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Spontaneous Formation of Highly Concentrated Water-in-Oil Emulsions (Gel-Emulsions) Hironobu Kunieda,*,† Yoji Fukui,† Hirotaka Uchiyama,‡ and Conxita Solans§ Department of Physical Chemistry, Division of Materials Science and Chemical Engineering, Yokohama National University, Tokiwadai 156, Hodogaya-ku, Yokohama 240, Japan, Research and Development Department, Procter & Gamble Far East, Inc., 17, Koyo-cho Naka 1-chome, Higashinada-ku, Kobe 658, Japan, and Departamento de Tensioactivos, CID, C.S.I.C., Jordi Girona 18-26, 08034 Barcelona, Spain Received September 11, 1995. In Final Form: January 16, 1996X Water-in-Oil-type gel-emulsions (or highly concentrated emulsions) are spontaneously formed from oil-swollen micellar solutions or oil-in-water (O/W) microemulsions in a water/tetraoxyethylene dodecyl ether/oil system with an abrupt increase in temperature. The phase change occurs from water-continuous microemulsion to water-in-oil (W/O) gel-emulsion via a lamellar liquid crystal and a bicontinuous surfactant phase (L3 phase). Hence, the spontaneous curvature of the surfactant layer is continuously changed with temperature change because the gel-emulsion consists of a reverse micellar solution and an excess water phase. In a narrow temperature range above the single L3-phase region, there is a two-phase region consisting of L3 and an excess water phase (W) in which emulsions are extremely unstable. The electroconductivity curve as a function of temperature monotonically decreases with increasing temperature when the final temperature is high and the temperature change is fast. If the temperature change is very slow, the electroconductivity has a maximum at the temperature where the L3 + W region is found because excess water is separated. In this case, the water droplet size in the final concentrated emulsion is very large. Therefore, it is important to change the temperature quickly to form fine gel-emulsions.
Introduction It is known that high-internal-phase-ratio emulsions (HIPREs) or highly concentrated emulsions can be formed in the water- and oil-rich regions under certain conditions.1-5 Due to their characteristic features such as large water content, high viscosity, and translucence, they are also referred to as gel-emulsions.6-11 HIPREs have been used for practical applications such as aviation fuels, emulsion explosvies, cosmetics, etc.12-15 Since the volume fraction of the dispersed phase in the HIPRE exceeds that for closest packing rigid spheres (Ostwald’s critical volume fraction), in general, it is difficult to form the gel-emulsions by a simple agitation method. In the past, the HIPREs were formed by adding the dispersed phase gradually or mixing the system with glass * To whom correspondence should be addressed. † Yokohama National University. ‡ Procter & Gamble Far East, Inc. § CID, C.S.I.C. X Abstract published in Advance ACS Abstracts, April 15, 1996. (1) Lissant, K. J. J. Colloid Interface Sci. 1966, 22, 462. (2) Lissant, K. J.; Mayhan, K. G. J. Colloid Interface Sci. 1973, 42, 201. (3) Groves, M. J.; Mustafa, R. M. A.; Carless, J. E. J. Pharm. Pharmacol. 1974, 26, 616. (4) Ali, A. A.; Mulley, B. A. J. Pharm. Pharmacol. 1978, 30, 205. (5) Princen, H. M. J. Colloid Interface Sci. 1979, 71, 55. (6) Solans, C.; Azemar, N.; Comelles, F.; Sanchez Leal, J.; Parra, J. L. Proceedings of the XVII Jornadas CED/AID, Madrid; AID: Barcelona, 1986; p 109. (7) Kunieda, H.; Solans, C.; Shida, N.; Parra, J. L. Colloids Surf. 1987, 24, 225. (8) Solans, C.; Azemar, N.; Parra, J. L. Prog. Colloid Polym. Sci. 1988, 76, 224. (9) Solans, C.; Dominguez, J. G.; Parra, J. L.; Heuser, J.; Friberg, S. E. Colloid Polym. Sci. 1988, 266, 570. (10) Kunieda, H.; Yano, N.; Solans, C. Colloids Surf. 1989, 36, 313. (11) Kunieda, H.; Evans, D. F.; Solans, C.; Yoshida, M. Colloids Surf. 1990, 47, 35. (12) Ishida, H.; Iwama, A. Combust. Sci. Technol. 1984, 37, 79. (13) Sagitani, H.; Hattori, T.; Nabeta, K.; Nagai, M. Nippon Kagaku Kaishi 1983, 1399. (14) Attwood, D.; Florence, A. T. Surfactant Systems; Chapman and Hall: New York, 1983. (15) Bampfield, A.; Cooper, J. In Encyclopedia of Emulsion Technology; Becher, P., Ed.; Marcel Dekker: New York, 1988; Vol. 3, p 381.
