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Spontaneous Emulsification of Oil in Aerosol-OT/Water/ Hydrocarbon Systems Taisei Nishimi† and Clarence A. Miller* Department of Chemical Engineering Rice University (MS 362), P.O. Box 1892, Houston, Texas 77251-1892 Received May 5, 2000. In Final Form: August 30, 2000 Drops containing a hydrocarbon, the anionic surfactant Aerosol-OT (AOT), and water were injected into water at 30 °C, and the resulting behavior was observed by videomicroscopy. Spontaneous emulsification of most of the injected oil yielding droplets a few microns in diameter was seen when n-octane was the hydrocarbon and when the initial drop contained at least 25 wt % AOT and no more than about 10 wt % water. Emulsification was also observed when the initial drop contained small amounts of NaCl solution instead of water and for suitable conditions when n-hexane was the hydrocarbon. However, much less emulsification occurred for n-decane and n-dodecane, probably because surfactant-rich phases in equilibrium with excess oil in these systems solubilized little hydrocarbon. The mechanism of emulsification is closely related to that of an earlier study by Rang and Miller (Prog. Colloid Polym. Sci. 1998, 109, 101) using drops containing n-hexadecane, the pure nonionic surfactant C12E6, and n-octanol. In both systems diffusion produced inversion from an oil-continuous to a water-continuous phase, leading to emulsification at locations where supersaturation in oil occurred. One difference in the present case was that inversion was not continuous but involved formation of the lamellar liquid crystalline phase. Another was that the shift to more hydrophilic conditions leading to inversion was caused by decreases in ionic strength of water in the injected drop which, in turn, was produced by diffusion of water into the drop and of AOT and, in some cases, NaCl out of the drop. Owing to this ionic strength effect, nearly complete emulsification to small oil droplets was observed in some ternary AOT/hydrocarbon/water systems in the present study but was not possible in ternary nonionic surfactant/hydrocarbon/water systems investigated previously.
Introduction When emulsions are formed by mixing oil and water phases, high shear rates are typically required to generate small drops having diameters on the order of 1 µm. When it is necessary or desirable to obtain small drops without high shear rates, spontaneous emulsification should be considered. Compositions of the initial oil and water phases are chosen so that small droplets form spontaneously when the phases are brought into contact, that is, no external energy of agitation is required. “Self-emulsification” refers to situations in which a small amount of agitation is supplied to achieve gentle mixing. Typically the droplets form spontaneously, and mixing serves mainly to disperse them throughout a large volume and to bring together portions of the oil and water phases that were not in contact initially. Self-emulsification of oils containing dissolved solutes is not only an intriguing phenomenon but is also of practical interest in the delivery of agricultural chemicals and drugs and during the use of cutting oils, bath oils, etc. A surfactant or a mixture of surfactants is added to the oil, so that it will emulsify spontaneously when contacted with water. The challenge is to understand the mechanism of spontaneous emulsification, so that a suitable surfactant or surfactant mixture can be chosen and its concentration in the oil optimized. Earlier literature on self-emulsification was summarized in previous articles by our group.1,2 One prior study found that low interfacial tension was not an adequate explanation for the phenomenon.3 Others indicated that phase behavior, especially formation of † Present address: Fuji Photo Film Co., Ltd., Ashigara Research Laboratories, 210, Nakanuma, Minami-Ashigarashi, Kanagawa 250-0193, Japan. * To whom correspondence should be addressed.
