Article pubs.acs.org/Langmuir
pH-Responsive Water-in-Water Pickering Emulsions Bach T. Nguyen,† Wenkai Wang,‡ Brian R. Saunders,‡ Lazhar Benyahia,† and Taco Nicolai*,† †
LUNAM, Université du Maine, IMMM UMR CNRS 6283, PCI, Le Mans, 72085 Cedex 9, France School of Materials, University of Manchester, Grosvenor Street, Manchester, M13 9PL, United Kingdom
‡
S Supporting Information *
ABSTRACT: The structure and stability of water-in-water emulsions was investigated in the presence of spherical, pHsensitive microgels. The emulsions were formed by mixing aqueous solutions 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 at least 1 week. The droplet size was visualized with confocal laser scanning microscopy and was found to be smallest in the stable pH range. Emulsions could be stabilized or destabilized by small changes of the pH. Addition of small amounts of salt led to a shift of the pH range where the emulsions were stable. The effects of varying the microgel concentration and the polymer composition were investigated.
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INTRODUCTION Water-in-water (W/W) emulsions are formed when two aqueous solutions of incompatible polymers are mixed.1−3 For such systems, the interface between the two phases is welldefined only on length scales larger than the correlation length of the polymer solutions. Smaller particles do not encounter an interface when they move from one polymer phase to the other. Therefore, contrary to oil-in-water emulsions, W/W emulsions cannot be stabilized by surfactants. However, it has been shown in recent years that they can be stabilized by different types of colloidal particles.4−12 These so-called Pickering emulsions have been investigated intensively for oil-in-water emulsion.13−15 When particles are situated at the interface they reduce the interface between the two incompatible polymer solutions, and therefore, the free energy of the system is reduced if the particles are less incompatible with either phase. The presence of the particles at the interface inhibits coalescence of droplets, leading to very efficient stabilization.16−18 Even though the interfacial tension between two aqueous polymer solutions is orders of magnitude smaller than between oil and water,19,20 it was shown that the reduction of the free energy when a colloidal particle enters the interface is still significantly larger than the thermal energy.4 Therefore, once the particles are at the interface, they do not leave spontaneously. In an earlier investigation of W/W emulsions formed by mixing aqueous solutions of dextran and poly(ethylene oxide) (PEO) it was shown that latex particles adsorbed spontaneously to the interface.4 Nevertheless, the droplets coalesced even if they were fully covered with particles, and during coalescence the particles were driven off the droplet surface into the solution. As a consequence, the presence of latex particles only reduced the rate of destabilization but did © 2015 American Chemical Society
not render the emulsions stable over an extended period of time. In a subsequent investigation it was found that protein microgels were much more effective in stabilizing the same emulsion even though they were smaller than the latex particles.10 A study by Hanazawa and Murray8 showed that small oil droplets spontaneously went to the interface of a W/W emulsion formed by mixing aqueous solutions of a protein and a polysaccharide. They found that the emulsions were less stable when fully liquid or fully solid oil droplets were used than with a mixture of these oils and suggested that this was related to the interaction between the oil droplets at the interface. In order to gain a better understanding of the parameters that control stabilization of W/W emulsions by colloidal particles, we have investigated the dextran−PEO emulsions in the presence of pH-sensitive microgels. The microgels are cross-linked polymer colloid particles that swell when the pH approaches the pKa of the constituent polymer.21,22 While stimuli-sensitive microgels have been used to stabilize oil-inwater emulsions,23−25 the present study is the first report of their use to stabilize W/W emulsions. The charged microgels comprised covalently cross-linked poly(ethyl acrylate-co-methacrylic acid-co-1,4-butanediol diacrylate) (PEA-MAA-BDDA) that showed a dramatic increase of their diameter when the pH approached the pKa of the particles.26,27 We will show that the microgels spontaneously enter the interface of W/W emulsions at all pH values but that their capacity to stabilize the emulsions depends on the pH. This allows stabilization and destabilization of W/W emulsions by varying the pH within a narrow range Received: December 18, 2014 Revised: March 4, 2015 Published: March 6, 2015 3605
