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Stability of Emulsions with Nonadsorbing Polymers Amit Meller and Joel Stavans* Department of Physics of Complex Systems, Weizmann Institute of Science, Rehovot 76100, Israel Received June 5, 1995. In Final Form: August 30, 1995X We present an experimental study of the effects of hydrophilic, nonadsorbing polymers on the stability of monodisperse oil-in-water emulsions. Above some threshold polymer concentration, the chains induce droplet segregation into fluid and solid phases due to the depletion effect. At higher polymer concentrations, polymers induce the formation of gel-like structures of droplets. Phase diagrams of droplet volume fraction versus polymer concentration are presented. At large droplet volume fractions where droplets are deformed, the polymers enhance considerably the stability of the emulsion against droplet coalescence. We compare our results with recent experiments and models of colloid-polymer mixtures.
I. Introduction Emulsions are dispersions of oil droplets in water (O/ W) or vice versa (W/O) and a surface active agent adsorbed at the oil-water interface.1 Droplets do not form spontaneously when these three ingredients are put together. Rather, their formation entails an expenditure of mechanical energy such as vigorous mixing or sonication to compensate for the cost of increasing the total interface area between the two liquids. Emulsions are therefore inherently metastable: when two droplets approach one another to a sufficiently small distance they may coalesce, thereby reducing the total interface area. This results in the eventual macroscopic separation of the oil and water. Besides droplet coagulation which is irreversible, emulsions exhibit reversible instabilities as a result of interdroplet interactions, as colloidal dispersions of hard particles. Intermolecular potentials can under certain conditions become attractive causing droplets to form flocs.2 For high droplet volume fractions φ, a cream is then formed while for low values of φ gels are obtained.3 Creaming and gel formation may also be effected by attractive depletion forces4 between oil droplets induced by surfactant micelles in solution.5 In fact, this process forms the basis of a fractionation method of preparing nearly-monodisperse O/W emulsions.6 In this paper we present a study of the stability behavior of nearly-monodisperse O/W emulsions upon the addition of nonionic hydrosoluble polymers to the water phase. Our results show that depletion forces due to the polymers induce creaming above a φ-dependent threshold polymer concentration and the formation of gel-like structures composed of large interconnected droplet flocs across the whole sample for larger polymer concentrations. The phase diagrams we obtain are very similar with those of other heterogeneous systems such as colloid-polymer7 and colloid-surfactant8 mixtures. The comparison of the X Abstract published in Advance ACS Abstracts, December 15, 1995.
(1) See for example: Becher, P. Emulsions; Theory and Practice, Rheinhold Publishing Corp.: New York, 1965. (2) Aronson, M. P. Langmuir 1989, 5, 494. (3) Bibette, J.; Mason, T. G.; Gang, Hu; Weitz, D. A. Phys. Rev. Lett. 1992, 69, 981. Bibette, J.; Mason, T. G.; Gang, H.; Weitz, D. A.; Poulin, P. Langmuir 1993, 9, 3352. (4) Asakura, S.; Oosawa, J. J. Polym. Sci. 1958, 32, 183. Vrij, A. Pure Appl. Chem. 1976, 48, 471. (5) Bibette, J.; Roux, D.; Nallet, F. Phys. Rev. Lett. 1990, 65, 2470. (6) Bibette, J. J. Colloid Interface Sci. 1991, 147, 474. (7) Poon, W. C. K.; Selfe, J. S.; Robertson, M. B.; Illet, S. M.; Pirie, A. D.; Pusey, P. N. J. Phys. II 1993, 3, 1075. Leal Calderon, F.; Bibette, J.; Biais, J. Europhys. Lett. 1993, 23, 653. Illet, S. M.; Orrock, A.; Poon, W. C. K.; Pusey, P. N. Phys. Rev. E 1995, 51, 1344. (8) Piazza, R.; Di Pietro, G. Europhys. Lett. 1994, 28, 445.