lyotropic liquid crystalline phases, was connected with the emulsification process, but the actual mechanism was not clarified.4,5 Rang and Miller1 studied self-emulsification of n-hexadecane/C12E6/n-octanol drops some 100 µm in diameter injected into water. Spontaneous emulsification yielding only small oil droplets was observed when the initial ratio of alcohol to hydrocarbon was greater than that in the excess oil phase in equilibrium with a microemulsion and water at the temperature of interest, that is, above the ratio existing at the phase inversion temperature (PIT), and when surfactant concentration was sufficiently high. The mechanism of emulsification is shown schematically in Figure 1. Initially water diffused rapidly into the oil phase, converting it to an oil-continuous microemulsion. However, as octanol diffused gradually into the aqueous phase, the ratio of alcohol to surfactant in the films covering the microemulsion droplets decreased, making them more hydrophilic and leading to an increased capability of the microemulsion to solubilize water and a decreased capability to solubilize oil. Eventually, the microemulsion was no longer able to solubilize all the oil present, and oil droplets nucleated. Moreover, the microemulsion itself inverted to become water-continuous and miscible with water, so that the final state was oil droplets dispersed in an aqueous phase, the size distribution of the droplets depending largely on their rate of coalescence. When enough surfactant was present, coa(1) Rang, M. J.; Miller, C. A. Prog. Colloid Polym. Sci. 1998, 109, 101. (2) Rang, M. J.; Miller, C. A. J. Colloid Interface Sci. 1999, 209, 179. (3) Lee, G. W. J.; Tadros, T. F. Colloids Surf. A 1982, 5, 105, 117, 129. (4) Groves, M. J. Chem. Ind. 1978, 17, 417. (5) Wakerly, M. G.; Pouton, C. W.; Meakin, B. J.; Morton, F. S. In Phenomena in Mixed Surfactant Systems; Scamehorn, J. F., Ed.; American Chemical Society: Washington, D. C., 1986; p 242.
10.1021/la0006521 CCC: $19.00 © 2000 American Chemical Society Published on Web 10/31/2000
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Figure 1. Schematic diagram showing mechanism of spontaneous emulsification for a drop of n-hexadecane containing suitable amounts of pure C12E6 and n-octanol.1
lescence was relatively slow, and the droplets remained small. No intermediate liquid crystalline phases were formed for this system. However, an intermediate lamellar phase was seen for systems in which the surfactant was tetradecyldimethylamine oxide (C14DMAO). Emulsions with small oil droplets were obtained with lower surfactant concentrations in this system, apparently because the lamellar phase reduced the rate of coalescence of the droplets. Nevertheless, both the liquid crystal and the microemulsion eventually became miscible with water as they lost alcohol by diffusion into the aqueous phase. This mechanism of emulsification may be summarized as causing the microemulsion to become supersaturated in oil by shifting it from lipophilic to hydrophilic conditions. However, achieving this behavior by diffusion of a mediumchain alcohol into the aqueous phase may not be desirable for some purposes. Also use of ionic surfactants instead of nonionic or zwitterionic surfactants may have advantages. For example, electrical repulsion may help stabilize the emulsion once formed. Accordingly, the present work uses the anionic surfactant Aerosol OT (AOT) with no alcohol and effects the shift toward hydrophilic conditions by a reduction in ionic strength. This reduction is produced by a combination of water diffusion into the microemulsion and of AOT (and, in some cases, NaCl) diffusion into the aqueous phase. The emulsions formed do not exhibit rapid coalescence rates although it is not clear whether this behavior is due to electrical repulsion between drops, the intermediate lamellar liquid crystalline phase formed during the emulsification process, or both. AOT was chosen because it has several favorable properties. First, it exhibits a high solubility in alkanes. Second, its double-chain structure makes it sufficiently lipophilic to produce emulsification without the addition of alcohols or cosurfactants. Indeed, emulsification can be produced in some ternary AOT/alkane/water systems, as shown below. Finally, AOT is available in pure form, and numerous studies of its equilibrium phase behavior, important in interpreting our videomicroscopy studies of emulsification, are available. Kunieda and Sato6 investigated phase behavior of the AOT/ benzene/water system. An important feature of Figure 2, their phase diagram at 50 °C, is that an oilin-water microemulsion Wm is seen at low AOT concentrations and a water-in-oil microemulsion Om is seen at high AOT concentrations. This change in microstructure occurs because adding AOT increases ionic strength of the system, making it more lipophilic and changing the sign of the spontaneous curvature of the surfactant films that separate oil from water. Also, a lamellar phase which