S0743-7463(95)00752-9 CCC: $12.00
balls or some textile fabrics to enhance the local agitation.7 Recently, it was found that water-in-oil-type gel-emulsions are spontaneously formed from oil-in-water (O/W) microemulsions or micellar solution phases in a water/ polyoxyethylene alkyl ether/oil system by simple temperature change.16 The hydrophile-lipophile property of the nonionic surfactant is largely influenced by temperature due to the conformation change of the hydrophilic polyoxyethylene chain.17-19 Hence, the nonionic surfactant tends to form aqueous micelles at lower temperatures whereas it forms reverse micelles at higher temperatures. At the transition temperature called the HLB temperature, the bicontinuous microemulsion (or surfactant phase) coexists with excess water and oil phases. Above the HLB temperature, water-in-oil-type highly concentrated emulsions form because the nonionic surfactant is lipophilic in a given water/oil system.7 It is considered that the spontaneous curvature of the surfactant layer changes during the spontaneous formation of gel-emulsions. However, the mechanism of spontaneous formation has not been studied due to the complicated phase changes. In this context, we have investigated the phase behavior of the nonionic surfactant and the change in electroconductivity to figure out the mechanism of spontaneous formation of gel-emulsions. Experimental Section Materials. Homogeneous tetraethylene glycol dodecyl ether (abbreviated as C12EO4) was obtained from Nikko Chemicals Co. Extrapure grade decane was obtained from Tokyo Kasei Kogyo Co. All the materials were used without further purification, and the water used was always doubly distilled. (16) Pons, R.; Carrera, I.; Erra, P.; Kunieda, H.; Solans, C. Colloids Surf. A 1994, 91, 259. (17) Kunieda, H.; Shinoda, K. Bull. Chem. Soc. Jpn. 1982, 55, 1777. (18) Kunieda, H.; Sato, Y. In Organized Solutions; Friberg, S. E., Lindman, B., Eds.; Surfactant Science Series; Marcel Dekker: New York, 1992; Vol. 44. (19) Karlstro¨m, G. J. Phys. Chem. 1984, 88, 4769.
© 1996 American Chemical Society
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Procedures. Procedures to Determine the Phase Diagram. The procedures used to determine the phase boundaries are described in previous papers.11,20 Preparation of Water-in-Oil-Type Gel-Emulsions (HIPREs). The oil-swollen aqueous micellar solution (microemulsion) was kept in a thermostat at lower temperature and was moved to another thermostat at higher temperature within 1 s. The final temperatures are 20, 30, and 50 °C. Gel-emulsions were produced without agitation. When the final temperature is high, the change in temperature of the system is fast. Electroconductivity Measurement. To distinguish watercontinuous from oil-continuous systems, 0.1 M NaCl was added to the water. The salt also enhances the stability of highly concentrated emulsions.10 The electroconductometer (Model CM40S, TOA Electronics, Ltd.) was used with a recorder. The temperature change was also monitored simultaneously by a thermocouple and was recorded continuously. While the temperature was changed, a magnetic stirrer was used to mix the system. The amount of sample was always 15 mL. In order to confirm the reproducibility, we repeated the same experiments at least three times. Since the present gel-emulsions are stable for more than one day, the reproducibility of the electroconductivity measurements is fairly good. VEM (Video-Enhanced Microscopy). A differential interference phase-contrast (Nomarski-type) microscope (Nikon X2F-NTF21) with a video-enhanced system (Hamamatsu Photonics Co., Argus 10) was used to observe gel-emulsions and vesicular dispersions.
Results and Discussion Phase Diagram of the Water/C12EO4/Decane System. The phase diagram of the 0.1 M aqueous NaCl/ C12EO4/decane system in the water-rich region was determined as a function of temperature and is shown in Figure 1. An O/W microemulsion or oil-swollen aqueous micellar solution (Wm) forms and coexists with excess oil at lower temperature. The lower boundary of the single microemulsion region corresponds to the solubilization curve of the oil. With increasing temperature, the solubilization increases and finally the microemulsion separates from the water and a three-phase body, III(W+D+O), appears in an oil-rich region. This temperature is called the HLB temperature, being that at which the hydrophile-lipophile property of the surfactant is considered to be balanced in a given water/oil system. On the other hand, surfactant aggregates are separated as a surfactant phase from the upper boundary of the single microemulsion region (Wm), at which the water phase coexists with the isotropic surfactant phase. However, the system is immediately changed to a LC + W region with a very slight increase in temperature. Judging from the phase rule, it is considered that there is a threephase region between the two two-phase regions. However, this three-phase region was not observed due to the narrowness. In the LC+W region, the lamellar liquid crystal is stably dispersed as vesicles in water. Figure 2 shows the VEM (video-enhanced microscopy) picture of vesicles in water. Since vesicles are spherical self-organizing structures, they do not show anisotropy while we observe the system under crossed polarizers. With increasing temperature, the system shows optical anisotropy in the LC region. Therefore, it is considered that the lamellar liquid crystal swollen with water is present at this temperature. The LC region can be considered to be a single lamellar liquid crystalline phase in which bimolecular layers are considerably separated by water. In the binary water/C12EO4 system, the cloud temperature is around 4 °C.21,22 Since the salt is present in the (20) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1985, 107, 107. (21) Saito, H. Nippon Kagaku Zasshi 1971, 92, 223.