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behavior we observe with that shown by these systems, and with theoretical models devised to describe them,9 is possible through our use of nearly-monodisperse emulsions. In contrast, previous investigations of the stability of mixed polymer-emulsion systems10 used polydisperse emulsions (polydispersity in itself may induce creaming as recent studies of emulsions consisting of droplets of two different sizes reveal11 ). We have also investigated how polymers affect the stability of our emulsions by studying their response to an applied osmotic stress. Our results provide strong evidence to the fact that polymers enhance considerably the stability of the emulsions against coalescence. II. Experimental Methods and Materials Raw emulsions were prepared from silicon oil, sodium dodecyl sulfate (SDS) and distilled, deionized water using the classical inversion method.1 The oil and surfactant were purchased from Fluka and Sigma respectively. The raw emulsion was then fractionated into nearly-monodisperse emulsions following Bibette’s method.6 Two stock emulsions of volume fraction φ ) 0.35, one consisting of 0.93 µm droplets and the other of 1.1 µm droplets, both with a polydispersity of 0.1 were then selected. Droplet size and polydispersity were measured by dynamic light scattering. Our emulsions show many similarities with the behavior of colloidal dispersions of particles interacting via a nearly hard-sphere potential.12 In particular they form crystalline structures as shown by their iridescence when indexmatched, at values of φ very near to 0.49, the value at which colloidal crystals of nearly hard-sphere particles form. The thermally-induced fluctuations of the droplets’ surface, whose relaxation time (of the order of 10-6 s) and amplitude (ca. 10 Å) are small,13 are important only at high volume fractions and do not affect the behavior we observe. All our emulsions were dialyzed against an SDS concentration of 1.5 critical micelle concentration. Samples were prepared by diluting the stock emulsion with ternary solutions of water, SDS, and poly(ethylene oxide) (PEO), a hydrophilic polymer purchased from Aldrich. The solutions were prepared with the same surfactant concentration as that of the stock emulsion and different polymer concentrations cp varying from 10-5 to 10-3 by weight. We used PEO chains of average molecular weight 100 000 and 900 000 (the radius of gyration Rg of our polymers is Rg ) 12.6 ( 0.7 nm (9) Gast, A. P.; Hall, C. K.; Russel, W. B. J. Colloid Interface Sci. 1983, 96, 251. Lekkerkerker, H. N. W.; Poon, W. C. K.; Pusey, P. N.; Stroobants, A.; Warren, P. B. Europhys. Lett. 1992, 20, 559. (10) Robins, M. M. In Microemulsions and Emulsions in Foods; ElNokaly, M., Cornell, D., Eds.; ACS Symposium Series 448; American Chemical Society: Washington, DC, 1991; Chapter 17. Fillery-Travis, A. J.; Bunning, P. A.; Hibberd, D. J.; Robins, M. M. J. Colloid Interface Sci. 1993, 159, 189. (11) Steiner, U.; Meller, A.; Stavans, J. Phys. Rev. Lett. 1995, 74, 4750. (12) See for example: Pusey, P. N.; van Megen, W. Nature 1986, 320, 340. (13) Gang, H.; Krall, A. H.; Weitz, D. Phys. Rev. Lett. 1994, 73, 3435.
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for the short chains, and Rg ) 47.2 ( 2.6 nm for the long chains, as determined from light scattering measurements). The 0.93 µm droplets were used with the small chains while the 1.1 µm droplets were used with the long chains. The corresponding values for the ratio ξ between Rg and the droplet radii are 0.027 ( 0.002 and 0.086 ( 0.006, respectively. We note that PEO couples weakly to SDS molecules in solution and that the change in conformation of the polymer chains due to the adsorption of surfactant micelles remaining in the aqueous phase is negligible.14 To map out the different phases of the system on the (φ,cp) plane, we prepared sample series of constant φ with varying cp by vigorous mixing. The state of the samples was monitored over time by time-lapse video. In all our experiments the typical time scale for phase separation turned out to be much shorter than the time scale for gravity-induced creaming. The composition of the samples was determined 2 days after their preparation (at this time no appreciable sedimentation occurred in the control sample with cp ) 0). The volume fraction of the upper (droplet rich) phase was determined by weighing before and after the evaporation of water in an oven. The polymer concentration in the lower phase was determined by first centrifuging to remove all oil droplets and then by comparing the scattered intensity of the polymer solution with samples of known composition. The value of φ in the lower phase and the polymer concentration in the upper phase were then calculated from the known overall volume fraction and polymer concentration, respectively. Our preliminary characterization of the gel phase obtained at high polymer concentrations was performed by small angle light scattering and optical microscopy. A partially index-matched sample with urea in the water phase (in order to avoid multiple scattering effects) was introduced into a cell 100 µm thick, and a series (120) of speckle patterns spanning the range 0.5-16° were recorded on a screen with a CCD camera. The patterns were averaged to extract the intensity of scattered light as a function of the angle. To probe the stability of dense emulsions against coalescence in the presence of polymers, we have applied osmotic pressure to our emulsions. As in previous studies,15 the osmotic stress technique16 was used in conjunction with optical microscopy to assess whether coalescence occurs or not. Each sample was enclosed in a dialysis bag and immersed in a large reservoir containing a mixture of water, SDS, and hydrophilic polymer (dextran, MW 500 000 purchased from Fluka). The dialysis bag, with molecular weight cut-off of 50 000, prevented both polymers and oil droplets from passing while letting the water and surfactant molecules pass relatively unhindered. The concentration of SDS in the reservoir was matched to that of the unstressed emulsion, and chemical equilibrium was attained a short time after the beginning of the experiment. The immersed samples reached a steady state after two days; nevertheless they were left inside the immersion medium for 4 days.