Figure 2. Phase diagram for AOT/benzene/water at 50 °C.6
can solubilize considerable amounts of oil is present between Wm and Om phases. Although they did not determine the complete ternary diagram when 2,2,4-trimethyl pentane (isooctane) was the oil, they did show that it had the same basic form as Figure 2. However, the corresponding phase boundaries were at higher AOT concentrations for isooctane at 30° than for benzene at 50°, that is, the former system was more hydrophilic and required a higher ionic strength (more AOT) to reverse the spontaneous curvature. One would expect phase behavior for n-octane, which was used in several of the experiments described below, to be similar to that for isooctane based on previous studies of microemulsion systems. Addition of NaCl also increases ionic strength and makes the system less hydrophilic. Kunieda and Sato6 and Maugey and Bellocq7 studied the effect of adding NaCl when the oil was isooctane. Kunieda and Shinoda8 presented information on phase behavior of AOT/alkane/ NaCl brine systems as a function of salinity and temperature. Ghosh and Miller9 provided additional information, especially on coexistence of a dilute lamellar phase with oil at low salinities. Kellay et al 10 discussed the conditions when the lamellar phase or isotropic surfactantrich phases coexisted with excess oil and NaCl brine for various alkanes at 20 °C and low AOT concentrations. They found a lamellar phase that solubilized considerable hydrocarbon for n-octane, a lamellar phase that solubilized (6) Kunieda, H.; Sato, T. Yukagaku 1979, 28, 627. (7) Maugey, M.; Bellocq, A. M. Langmuir 1999, 15, 8602. (8) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1980, 75, 601. (9) Ghosh, O.; Miller, C. A. J. Phys. Chem. 1987, 91, 4528. (10) Kellay, H.; Binks, B. P.; Hendrikx, Y.; Lee, L. T.; Meunier, J. Adv. Colloid Interface Sci. 1994, 49, 85.
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Figure 3. Video frames for a drop with proportions 60/30/10 by weight of n-octane/AOT/water injected into water at 30 °C. Times are indicated in the upper part of the frame (hours:minutes:seconds:frame).
much less hydrocarbon for n-decane, and an isotropic (sponge) phase that solubilized minimal hydrocarbon for n-dodecane. Ghosh and Miller,9 working at 30 °C and higher surfactant concentrations, also found low solubilization for the n-dodecane system. Experimental Section Aerosol OT [AOT; sodium bis(2-ethylhexyl) sulfosuccinate] was obtained from American Cyanamid (now Cytec). Sources for the hydrocarbons, all at least 99% pure, were Humphrey Chemical Co. for n-octane and n-dodecane, TCI America for 2,2,4-trimethyl pentane (isooctane), Aldrich for n-decane, and Baxter for nhexane. Sodium chloride was reagent grade. Water was distilled and deionized with a SYBRON Barnstead glass still and Nanopure II system. The emulsification process was studied by videomicroscopy. A small drop of hydrocarbon containing AOT and sometimes solubilized water or NaCl solution was injected into a rectangular glass capillary cell having a thickness of 400 µm and nearly filled with water or NaCl brine. A Mettler thermal stage maintained constant temperature. The injection procedure was different from that used previously1 in that drops were injected through glass micropipets with radii between 30 and 100 µm instead of through a thin hypodermic needle with an outside diameter of 210 µm. The micropipets were pulled by gravity from capillaries with outside diameters of 1 mm. The use of micropipets with a gas injection system (Picospritzer II, Parker Hannifin Company) assured reproducibility of the volume of liquid injected. A few experiments were performed with the same cells but in a microscope with a vertically oriented stage as in previous work.11 In this case, the oil was carefully layered on water or a NaCl solution and development of intermediate phases followed as a function of time.
Results Ternary AOT/n-Octane/Water Systems. Figure 3 presents a series of videoframes showing the behavior observed when a drop containing proportions 60/30/10 by weight of n-octane/AOT/water was injected into water at 30 °C. Upon injection the drop became cloudy and highly nonspherical. In addition, its surface appeared rough and exhibited numerous “bulges” (Figure 3a,b), which grew rapidly outward and then just as rapidly shrank, leaving (11) Mori, F.; Lim, J. C.; Raney, O. G.; Elsik, C. M.; Miller, C. A. Colloids Surf. 1989, 40, 323.