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The microgels were labeled with rhodamine B by adding 5 ppm of the fluorescent probe and allowing the dispersion to mix for at least 30 min at the required pH. No rhodamine was found in the water phase after removal of the microgels by ultracentrifugation or filtration, implying that all the rhodamine B adsorbed strongly to the microgels, at least in the pH range investigated here (4−8.5). A small amount of dextran labeled with fluorescein isothiocyanate (FITC) was added to a few solutions in order to identify the dextran phase within the emulsions. No effect of fluorescent labeling on the stability of the emulsions was observed. Emulsions were formed by hand-shaking or using a minishaker. The speed of mixing and the order of mixing the three components were found to be unimportant. The reason for this insensitivity is most likely that due to the very low interfacial tension during mixing, all ingredients were homogeneously distributed. The pH was set by adding aliquots of 0.1 or 1 M NaOH or HCl. Confocal laser scanning microscopy (CLSM) observations were made with a Leica TCS-SP2 (Leica Microsystems, Heidelberg, Germany) in an air-conditioned room at 20 °C. Images of 512 × 512 pixels were produced at different magnifications with a water immersion objective (HCX PL APO 63× NA = 1.2). The incident light was emitted by a laser beam at 543 and/or 488 nm. The fluorescence intensity was recorded between 560 and 700 nm. Care was taken not to saturate the fluorescence signal so that it was proportional to the concentration of the probes. The hydrodynamic radius of dilute solutions of the microgel particles was determined by dynamic light scattering (DLS) using a commercial apparatus (ALV, Langen, Germany). The light source was a He−Ne laser with wavelength λ = 632 nm. The temperature was controlled by a thermostat bath to within ±0.2 °C. Measurements were made at angles of observation between 12° and 150° at 20 °C. In all cases, the autocorrelation functions could be described in terms of a narrow relaxation time distribution. The average relaxation rate (Γ) was found to be proportional to q2. We checked that the microgels were sufficiently dilute so that the effect of interactions on the measured diffusion coefficient could be neglected. In dilute solutions, the relaxation is caused by self-diffusion of the particles and Γ is related to the diffusion coefficient (D): Γ = q2D. The average hydrodynamic radius (Rh) may be calculated using the Stokes−Einstein equation
close to neutral, which may be potentially useful in the long term for delivery applications. The effect of screening electrostatic interactions by adding NaCl was also investigated.
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MATERIALS AND METHODS
Materials. The dextran and PEO samples used for this investigation were purchased from Sigma-Aldrich. The nominal weight-average molar mass was Mw = 5 × 105 g/mol for the dextran and Mw = 2 × 105 g/mol for the PEO. For this study, rather high molar masses were chosen in order to delay creaming (for PEO-in-dextran emulsions) or sedimentation (for dextran-in-PEO emulsions) of the emulsion droplets. Dextran labeled with the fluorophore fluorescein isothiocyanate (FITC) (Mw = 5 × 105 g/mol) was purchased from Sigma-Aldrich. Dextran was used without further purification, but the PEO sample contained a small amount of silica particles that were removed by centrifugation before use. Solutions of dextran and PEO were prepared by dissolving the powder in salt-free water (Milli-Q) at neutral pH with stirring. Concentrations of PEO (CPEO) and dextran (Cdex) are indicated as weight percentages. The microgel particles were synthesized as described elsewhere.26 Briefly, an aqueous sodium dodecyl sulfate solution (518 g, 0.35 wt %) was added to a reaction flask, heated to 80 °C in a water bath, and purged with nitrogen gas for 30 min. A comonomer solution was prepared containing ethyl acrylate (EA) (189.90 g, 1.90 mol), methacrylic acid (MAA) (97.06 g, 1.13 mol), and 1,4-butanediol diacrylate (BDDA) (2.88 g, 0.0145 mol). A portion of the comonomer solution (31.5 g) was added to the flask along with K2HPO4 (3.15 g of a 7 wt % solution) and ammonium persulfate (APS, 3.5 g of 5 wt % solution) and the seed stage formed. After a further 30 min, the remaining monomer mixture was added at a constant rate to the reaction vessel over a period of 90 min. After a further 2 h, the dispersion was cooled and dialyzed extensively using high-purity water. Methods. W/W emulsions were formed by mixing PEO and dextran solutions. Depending on the composition, dextran-in-PEO or PEO-in-dextran emulsions were formed. Phase inversion occurred approximately at the composition when the volume fraction of the two phases was equal. The phase diagram of this system at 20 °C has been reported before and does not depend on the pH (see Figure 1). The binodal was obtained by inspecting confocal laser scanning microscopy images for signs of phase separation. For most phase-separated mixtures, separation was practically complete, so tie-lines could be straightforwardly deduced from the phase volumes with small corrections for the differences in the densities of the two phases, which were taken from the literature.28
D=
kT 6πηR h
(1)
where η is the viscosity, k is Boltzman’s constant, and T is the absolute temperature.