Results The typical behavior we observe is portrayed in Figure 1, where we show snapshots of a series of samples prepared with the same value of φ ) 0.25 and variable cp at different times. For cp ) 0 (sample a) or small values of cp, samples are in a homogeneous fluid phase and barely sediment due to gravity over the time of the run. Above a threshold value ccp(φ), the samples phase separate into a dropletrich, supernatant phase and a droplet-poor phase (samples b-e). As cp is increased above ccp(φ), the degree of droplet segregation is larger, the lower phase becoming virtually transparent. At the same time a reentrant behavior is observed: the compactivity of the droplet-rich phase decreases with increasing cp. This trend continues until further increases of cp do not induce visible phase separation. In fact, at longer times sample e shows a much slower gravity-induced creaming than samples with no polymer. The rate of creaming is even slower than (14) Cabane, B.; Duplessix, R. J. Phys. (Paris) 1982, 43, 1529. (15) Bibette, J.; Morse, D. C.; Witten, T. A.; Weitz, D. A. Phys. Rev. Lett. 1992, 69, 2439. (16) Parsegian, V. A.; Rand, R. P.; Fuller, N. L.; Rau, D. C. Methods Enzymol. 1986, 127, 400.
Figure 1. Snapshots at different times during the evolution of a series of samples with fixed droplet volume fraction φ ) 0.25 and varying cp. The first snapshot (top) was taken just after sample preparation whereas the other snaphots were taken 1, 3, and 17 h (top to bottom) after the preparation of the samples.
Stability of Emulsions
Figure 2. Scattered intensity I(q) at small angles for emulsions with polymer (a) and without polymer (b). The solid line in the bottom part is the Rayleigh-Gans form factor of the droplets.
expected by the increase in viscosity due to the polymers. To understand the origin of this behavior, we performed small-angle light scattering measurements on a partially index-matched sample at φ ) 0.15 in this regime of cp. We show the results in Figure 2a, where we plot the intensity of scattered light I(q) as a function of wavevector q ) (4πn/λ) sin(θ/2). The main feature is a broad peak whose maximum at qmax ) 0.35 µm-1 corresponds to a characteristic length scale d ) 2π/qmax ) 20 µm. In contrast, no such peak is observed when light is scattered by a sample with the same value of φ but without polymer (Figure 2b). In fact, in this case I(q) can be fitted well by the RayleighGans form factor of the droplets (solid line). The fit yields 845 ( 5 nm for the diameter of the droplets in agreement with the dynamic light scattering measurements. The origin of the characteristic length scale d becomes clear upon close scrutiny of the sample under a microscope. This reveals a structure which is far from homogeneous shown in Figure 3a. Droplets form a gel-like structure of interconnected flocs, very similar to gels of adhesive emulsions.3 The typical size of the flocs observed in the picture is about 25 µm, in good agreement with the light scattering measurements. For comparison, we show in Figure 3b the homogeneous structure of an emulsion with the same value of φ but without polymer. Further information about the nature of the gels is furnished by their response to an applied osmotic stress. In Figure 4 we plot φf , the value of the volume fraction attained after applying a given osmotic pressure π on emulsions in the gel phase in the case of long polymers. Also shown for comparison are the results for an emulsion without polymer. The salient features in the figure are as follows: First, under the smallest applied pressures, emulsions reach values of φ above that of a closed-packed
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Figure 3. Photomicrographs of emulsions at φ ) 0.15 corresponding to the same conditions as in Figure 2: (a, top) emulsion gel obtained at cp ) 7.7 × 10-4; (b, bottom) emulsion without polymer in a fluid state.
Figure 4. Volume fraction φf attained by emulsions with different polymer concentrations cp, after being subjected to an osmotic pressure π for 4 days. The data correspond to cp ) 0.00 (full squares), cp ) 7 × 10-4 (full circles), and cp ) 9.5 × 10-4 (empty squares).
configuration of spheres, and thus droplets are by necessity deformed. For larger values of π, φf levels off at about 0.9, and further compression in such a highly packed and deformed droplet structure is hard to attain. Examination of the droplets with the microscope reveals that even at this high state of compression and values of π reaching 0.5 atm, droplets still keep their identity and the dense emulsion can be redispersed by dilution with no change in polydispersity. In contrast, for an emulsion without polymer, droplet coalescence is already observed at about 0.12 atm, a fact which precludes measurements above this value. Furthermore note that this value is consistent with the one obtained by Bibette for droplets of the same size.6 The observed polymer-induced enhancement of emulsion stability against coalescence can be rationalized if one assumes that polymer chains remain in the thin films separating the oil droplets. This assumption is consistent
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a
Figure 5. Polymer concentration cpeq in equilibrium with an applied osmotic pressure π in an emulsion (full circles) and in a polymer-water solution (empty circles).