behind small oil droplets (diameters of about 1 µm) in the aqueous phase and possibly some vesicles or particles of the lamellar phase. The droplets moved radially outward across the lower part of the cell’s 400-µm thickness (Figure 3b-d). At the same time, oil that had not been emulsified and remained in the upper part of the cell experienced a radially inward flow to form a drop that was larger than the emitted droplets but much smaller than the initial drop (Figure 3c,d). This drop remained at the end of the experiment along with many small oil droplets but no visible particles of the lamellar phase. Similar behavior was seen for injected drops that initially contained 10 wt % water and various amounts of AOT between 25 wt % and 65 wt %. However, when initial AOT content in the drop was reduced to 20 wt % or less in the absence of water or with 10 wt % water, convection, presumably driven by Marangoni flow, was observed in the vicinity of the drop but little emulsification. The behavior of drops containing 21 wt % AOT and 5.3 wt % water was interesting. Minimal emulsification was observed for small drops. However, substantial emulsification was seen for large drops. Evidently this AOT concentration is very near the minimum required to emulsify a large portion of the oil. The initial water content of the injected drop had a significant influence on behavior. With AOT concentration in the drop maintained at 25 wt %, vigorous emulsification was seen when it contained no water or 10 wt % water. However, no significant emulsification was observed for drops containing 25 wt % water. Experiments with the vertical stage microscope were also conducted in which n-octane/AOT/water mixtures were carefully placed on water and behavior near the interface observed. An intermediate lamellar liquid crystalline phase formed just below oils with proportions 60/ 30/10 and 70/20/10 by weight. Beneath it a layer of a concentrated oil-in-water emulsion was observed. For an oil containing only 10 wt % AOT, that is, having proportions 80/10/10, some emulsion was seen but no liquid crystal. Effect of NaCl. For oil drops containing small amounts (10 wt %) of water, emulsification of oil was promoted by replacing the water with NaCl brine. For example, a drop containing proportions 70/20/10 by weight of n-octane/
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Figure 4. Video frames for a drop with proportions 70/20/10 by weight of n-octane/AOT/water injected into 0.25 wt % NaCl at 30 °C. Times are as indicated.
Figure 5. Video frames for a drop with proportions 70/20/10 by weight of n-octane/AOT/water injected into 0.50 wt % NaCl at 30 °C. Times are as indicated.
AOT/water exhibited only slight emulsification, as indicated previously, whereas a drop with the same amount of 0.75 wt % NaCl instead of water showed extensive emulsification comparable with that of Figure 3. Emulsification was also seen for drops of 60/30/10 mixtures injected into water for NaCl concentrations in the solubilized water ranging from 0 to 10 wt % (the highest value investigated). A small drop of oil that had not been emulsified, similar to that shown in Figure 3d, remained at the end of these experiments. In contrast, adding NaCl to the external aqueous phase inhibited emulsification, at least initially. When a 70/ 20/10 drop without NaCl was injected into 0.25 wt % NaCl, myelinic figures of the lamellar liquid crystalline phase were observed growing outward from the drop surface (Figure 4a,b). Approximately 1.5-2 min after injection, the myelins had largely disappeared and many droplets were seen within and at the surface of the drop (Figure 4c). Soon these droplets were emitted into the aqueous
phase (Figure 4d). At the end of the experiment many small droplets and one or a few somewhat larger drops were present. The latter were much smaller than the injected drop. When drops of the same initial composition were injected into 0.50 wt % and 0.75 wt % NaCl, a few droplets formed within the drop or at its surface (Figures 5a,b). The droplets were in motion, indicating relatively strong convection. The number of droplets within the larger drop increased with time (Figure 5c,d), but no emulsification of oil in the aqueous phase occurred. A few experiments were conducted with NaCl in both the drop and the aqueous phase into which it was injected. When 70/20/10 drops containing 0.50 wt % and 0.75 wt % NaCl brine were injected into 0.25 wt % NaCl brine, myelinic figures were observed. However, they persisted and no spontaneous emulsification occurred, in contrast to the experiment of Figure 4, where no NaCl was present in the injected drop. Finally, 70/20/10 drops containing
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Figure 6. Video frames for a drop with proportions 50/40/10 by weight of n-dodecane/AOT/2 wt % NaCl injected into water at 30 °C. Times are as indicated.
Figure 7. Video frames for a drop with proportions 50/40/10 by weight of n-dodecane/AOT/2 wt % NaCl injected into 1 wt % NaCl at 30 °C. Times are as indicated.
