Phase Inversion of Silica Particle-Stabilized Water-in-Water Emulsions

2 days ago - An aqueous two-phase system (ATPS) is of great value in low calorie foods or oil-free cosmetics and pharmaceuticals. In contrast to the r...
0 downloads 0 Views 12MB Size
Download PDF
Article Cite This: Langmuir XXXX, XXX, XXX−XXX

pubs.acs.org/Langmuir

Phase Inversion of Silica Particle-Stabilized Water-in-Water Emulsions Bernard P. Binks* and Hui Shi Department of Chemistry and Biochemistry, University of Hull, Hull HU6 7RX. U.K.

Langmuir Downloaded from pubs.acs.org by UNIV OF TEXAS AT DALLAS on 03/09/19. For personal use only.

S Supporting Information *

ABSTRACT: An aqueous two-phase system (ATPS) is of great value in low calorie foods or oil-free cosmetics and pharmaceuticals. In contrast to the recent work on polymer/polymer ATPSs, a simple polymer/salt ATPS (polyethylene glycol/Na2SO4) was chosen to study water-in-water (w/w) emulsions stabilized by solid particles. The binodal curve and the tie lines were first determined for the mixture at room temperature. Above the binodal curve, two water-based phases coexist; the upper phase is rich in polymer, whereas the lower phase is rich in salt. Within the two-phase region, we attempted to prepare w/w emulsions with or without the addition of common emulsifiers. Ionic and nonionic surfactants, a polymer, and various solid particles (hydrophilic calcium carbonate particles of different sizes and shapes, wax microspheres) were selected, but no stable emulsion was possible. However, stable w/w emulsions of both types (polymer-insalt and salt-in-polymer) were formed using dichlorodimethylsilane-modified nanosilica particles. Using partially hydrophobic fumed silica as the emulsifier, emulsions remained fully emulsified for over 1 year and we link the extent of hydrophobization of particles to the properties of the emulsions via contact angle measurements. Furthermore, systematic emulsion studies were conducted at different overall compositions such that changes in emulsion type and stability were mapped onto the phase diagram. Catastrophic phase inversion of emulsion type and evolution of emulsion stability were monitored along the tie lines. Importantly, stability to coalescence was found to decrease approaching conditions of phase inversion.

1. INTRODUCTION Aqueous solutions containing two distinct dissolved polymers at certain concentrations may separate into two water-based phases, each phase being rich in one of the polymers.1−3 This is the so-called aqueous two-phase system (ATPS) resulting from the incompatibility between the polymer pair.4−6 Amongst the various ATPSs explored, those containing polymer/polymer or even polymer/salt are the most studied.7−9 Phase separation and behavior in the ATPS depend inter alia on polymer concentration, polymer molecular weight, salt type and concentration, pH, and temperature.10−12 ATPSs gained great attention in the 1980s because of their practical applications in the extraction, separation, purification, and enrichment of biological materials.13,14 Since the early 2000s, they were developed to remove heavy metal ions from waste water.15,16 However, ATPSs have shown potential in applications such as low calorie foods or oil-free cosmetics and pharmaceuticals.17 If the two phases of an ATPS are mixed, a water-in-water (w/w) emulsion may potentially form which meets the demands of a completely water-based system.18 However, there are limited reports on the stabilization of w/w emulsions owing primarily to the difficulty in achieving longterm stability. ATPSs exhibit ultralow interfacial tensions, typically 1 μN m−1, which is over 4 orders of magnitude lower than that of alkane−water interfaces (around 50 mN m−1).19,20 Although this enables effective drop breakup during © XXXX American Chemical Society

emulsification, it is detrimental to drop stability as molecular emulsifiers desorb and adsorb very easily. In the case of particulate emulsifiers, the low tension results in a noticeably reduced energy of desorption from the interface, implying greater difficulty in preparing stable emulsions. In addition, the water−water interface of an ATPS is calculated to be much thicker than that of an oil−water interface, being a few hundred nanometers.21 Surfactant molecules of around 1 nm in length22 are incapable of straddling such an interface. These factors result in the notorious difficulty in selecting an effective emulsifier for ATPS.18 Recently, colloidal particles have been considered as promising emulsifiers for ATPS. They include those of fat,23 protein,24−27 liposome,28 polystyrene spheres,29,30 Al(OH)3 plates,31 polymer platelets,32 polymer microgel,33 polyelectrolyte complexes,34,35 cellulose nanocrystals,36 and polymer− protein conjugates37 among others. However, unlike the wellstudied Pickering emulsions of oil and water containing particles, there appears to be no explicit criteria for the selection of a particulate emulsifier to stabilize w/w emulsions. For example, polyethylene glycol (PEG)/dextran mixtures of different molecular weights and overall compositions have been emulsified to varying levels of success with latex Received: December 14, 2018 Revised: January 31, 2019

A

DOI: 10.1021/acs.langmuir.8b04151 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir particles,29,30 cellulose nanocrystals,36 polymer platelets,32 polyelectrolyte complexes,34,35 polymer microgel,33 liposome,28 or protein particles of different types, shapes, and sizes.24,27 The particles appeared to have been selected randomly without justification. A detailed understanding of w/w emulsions is lacking, including the influence of particle wettability, size, and shape on emulsion properties. Almost all of the studies dealing with emulsification of ATPS involve two polymers, such as the most popular PEG/dextran system,28 dextran/gelatin system,26,29,31 xyloglucan/amylopectin system,25 and dextran/methylcellulose system. Elucidating the mechanism of emulsion stabilization by particles is quite complex here as potentially one or both of the polymers may adsorb on particle surfaces changing their wettability in situ. This concern has not been addressed sufficiently so far. In an attempt to simplify the system, we investigate here an ATPS composed of only one polymer (PEG 8000) and a simple electrolyte (Na2SO4).38 We aimed to generate w/w emulsions of long-term stability. Preliminary findings were already reported.18,39 Systematic emulsion studies are carried out to establish the general pattern of stabilization in the presence of different kinds of emulsifier. Bearing in mind the excellent stability of particle-stabilized emulsions, we focus on the emulsifying ability of fumed silica particles and describe the effect of particle wettability. Unlike previous studies, this is a novel aspect in our work and one which yielded great understanding of oil−water emulsions in the past.18 In particular, the effect of the water/water volume ratio along a tie line is investigated in which the catastrophic phase inversion of emulsions can be induced. On the basis of contact angle measurements, the effect of the adsorption of PEG 8000 on particle wettability is discussed and related to the emulsion behavior.