both with polymer partitioning in depletion-induced phase separating polymer-colloid systems7,9 and with recent dynamic light scattering studies of free-standing SDS soap films containing nonionic polymers17 (note that these films have essentially the same structure as the films separating the oil droplets in our emulsions). In particular, these latter studies have shown that the relaxation time of the peristaltic thickness fluctuations believed to be responsible for film rupture is increased by the presence of polymers in the film core, and therefore these fluctuations are hindered. Furthermore, the polymers squeezed between the droplets exert a repulsive force of steric character, preventing the approach of the droplet interfaces and therefore film instability. Going back to Figure 4, it might be hypothesized that the polymer chains left out in the voids outside the compact droplet network which constitutes the gel are responsible for balancing out the osmotic pressure exerted by the dextran reservoir, thereby determining φf. This is in fact not so. The external osmotic pressure is offset by the disjoining pressure exerted by the films between droplets, together with elastic forces due to droplet deformation. To justify this claim, we calculate from the values of φf and π in Figure 4 the value of the equilibrium polymer concentration cpeq(π) in the voids (the amount of polymer remaining within the films is negligible) and compare it to the equilibrium value of cp reached when a polymer/ water solution prepared with cp ) 1 × 10-3 and the same SDS concentration as in our emulsions is subjected to an applied osmotic pressure. This value of cp is already higher than the polymer concentration in the voids before subjecting the emulsions to an osmotic pressure. We plot in Figure 5 the results. The value of cpeq in the emulsion is consistently smaller than the polymer concentration in the polymer/water solution by a factor of nearly 5 for any given value of π. Thus the compressibility of the polymer/ water solution is much larger than that of the emulsion, and clearly the contribution of the polymers in the voids toward balancing the pressure exerted by the reservoir is not significant. The full phase behavior of the emulsion-polymer mixtures is contained in the (φ,cp) planes shown in Figure 6, for long (top) and short (bottom) polymer chains, respectively. The solid lines in each phase diagram, which are guides to the eye, separate one-phase regions from an unstable region where samples phase separate. The stable phase in the left side of the diagrams is a fluid one whereas the stable phase in the right side corresponds to a solid one. In fact, the solid phase has a crystalline structure as evidenced by the iridescent colors it exhibits when index matching our emulsions with urea. The dashed curves, (17) Krichevsky, O.; Stavans, J. Phys. Rev. Lett. 1994, 73, 696.
b
Figure 6. Phase diagrams of polymer concentration cp as a function of droplet volume fraction φ for size ratios 0.086 ( 0.006 (a) and 0.027 ( 0.002 (b). The different states of our samples are liquid (empty circles), solid (full triangles), and gel (empty diamonds). The solid squares represent samples that phase separate. The droplet diameter is 1.1 µm (a) and 0.93 µm (b). Solid and dashed lines are guides to the eye and denote respectively the onset of phase separation and the values of cp above which gravity-induced settling is slower than samples without polymer.
also guides to the eye, denote the values of cp of samples whose gravity-induced creaming is slower than emulsions without polymer. Above these lines we observe the interconnected gel-like structures across the samples described previously. While phase separation is observed at slightly larger values of cp in the case of long chains, the gel phase appears at considerably larger values of values of cp in the case of the short chains. Note that for both diagrams polymer concentrations are well below the onset of the semidilute regime (the polymer overlap concentrations are c* ) 4 × 10-3 and 14 × 10-3 g/cm3 for the long and short chains, respectively). These phase diagrams share many features in common with those of colloid-polymer mixtures.7,9 In particular, our phase diagram for ξ ) 0.086 ( 0.006 compares favorably with the experimental diagram published by Poon et al.7 for ξ ) 0.08 and the theoretical phase diagram of Lekkerkerker et al.,9 even though absolute sizes differ by nearly a factor of 3. Near the onset of phase separation, the values of the volume fraction occupied by the polymer chains, regarded as spheres of radius Rg differ by a factor of less than 2 in both cases. This discrepancy can be accounted for by the larger degree of polydispersity of our polymers. We intend to carry out studies of the crystalline structure of the solid phase, larger size ratios, and small-angle light scattering studies of the depletion potential in the near future. We thank U. Steiner and O. Krichevsky for helpful discussions. A.M. acknowledges support from the Israeli Ministry of Arts and Sciences. This study was supported by Grant No. 92-00093/2 from the United States-Israel Binational Science Foundation (BSF), Jerusalem, Israel, and the Minerva Foundation. LA950440H