0.50 wt % NaCl brine injected into 0.50 wt % and 0.75 wt % NaCl brine exhibited some emulsification within the drop but none outside it. Behavior with Other Hydrocarbons. Several experiments were conducted with 2,2,4-trimethyl pentane (isooctane) as the hydrocarbon. Behavior was the same as observed for the same compositions with n-octane. When the hydrocarbon was n-dodecane, no experiments yielded extensive emulsification of the oil in water. Convection in and near the drop and some emulsification with a large drop remaining at the end of the experiment were observed even for injected drops with high AOT contents, that is, with proportions 50/40/10 and 56/44/0 by weight of n-dodecane/AOT/water. The same was true when water in the 50/40/10 drop was replaced by 2 wt % NaCl. Video frames a and b of Figure 6 show the initial convection and droplet formation, whereas frame c shows some droplets being released to the aqueous phase and frame d the final large drop. When similar drops, that is,
containing 2 wt % NaCl, were injected into 1 wt % NaCl instead of water, myelinic figures developed at the interface but only after approximately 5 min (Figure 7). This behavior is similar to that observed previously in a nonionic surfactant system12 and is discussed further below. For n-decane the amount of emulsification for 50/40/10 and 56/44/0 drops injected into water was somewhat greater than that observed for n-dodecane but considerably less than that for n-octane. Replacing water by 1 wt % NaCl in the 50/40/10 drop increased the amount of emulsification slightly, but a large drop still remained at the end of the experiment. Injecting 50/40/10 drops containing 1 wt % NaCl into 0.25 wt % NaCl brine yielded myelinic figures initially. However, the myelins dissolved in a few minutes, and only a large oil drop remained at the end of the experiment. (12) Lim, J. C.; Miller, C. A. Langmuir 1991, 7, 2021.
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Figure 8. Video frames for a drop with proportions 70/20/10 by weight of n-hexane/AOT/water injected into water at 30 °C. Times are as indicated.
Drops containing 70/20/10 n-hexane/AOT/water exhibited myelinic figures at their surfaces upon injection into water. They also assumed highly irregular shapes (Figure 8b-d), indicating that interfacial tension was very low and that a hydrodynamic instability developed as the drop flattened and spread at the top surface of the cell. Later the drop retracted leaving some emulsified oil drops behind and ultimately forming a single residual drop (Figure 8f) in a manner similar to that observed with n-octane. Discussion Ternary AOT/n-Octane/Water Systems. As indicated previously, the strategy of these experiments was to produce spontaneous emulsification of oil drops in water by choosing compositions that would cause the drops to become more hydrophilic after injection owing to uptake of water from the aqueous phase and loss of AOT and sometimes NaCl to this phase. Thus, the initial drops would either be water-in-oil (w/o) microemulsions or sponge phases or would be rapidly converted to such phases as they took up water. Moreover, their surfactant films would experience an ionic strength that decreased with time as the ratios of AOT/water (and NaCl/water when NaCl was present) in the microemulsions decreased. Eventually, inversion to a water-continuous phase would occur with accompanying local supersaturation in oil and hence nucleation of oil droplets. For the nonionic surfactant system studied previously1 the microemulsion inverted continuously from w/o to o/w, passing through compositions where it was bicontinuous. In contrast, inversion in the ternary n-octane/AOT/water system investigated here involved formation of the lamellar liquid crystalline phase because, as indicated previ-
ously, phase behavior is similar to that of Figure 2 for benzene. Nevertheless, the basic strategy of making the drop become more hydrophilic was successful in producing extensive emulsification whenever the initial AOT content was at least 25 wt % AOT and that of water no more than 10 wt %. At the end of experiments with such drops, nearly all the oil phase existed as small droplets on the order of 1 µm in diameter although a single somewhat larger drop remained, for example, as in Figure 3d. But only minimal emulsification was seen for drops with low initial concentrations of AOT. Further insight into this behavior can be obtained by considering the diffusion path, that is, the set of compositions produced by diffusion in each phase (see Miller and Neogi13). Here the diffusion path approach is applicable when an oil drop first contacts water and remains valid as long as some oil in the interior of the drop remains at its initial composition. When the injected oil contains only small amounts of AOT and water (e.g., point P of Figure 9), the initial diffusion path is POQW, shown as three dashed-line segments with arrows in Figure 9. PO and QW represent compositions in the L2 and L1 phases, respectively, whereas OQ is a tie-line. (See the appendix for further information on this calculation and others mentioned below.) In accordance with current usage L1 and L2 have been used to denote aqueous and oleic phases instead of Wm and Om as in Figure 2. For this path no intermediate liquid crystalline phase is formed, so that no emulsification occurs as the result of an inversion process as outlined above. Nevertheless, because compositions along QW in (13) Miller, C. A.; Neogi, P. Interfacial Phenomena; Marcel Dekker: New York, 1985.