2.2. Methods. All of the experiments in this study were conducted at room temperature (23 ± 2 °C). The weight fractions of polymer, wPEG8000, and salt, wNa2SO4, herein equal the mass of PEG 8000 or Na2SO4 divided by the total mass of PEG 8000, Na2SO4, and water in the PEG 8000/Na2SO4 aqueous mixtures. 2.2.1. Determination of Binodal Curve. The binodal curve of the PEG 8000/Na2SO4 ATPS was determined with a method following the same principle as the cloud point method.40 A series of PEG 8000/Na2SO4 aqueous mixtures were prepared at a fixed wNa2SO4 and increasing wPEG8000. Aqueous mixtures became turbid at a particular wPEG8000 forming two phases when left to settle. The overall composition, just prior to the formation of two phases, provided a point on the binodal curve. This procedure was repeated for other values of wNa2SO4 to generate the binodal curve. 2.2.2. Determination of Tie Lines. Calibration curves of refractive index and density were first established for three sets of homogeneous solutions of PEG 8000 in water with and without the addition of Na2SO4 (wNa2SO4 = 0, 0.02, and 0.0375) using an Abbé refractometer (Hilger & Watts) and a portable density meter DMA 35N (Anton Paar GmbH; see Figures S2 and S3). Regression equations were then obtained after fitting the data.41,42 Subsequently, PEG 8000/Na2SO4 aqueous mixtures were prepared at varying overall composition and kept at room temperature for 2 days to allow separation into upper and lower phases. The densities and refractive indexes of both phases were measured after two- and fivefold dilution, respectively. The weight fractions of PEG 8000 and Na2SO4 in both phases were determined using the regression equations (Figure S4). This is the densitometric-refractometric method for equilibrium phase composition.43,44 Tie lines were verified using a second method by fitting the binodal curve to the empirical equation and applying the mass balance for PEG 8000 and Na2SO4 using the relationship between the composition of a phase and the overall composition (Figures S5 and S6).45−47 2.2.3. Silica Dispersions in Water, Aqueous PEG 8000, and Aqueous Na2SO4. Aqueous PEG 8000 solutions and aqueous Na2SO4 solutions were prepared at a solute weight fraction of 0.2 (20 wt %). Fumed silica particles (0.5 or 2 wt %) of intermediate hydrophobicity (75% SiOH) were dispersed in each of pure water, aqueous PEG 8000 solution, and aqueous Na2SO4 solution. These mixtures were thoroughly mixed using an Ultra Turrax T25 (IKA) rotor-stator homogenizer with a 0.8 cm head operating at 13 000 rpm for 2 min. Photos of glass vials containing these silica suspensions were taken over time to evaluate their stability. Because of the fractal nature of fumed silica, we did not determine the adsorption isotherm of PEG 8000 on these particles which is further complicated by the presence of different amounts of salt. 2.2.4. Preparation and Characterization of w/w Emulsions. PEG 8000/Na2SO4 aqueous mixtures were prepared at known wPEG8000 (0−0.40) and wNa2SO4 (0−0.18) in 14 mL screw cap glass vials (i.d. 2.3 cm, h. 5.8 cm). The mixture was stained with a trace amount of mPEG-FITC prior to emulsion preparation to aid visualization of the PEG-rich phase in emulsions. Subsequently, surfactants (above the critical micelle concentration) or solid particles (varying wt %) were added to the mixture as an emulsifier which was homogenized at 13 000 rpm for 2 min. Photographs of the glass vial containing the emulsion were taken using a Nikon COOLPIX P7700 camera immediately after homogenization and over time to record changes in emulsion stability. Emulsion droplets were observed on a glass microscope slide with a single cavity well (Academy Science) immediately after preparation using an Olympus BX51 optical microscope equipped with a 16-bit Olympus camera (DP70) and Image-Pro Plus 6.0 software (Media Cybernetics). Optical microscope images were taken at different magnifications, and the droplet diameter was measured with software ImageJ. Fluorescence microscopy was used to determine emulsion type using the UWIBA filter set with the same microscope (excitation wavelength = 460−490 nm, emission wavelength = 515−550 nm). This excited green light emission from mPEG-FITC with blue light is to detect the

2. EXPERIMENTAL SECTION 2.1. Materials. The chemicals used in this work include PEG (PEG 8000, Mw = 7000−9000 g mol−1, Sigma-Aldrich), anhydrous sodium sulfate (Na2SO4, 99%, Fisher Scientific), mPEG-FITC (methoxyPEG functionalized with fluorescein isothiocyanate dye, Mw = 10 285 g mol−1, Creative PEGWorks, see Figure S1), sodium dodecyl sulfate (SDS, 99%, Sigma-Aldrich), cetyltrimethylammonium bromide (CTAB, 99%, Acros Organics), pentaethylene glycol monododecyl ether (C12E5, 99%, Nikko Chemicals), and polyvinyl alcohol (PVA, Mw 89 000−98 000 g mol−1, Sigma-Aldrich). Three kinds of hydrophilic calcium carbonate particles of different shapes and sizes were used. Calofort U (Specialty Minerals Inc.) contains spherical particles of primary diameter equal to 70 nm, whereas Cube80 KAS and Wiscal A contain cubic and fibrous particles of average size 8 and 20 μm (Maruo Calcium Co. Ltd.), respectively. Wax microspheres (Florabeads JOJOBA 60/100, mean diameter 200 μm) are natural Jojoba esters (C38−C44) supplied by Chesham Specialty Ingredients. Quasi-spherical-fumed silica particles (primary diameter 20 nm) were obtained from Wacker-Chemie. They are produced via flame hydrolysis and endowed with different surface hydrophobicities by reacting silanol groups (SiOH) with dichlorodimethylsilane (DCDMS, Sigma >99.5%). The residual silanol content of the fumed silica particles used in this work ranged from 100% (very hydrophilic) to 15% (very hydrophobic). Sodium hydroxide (>97%), cyclohexane (>99%), and chloroform (>99.8%) were from Fisher Scientific, whilst isopropanol (>99.8%) and ethanol (>99.8%) were from VWR. Water used in this work was purified by passing through an Elgastat Prima reverse osmosis unit followed by a Millipore Milli-Q reagent water system. Its surface tension measured with a Krüss K11 tensiometer and a du Noüy ring was 72.0 mN m−1 at 25 °C, and its pH measured with a Jenway 3510 pH meter was around 6. All chemicals were used as received without further purification. B

DOI: 10.1021/acs.langmuir.8b04151 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir PEG-rich phase. Thus, a fluorescent PEG-rich phase appeared green and a salt-rich phase stayed nonfluorescent and black, indicating whether the PEG-rich phase was the dispersed phase or the continuous phase. The process was repeated to examine the type and stability of emulsions prepared at varying overall compositions in the presence of different emulsifiers. We found that the conductivity method was not suitable to determine emulsion type because the conductivity of the two separated phases only differ by up to a factor of 10 and emulsion conductivities were intermediate between these values also. 2.2.5. Contact Angle Measurement. Glass microscope slides were cleaned by soaking in 30 wt % aqueous NaOH solution overnight and then rinsed with water. After drying with air, 10 μL of aqueous PEG solution at different concentrations was placed onto a cleaned glass slide in air. The contact angle was measured using a Krüss DSA10 Mk2 drop shape analyzer of the sessile drop. To obtain glass slides with different hydrophobicities, 250 mL volumetric flasks were first pretreated with 0.5 mL of DCDMS to hydrophobize the inner surfaces. They were then washed with chloroform, isopropanol, and ethanol and then left to dry. Solutions of DCDMS in anhydrous cyclohexane were prepared in the flasks ranging in concentration from 3.3 × 10−7 to 0.1023 M. Clean glass slides were immersed in DCDMS/cyclohexane solutions in Teflon holders for 1 h. Excess solution was removed, and the hydrophobized glass slides were then washed with fresh chloroform, isopropanol, and ethanol. After drying in air, contact angles of water in air and salt-rich phase (drop) under PEG-rich phase were measured on the slides, as described in the Supporting Information.48,49

Figure 1. Phase diagram of PEG 8000/Na2SO4 ATPS determined at room temperature (T = 23 ± 3 °C). Inset: photo of vessel containing a mixture containing wPEG8000 = 0.12 and wNa2SO4 = 0.075 (ϕPEG‑rich = 0.5, filled black square in two-phase region) after complete phase separation. The binodal curve (solid black line) was determined using the cloud point method, and the red dashed line was from ref 50. Tie lines (TL, dashed lines) were determined using two different methods.

