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Langmuir 1996, 12, 872-874
Aggregation Phenomena in Water-in-Oil Emulsions F. Leal-Calderon,† B. Gerhardi,† A. Espert,† F. Brossard,‡ V. Alard,‡ J. F. Tranchant,‡ T. Stora,† and J. Bibette*,† Centre de Recherche Paul Pascal, Av. A. Schweitzer, 33600 Pessac, France, and Laboratoire de physique, Parfums Christian Dior, 45804 St Jean de Braye Ce´ dex, France Received July 24, 1995. In Final Form: October 9, 1995X We report the phase behavior of monodisperse water-in-oil emulsion droplets. We show that depending upon the droplet size, the surfactant concentration within the continuous oil phase, the chemical nature of the oil, and the temperature, the emulsion droplets may remain dispersed or reversibly aggregated. Close to the phase transition threshold, we observe coexisting phases, whereas much above, we preferentially observe gels. Since such phase transitions exhibit the same trends than what was previously observed in direct oil-in-water emulsions, a size purification process based on the same principles is accessible in the coexisting regime. We present phase diagrams for two different droplet sizes that describe the main qualitative features of these systems.
Introduction Emulsions are commonly known to be metastable systems that are obtained by dispersing two immiscible fluids in the presence of surfactant. These systems are widely used in various industrial processes such as paint, coating, road surfacing, oil recovery, cosmetics, food, and medicine.1 Direct emulsions consist of oil droplets dispersed in an aqueous continuous phase. By contrast, inverse emulsions are made of water droplets in oil. In this paper, we establish some basic phenomenological rules that govern the aggregation of inverse emulsions. The role of excess surfactant is to induce a fluid-solid like phase transition. In the coexistence regime, we observe compact clusters in equilibrium with Brownian droplets. Such clusters are very reminiscent of those observed in direct emulsions in the presence of a water-soluble surfactant when above its critical micellar concentration.2 Since such a phase transition exhibits the same trends as previously observed in direct systems, we identically perform the same size fractionation3 and obtain a set monodisperse water droplets. These well-controlled systems are further used to explore the behavior of the inverse emulsion droplets in various oil continuous phases. We find that droplets stabilized with Span 80 (glycerol monooleate, Sigma) may be either aggregated or dispersed, depending upon the nature of the oil phase and the droplet diameter. Small water droplets in mineral oil (n-alkanes) lead more easily to dispersed systems whereas larger droplets in vegetable (fatty triglycerides), silicone, and some functionalized oils lead to gels made of strongly aggregated droplets. These observations are in agreement with the previously reported adhesion between large droplets revealed by the existence of contact angles in the same conditions.4 These gels are also very similar to those previously observed in direct emulsions stabilized with anionic surfactant in the presence of electrolytes.5 Moreover these gels are very stable against coalescence and lead to stable and highly viscoelastic materials that may contain more than 90% of oil. †
Centre de Recherche Paul Pascal. Laboratoire de physique, Parfums Christian Dior. X Abstract published in Advance ACS Abstracts, December 15, 1995. ‡
(1) Becher, P. Emulsions: Theory and Practice; Reinhold: New York, 1965. (2) Bibette, J.; Roux, D.; Pouligny, B. J. Phys. II 1992, 2, 401. (3) Bibette, J. J. Colloid Interface Sci. 1991, 147, 474. (4) Leemarkers, F. A. M.; Sdranis, Y. S.; Lyklema, J.; Groot, R. D. Colloids Surf. 1994, 85, 135. (5) Bibette, J.; Mason, T. G.; Gang, H.; Weitz, D. A. Phys. Rev. Lett. 1992, 69, 981. Bibette, J.; Mason, T. G.; Gang, H.; Weitz, D. A.; Poulin, P. Langmuir 1992, 8, 3180.