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Figure 9. Schematic diffusion path POQW for n-octane with a low concentration of AOT contacting water.
the L1 phase are supersaturated in oil, some initial spontaneous emulsification of oil is expected.13 However, the bulk composition of a small injected drop changes rapidly, so that the criterion of the preceding paragraph is violated shortly after injection. Indeed, the drop rapidly becomes depleted in surfactant, so that in the final state it contains very little AOT and is in equilibrium with a dilute oil-in-water microemulsion. Under these conditions a relatively small overall amount of emulsification would be expected, as, in fact, was observed experimentally when initial AOT concentration was low. The highest surfactant concentration at P for which an initial diffusion path such as POQW is applicable at 30 °C cannot be determined precisely from available information for the AOT/octane/water system, because the limiting tie-line of the L1-L2 region, that is, the L1-L2 side of the L1-L2-LR three-phase triangle of Figure 9, is not known. However, using the available information on phase behavior, we have made diffusion path calculations which indicate that the limiting AOT concentration is in the range of 15-20 wt %. This estimate is consistent with results of the vertical cell experiments described above in which an intermediate phase of LR was seen when the oil phase contained 20 wt % AOT but not 10 wt % AOT. In addition, the calculations show that, whatever its exact value, the limiting concentration decreases as the initial oil phase is made more lipophilic and increases if the diffusion coefficient in the oil phase decreases, as would occur as water content is raised owing to an increase in size of the water droplets present in the initial microemulsion. The former trend is consistent with the observation of emulsification for a drop having proportions 70/ 20/10 of n-octane/AOT/0.75 wt % NaCl, but not for one having the same proportions of n-octane/AOT/water. The latter trend is consistent with the observation of emulsification for salt-free drops with proportions 65/25/10 but not for drops with proportions 50/25/25. For injected drops with AOT concentrations greater than the limiting value the precise form of the diffusion path cannot be determined from the available information. However, Figure 9 indicates that it must include formation of an intermediate lamellar (LR) phase between the L1 and L2 phases. This change in diffusion path apparently is associated with the change in behavior from minimal
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emulsification to extensive emulsification. In fact, the vertical cell experiments described previously did exhibit a lamellar intermediate phase for oil compositions in which extensive emulsification of injected drops occurred. One reason for the additional emulsification is that it results not only from local supersaturation in the L1 phase as in Figure 9, but also from that in the LR phase as it loses surfactant by diffusion into the L1 phase. The data of Kunieda and Sato6 indicate a surfactant/oil ratio of at least 0.85 in the LR phase in equilibrium with L2, which suggests that, except for drops with very high initial concentrations of AOT, not all of the L2 phase will be converted to liquid crystal, and an oil drop will remain at the end of the experiment as observed. As shown in Figure 3, this remaining drop is frequently small. An exception was 70/20/10 drops where some LR was formed on initial contact, according to the vertical cell experiments, but the final drop was large. As indicated above, this composition is just above that where the transition between diffusion paths occurs. Apparently, the AOT concentration in the drop quickly fell below the transition value after injection, so that little lamellar phase was formed. None was seen in the drop injection experiments for this oil composition. The higher initial surfactant concentrations associated with extensive liquid crystal formation produced lower interfacial tensions. Hence, when drops with high AOT concentrations rose to the upper surface of the cell after injection because of their low density, gravity caused much more flattening than for drops containing less AOT, producing a generally radial outward flow. Although some aspects of the behavior shown in Figure 3 are not fully understood, the bulges indicate existence of a hydrodynamic instability associated with the flattening/spreading, as does the similarly striking behavior for n-hexane shown in Figure 8. Instabilities caused by Marangoni flow have been observed during spreading of aqueous surfactant solutions on thin layers of water. Analysis has clarified some aspects of this instability,14 although its development remains incompletely understood. The instabilities seen here may be related, but the situation is more complex because the drop is not miscible with the water on which it is spreading and