3. RESULTS AND DISCUSSION 3.1. Study of PEG 8000/Na2SO4 ATPS. The binodal curve and tie lines are the key features of a phase diagram. The binodal curve separates the component concentrations which form two immiscible aqueous phases (above the curve) from those that make one phase (below the curve). A tie line connects two nodes which lie on the binodal curve and denotes the equilibrium compositions of the two phases at a certain temperature. All of the potential systems have the same upper phase and lower phase equilibrium composition because of being on the same tie line. The partial phase diagram of the PEG 8000/Na2SO4 ATPS is shown in Figure 1. It matches well with that reported in the literature50 and is similar to that for the PEG 4000 + sodium sulfite system.51 The critical concentration of polymer to form two-phase systems decreases with an increase in salt concentration. Subsequently, four tie lines were determined using a combination of methods.52,53 We verified that the tie lines obtained from the two methods were in good agreement (Figure S7). In the process of tie line determination, the density and refractive index of the two phases were measured for PEG 8000/Na2SO4 aqueous mixtures of increasing wPEG8000 and fixed wNa2SO4 of 0.063 (Figure S8). The density difference between the two phases increased from 0.015 to 0.15 g cm−3, whilst the refractive index difference increased from 0.010 to 0.045 between wPEG8000 of 0.1 and 0.4. The density difference is much smaller than that of alkane and water (around 0.3 g cm−354), but it increased with polymer concentration owing to the increasing exclusion between polymer and salt.5,6 According to conductivity measurements, the upper phase of a PEG 8000/Na2SO4 mixture was determined to be PEG-rich with a relatively low conductivity of 8.3 mS cm−1 and the lower phase was salt-rich with a high conductivity of 77 mS cm−1. The values refer to a mixture prepared at an overall composition of wPEG8000 = 0.12 and wNa2SO4 = 0.075 which separates into two phases of equal volume.

In anticipation of our work on silica-stabilized w/w emulsions, we investigated the stability of partially hydrophobic fumed silica particle suspensions in pure water, in 20 wt % aqueous Na2SO4 solution and in 20 wt % aqueous PEG 8000, these being close to typical polymer and salt concentrations in two-phase systems (Figure S9). We observed different suspension stabilities depending on the dispersion medium and the concentration of particles. Particles sedimented fast in pure water forming a concentrated layer at the bottom of the vial because of the large density difference between silica and water.55 For silica suspensions in aqueous Na2SO4 solutions, air bubbles and increased viscosity were observed.56 Salt cations screen the negative charge on silica surfaces increasing the particle hydrophobicity and enhancing their affinity to air.57,58 Further, the particles formed a gel-like network at high ionic strength leading to a dramatic increase in suspension viscosity and trap air bubbles during homogenization.55,59 Meanwhile, salt also caused flocculation of the particles with flocs sedimenting over time. For silica suspensions in aqueous PEG 8000, PEG molecules are capable of adsorbing on silica surfaces via hydrogen bonds between the silanol groups and the ether oxygen of PEG.60 The dispersion of silica in aqueous PEG 8000 separates into a concentrated floc phase and a clear supernatant containing remaining polymer within 1 day owing to polymer bridging.61 The supernatant volume increases with time as already reported.62 In addition, a higher particle concentration improved the dispersion stability. The supernatant appeared either faster (in Na2SO4) or in larger volume (in PEG 8000) at 0.5 wt % particles compared with that at 2 wt %. This is understandable C

DOI: 10.1021/acs.langmuir.8b04151 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 2. Photos of PEG 8000/Na2SO4 aqueous mixtures at an overall composition of wPEG8000 = 0.12 and wNa2SO4 = 0.075 (ϕPEG‑rich = 0.5) before and after homogenization over time without any emulsifier and with common surfactants (SDS, CTAB, and C12E5) and a polymeric surfactant (PVA) as an emulsifier. The surfactant at 20× cmc and 1 wt % of PVA was predissolved in water. Excess PVA remained in the aqueous mixture.

(0.07, 8, and 20 μm), wax microspheres (200 μm), and fumed silica particles of different hydrophobicities. The appearance of the vessels with time using 1 wt % particles is shown in Figure 3 and in Figures S10 and S11. The hydrophilic calcium carbonate particles sedimented to the bottom within 5 min regardless of the shape and size, and no emulsion prevailed after 1 h (Figure S10). In the case of wax microspheres, only a portion of the particles entered the liquid mixture, with the rest remaining on the surface (Figure S10). Phase separation occurred immediately after homogenization with all of the particles eventually floating on the air/water surface. As for fumed silica particles at 1 wt %, we investigated the influence of particle hydrophobicity on the ability to stabilize emulsions. For the more hydrophobic particles (SiOH content of 15, 23, and 35%), no emulsion was possible because the powdered silica did not enter the liquid being too hydrophobic (Figure S11). However, for particles of SiOH content between 50 and 100%, although emulsification was incomplete, a volume of emulsion remained after 1 month between two clear aqueous phases (Figures S11 and 3). Optical microscopy of the turbid layer revealed the presence of very small (approx. 10 μm) emulsion droplets in each case (Figure S12). It is not easy to ascertain why solid particles of similar size and hydrophilicity

because an increase in particle concentration leads to higher viscosity which can enhance suspension stability.63,64 3.2. Emulsion Stabilization by Surfactants, Polymer, or Selected Solid Particles. Although it seems well accepted that stabilizing emulsions in the ATPS is very challenging/ impossible, we found no specific evidence to support this in the literature. Figure 2 shows the appearance of PEG 8000/ Na2SO4 aqueous mixtures (wPEG8000 = 0.12, wNa2SO4 = 0.075, and ϕPEG‑rich = 0.5) with or without the addition of common emulsifiers at different times since preparation. The control emulsion (no emulsifier) exhibited complete phase separation within 5 min. Anionic (SDS), cationic (CTAB), and nonionic (C12E5) surfactants together with a polymeric surfactant (PVA) were used to attempt to emulsify PEG 8000/Na2SO4 aqueous mixtures at up to 20 times their critical micelle concentration in water or 1 wt % (polymer). All of the temporarily formed emulsions separated into two phases within 5 min with the phases becoming clear after 3 h. This is similar to the control emulsion and reveals that these conventional emulsifiers cannot emulsify such ATPS. Various kinds of solid particle were selected to emulsify PEG 8000/Na2SO4 ATPS, including hydrophilic calcium carbonate particles of different shapes (sphere, cube, and fiber) and sizes D

DOI: 10.1021/acs.langmuir.8b04151 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 3. Photos of PEG 8000/Na2SO4 aqueous mixtures at an overall composition of wPEG8000 = 0.12 and wNa2SO4 = 0.075 (ϕPEG‑rich = 0.5) before and after homogenization over time with 1 wt % of fumed silica particles as an emulsifier. Particles were modified to different extents with DCDMS with residual silanol content ranging from 100 to 63%. Emulsions are all salt-in-PEG type.