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RESULTS AND DISCUSSION Effect of the pH. It was already reported elsewhere that the poly(EA-MAA-BDDA) microgels dramatically swell when the pH is increased.26 The hydrodynamic radius of the microgels used for this study was determined as a function of the pH using DLS as described in the methods section (Figure 2). Rh increased from 60 nm for pH 7.5, implying that the density of the microgel particles decreased by a factor 50 in this narrow pH range as the pH was increased. This result can be explained by the strong increase of the charge density that occurred when the pH approached the pKa, leading to a strong increase of the osmotic pressure of the counterions. The value of the pKa was 6.5 in 0.1 M NaCl and 7.5 in salt-free water, as determined by potentiometric titration (see the Supporting Information, Figure S1). We studied the effect of the pH for an emulsion of dextranrich droplets in a continuous PEO-rich phase and an emulsion of PEO-rich droplets in a continuous dextran-rich phase. The dextran-in-PEO and PEO-in-dextran emulsions with PEO/ dextran compositions of 6.2/4.0 and 3.3/9.5 (wt %/wt %), respectively, are situated on the same tie-line in the phase diagram (see Figure 1). Therefore, the polymer concentration in each phase is the same for both emulsions and they have the
Figure 1. Phase diagram of aqueous mixtures of PEO/dextran mixture reproduced from ref 4. (Copyright 2012 American Chemical Society.) The binodal, 50/50 volume fraction, and a few tie lines are indicated by the solid, dashed, and dashed−dotted lines, respectively. The filled circles indicate the compositions of the PEO-in-dextran and dextranin-PEO emulsions that were studied in detail. 3606
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intensity. Figure 4 shows the microgel concentration in the dextran phase divided by that in the PEO phase (R), which
Figure 2. Dependence of the hydrodynamic radius of the microgel particles on the pH. Figure 4. Concentrations of microgel in the dextran phase divided by that in the PEO phase as a function of the pH for dextran-in-PEO (triangles) and PEO-in-dextran emulsions (circles).
same interfacial tension (γ = 75 μN/m).4 Under these conditions, the PEO phase contains CPEO = 8.2 wt % and almost no dextran, whereas the dextran phase contains Cdex = 15.8 wt % and almost no PEO. The weight fraction of each phase is simply given by the ratio between the overall polymer concentration in the mixture and the polymer concentration in the phase. The weight fraction of the PEO phase is thus 6.2/8.2 = 0.75 for the dextran-in-PEO emulsion and 3.3/8.2 = 0.4 for the PEO-in-dextran emulsion. Using the specific volumes of dextran and PEO in water at 20 °C reported by Kang and Sandler,28 we find that the density of the dextran and PEO phase was 1.10 g/mL and 1.02 g/mL respectively. Given the small difference in the densities, the volume fraction is approximately equal to the weight fraction. CLSM images of dextran-in-PEO and PEO-in-dextran emulsions with 0.05 wt % microgels are shown in Figure 3 at
corresponds to the contrast of the coloring in the images, as a function of the pH. The partition had a curious nonmonotonous pH-dependence. Excess microgels strongly preferred the PEO phase at lower pH values up to pH 6.8. At higher pH, an increasing fraction of microgels went to the dextran phase up to pH 7.5, where the microgels showed a preference for the dextran phase. However, at pH 8.0 the concentration of microgels was again slightly higher in the PEO phase. The absolute values of R, which could be determined with an accuracy of about 10%, were not quite the same for the two types of emulsions, perhaps because the droplet size and therefore the fraction of microgels at the interface were not the same. However, one should realize that the results are highly sensitive to the pH, which could be determined with an accuracy of about 0.05 units. In addition, we will show below that the results are sensitive to even very small amounts of salt, which may cause some additional scatter in the data that is difficult to quantify. Therefore, though we believe that the maximum in the values of R and the inversion of the preference are robust results, one has to be careful with any interpretation of the detailed features. The evolution of the emulsions with time at different pH values is shown in Figure 5. When considering the stability of emulsions, it is important to clearly distinguish the formation of a layer of the continuous phase due to creaming or sedimentation of emulsion droplets from the formation of a layer of the droplet phase due to coalescence of droplets. The rate of creaming or sedimentation depends on the droplet size, the difference in density of the two phases, and the viscosity of the continuous phase.10 Here, we will consider that the emulsion is destabilized only when the droplets coalesce and a pure layer of the droplet phase is formed. In the absence of microgels, the emulsions phase separated in less than 1 h, with a layer of the dextran phase at the bottom and a layer of the PEO phase on top. We found that the stability in the presence of microgels was extremely sensitive to the pH. At pH ≥7.8, the emulsions rapidly completely destabilized, whereas they were stable for at least 1 week between 7.0 and 7.5. However, at lower pH the emulsions again
Figure 3. CLSM images (160 × 160 μm) of dextran-in-PEO (top) and PEO-in-dextran (bottom) emulsions containing 0.05 wt % microgels at different pH values.