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Experimental Observations The initial concentrated inverse emulsion is prepared as reported in ref 6. A 2% magnesium sulfate aqueous solution is slowly added under shear to a mixture of oil and surfactant. Typically 80 g of electrolyte solution is added to a mixture of 12 g of oil and 8 g of surfactant. We use Span 80 as a low HLB1 surfactant and dodecane (Sigma) as the oil continuous phase. As reported,6 adding salt to the dispersed aqueous phase leads to extremely stable droplets in terms of coarsening processes. An emulsion made of droplets that range from 0.2 to 1 µm in diameter is easily obtained and remains stable for months in a concentrated state. However, in order to study the phase behavior, the emulsion has to be diluted to a droplet volume fraction of about 10%. Surprisingly, we find that in many different oils, most of the droplets remain aggregated whatever the water volume fraction is. However, in mineral oils (from octane to hexadecane) we find that after a few hours of settling, a milky solution coexists with a denser sediment. By contrast, we systematically find a complete aggregation and formation of a clear supernatant in many kinds of oils (vegetable, silicone, fatty esters). Finally, we note that when present, free droplets are essentially the smaller ones, i.e. below 0.5 µm in diameter. Since a certain amount of water droplets remain dispersed in dodecane, we take advantage of this property and dilute our concentrated initial emulsion to about 10% in volume. After several hours, sedimentation is achieved and we collect the dilute phase that contains the smaller droplets with a droplet volume fraction lower than 1%. The sediment was also diluted in dodecane, and since it did not exhibit any redispersibility, we did abandon it. In order to concentrate the Brownian droplets of the dilute phase, we use a classical centrifugation technique and obtain an emulsion of about 50% of water volume fraction. This emulsion is the starting material to which is applied the size fractionation technique. This technique was previuosly described for the case of direct emulsions3 and is identically used. Within several decantation steps, we were able to separate two different monodisperse emulsion droplets of diameters 0.21 and 0.51 µm (deduced from quasi-elastic light scattering measurements). The surfactant concentration used during these decantation steps ranges from 2 to 15 wt %; the critical micellar concentration of Span 80 in dodecane is (6) Aronson, M. P.; Petko, M. F. J. Colloid Interface Sci. 1993, 159, 134.
© 1996 American Chemical Society
Aggregation Phenomena in Water-in-Oil Emulsions
Figure 1. Monodisperse emulsion (0.21 µm droplet diameter) in the flocculated state due to depletion forces (12 wt % Span 80). Aggregates coexist with single Brownian droplets.
deduced from surface tension measurements and is found equal to about 0.03 wt %. Above this concentration, inverted micelles are present within the continuous oil phase, thus allowing micellar depletion forces to induce reversible flocculation and further sedimentation, as a required condition for the purification technique to be successfully applied. Above a certain surfactant concentration, the emulsion undergoes a fluid-solid like phase transition which leads to the coexistence of free droplets and dense clusters as shown in Figure 1. The water droplet volume fraction is 5%, the surfactant concentration is around 400 times the critical micellar concentration, the droplet diameter is 0.21 µm. This threshold concentration is reduced as the droplet diameter is increased due to the effect of depletion forces. Moreover, we find the same dependence that was reported in direct systems2 of the droplet volume fraction on the flocculation threshold as more evidence of the equilibrium phase transition induced by excess inverted micelles. These two samples are then exploited to characterize the role of the oil continuous phase in inducing aggregation. From our previous investigations, we learn that these droplets are redispersable in dodecane and remain aggregated in many different oils. Hence, we decided to identify and characterize the aggregation threshold as a function of the continuous phase composition, made of a mixture of dodecane and isopropyl myristate (Sigma). The aggregation threshold is identified for the two different sizes, both using direct microscopic observations and sedimentation of macroscopic samples. The system changes continuously from a Brownian emulsion to a gel within the addition of isopropyl myristate. For intermediate solvent compositions, we observe coexisting states. We also note that this threshold depends upon the droplet size. Indeed, aggregation is found at lower isopropyl myristate content as the droplet diameter is increased. In other words, a smaller amount of the so-called bad solvent is required for larger droplets to phase separate. Figure 2 shows a microscopic picture of an emulsion in which the solvent composition is much above the aggregation threshold. The bad solvent content is 70 wt %. As shown by this picture, the shape of those clusters is very tenuous. These clusters are very reminiscent of those observed in direct emulsions undergoing gelation upon addition of electrolytes.5 Such an observation suggests that the
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Figure 2. Ramified clusters observed in the presence of a great amount of bad solvent (0.21 µm droplet diameter, 70 wt % isopropyl myristate, 2 wt % Span 80).