mixing,68,69 we decided to prepare silica-stabilized emulsions by vigorous hand shaking for 30 s with 2 wt % of particles. The appearance of a free PEG-rich layer 1 month after preparation (Figure 4, bottom row) shows that low-shear mixing is not beneficial for the stability of w/w emulsions in this case. Homogenization is more powerful and generates smaller droplets than handshaking which can slow down the sedimentation rate.70,71 On the basis of the above results, we decided to pursue investigations of the system containing 2 wt % fumed silica in which emulsions were prepared by homogenization at 13 000 rpm for 2 min. Given that our aim is to generate stable w/w emulsions, emulsions exhibiting coalescence were only possible at the composition stated above (ϕPEG‑rich = 0.5) with either of the three types of silica. We thus decided to vary the overall composition which allows us to vary ϕPEG‑rich. The appearance of emulsions after 1 month prepared at ϕPEG‑rich of 0.3, 0.5, and 0.7 is seen in Figure 5. The emulsion type is determined via fluorescence microscopy whose images are given in Figure 6. At any ϕPEG‑rich, the emulsion type remains the same for different initial particle hydrophobicities (100 to 50% SiOH) and so we witness no evidence of transitional phase inversion.72,73 By contrast, at ϕPEG‑rich = 0.3, PEG-in-salt emulsions were preferred, whereas salt-in-PEG emulsions were preferred at ϕPEG‑rich of 0.5 and 0.7, that is, catastrophic phase inversion is evidenced upon increasing ϕPEG‑rich. Despite the same emulsion type, emulsion stability varied with particle hydrophobicity for each value of ϕPEG‑rich, as seen

(Calofort U and 100% SiOH fumed silica) exhibit different behaviors. We suggest that it is most likely due to the network of aggregated fumed silica particles arising from hydrogen bonding between silanol groups which form in certain liquids.65 This has been observed in various oil−water emulsions contributing to stability.55,56 This possibility cannot occur in the case of calcite particles. 3.3. Water-in-Water Emulsions Stabilized by Fumed Silica Nanoparticles. 3.3.1. Effect of Particle Hydrophobicity. At the same overall composition (wPEG8000 = 0.12, wNa2SO4 = 0.075, and ϕPEG‑rich = 0.5), emulsion stability was examined to determine the optimum emulsifying conditions using fumed silica particles (50, 75, and 100% SiOH) as a stabilizer. It is expected that emulsions become more stable upon increasing the particle concentration.66 The concentration of particles was increased from 1 to 2 wt % (with high shear), and the appearance of emulsions 1 month after preparation is shown in Figure 4, middle row. The three emulsions released the salt-rich phase below, and two of them were more stable at 2 wt % particles as no PEG-rich phase was released above. Making use of mPEG-FITC and fluorescence microscopy, the emulsion type was determined to be salt-inPEG (later). Although coalescence was not completely prevented at 2 wt % particles, the emulsion viscosity increased hindering the sedimentation of the droplets.67 Because certain surfactant systems yield emulsions of either higher stability or of the inverted type under low shear E

DOI: 10.1021/acs.langmuir.8b04151 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 4. Photos of PEG 8000/Na2SO4 emulsions at an overall composition of wPEG8000 = 0.12 and wNa2SO4 = 0.075 (ϕPEG‑rich = 0.5) with 100, 75, and 50% SiOH-fumed silica particles as an emulsifier 1 month after homogenization at particle concentrations of 1 or 2 wt %. Aqueous mixtures with 2 wt % particles were also emulsified by hand shaking for 30 s. All emulsions are salt-in-PEG type.

Figure 5. Photos of PEG 8000/Na2SO4 emulsions at three different overall compositions of wPEG8000 = 0.06, wNa2SO4 = 0.075 (ϕPEG‑rich = 0.3, PEG-in-salt), wPEG8000 = 0.12, wNa2SO4 = 0.075 (ϕPEG‑rich = 0.5, saltin-PEG), and wPEG8000 = 0.12, wNa2SO4 = 0.0575 (ϕPEG‑rich = 0.7, salt-inPEG) with 2 wt % of 100, 75, and 50% SiOH-fumed silica particles as an emulsifier 1 month after preparation.

in Figure 5. Because of the appearance of resolved dispersed phase, coalescence occurred in the emulsion with 100% SiOHfumed silica at ϕPEG‑rich of 0.3, in all emulsions at ϕPEG‑rich of 0.5 and in the emulsion with 50% SiOH-fumed silica at ϕPEG‑rich of 0.7. Creaming ensued in the emulsion with 50% SiOH-fumed silica at ϕPEG‑rich of 0.3 leading to the resolved salt-rich phase, whereas the resolved PEG-rich phase formed in the emulsion with 100% SiOH-fumed silica at ϕPEG‑rich of 0.7 owing to sedimentation. Two emulsions stable to both coalescence and creaming/sedimentation were formed with 75% SiOH-fumed silica at ϕPEG‑rich = 0.3 and 0.7. Their average droplet diameters were relatively small, ca. 10 μm, and exhibited no change over 1 month (Figure S13a). They also remained fully emulsified for at least 1 year (Figure S13b). Although we do not know the thickness of the water−water interface in our system, we recall that fumed silica primary particles fuse irreversibly into aggregates of several hundred nanometers55,65 potentially capable of straddling the interface. Although the physisorption of PEG on silica particles in water via hydrogen bonding has been reported for completely hydrophilic particles, no study appears to exist for silica particles of increasing hydrophobicity. In the former case, the magnitude of the negative zeta potential of particle dispersions decreases with polymer concentration and, at sufficiently high levels, bridging of particles may occur enhancing flocculation. This scenario is equally possible for less hydrophilic particles which possess free silanol groups. In order to assess the impact of PEG adsorption on particle surfaces, we have measured contact angles on glass microscope slides (mimic for particles)

treated progressively with the same silanising agent as on particle surfaces. The contact angles of a water drop containing different concentrations of PEG 8000 in air on hydrophilic glass increase from 90°. However, emulsions are all of the salt-in-PEG type using silica particles possessing 100, 75, or 50% SiOH (Figure 6b). Unfortunately, we lack the relation between % SiOH on particle surfaces and F

DOI: 10.1021/acs.langmuir.8b04151 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 6. Fluorescence microscope images of w/w emulsions just after homogenization formed from PEG 8000/Na2SO4 aqueous mixtures at three different overall compositions of (a) wPEG8000 = 0.06, wNa2SO4 = 0.075 (ϕPEG‑rich = 0.3), (b) wPEG8000 = 0.12, wNa2SO4 = 0.075 (ϕPEG‑rich = 0.5), and (c) wPEG8000 = 0.12, wNa2SO4 = 0.0575 (ϕPEG‑rich = 0.7) with 2 wt % of 50, 75, and 100% SiOH-fumed silica particles as an emulsifier. With the addition of trace amount of mPEG-FITC, the PEG-rich phase is fluorescent and green, whereas the salt-rich phase is nonfluorescent and black. There is no obvious difference in average droplet size of emulsions stabilized by silica particles of different hydrophobicities. The large fluorescent objects in the far right images in (b,c) are air bubbles trapped in emulsions which have an affinity for the tracer.

