimentation. Thus artefactual broadening of the droplet size distribution is anticipated, rather than artefactual narrowing.

      There is no corresponding record for this reference.
    39. 39
      Ilavsky, J.; Jemian, P. R. Irena: tool suite for modeling and analysis of small-angle scattering. J. Appl. Crystallogr. 2009, 42, 347– 353,  DOI: 10.1107/s0021889809002222
      39
      Irena: tool suite for modeling and analysis of small-angle scattering
      Ilavsky, Jan; Jemian, Peter R.
      Journal of Applied Crystallography (2009), 42 (2), 347-353CODEN: JACGAR; ISSN:0021-8898. (International Union of Crystallography)
      Irena, a tool suite for anal. of both x-ray and neutron small-angle scattering (SAS) data within the com. Igor Pro application, brings together a comprehensive suite of tools useful for studies in materials science, physics, chem., polymer science and other fields. In addn. to Guinier and Porod fits, the suite combines a variety of advanced SAS data evaluation tools for the modeling of size distribution in the dil. limit using max. entropy and other methods, dil. limit small-angle scattering from multiple noninteracting populations of scatterers, the pair-distance distribution function, a unified fit, the Debye-Bueche model, the reflectivity (x-ray and neutron) using Parratt's formalism, and small-angle diffraction. There are also a no. of support tools, such as a data import/export tool supporting a broad sampling of common data formats, a data modification tool, a presentation-quality graphics tool optimized for small-angle scattering data, and a neutron and x-ray scattering contrast calculator. These tools are brought together into one suite with consistent interfaces and functionality. The suite allows robust automated note recording and saving of parameters during export.
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    40. 40
      McAuliffe, C. Solubility in water of C1-C9 hydrocarbons. Nature 1963, 200, 1092,  DOI: 10.1038/2001092a0
      40
      Solubility in water of C1-C9 hydrocarbons
      McAuliffe, C.
      Nature (London, United Kingdom) (1963), 200 (4911), 1092-3CODEN: NATUAS; ISSN:0028-0836.
      The solubilities in H2O at 25 ± 1.5° of methane, ethane, propane, butane, isobutane, pentane, isopentane, hexane, 2-methylpentane, 2,2-dimethylbutane, n-heptane, 2,4-dimethylpentane, n-octane, 2,2,4-trimethylpentane, cyclopentane, cyclohexane, methylcyclopentane, methylcyclohexane, benzene, toluene, o-xylene, ethylbenzene, and isopropylbenzene were detd.
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    41. 41
      Taylor, P. Ostwald ripening in emulsions: estimation of solution thermodynamics of the disperse phase. Adv. Colloid Interface Sci. 2003, 106, 261– 285,  DOI: 10.1016/s0001-8686(03)00113-1
      41
      Ostwald ripening in emulsions. Estimation of solution thermodynamics of the disperse phase
      Taylor, P.
      Advances in Colloid and Interface Science (2003), 106 (), 261-285CODEN: ACISB9; ISSN:0001-8686. (Elsevier Science B.V.)
      The rate of Ostwald ripening was measured for small emulsions prepd. by the diln. of O/W microemulsions contg. a range of n-alkanes. The rates were detd. as a function of temp. for decane, undecane, dodecane, and tridecane using photon correlation spectroscopy. The aq. solubilities of these oils were estd. from the rates of ripening using the Lifshitz, Slyozov and Wagner theory of Ostwald ripening. These solubilities were used to calc. the free energy, enthalpy and entropy of soln. for the 4 alkanes. Values obtained were in good agreement with literature values. The free energy of soln. of hexadecane and octadecane were also detd. from the rate of ripening of mixed oil phase emulsions with dodecane using the treatment suggested by Kabalnov. The values obtained agreed well with extrapolated values of the free energy from lower alkanes, suggesting that coiling or aggregation of the longer chain mols. in soln. was not affecting the soly.
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    42. 42
      Binks, B. P.; Cho, W.-G.; Fletcher, P. D. I.; Petsev, D. N. Stability of oil-in-water emulsions in a low interfacial tension system. Langmuir 2000, 16, 1025– 1034,  DOI: 10.1021/la990952m
      42
      Stability of Oil-in-Water Emulsions in a Low Interfacial Tension System
      Binks, B. P.; Cho, W-G.; Fletcher, P. D. I.; Petsev, D. N.