different pH values between 6.5 and 8.0, i.e., in the range where the microgel size changed dramatically. All images shown in this paper were taken shortly after mixing. The red color corresponds to fluorescence of the rhodamine B adsorbed to the microgels, and the intensity is proportional to the local density of the microgels. In some images, one can clearly see that the microgels are preferentially situated at the interface, but excess microgels are found in both phases. The droplet size was polydisperse and decreased with increasing pH up to pH 7.2, after which it increased again. Partition of the excess microgels between the PEO and the dextran phases can be quantified by measuring the fluorescence 3607
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addition of HCl allowed again formation of stable emulsions. Stabilization and destabilization of the PEO-in-dextran emulsion by pH-cycling could be repeated several times. However, dextran-in-PEO emulsions were no longer stable at pH 7.2 after the second pH-cycling. The origin of this behavior may be the sensitivity of the dextran-in-PEO emulsion to the ionic strength, which increases after each pH-cycling. The total amount of titratable MAA groups in the microgels was 4.1 mmol/g. This means that for a solution containing 0.05 wt % microgels the salt concentration in the form of counterions can be increased by up to 2 mmol/L when the pH is increased. The same amount in the form of excess salt will be produced when the pH is subsequently reduced. The effect of the ionic strength will be discussed in detail below. Effect of the Microgel Concentration and the Interfacial Tension. Before discussing the effect of adding monovalent salt on the stability of the emulsions, we briefly mention the effects of varying the microgel concentration and the interfacial tension at a fixed pH (7.2). The former was studied by varying the microgel concentration between 0.01 and 0.05 wt % for a given interfacial tension. The latter was studied by varying the interfacial tension between pure dextran and PEO phases, while the relative volume fractions and the microgel concentration were kept constant. The results of this investigation are given in the Supporting Information. The droplet size did not depend much on the microgel concentration, except at 0.01 wt %, where they were larger (see the Supporting Information, Figure S3). Adding fewer microgels decreased the stability of the emulsions, but the effect was important only if the concentration was less than 0.03 wt % (see the Supporting Information, Figure S4). The interfacial tension between the pure PEO and dextran phases can be decreased without changing the volume fraction of each phase by dilution with water, as was described in ref 10. Here, we kept the microgel concentration fixed at 0.03 wt %. A decrease of the interfacial tension led to an increase of the droplet size (see the Supporting Information, Figure S5). In addition, the microgel layer at the interface became less visible, indicating that its density had decreased. As expected, the stability of the emulsions decreased with increasing dilution, i.e. decreasing interfacial tension (see the Supporting Information, Figure S6). The undiluted sample showed signs of destabilization only after 1 week, while the most diluted sample fully destabilized within an hour, i.e., similar to mixtures without added microgels. These results on the effects of varying the particle concentration or the interfacial tension are similar to those reported for the same emulsions stabilized with protein microgels.10 Effect of the Ionic Strength. In the first instance, the effect of the ionic strength on the emulsions was studied by adding NaCl to a PEO-in-dextran and a dextran-in-PEO emulsion at fixed pH (7.2). The effect of adding monovalent salt on the hydrodynamic radius of the microgels was small. It decreased weakly from 220 nm in the absence of added salt to 190 nm in the presence of 2 mM NaCl and remained constant when more NaCl was added up to 10 mM. CLSM images of the emulsions at different NaCl concentrations showed that addition of salt led to somewhat larger droplet sizes (see Figure6). Remarkably, addition of as little as 1 mM NaCl resulted in an inversion of the partition of the microgels from being preferentially in the PEO phase toward being preferentially in the dextran phase.