because the intermediate lamellar phase is present. The later retraction of oil that had not been emulsified (Figure 3c) was probably the result of an increase in interfacial tension caused by a decrease in AOT concentration in the oil. With the higher tension and smaller volume of this remaining oil, gravity no longer dominated interfacial effects and free energy was minimized by formation of a nearly spherical drop. The retraction in the upper part of the cell to form this drop produced the outward flow of the oil droplets in the lower part seen in Figures 3c and 3d. Whereas nearly complete self-emulsification to small oil droplets was observed for ternary AOT/water/hydrocarbon systems with proper choice of hydrocarbon (e.g., n-octane), such behavior was not possible for ternary nonionic/water/hydrocarbon systems.1 The reason is the difference in phase behavior. More specifically, the fact that AOT concentration affects ionic strength, which, in turn, strongly influences spontaneous curvature of the surfactant films, allows diffusion to shift conditions in a microemulsion from lipophilic to hydrophilic, causing local supersaturation and spontaneous emulsification of oil. No analogous mechanism exists for nonionic surfactants. (14) Matar, O. K.; Troian, S. M. Phys. Fluids A 1997, 9, 3645.
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Effects of NaCl and Oil Composition. According to Kunieda and Shinoda,8 a bicontinuous microemulsion with balanced hydrophilic and lipophilic properties forms for a system containing n-octane, AOT, and 0.4 wt % NaCl brine at 30 °C. As a result, injection of a salt-free 70/20/10 drop of n-octane/AOT/water into 0.25 wt % NaCl, although initially yielding myelinic figures, ultimately produced an oil-in-water emulsion (Figure 4), because the salinity was below 0.4 wt % and the surfactant films were hydrophilic in their final state. In contrast, when a similar drop was injected into 0.50 and 0.75 wt % NaCl solutions, the surfactant films experienced somewhat lipophilic conditions. Because no inversion to hydrophilic conditions occurred, no significant emulsification was expected and indeed none was observed, as indicated previously. The experiment shown in Figure 5 is for a situation in which the drop contained water but the external phase was 0.50 wt % NaCl, that is, slightly above the salinity for the balanced state. The behavior seen is similar to that observed near the balanced state for drops of n-hexadecane/oleyl alcohol injected into dilute solutions of pure nonionic surfactants.15 Only a few droplets of oil are formed, and the large drop never becomes hydrophilic enough to be miscible with the aqueous phase. In connection with the bulges and other dynamic phenomena described above, somewhat different but equally intriguing phenomena involving spontaneous emulsification were observed by Shahidzadeh et al.16,17 In their experiments several hydrocarbons including n-octane contacted a dispersion of AOT vesicles dispersed in NaCl brine of suitable concentration. That is, AOT was initially in the aqueous phase instead of in the oil phase as in the present work. The lamellar phase played a key role in the emulsification process in the experiments of both groups. Ghosh and Miller9 and Kellay et al.10 reported that the surfactant-rich liquid phase, which coexisted at some conditions with excess oil and brine phases in the n-dodecane/AOT/NaCl brine system, was unable to solubilize significant amounts of hydrocarbon. Somewhat more solubilization was observed for n-decane. For n-octane, however, a lamellar phase that coexisted with excess oil and water phases exhibited much greater solubilization.10 For an intermediate phase to produce significant spontaneous emulsification of oil when it is made more hydrophilic, it must contain a substantial amount of solubilized oil. Because this was not the case for n-dodecane and n-decane, it is not surprising that much more spontaneous emulsification was observed for n-octane and n-hexane than for n-decane and n-dodecane. The behavior seen in Figure 7, that is, development of myelinic figures several minutes after drop injection, is similar to that observed by Lim and Miller12 for rather lipophilic drops of n-hexadecane/oleyl alcohol injected into dilute nonionic surfactant solutions which were more hydrophilic. The situation of Figure 7 is similar in that the drop is lipophilic (salinity above 1.5 wt %, the approximate value for the balanced state9), whereas the external phase is hydrophilic (salinity below 1.5 wt %). Composition of the drop follows along the L2 coexistence curve with L1 until the boundary of the L1/L2/LR region is reached, at which time myelinic figures begin to form. Summary Spontaneous emulsification yielding small oil droplets (15) Lim, J. C. and Miller, C. A. Unpublished results. (16) Shahidzadeh, N.; Bonn, D.; Meunier, J. Europhys. Lett. 1997, 40, 459. (17) Shahidzadeh, N.; Bonn, D.; Aguerre-Chariol, O.; Meunier, J. Colloids Surf. A 1999, 147, 375.