Figure 3. Phase diagrams obtained for two monodisperse emulsions. The surfactant concentration is always greater than 2 wt % in order to ensure stability against coalescence: O, Brownian droplets; y, coexistence between aggregated and free Brownian droplets; b, total aggregation (gel).
attractive interaction between water droplets induced by the addition of the second solvent may become much larger than thermal energy. Above 1% in droplet volume fraction, the gel rapidly forms after shaking and further contracts upon the effect of gravity. However, above roughly 30%, the contraction is very slow (days). Figure 3 shows the phase diagrams obtained when both the surfactant and the second solvent (isopropyl myristate) contents are varied. The water droplet volume fraction is kept constant at 5%. The lines are guide to the eyes and give the form of the stability domains in terms of flocculation. The continuous line corresponds to the onset
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Figure 4. Influence of the temperature on the onset of phase transitions.
of phase transition which leads to coexisting states. The broken line corresponds to virtual complete aggregation conditions. Figure 4 shows the role of temperature on the onsets of phase transitions. As observed on that phase diagram, lowering the temperature from 25 to 5 °C shifts the solventinduced transition toward smaller amounts of the so-called bad solvent. The surfactant-induced transition is essentially not sensitive to the temperature even though we were able to detect a shift of about 1% for a change in temperature of 20 °C.
Leal-Calderon et al.
agreement with what was reported previously concerning direct emulsions.2 Similar conclusions about depletion flocculation of water droplets by swollen micelles in microemulsions have been reported.7 The microscopic mechanism of the interaction driven by the presence of the second solvent is more speculative. Even though this interaction obviously originates in the mixing preference of the surfactant chains for the solvent mixture or for themselves, the microscopic mechanism remains obscure. The presence of critical points in inverse micellar solutions8 and the aggregation of grafted silica particles immersed in organic solvents9 are certainly due to similar effects; large contact angles in inverse emulsions have been attributed to the solvent depletion within the thin adhesive film.4 From empirical considerations, we deduce that surfactant chains mix more easily with mineral oil than with vegetable, silicone, and some functionalized oils. However, the size dependence of such a mechanism, reflected by the shifts in the phase transition thresholds, is certainly due to the increasing contact surface between larger droplets. We checked that the size dependence of such an interaction allows also performance of nice and efficient size fractionation on the same basis that was originally achieved by using depletion forces.2 Conclusion
The microscopic mechanism of the two kinds of interaction is now discussed. The one induced by the excess surfactant concentration possesses all the characteristics of a micellar depletion mechanism. As shown in Figure 3, the phase transitions are governed by both the surfactant concentration and the droplet diameter. This property is in agreement with a depletion mechanism and supports the possibility of purifying in sizes a crude polydisperse inverted emulsion. Moreover, the strength of such a depletion potential may be easily evaluated at the onset of flocculation for the three different samples. For two particles in contact, its expression is:
The microscopic mechanism leading to inverted emulsion gels is an open question. The microscopic origin of this phenomenon is currently explored and may have some implications in bilayers properties. Indeed, adhesive water droplets lead to the formation of surfactant bilayer separating aqueous domains. The liquid nature of the interface and the intrinsic metastability of emulsions may also lead to some new and unexplored aspects of surfactants bilayers. As an example, the surprising stability against coarsening shown by these aggregates is a puzzling challenge since they form in the presence of a so-called bad solvent which should be inefficient in stabilizing the interfaces. Moreover, their extensive industrial use in both cosmetic and food industry might be served by this guidance.
dd 3 µc ) - φm kT 2 dm
Acknowledgment. The authors thank C. Dior Company for partial financial support.
Discussion
where φm is the micelle volume fraction and dd and dm are the droplet and the micellar diameters, respectively. We find a value for the contact potential at the phase transition threshold which is about 5kT for the two samples (in pure dodecane), assuming a micellar diameter of 50 Å. This gives a fairly good certitude for the origin of this transition. Moreover the structure of the aggregates are also in perfect
LA950615N (7) Binks, B. P.; Fletcher, P. D. I.; Horsup, D. I. Colloids Surf. 1991, 61, 291. (8) Huang, S.; Safran, S. A.; Kim, M. W.; Grest, G. S.; Kotlarckyk, M.; Quirke, N. Phys. Rev. Lett. 1984, 53, 592. (9) Jansen, J. W.; De Kruif, C. G.; Vrij, A. J. Colloid Interface Sci. 1986, 114, 471.