Figure 5. Dextran-in-PEO (left) and PEO-in-dextran (right) emulsions at different times after preparation. The emulsions contained 0.05 wt % microgel. The pH increases from left to right (6.5, 6.8, 7.0, 7.2, 7.5, 7.8, and 8.0).
showed signs of destabilization, especially the PEO-in-dextran emulsions, which completely phase separated after 1 day at pH 6.5. Destabilization of dextran-in-PEO emulsions was still slow at pH 6.5, but at lower pH, it was rapid also for this type of emulsion. We have found that microgels also swell when the pH is increased in the concentrated PEO or dextran phases and that it occurs at approximately the same pH as in pure water (see the Supporting Information, Figure S2). The size of the swollen microgels was about the same in the both polymer solutions, but the microgels were significantly less swollen than in pure water due to osmotic deswelling of microgel particles by nonabsorbing free polymer, as has been reported elsewhere.29 One could therefore argue that stabilization of the emulsions above pH 6.5 is related to strong swelling of the microgels. However, the increase in size above pH 6.5 cannot explain why the emulsions destabilized rapidly for pH >7.5 without significant change in their size. Another possibility is that changes in the partitioning of the microgels between the two phases lead to changes in the contact angle of the microgels at the interface. However, we found very different behavior of the emulsions at pH 7.8 and 7.2, while R was similar. Therefore, we believe that interactions between the particles at the interface play a crucial role. It is clear that both the droplet size and the stability were different for dextran-in-PEO and PEO-in-dextran emulsions, even though the interfacial tension was the same. Differences in behavior between the two types of emulsions were also found when they were stabilized with protein microgels.10 In that case, the PEO-in-dextran emulsions were systematically more stable than the dextran-in-PEO emulsions, whereas here we find the opposite behavior. Different behavior can be expected if the particles are not symmetrically positioned at the interface, so the direction of the curvature of the interface matters. A consequence of the sensitivity of the system to the pH is that stable emulsions at intermediate pH could be destabilized by decreasing or increasing the pH. We have tested this by adding small amounts of NaOH to the solution at pH 7.2 to raise the pH to 8.0 and observed, as expected, rapid destabilization. Subsequent decrease of the pH to 7.2 by 3608
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Figure 8. CSLM images (160 × 160 μm) of dextran-in-PEO (top) and PEO-in-dextran (bottom) emulsions containing 0.05 wt % microgels and 10 mM NaCl at different pH values.
Figure 6. CLSM images (160 × 160 μm) of dextran-in-PEO emulsions (top) and PEO-in-dextran emulsions (bottom) (pH 7.2, 0.03 wt % microgels) at different NaCl concentrations.
The effect of adding salt on the stability of the emulsions is shown in Figure7. Salt-free emulsions showed little destabiliza-
Figure 9. Dextran-in-PEO (left) and PEO-in-dextran (right) emulsions at different times after preparation. The emulsions contained 0.05 wt % microgel. The pH increases from left to right (6.5, 6.8, 7.0, 7.2, 7.5, 7.8, and 8.0).
which were stable in pure water. However, it led to stabilization of these emulsions at pH 6.5 and 6.8, which destabilized rapidly in pure water. The effect of adding salt was less important for PEO-in-dextran emulsions, confirming the results obtained at pH 7.2. The strong effect of salt on the dextran-in-PEO emulsions means that we cannot neglect the effect of the increase of the ionic strength when we adjust the pH of the microgels. This explains why the dextran-in-PEO emulsions could no longer be stabilized by reducing the pH from 8 to 7.2 after the second pH-cycling, because repeated pH-cycling is equivalent to adding a few micromoles of NaCl. As was mentioned above, the pKa shifted to a lower value after addition of salt, which may be related to the shift of the stable pH range to lower values. However, the pH-induced swelling of the microgels was similar with and without 10 mM NaCl (see the Supporting Information, Figure S2). Therefore, the sensitivity of the stability to addition of NaCl cannot be explained by a change in the size of the microgels, but it is most likely related to changes in the interaction between the microgels at the interface and, hence, interface mechanical properties. Furthermore, both attraction due to hydrophobic interaction and repulsion due to electrostatic interaction between particles at the interface can potentially inhibit contact of bare interfaces between two droplets. It is clear that much research is still needed to fully understand the new phenomenon of stabilization of W/W emulsions by microgel particles. Because PEO and dextran are biocompatible polymers
Figure 7. Dextran-in-PEO (left) and PEO-in-dextran (right) emulsions (pH 7.2, 0.03 wt % microgel) at different times after preparation. The NaCl concentration increases from left to right (0, 0.5, 1, 1.5, 2, 2.5, 3, and 10 mM). Notice that the dextran bottom layer was labeled with fluorescent dextran only in the left panel.