Figure 1. Phase diagram of the 0.1 M aqueous NaCl/C12EO4/ decane system as a function of temperature. Decane was added to 3 wt % C12EO4 aqueous solution, and the weight percent of decane in the system is plotted horizontally. (a) Phase diagram over a wide range of temperature. (b) Detailed phase diagram around the HLB temperature. Wm; oil-swollen micellar solution (O/W-type microemulsion). Om, water-swollen reverse micellar solution (W/O-type microemulsion); D, surfactant phase (middle-phase microemulsion); L3, bicontinuous surfactant phase; LC, lamellar liquid crystal; W and O, excess water and oil phases, respectively. I, II, and III indicate one-, two-, and three-phase regions.
system, the cloud temperature is lowered and there is a two-isotropic-phase region at lower temperature on the left-hand axis in Figure 1. It is known that the two-phase region is changed to a vesicular dispersion at higher temperature in the binary system. With the further increase in temperature, the lamellar liquid crystal is melted and an isotropic phase appears, as is shown in Figure 1. In the water-rich region, the isotropic single phase goes up to higher temperature and is connected to the left-hand axis. In the binary water/ surfactant system, there is an isolated isotropic phase called the L3 phase in which the surfactant molecules form a bicontinuous network. Therefore, the isotropic phase (22) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 975.
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Figure 2. VEM picture of vesicular dispersion in the LC + W region at 25 °C in Figure 1. The decane concentration is 0.4 wt % in the system. The bar indicates 10 µm.
above the LC temperature is considered to be the L3 phase, which is shifted to lower temperature upon addition of decane. Above the single-phase L3 region, excess water (or brine) is separated. In a narrow temperature range, the emulsions are extremely unstable and are separated into two isotropic phases within 15 min. Judging from the former NMR self-diffusion data,23 the L3 phase coexists with the excess water phase in this two-phase region. With the increase in temperature, the L3 phase is continuously changed to a reverse micellar solution without phase separation.23 Therefore, the L3 + W region is also continuously changed to the Om + W region, in which emulsions are very stable. It is clear from Figure 1 that the spontaneous curvature of surfactant aggregates is continuously changed from convex to concave toward water. The type of aggregation is changed from oil-swollen aqueous micelles to waterswollen reverse micelles via vesicles, the lamellar liquid crystal, and the bicontinuous L3 phase. Electroconductivity in Each Phase. The electroconductivity was measured in a stationary state at each temperature. The result is shown in Figure 3. In Figure 3, thermal equilibrium is attained at each temperature and the system is mixed by a magnetic stirrer while measuring the electroconductivity. In the single Wm region, the electroconductivity is high because it has a water-continuous structure. When vesicles are formed at higher temperature, the electroconductivity is decreased due to the presence of larger aggregates. With increasing temperature, vesicles are changed to the flat lamellar liquid crystal and the electroconductivity is decreased. The electroconductivity starts increasing when the L3 phase appears. Above the LC temperature, a surfactantcontinuous L3 phase forms in a very narrow temperature range. At the temperature of the single L3 phase region, the electroconductivity is 3.6 and 3.13 mS/cm at the compositions of arrows A and B, respectively, in Figure 1. The L3 phase has a three-dimensional continuous bilayer structure, and the electroconductivity is low compared with that of the micellar solution due to the obstruction effect of the surfactant layer.24 It is known that the electroconductivity of the L3 phase is approximately 60% of that in the Wm phase of the same composition.25,26 The (23) Solans, C.; Pons, R.; Zhu, X.; Davis, H. T.; Evans, D. F.; Nakamura, K.; Kunieda, H. Langmuir 1993, 9, 1479. (24) Strey, R.; Schomacker, R.; Roux, D.; Nallet, F.; Olsson, U. J. Chem. Soc., Faraday Trans. 1990, 86, 2253.