      Langmuir (2000), 16 (3), 1025-1034CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      The stability of oil-in-water emulsions to both creaming and coalescence was measured as a function of salt concn. in heptane + water mixts. stabilized by sodium bis(2-ethylhexylsulfosuccinate) (AOT). Emulsions were prepd. from pre-equilibrated phases in Winsor I systems. Up to 0.035 M NaCl, the creaming rate decreases with salt concn., with no visible sign of coalescence. Above 0.035 M and approaching the phase inversion salt concn. of 0.055 M, the creaming rate increases quite markedly and coalescence becomes appreciable. The creaming at low salt concns. is due mainly to the buoyancy motion of single drops. A simple model for the time evolution of resolved water is developed which successfully describes the behavior. The drop size changes obsd. are due to Ostwald ripening, the rate of which decreases with salt concn. Exptl. ripening rates are consistent with a mechanism by which oil is transported between emulsion drops via microemulsion droplets present in the continuous phase. We calc. the energy of interdrop interaction allowing for drop deformation using exptl. detd. parameters of interfacial tension, drop radius, and zeta potential. At high [NaCl], due mainly to the low interfacial tension, the drops can deform and the attraction between them becomes significant. As a result, flocculation occurs which leads to coalescence instability.
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    43. 43
      Binks, B. P.; Clint, J. H.; Fletcher, P. D. I.; Rippon, S.; Lubetkin, S. D.; Mulqueen, P. J. Kinetics of swelling of oil-in-water emulsions. Langmuir 1998, 14, 5402– 5411,  DOI: 10.1021/la980522g
      43
      Kinetics of Swelling of Oil-in-Water Emulsions
      Binks, B. P.; Clint, J. H.; Fletcher, P. D. I.; Rippon, S.; Lubetkin, S. D.; Mulqueen, P. J.
      Langmuir (1998), 14 (19), 5402-5411CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      The kinetics of swelling of squalane-in-water emulsion drops are investigated by the addn. of decane-in-water emulsion drops. Squalane is sufficiently insol. in the aq. continuous phase that it cannot transfer between oil drops. Decane is able to transfer between drops and swells the squalane drops. The mixed emulsions were stabilized by the nonionic surfactant n-dodecyl octaoxyethylene glycol ether (C12E8) and were stable with respect to drop coalescence. A systematic series of expts. were made in which the swelling rates were detd. as functions of the initial drop radii, vol. fractions, and oil compns. of both types of emulsions. Using a theor. model originally developed by Ugelstad et al., the entire data set was successfully fitted with a single adjustable parameter equal to the product of the soly. of decane (C∞) and its diffusion coeff. in the aq. continuous phase (D). The measured value of C∞D was consistent with a mechanism of decane transport in which micelles of C12E8 act as carriers facilitating decane transport between emulsion drops. Also in agreement with this mechanism, it was obsd. that increasing the aq. phase concn. of C12E8 increased the swelling rate. Ostwald ripening rates of the decane-in-water emulsions gave values of C∞D consistent with those derived from swelling expts.
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    44. 44
      Binks, B. P.; Clint, J. H.; Fletcher, P. D. I.; Rippon, S.; Lubetkin, S. D.; Mulqueen, P. J. Kinetics of swelling of oil-in-water emulsions stabilized by different surfactants. Langmuir 1999, 15, 4495– 4501,  DOI: 10.1021/la990054q
      44
      Kinetics of Swelling of Oil-in-Water Emulsions Stabilized by Different Surfactants
      Binks, B. P.; Clint, J. H.; Fletcher, P. D. I.; Rippon, S.; Lubetkin, S. D.; Mulqueen, P. J.
      Langmuir (1999), 15 (13), 4495-4501CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      The effects of different concns. of a range of surfactants on the kinetics of swelling of squalane-in-water emulsion drops by the addn. of decane-in-water emulsion drops were obsd. Using a model described in detail in an earlier paper (Langmuir 1998, 14, 5402), kinetic measurements were used to det. the product of the decane soly. C∞ (expressed as a dimensionless vol. fraction) and effective diffusion coeff. D for the transport of decane between drops across the aq. continuous phase. The issue is addressed as to whether the decane transfer occurs by "mol." transport or whether the surfactant micelles present in the continuous aq. phase act as "carriers" for the decane. Swelling rates were investigated for different concns. of two nonionic surfactants, dodecyl hexaoxyethylene glycol ether (C12E6) and dodecyl octaoxyethylene glycol ether (C12E8), the cationic surfactant tetradecyl trimethylammonium bromide (TTAB) and the anionic species sodium dodecyl sulfate (SDS). The solubilization of decane was detd. for aq. solns. of all the surfactants. For mol. transport, the product C∞D is predicted to be of the order of 10-17 m2 s-1 and independent of the surfactant concn. in the continuous aq. phase. From the swelling expts., the product C∞D is found to be on the order of 10-14-10-13 m2 s-1 and to increase with surfactant concn. for all species except SDS. The measured magnitudes of C∞D for all the surfactants are consistent with oil transfer occurring by the "micelle carrier" mechanism with negligible energy barrier to the transport process.