tion after a week, while in the presence of 10 mM NaCl dextran-in-PEO emulsions destabilized completely within 1 day. Remarkably, the effect of adding salt was visible even when as little as 0.5 mM NaCl was added. The PEO-in-dextran emulsions remained more stable in the presence of salt, but a small clear PEO top phase appeared when 2 mM or more NaCl was added, indicating that also PEO droplets were less stable in the presence of salt. The effect of salt on the pH-dependence was investigated by adding 10 mM NaCl to emulsions at different pH values. CLSM images are shown in Figure 8 and can be compared with those of the salt-free emulsions shown in Figure 3. At lower pH (6.5 and 6.8), the droplets were smaller in the presence of salt and the adsorption of microgels at the interface can be seen much more clearly, whereas at higher pH (7.2 and 7.5) the droplets were larger. The effect of adding 10 mM NaCl on the stability of the emulsions depended on the pH (see Figure 9). It led to destabilization of dextran-in-PEO emulsions at pH 7.2 and 7.5, 3609
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(6) Firoozmand, H.; Murray, B. S.; Dickinson, E. Interfacial structuring in a phase-separating mixed biopolymer solution containing colloidal particles. Langmuir 2009, 25 (3), 1300−1305. (7) Firoozmand, H.; Rousseau, D. Tailoring the morphology and rheology of phase-separated biopolymer gels using microbial cells as structure modifiers. Food Hydrocolloids 2014, 42, 204−214. (8) Hanazawa, T.; Murray, B. S. The influence of oil droplets on the phase separation of protein−polysaccharide mixtures. Food Hydrocolloids 2012, 34, 128−137. (9) Murray, B. S.; Phisarnchananan, N. The effect of nanoparticles on the phase separation of waxy corn starch + locust bean gum or guar gum. Food Hydrocolloids 2014, 42, 92−99. (10) Nguyen, B. T.; Nicolai, T.; Benyahia, L. Stabilization of water-inwater emulsions by addition of protein particles. Langmuir 2013, 29 (34), 10658−10664. (11) Poortinga, A. T. Microcapsules from self-assembled colloidal particles using aqueous phase-separated polymer solutions. Langmuir 2008, 24 (5), 1644−1647. (12) Buzza, D. M. A.; Fletcher, P. D.; Georgiou, T. K.; Ghasdian, N. Water-in-water emulsions based on incompatible polymers and stabilized by triblock copolymersTemplated polymersomes. Langmuir 2013, 29 (48), 14804−14814. (13) Ramsden, W. Separation of solids in the surface-layers of solutions and ‘suspensions’ (observations on surface-membranes, bubbles, emulsions, and mechanical coagulation)Preliminary account. Proc. R. Soc. London 1903, 72, 156−164. (14) Pickering, S. U. CXCVI.Emulsions. J. Chem. Soc., Trans. 1907, 91, 2001−2021. (15) Binks, B. P.; Horozov, T. S. Colloidal Particles at Liquid Interfaces. Cambridge Univ Press: Cambridge, UK, 2006. (16) Arditty, S.; Whitby, C. P.; Binks, B. P.; Schmitt, V.; LealCalderon, F. Some general features of limited coalescence in solidstabilized emulsions. Eur. Phys. J. E 2003, 11 (3), 273−281. (17) Destribats, M.; Rouvet, M.; Gehin-Delval, C.; Schmitt, C.; Binks, B. P. Emulsions stabilised by whey protein microgel particles: Towards food-grade Pickering emulsions. Soft Matter 2014, 10, 6941− 6954. (18) Aveyard, R.; Binks, B. P.; Clint, J. H. Emulsions stabilised solely by colloidal particles. Adv. Colloid Interface Sci. 2003, 100, 503−546. (19) Ding, P.; Wolf, B.; Frith, W. J.; Clark, A. H.; Norton, I. T.; Pacek, A. W. Interfacial tension in phase-separated gelatin/dextran aqueous mixtures. J. Colloid Interface Sci. 2002, 253 (2), 367−376. (20) Liu, Y.; Lipowsky, R.; Dimova, R. Concentration dependence of the interfacial tension for aqueous two-phase polymer solutions of dextran