Figure 7. (a) Images of a drop of aqueous PEG 8000 at different concentrations (wPEG8000 = 0−0.5) on clean hydrophilic glass slides in air; (b) variation of static contact angle of a water drop in air (θwater,air, dashed line) and of a drop of salt-rich phase in the PEG-rich phase (θPEG,salt and θsalt,PEG, solid lines) on DCDMS-coated glass slides as a function of [DCDMS]. The three-phase contact angles through the salt-rich phase (θPEG,salt and θsalt,PEG) were measured by wetting the glass slide first with PEG-rich phase or salt-rich phase, respectively. PEG-rich phase and salt-rich phase were separated from a PEG 8000/ Na2SO4 aqueous mixture of wPEG8000 = 0.12 and wNa2SO4 = 0.075 (ϕPEG‑rich = 0.5).

the concentration of DCDMS used to hydrophobize glass slides. SiOH-fumed silica (100%) in water is completely hydrophilic, and one would predict that it would stabilize a PEG-in-salt emulsion. However, in the presence of a relatively high PEG concentration (wPEG8000 = 0.12) inducing polymer adsorption and Na2SO4 concentration reducing the particle surface charge, such particles become hydrophobized. The hydrophobization effect of adsorbed PEG on silica surfaces has been reported before.74,75 3.3.2. Emulsions with 75% SiOH Silica Particles. Because stable w/w emulsions of both types (PEG-in-salt and salt-inPEG) were achieved using partially hydrophobic fumed silica particles (75% SiOH), further studies were focused on these. Within the two-phase region, we evaluated the effect of increasing wPEG8000 at fixed wNa2SO4 and the effect of increasing wNa2SO4 at fixed wPEG8000 on PEG 8000/Na2SO4 ATPS and w/w emulsions (red arrows in Figure 1). Figures 8 and S15 show the appearance of the aqueous mixtures and emulsions for the former and latter, respectively, at two different times. Microscopy images of emulsions just after preparation are given in Figures 9 and S16. At fixed wNa2SO4, increasing wPEG8000 from 0.06 to 0.3 resulted in an increase in the volume fraction of the PEG-rich phase from 0.3 to 0.67 (Figure 8). All of the corresponding emulsions were of the type salt-in-PEG of similar coalescence stability except for that at wPEG8000 = 0.06 which was a stable PEG-in-salt emulsion. In line with other Pickering emulsion systems, this is an example of catastrophic phase inversion induced by increasing ϕPEG‑rich.76 However,

there is a slight difference here in that equilibrium phase compositions also varied with ϕPEG‑rich. In the case of increasing wNa2SO4 from 0.0575 to 0.15 at wPEG8000 of 0.12, the volume fraction of salt-rich phase increased from 0.3 to 0.68. All of the corresponding emulsions were of the salt-inPEG type but the extent of coalescence increased progressively. In order to study further the stabilization of w/w emulsions within the two-phase region, a substantial number of overall compositions were prepared in the presence of 2 wt % of 75% SiOH-fumed silica. A summary of the findings is shown in Figure 10. The data are split into six kinds of behavior dividing the phase diagram into three regions: stable emulsions, unstable emulsions, and aqueous gel (without silica). A major new finding is that emulsions of both types stable to coalescence form at compositions close to the binodal curve. In the case of compositions at the end of long tie lines, a possible explanation is that due to the low volume fraction of droplets in either emulsion, their collision frequency is reduced rendering emulsions stable. However, in the case of compositions at the end of the shortest tie line near the plait point, the volume fraction of dispersed phase in either emulsion is appreciable (>50%) and we suggest that the higher emulsion viscosity contributes to emulsion stability. Stable salt-in-PEG emulsions also form toward the top left part of the diagram for wNa2SO4 from 0.02 to 0.06 and wPEG8000 from 0.08 to 0.4. Stable PEG-in-salt emulsions appear in the bottom G

DOI: 10.1021/acs.langmuir.8b04151 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 8. (Upper) Photos of PEG 8000/Na2SO4 aqueous mixtures upon increasing wPEG8000 at fixed wNa2SO4 of 0.075 and photos of corresponding w/w emulsions just after homogenization (middle) and after 1 month (lower) stabilized by 2 wt % of 75% SiOH-fumed silica. Emulsion type was determined by adding a trace amount of mPEG-FITC and observing with a fluorescence microscope, as shown in Figure 9.

Figure 10. Variation of w/w emulsion type and stability after 1 month for PEG 8000/Na2SO4 aqueous mixtures at different overall compositions. Emulsions were stabilized by 2 wt % of 75% SiOHfumed silica. Filled black squaresgel present and no emulsion attempted; open black squaresgel formed after emulsification.

Emulsions of both types become unstable to coalescence on moving away from the binodal curve (Figure S17). At higher salt concentrations and close to conditions of phase inversion, we noticed the formation of a gelled phase after homogenization for several compositions (open black squares in Figure 10). Microscopy revealed that there were no emulsion droplets but agglomerates of gelled PEG-rich phase (Figure S18). In the absence of silica particles, the gel also formed before emulsification at compositions further away from the binodal curve (filled black squares in Figure 10). Emulsion preparation was not attempted in these cases. 3.3.3. Catastrophic Phase Inversion of w/w Emulsions along Tie Lines. Because PEG 8000/Na2SO4 aqueous mixtures on a tie line have the same equilibrium phase composition (but different volumes of the two phases), we thought it worth exploring how emulsion type and stability vary along a tie line. In Figure 11, the findings regarding the type and qualitative coalescence stability after 1 month of emulsions stabilized by 2 wt % of 75% SiOH-fumed silica along four tie lines (TL-1-4) are given. The appearance of all of these emulsions is shown in Figure S19. Catastrophic emulsion phase inversion from saltin-PEG to PEG-in-salt occurred on decreasing ϕPEG‑rich along a tie line. Apart from TL-1 for which all emulsions were stable (close to the plait point), stable emulsions ensued away from phase inversion conditions, whereas coalescence became more prevalent approaching phase inversion from either side for TL2-4. For TL-3 and TL-4, the formation of a PEG gel occurred near phase inversion (not emulsion). All of these findings are in line with those reported earlier in oil−water emulsions stabilized by fumed silica particles.72 We quantified emulsion stability by determining the volume fraction of residual stable emulsion (ϕemulsion) after 1 month relative to the total volume. A value of zero indicates complete instability, whereas a value of unity signifies an emulsion stable

Figure 9. (left) Optical and (right) fluorescence micrographs of w/w emulsions formed from PEG 8000/Na2SO4 aqueous mixtures at different wPEG8000 and fixed wNa2SO4 of 0.075 stabilized by 2 wt % of 75% SiOH-fumed silica just after homogenization.

right of the diagram at low wPEG8000 from 0.02 to 0.08 but a wider range of wNa2SO4 from 0.07 to 0.18. For wPEG8000 ≤ 0.08, the amount of PEG present far exceeds that required to form a monolayer on all particles present60−62 at least in the absence of salt. Close to the binodal curve, catastrophic phase inversion occurred around wPEG8000 of 0.085 and wNa2SO4 of 0.068. H

DOI: 10.1021/acs.langmuir.8b04151 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

emulsion but a PEG gel agglomerate forms at ϕPEG‑rich = 0.24. We note that catastrophic phase inversion occurs around ϕPEG‑rich equal to 0.25 when the volume fraction of dispersed phase reaches the close-packing limitation for spheres of around 0.75 as reported in particle-stabilized oil−water emulsions.76 The variation of emulsion stability for the other long tie lines (TL-2 and TL-4) is plotted in Figure S20 and displays a similar trend.

4. CONCLUSIONS In the PEG 8000/Na2SO4 ATPS, conventional surfactants and a range of hydrophilic or hydrophobic particles were found ineffective in stabilizing emulsions in the two-phase region. Fumed silica particles which were either hydrophilic (100% SiOH) or partially hydrophobic however (≥50% SiOH) were effective emulsifiers with both salt-in-PEG and PEG-in-salt emulsions being stabilized. These w/w emulsions were stable to coalescence for over a year. At equal volumes of the two phases, emulsions were of the salt-in-PEG type independent of the inherent wettability of the particles. We mapped the variation of emulsion type and stability within the two-phase region for over 60 different compositions with 2 wt % of 75% SiOH-fumed silica as the stabilizer. Emulsions of both types were most stable close to the binodal but became unstable further away from it. Along a number of tie lines, catastrophic phase inversion was effected by increasing the volume fraction of the dispersed phase. The stability of emulsions decreased progressively approaching phase inversion from either side. In line with earlier studies on w/w emulsions, we find that particles exhibiting partial hydrophobicity are effective stabilizers and that phase inversion can be effected along a tie line. A comprehensive mapping of emulsion type and stability within the two-phase region has not appeared before and is useful in guiding the selection of the overall composition to yield emulsions of use in applications. Emulsions of both types are ultrastable close to the plait point.