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    45. 45
      Walter, J.; Thajudeen, T.; Süβ, S.; Segets, D.; Peukert, W. New possibilities of accurate particle characterisation by applying direct boundary models to analytical centrifugation. Nanoscale 2015, 7, 6574– 6587,  DOI: 10.1039/c5nr00995b
      45
      New possibilities of accurate particle characterisation by applying direct boundary models to analytical centrifugation
      Walter, Johannes; Thajudeen, Thaseem; Sueβ, Sebastian; Segets, Doris; Peukert, Wolfgang
      Nanoscale (2015), 7 (15), 6574-6587CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Anal. centrifugation (AC) is a powerful technique for the characterization of nanoparticles in colloidal systems. As a direct and abs. technique it requires no calibration or measurements of stds. Moreover, it offers simple exptl. design and handling, high sample throughput as well as moderate investment costs. However, the full potential of AC for nanoparticle size anal. requires the development of powerful data anal. techniques. In this study we show how the application of direct boundary models to AC data opens up new possibilities in particle characterization. An accurate anal. method, successfully applied to sedimentation data obtained by anal. ultracentrifugation (AUC) in the past, was used for the first time in analyzing AC data. Unlike traditional data evaluation routines for AC using a designated no. of radial positions or scans, direct boundary models consider the complete sedimentation boundary, which results in significantly better statistics. We demonstrate that meniscus fitting, as well as the correction of radius and time invariant noise significantly improves the signal-to-noise ratio and prevents the occurrence of false positives due to optical artifacts. Moreover, hydrodynamic non-ideality can be assessed by the residuals obtained from the anal. The sedimentation coeff. distributions obtained by AC are in excellent agreement with the results from AUC. Brownian dynamics simulations were used to generate numerical sedimentation data to study the influence of diffusion on the obtained distributions. Our approach is further validated using polystyrene and silica nanoparticles. In particular, we demonstrate the strength of AC for analyzing multimodal distributions by means of gold nanoparticles.
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    46. 46
      Beltramo, P. J.; Gupta, M.; Alicke, A.; Liascukiene, I.; Gunes, D. Z.; Baroud, C. N.; Vermant, J. Arresting dissolution by interfacial rheology design. Proc. Natl. Acad. Sci. U.S.A. 2017, 114, 10373,  DOI: 10.1073/pnas.1705181114
      46
      Arresting dissolution by interfacial rheology design
      Beltramo, Peter J.; Gupta, Manish; Alicke, Alexandra; Liascukiene, Irma; Gunes, Deniz Z.; Baroud, Charles N.; Vermant, Jan
      Proceedings of the National Academy of Sciences of the United States of America (2017), 114 (39), 10373-10378CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      A strategy to halt dissoln. of particle-coated air bubbles in water based on interfacial rheol. design is presented. Previously a dense monolayer was believed to be required for such an armored bubble to resist dissoln.; in fact, engineering a two-dimensional yield stress interface achieves such performance at sub-monolayer particle coverage. A suite of interfacial rheol. techniques characterized spherical and ellipsoidal particles at an air/water interface as a function of surface coverage. Bubbles with varying particle coverage were made and their resistance to dissoln. evaluated using a microfluidic technique. A bare bubble only had a single pressure at which a given radius was stable; the authors detd. a range of pressures over which armored bubble dissoln. was arrested. The link between interfacial rheol. and macroscopic dissoln. of ∼100 μm bubbles coated with ∼1 um particles is presented and discussed. Generic design rationale was confirmed using non-spherical particles, which develop significant yield stress at even lower surface coverages. Thus, it can be used to successfully inhibit Ostwald ripening in a multitude of foam and emulsion applications.
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    47. 47
      Izquierdo, P.; Esquena, J.; Tadros, T. F.; Dederen, C.; Garcia, M. J.; Azemar, N.; Solans, C. Formation and stability of nano-emulsions prepared using the phase inversion temperature method. Langmuir 2002, 18, 26– 30,  DOI: 10.1021/la010808c
      47
      Formation and Stability of Nano-Emulsions Prepared Using the Phase Inversion Temperature Method
      Izquierdo, P.; Esquena, J.; Tadros, Th. F.; Dederen, C.; Garcia, M. J.; Azemar, N.; Solans, C.
      Langmuir (2002), 18 (1), 26-30CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Formation of O/W nano-emulsions was studied in H2O/‾C12‾E4/oil systems by the phase inversion temp. emulsification method. Emulsification was carried out at the corresponding HLB (hydrophilic-lipophilic balance) temp., and then the emulsions were cooled fast to 25°. The influence of surfactant concn. and oil soly. on HLB temp., nano-emulsion droplet size, and stability also was studied. Droplet size was detd. by dynamic light scattering, and nano-emulsion stability was assessed, measuring the variation of droplet size as a function of time. The results obtained showed that the breakdown process of nano-emulsions studied could be attributed to Ostwald ripening. An increase of nano-emulsion instability with the increase in surfactant concn. and oil soly. was also found.
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