and polyethylene glycol. Langmuir 2012, 28 (8), 3831−3839. (21) Richtering, W.; Saunders, B. R. Gel architectures and their complexity. Soft Matter 2014, 10 (21), 3695−3702. (22) Hoare, T.; Pelton, R. Characterizing charge and crosslinker distributions in polyelectrolyte microgels. Curr. Opin. Colloid Interface Sci. 2008, 13 (6), 413−428. (23) Richtering, W. Responsive emulsions stabilized by stimulisensitive microgels: Emulsions with special non-Pickering properties. Langmuir 2012, 28 (50), 17218−17229. (24) Destribats, M.; Eyharts, M.; Lapeyre, V.; Sellier, E.; Varga, I.; Ravaine, V.; Schmitt, V. Impact of pNIPAM microgel size on its ability to stabilize Pickering emulsions. Langmuir 2014, 30 (7), 1768−1777. (25) Ngai, T.; Behrens, S. H.; Auweter, H. Novel emulsions stabilized by pH and temperature sensitive microgels. Chem. Commun. 2005, 3, 331−333. (26) Lally, S.; Freemont, T. J.; Cellesi, F.; Saunders, B. R. pHresponsive microgels containing hydrophilic crosslinking co-monomers: Shell-exploding microgels through design. Colloid Polym. Sci. 2011, 289 (5-6), 647−658. (27) Rodriguez, B.; Wolfe, M.; Fryd, M. Nonuniform swelling of alkali swellable microgels. Macromolecules 1994, 27 (22), 6642−6647. (28) Kang, C.; Sandler, S. Phase behavior of aqueous two-polymer systems. Fluid Phase Equilib. 1987, 38 (3), 245−272. (29) Saunders, B. R.; Crowther, H. M.; Vincent, B. Poly[(methyl methacrylate)-co-(methacrylic acid)] microgel particles: Swelling
and biocompatibility studies involving poly(EA-MAA-BDD) microgels have been favorable,30,31 it follows that advances in the understanding and control of these stimulus-responsive W/ W emulsions may lead to new delivery opportunities in the future.
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CONCLUSION pH-sensitive microgels can be used to stabilize W/W emulsions in a specific pH range, and it is possible to stabilize or destabilize the emulsions by varying the pH. The microgels spontaneously covered the interface between the dextran and the PEO phase at any pH at least in the range 5−8. However, they only stabilized the W/W emulsions effectively in a narrow pH range, which happened to be close to physiological pH. The pH range where the emulsions were stable shifted to lower pH when a small amount of monovalent salt was added. The microgels strongly swell above a critical pH, but the effects of the pH and the ionic strength on the emulsion stability cannot solely be explained in terms of size change. Interaction between the microgels at the interface and therefore the mechanical properties of the microgel layer may play a crucial role. In addition, the stability is not the same for dextran-in-PEO and PEO-in-dextran emulsions, which shows that the curvature of the interface is also an important parameter. Furthermore, it was shown that varying the pH or addition of electrolyte could cause the nonadsorbed microgel particles to spontaneously migrate from the PEO to dextran phase. If the microgel particles were to contain an additive, this effect may enable triggered delivery in the future.
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ASSOCIATED CONTENT
S Supporting Information *
CLSM images of emulsions at different microgel concentrations and different interfacial tensions; photographs of the emulsions at different microgel concentrations and different interfacial tensions shown as a function of the waiting time after preparation; and the effect of PEO, dextran, and 10 mM NaCl on the pH-dependent swelling of the microgels. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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DOI: 10.1021/la5049024 Langmuir 2015, 31, 3605−3611