Figure 11. Variation of emulsion type and stability after 1 month for PEG 8000/Na2SO4 aqueous mixtures along four tie lines with 2 wt % of 75% SiOH-fumed silica as an emulsifier.

to both coalescence and creaming/sedimentation. Figure 12 is a plot of ϕemulsion against the volume fraction of PEG-rich phase initially, ϕPEG‑rich, along TL-3 as an example. The decrease in stability approaching inversion from the left (1.00 to 0.90) is not as drastic as that approaching from the right (1.00 to 0.33) but a clear minimum in stability can be evidenced. No

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b04151. Materials; chemical structure of mPEG-FITC and photograph of a glass vial containing PEG 8000/ Na2SO4 aqueous mixture; methods; calibration curves of refractive index for PEG 8000 + Na2SO4 + H2O system; calibration curves of density for PEG 8000 + Na2SO4 + H2O system; tie lines of PEG 8000/Na2SO4 ATPS determined at room temperature; binodal curve of PEG 8000/Na2SO4 ATPS; comparison of tie lines obtained with two different methods; variation of density and refractive index of the upper and lower phases; suspensions of 0.5 and 2 wt % of 75% SiOHfumed silica in pure water, in 20 wt % aqueous Na2SO4 solution and in 20 wt % aqueous PEG 8000 solution; photos of PEG 8000/Na2SO4 aqueous mixtures; microscope images of w/w emulsions; schematic diagram of the two ways to measure the three-phase contact angle; optical and fluorescence micrographs of w/w emulsions; photos of w/w emulsions stabilized by 2 wt % of 75% SiOH-fumed silica; and variation of volume fraction of residual emulsion (PDF)

Figure 12. Variation of ϕemulsion with ϕPEG‑rich for w/w emulsions formed from PEG 8000/Na2SO4 aqueous mixtures stabilized by 2 wt % of 75% SiOH-fumed silica on TL-3 and 1 month after preparation. Dashed line denotes catastrophic phase inversion. At ϕPEG‑rich = 0.24, no emulsion but PEG gel agglomerates formed after homogenization. I

DOI: 10.1021/acs.langmuir.8b04151 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

■

Polyethylene Glycol-Sodium Sulphate two-Phase Systems in the presence of Iodide and Thiocyanate Ions. Anal. Chim. Acta 2001, 427, 293−300. (16) Hamta, A.; Dehghani, M. R. Application of Polyethylene Glycol Based Aqueous Two-Phase Systems for Extraction of Heavy Metals. J. Mol. Liq. 2017, 231, 20−24. (17) Nishinari, K.; Doi, E. Food Hydrocolloids: Structures, Properties, and Functions; Plenum Press: New York, 1993; p 330. (18) Binks, B. P. Colloidal Particles at a Range of Fluid-Fluid Interfaces. Langmuir 2017, 33, 6947−6963. (19) de Oliveira, C. C.; Coimbra, J. S. d. R.; Zuniga, A. D. G.; Martins, J. P.; Siqueira, A. M. d. O. Interfacial Tension of Aqueous Two-Phase Systems Containing Poly(ethylene glycol) and Potassium Phosphate. J. Chem. Eng. Data 2012, 57, 1648−1652. (20) Zeppieri, S.; Rodríguez, J.; López de Ramos, A. L. Interfacial Tension of Alkane + Water Systems. J. Chem. Eng. Data 2001, 46, 1086−1088. (21) Scholten, E.; Sagis, L. M. C.; van der Linden, E. Bending Rigidity of Interfaces in Aqueous Phase-separated Biopolymer Mixtures. J. Phys. Chem. B 2004, 108, 12164−12169. (22) Duplâtre, G.; Ferreira Marques, M. Size of Sodium Dodecyl Sulfate Micelles in Aqueous Solutions as Studied by Positron Annihilation Lifetime Spectroscopy. J. Phys. Chem. 1996, 100, 16608−16612. (23) Poortinga, A. T. Microcapsules from Self-Assembled Colloidal Particles Using Aqueous Phase - Separated Polymer Solutions. Langmuir 2008, 24, 1644−1647. (24) Nguyen, B. T.; Nicolai, T.; Benyahia, L. Stabilization of Waterin-Water Emulsions by Addition of Protein Particles. Langmuir 2013, 29, 10658−10664. (25) de Freitas, R. A.; Nicolai, T.; Chassenieux, C.; Benyahia, L. Stabilization of Water-in-Water Emulsions by Polysaccharide-Coated Protein Particles. Langmuir 2016, 32, 1227−1232. (26) Chatsisvili, N.; Philipse, A. P.; Loppinet, B.; Tromp, R. H. Colloidal Zein Particles at Water-Water Interfaces. Food Hydrocolloids 2017, 65, 17−23. (27) Gonzalez-Jordan, A.; Nicolai, T.; Benyahia, L. Influence pf the Protein Particle Morphology and Partitioning on the Behavior of Particle-Stabilized Water-in-Water Emulsions. Langmuir 2016, 32, 7189−7197. (28) Dewey, D. C.; Strulson, C. A.; Cacace, D. N.; Bevilacqua, P. C.; Keating, C. D. Bioreactor Droplets from Liposome-Stabilized AllAqueous Emulsions. Nat. Commun. 2014, 5, 4670. (29) Firoozmand, H.; Murray, B. S.; Dickinson, E. Interfacial Structuring in A Phase-Separating Mixed Biopolymer Solution Containing Colloidal Particles. Langmuir 2009, 25, 1300−1305. (30) Balakrishnan, G.; Nicolai, T.; Benyahia, L.; Durand, D. Particles Trapped at the Droplet Interface in Water-in-Water Emulsions. Langmuir 2012, 28, 5921−5926. (31) Vis, M.; Opdam, J.; van ’t Oor, I. S. J.; Soligno, G.; van Roij, R.; Tromp, R. H.; Erné, B. H. Water-in-Water Emulsions Stabilized by Nanoplates. ACS Macro Lett. 2015, 4, 965−968. (32) Inam, M.; Jones, J. R.; Pérez-Madrigal, M. M.; Arno, M. C.; Dove, A. P.; O’Reilly, R. K. Controlling the Size of Two-Dimensional Polymer Platelets for Water-in-Water Emulsions. ACS Cent. Sci. 2018, 4, 63−70. (33) Nguyen, B. T.; Wang, W.; Saunders, B. R.; Benyahia, L.; Nicolai, T. pH-Responsive Water-in-Water Pickering Emulsions. Langmuir 2015, 31, 3605−3611. (34) Hann, S. D.; Niepa, T. H. R.; Stebe, K. J.; Lee, D. One-Step Generation of Cell-Encapsulating Compartments via Polyelectrolyte Complexation in an Aqueous Two Phase System. ACS Appl. Mater. Interfaces 2016, 8, 25603−25611. (35) Ma, Q.; Song, Y.; Kim, J. W.; Choi, H. S.; Shum, H. C. Affinity Partitioning-Induced Self-Assembly in Aqueous Two-Phase Systems: Templating for Polyelectrolyte Microscapsules. ACS Macro Lett. 2016, 5, 666−670.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Bernard P. Binks: 0000-0003-3639-8041 Notes

The authors declare no competing financial interest. Since submitting our paper, we were alerted to the paper by Griffith, C.; Daigle, H. On the shear stability of water-in-water Pickering emulsions stabilized with silica nanoparticles, J. Colloid Interface Sci., 2018, 532, 83−91. Their ATPS was PEG 20 000-MgSO4 and emulsions were stabilized by PEGylated nanosilica particles (6 and 50 nm). Flocculated particles were evident around emulsion drops. Under shear, salt-in-PEG emulsions containing the larger particles were more stable.