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was observed for n-octane/AOT/water mixtures injected into water when initial AOT concentration exceeded approximately 25 wt % and initial water content was less than approximately 10 wt %. An intermediate lamellar liquid crystalline phase that solubilized considerable octane formed on contact, and local supersaturation occurred in the aqueous phase, leading to nucleation of oil droplets there. As AOT continued to diffuse into the aqueous phase the lamellar phase was itself converted to a microemulsion, causing emulsification of much of its solubilized oil. Intriguing hydrodynamic effects were typically observed along with emulsification in the videomicroscopy experiments. Low interfacial tensions caused the drop to flatten near the upper surface of the experimental cell and spread radially shortly after injection. In some experiments a hydrodynamic instability led to fingering during this process. Later when most of the surfactant had diffused into the aqueous phase, the small amount of oil that had not been emulsified retracted as interfacial tension increased, forming a small residual drop of oil. Appendix When two semi-infinite phases with uniform compositions in a ternary system are contacted without mixing at a planar interface, the time-dependent diffusion equations take the form
(∂ωj/∂t) ) Dj (∂ωj/∂x2), (∂ωj′/∂t) ) Dj′ (∂ωj′/∂x2),
j ) 1,2 for oleic phase j ) 1,2 for aqueous phase
where ω1 (ω1′) and ω2 (ω2′) are mass fractions of AOT and oil in the oleic (aqueous) phase. D1 (D1′) and D2 (D2′) are the corresponding diffusion coefficients. These equations can be solved, subject to plausible assumptions, using a similarity transformation.13 The result is
ωj ) Aj + Bj erf[x/(4Djt)1/2], ωj′ ) Aj′ + Bj′ erf[x/(4Dj′t)1/2],
j ) 1,2 j ) 1,2
The mass fractions ω3 and ω3′ of water follow from the requirement that the mass fractions in each phase sum to unity. The arbitrary constants Aj, Aj′, Bj, and Bj′ are found by requiring that (a) the initial compositions in the two phases exist for x ) (∞, (b) local equilibrium as specified by tie-lines on the equilibrium phase diagram exist at the interface, and (c) a mass balance for each component be satisfied at the interface. The mass balance conditions require the position of the interface to vary as t1/2, where is a constant. With this solution the set of compositions in each phase is independent of time and can be plotted as the “diffusion path” on the equilibrium phase diagram.13 If the diffusivities of all species in a given phase are equal, as is assumed here, the diffusion path consists of two straight line segments in the phases joined by an equilibrium tie-line as in Figure 9. For the calculations mentioned above, one end of the tie-line was taken as the oil vertex, that is, solubility of AOT and water were neglected, a reasonable assumption. To find the maximum initial AOT content in the oil phase consistent with a diffusion path of this type, the limiting tie-line of the L1-L2 region is needed. This tie-line is not known. However, a lower limit on the maximum AOT content can be obtained by choosing the L1 phase to have proportions 35/18/47 by weight of n-octane/AOT/water, the point on the L1 coexistence curve with the highest oil
Oil Emulsification in AOT/Water/Hydrocarbon System
content reported by Kunieda and Sato6 for the isooctane system at 30 °C. With this tie-line and with diffusivities D2 and D1 in the oil and water phases assumed equal, this procedure yields an AOT concentration of approximately 16 wt % in the initial oil phase. If D2/D1 ) 0.25, for example, if diffusivity in the oil is decreased because of a higher concentration
Langmuir, Vol. 16, No. 24, 2000 9241
of water, which increases the size of the microemulsion droplets, the corresponding result is approximately 20 wt %. If the system were more lipophilic with the L1 end of the tie-line having less AOT and more water (e.g., 35/ 12/53 n-octane/AOT/water), the limiting AOT content falls to 11 wt %. LA0006521