■

ACKNOWLEDGMENTS The authors thank the University of Hull for a PhD Scholarship to H.S. We thank the referees for their useful suggestions.

■

REFERENCES

(1) Albertsson, P. Å. Partition of Cell Particles and Macromolecules, 3rd ed.; Wiley-Interscience: New York, 1986. (2) Hatti-Kaul, R. Aqueous Two-Phase Systems: Methods and Protocols. Methods Biotechnology; Humana Press Inc.: New Jersey, 2000; pp 11−21. (3) Albertsson, P.-Å. Partition of Cell Particles and Macromolecules in Polymer Two-Phase Systems. Advances in Protein Chemistry; Elsevier, 1970; Vol. 24, pp 309−341. (4) Beijerinck, M. W. Parasiten und Infektionskrankenheiten. Zentralbl. Bakteriol. 1896, 2, 697−699. (5) Asenjo, J. A.; Andrews, B. A. Aqueous Two-Phase Systems for Protein Separation: A Perspective. J. Chromatogr. A 2011, 1218, 8826−8835. (6) Albertsson, P. Å.; Johansson, G.; Tjerneld, F. Aqueous TwoPhase Separations. Separation Processes in Biotechnology; Marcel Dekker: New York, 1990. (7) Iqbal, M.; Tao, Y. F.; Xie, S. Y.; Zhu, Y. F.; Chen, D. M.; Wang, X.; Huang, L. L.; Peng, D. P.; Sattar, A.; Shabbir, M. A. B.; Hussain, H. I.; Ahmed, S.; Yuan, Z. H. Aqueous Two-Phase System (ATPS): An Overview and Advances in Its Applications. Biol. Proced. Online 2016, 18, 1−18. (8) Diamond, A. D.; Hsu, J. T. Protein Partitioning in PEG/Dextran Aqueous Two-Phase Systems. AIChE J. 1990, 36, 1017−1024. (9) Rosa, P. A. J.; Ferreira, I. F.; Azevedo, A. M.; Aires-Barros, M. R. Aqueous Two-Phase Systems: A Viable Platform in the Manufacturing of Biopharmaceuticals. J. Chromatogr. A 2010, 1217, 2296−2305. (10) Kim, C.-W.; Rha, C. Phase Separation of Polyethylene Glycol/ Salt Aqueous Two-Phase Systems. Phys. Chem. Liq. 2000, 38, 181− 191. (11) Snyder, S. M.; Cole, K. D.; Szlag, D. C. Phase Compositions, Viscosities, and Densities for Aqueous Two-Phase Systems Composed of Polyethylene Glycol and Various Salts at 25°C. J. Chem. Eng. Data 1992, 37, 268−274. (12) Cesi, V.; Katzbauer, B.; Narodoslawsky, M.; Moser, A. Thermophysical Properties of Polymers in Aqueous Two-Phase Systems. Int. J. Thermophys. 1996, 17, 127−135. (13) Raja, S.; Murty, V. R.; Thivaharan, V.; Rajasekar, V.; Ramesh, V. Aqueous Two-Phase Systems for the Recovery of Biomolecules − A Review. J. Sci. Technol. 2012, 1, 7−16. (14) Yau, Y. K.; Ooi, C. W.; Ng, E.-P.; Lan, J. C.-W.; Ling, T. C.; Show, P. L. Current Applications of Different Type of Aqueous TwoPhase Systems. Bioresour. Bioprocess. 2015, 2, 49. (15) Shibukawa, M.; Nakayama, N.; Hayashi, T.; Shibuya, D.; Endo, Y.; Kawamura, S. Extraction Behaviour of Metal Ions in Aqueous J

DOI: 10.1021/acs.langmuir.8b04151 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir (36) Peddireddy, K. R.; Nicolai, T.; Benyahia, L.; Capron, I. Stabilization of Water-in- Water Emulsions by Nanorods. ACS Macro Lett. 2016, 5, 283−286. (37) Xue, L.-H.; Xie, C.-Y.; Meng, S.-X.; Bai, R.-X.; Yang, X.; Wang, Y.; Wang, S.; Binks, B. P.; Guo, T.; Meng, T. Polymer-Protein Conjugate Particles with Biocatalytic Activity for Stabilization of Water-in-Water Emulsions. ACS Macro Lett. 2017, 6, 679−683. (38) Ananthapadmanabhan, K. P.; Goddard, E. D. Aqueous Biphase Formation in Polyethylene Oxide-Inorganic Salt Systems. Langmuir 1987, 3, 25−31. (39) Binks, B. P.; Shi, H. Water-in-water emulsions stabilised by silica particles, oral presentation at IACIS: Rotterdam, May 21−25, 2018. (40) Hatti-Kaul, R. Aqueous Two-Phase Systems: Methods and Protocols; Humana Press: Totowa, 2000; p 15. (41) Zafarani-Moattar, M. T.; Sadeghi, R. Phase Behavior of Aqueous Two-Phase PEG + NaOH System at Different Temperature. J. Chem. Eng. Data 2004, 49, 297−300. (42) Yin, G.; Li, S.; Zhai, Q.; Jiang, Y.; Hu, M. Phase Behavior of Aqueous Two-Phase Systems Composed of 1-alkyl-3-methylimidazolium Bromide+Rb2CO3/Cs2CO3+Water. Thermochim. Acta 2013, 566, 149−154. (43) Zhang, W.; Hu, Y.; Wang, Y.; Han, J.; Ni, L.; Wu, Y. LiquidLiquid Equilibrium of Aqueous Two-Phase Systems Containing Poly(ethylene glycol) of Different Molecular Weights and Several Ammonium Salts at 298.15 K. Thermochim. Acta 2013, 560, 47−54. (44) Claros, M.; Taboada, M. E.; Galleguillos, H. R.; Jimenez, Y. P. Liquid-Liquid Equilibrium of the CuSO4+PEG 4000+H2O System at Different Temperatures. Fluid Phase Equilib. 2014, 363, 199−206. (45) Ferreira, L. A.; Teixeira, J. A. Salt Effect on the Aqueous TwoPhase System PEG 8000-Sodium Sulfate. J. Chem. Eng. Data 2011, 56, 133−137. (46) Silvério, S. C.; Wegrzyn, A.; Lladosa, E.; Rodríguez, O.; Macedo, E. A. Effect of Aqueous Two-Phase System Constituents in Different Poly(ethylene glycerol)-Salt Phase diagram. J. Chem. Eng. Data 2012, 57, 1203−1208. (47) Huddleston, J. G.; Willauer, H. D.; Rogers, R. D. Phase Diagram Data for Several PEG + Salt Aqueous Biphasic Systems at 25 °C. J. Chem. Eng. Data 2003, 48, 1230−1236. (48) Reed, K. M. Wettability of Solid Particles in relation to ParticleStabilised Foams and Emulsions. University of Hull, Ph.D. Thesis, 2011. (49) Armistead, C. G.; Tyler, A. J.; Hambleton, F. H.; Mitchell, S. A.; Hockey, J. A. Surface hydroxylation of silica. J. Phys. Chem. 1969, 73, 3947−3953. (50) González-Amado, M.; Rodil, E.; Arce, A.; Soto, A.; Rodríguez, O. The Effect of Temperature on Polyethylene Glycol (4000 or 8000)-(Sodium or Ammonium) Sulfate Aqueous Two-Phase Systems. Fluid Phase Equilib. 2016, 428, 95−101. (51) Alvarenga, B. G.; Virtuoso, L. S.; Lemes, N. H. T.; Da Silva, L. A.; Mesquita, A. F.; Nascimento, K. S.; Hespanhol da Silva, M. C.; Mendes da Silva, L. H. Measurement and Correlation of the Phase Equilibrium of Aqueous Two-Phase Systems Composed of Polyethylene Glycol 1500 or 4000 + Sodium Sulfite + Water at Different Temperatures. J. Chem. Eng. Data 2014, 59, 382−390. (52) Murari, G. F.; Penido, J. A.; Machado, P. A. L.; Lemos, L. R. d.; Lemes, N. H. T.; Virtuoso, L. S.; Rodrigues, G. D.; Mageste, A. B.; Mageste, A. B. Phase Diagrams of Aqueous Two-Phase Systems Formed by Polyethylene Glycol+Ammonium Sulfate+Water: Equilibrium Data and Thermodynamic Modelling. Fluid Phase Equilib. 2015, 406, 61−69. (53) Atefi, E.; Fyffe, D.; Kaylan, K. B.; Tavana, H. Characterization of Aqueous Two-Phase Systems from Volume and Density Measurements. J. Chem. Eng. Data 2016, 61, 1531−1539. (54) Doolittle, A. K.; Peterson, R. H. Preparation and Physical Properties of a Series of n-Alkanes. J. Am. Chem. Soc. 1951, 73, 2145− 2151.

(55) Horozov, T. S.; Binks, B. P.; Gottschalk-Gaudig, T. Effect of Electrolyte in Silicone Oil-in-Water Emulsions Stabilised by Fumed Silica Particles. Phys. Chem. Chem. Phys. 2007, 9, 6398−6404. (56) Binks, B. P.; Lumsdon, S. O. Stabiility of Oil-in-Water Emulsions Stabilised by Silica Particles. Phys. Chem. Chem. Phys. 1999, 1, 3007−3016. (57) Kostakis, T.; Ettelaie, R.; Murray, B. S. Effect of High Salt Concentrations on the Stabilization of Bubbles by Silica Particles. Langmuir 2006, 22, 1273−1280. (58) Dishon, M.; Zohar, O.; Sivan, U. Effect of Cation Size and Charge on the Interaction between Silica Surfaces in 1:1, 2:1, and 3:1 Aqueous Electrolytes. Langmuir 2011, 27, 12977−12984. (59) Mondragon, R.; Julia, J. E.; Barba, A.; Jarque, J. C. Characterization of Silica-Water Nanofluid Dispersed with An Ultrasound Probe: A Study of Their Physical Properties and Stability. Powder Technol. 2012, 224, 138−146. (60) Mathur, S.; Moudgil, B. M. Adsorption Mechanism(s) of Poly (Ethylene Oxide) on Oxide Surfaces. J. Colloid Interface Sci. 1997, 196, 92−98. (61) Liu, H.; Xiao, H. Adsorption of Poly (Ethylene Oxide) with Different Molecular Weights on the Surface of Silica Nanoparticles and the Suspension Stability. Mater. Lett. 2008, 62, 870−873. (62) Lafuma, F.; Wong, K.; Cabane, B. Bridging of Colloidal Particles through Adsorbed Polymers. J. Colloid Interface Sci. 1991, 143, 9−21. (63) Sen, S.; Govindarajan, V.; Pelliccione, C. J.; Wang, J.; Miller, D. J.; Timofeeva, E. V. Surface Modification Approach to TiO2 Nanofluids with High Particle Concentration, Low Viscosity, and Electrochemical Activity. ACS Appl. Mater. Interfaces 2015, 7, 20538− 20547. (64) Yu, F.; Chen, Y.; Liang, X.; Xu, J.; Lee, C.; Liang, Q.; Tao, P.; Deng, T. Dispersion Stability of Thermal Nanofluids. Prog. Nat. Sci.: Mater. Int. 2017, 27, 531−542. (65) Successful Use of AEROSIL Fumed Silica in Liquid Systems; Technical Information 1279; Evonik Industries, 2017. (66) Nguyen, B. T.; Nicolai, T.; Benyahia, L. Stabilization of Waterin-Water Emulsions by Addition of Protein Particles. Langmuir 2013, 29, 10658−10664. (67) Binks, B. P.; Whitby, C. P. Silica Particle-Stabilized Emulsions of Silicone Oil and Water: Aspects of Emulsification. Langmuir 2004, 20, 1130−1137. (68) Kloet, J. V.; Schramm, L. L. The Effect of Shear and Oil/Water Ratio on the Required Hydrophile - Lipophile Balance for Emulsification. J. Surfactants Deterg. 2002, 5, 19−24. (69) Goyal, A.; Sharma, V.; Upadhyay, N.; Singh, A. K.; Arora, S.; Lal, D.; Sabikhi, L. Development of Stable Flaxseed Oil Emulsions as a Potential Delivery System of ω-3 Fatty Acids. J. Food Sci. Technol. 2015, 52, 4256−4265. (70) McClements, D. J. Food Emulsions: Principles, Practices, and Techniques, 3rd ed.; CRC Press: Boca Raton, 2015; pp 245−284. (71) Frising, T.; Noïk, C.; Dalmazzone, C. The Liquid/Liquid Sedimentation Process: From Droplet Coalescence to Technologically Enhanced Water/Oil Emulsion Gravity Separators: A Review. J. Dispersion Sci. Technol. 2006, 27, 1035−1057. (72) Binks, B. P.; Lumsdon, S. O. Transitional Phase Inversion of Solid-Stabilized Emulsions Using Particle Mixtures. Langmuir 2000, 16, 3748−3756. (73) Binks, B. P.; Murakami, R. Phase Inversion of ParticleStabilized Materials from Foams to Dry Water. Nat. Mater. 2006, 5, 865−869. (74) Derkaoui, N.; Said, S.; Grohens, Y.; Olier, R.; Privat, M. Polyethylene Glycol Adsorption on Silica: From Bulk Phase Behavior to Surface Phase Diagram. Langmuir 2007, 23, 6631−6637. (75) Yang, R.; Yang, X. R.; Evans, D. F.; Hendrickson, J. Scanning Tunneling Microscopy Images of Poly(ethylene oxide) Polymers: Evidence for Helical and Superhelical Structures. J. Phys. Chem. 1990, 94, 6123−6125. K

DOI: 10.1021/acs.langmuir.8b04151 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir (76) Binks, B. P.; Lumsdon, S. O. Catastrophic Phase Inversion of Water-in-Oil Emulsions Stabilised by Hydrophobic Silica. Langmuir 2000, 16, 2539−2547.

L

DOI: 10.1021/acs.langmuir.8b04151 Langmuir XXXX, XXX, XXX−XXX