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Figure 3. Change in electroconductivity as a function of temperature. The electroconductivity was measured in a stationary state at each temperature while the system was stirred by magnetic stirrer. b, 4 wt % decane (arrow A in Figure 1); 0, 8 wt % decane (arrow B in Figure 1); 3, 12 wt % decane (arrow C in Figure 1).
electroconductivities of the Wm phase are 5.7 and 5.76 mS/cm at 6.5 and 10.2 °C at the compositions of arrows A and B in Figure 1, respectively. By taking account of the temperature effect, the electroconductivities of the L3 phase can be calculated to be 4.21 and 3.68 mS/cm at 13 °C, respectively. These values are in fairly good agreement with the measured values. The obstruction effect on the electroconductivity in the lamellar liquid crystal is considered to be larger than that of the L3 phase. In fact, a similar change in electroconductivity from the micellar phase to the L3 phase via the LC phase is observed in a binary water/polyoxyethylene-type nonionic surfactant system.24 However, the conductivity in the LC phase cannot be directly compared with that in an isotropic phase, since the LC phase is anisotropic. Above the single L3 phase temperature, the electroconductivity still increases and reaches its maximum in the L3 + W region, where the emulsion is extremely unstable. To be exact, it is difficult to form emulsions due to the very fast coalescence of droplets. At this temperature, the electroconductivity still increases although the system is being agitated. It is considered that a large bulk water phase is present and the electroconductivity is high. Above the L3 + W region, the electroconductivity decreases again because fine stable water droplets are formed in the Om + W region. The system is oilcontinuous because the electroconductivity of the emulsion in the Om + W region is much less than 1 µS/cm. Change in Electroconductivity with Changing Temperature. We measured the continuous change in electroconductivity as a function of temperature. Initially, a single microemulsion phase was prepared at lower temperature, and then, we transferred the microemulsion to another thermostat at higher temperature than the HLB temperature. The changes in electroconductivity with temperature were monitored while the system was agitated by stirrer. The results are shown in Figure 4. In Figure 4d, we started at 9 °C, at which the two-phase region is present instead of a single phase. When the final temperature is high, the temperature change is fast and the electroconductivity is monotonically decreased with increasing temperature. On the other hand, in the (25) Nilsson, P. G.; Lindman, B. J. Phys. Chem. 1984, 88, 4769. (26) Yoshida, M.; Kunieda, H. Yukagaku (J. Jpn. Oil Chem. Soc.) 1991, 40, 657.
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Figure 5. Change in electroconductivity at arrow B in Figure 1 by changing temperature from 8 to 20 °C (s) and from 8 to 50 °C (- ‚ -) without agitation.
Figure 4. (a) Change in electroconductivity at arrow A in Figure 1 by changing temperature from 6.5 to 20 °C (s), from 6.5 to 30 °C (- - -), and from 6.5 to 50 °C (- ‚ -). (b) Change in electroconductivity at arrow B in Figure 1 by changing temperature from 10 to 20 °C (s), from 10 to 30 °C (- - -), and from 10 to 50 °C (- ‚ -). (c) Change in electroconductivity at arrow C in Figure 1 by changing temperature from 11.5 to 20 °C (s), from 11.5 to 30 °C (- - -) and from 11.5 to 50 °C (- ‚ -). (d) Change in electroconductivity at arrow D in Figure 1 by changing temperature from 9 to 20 °C (s) and from 9 to 50 °C (- ‚ -).
case that the temperature change is slow, there is a maximum on the electroconductivity curve, as is shown in Figure 4. According to the result shown in Figure 3, the formation of the L3 phase and the rapid coalescence of water droplets in the L3 + W region causes the maximum in electroconductivity data in Figure 4 when temperature
is slowly changed. In this region, even if the system is agitated, the coalescence of water droplets is very fast. Hence, if the temperature change is very fast, the coalescence of water droplets is not possible when the system passes through the L3 + excess water phase region. In a relatively oil-rich region, the electroconductivity curve does not have a maximum even if the temperature is changed slowly because no extremely unstable emulsion region is present, as is shown in Figures 1 and 4d. This result also suggests that the L3 phase and the L3 + W region cause the maximum in electroconductivity. Spontaneous Formation of Gel-Emulsions. Gelemulsions or HIPREs are spontaneously formed from the oil-swollen micellar solution by changing the temperature without vigorous agitation. Figure 5 shows the change in electroconductivity at arrow B without agitation. In this case, the single microemulsion at 10 °C is just shifted to another thermostat at 20 and 50 °C, respectively. In both cases, the electroconductivity decreases monotonically and W/O-type emulsions are formed. However, the former sample is less viscous than the latter sample. It is suggested that the emulsion droplets in the former sample are larger. As discussed before, when the temperature change is slow, it is possible that water droplets are coalesced in the two-phase region including L3 and excess water. Figure 6 shows the VEM picture of highly concentrated emulsions formulated by the present spontaneous-formation method. After reaching the final temperatures, we observed the gel-emulsions at room temperature. In the case that the final temperature is high (50 °C), emulsion droplets are small, as is shown in Figure 6b. On the other hand, emulsion droplets are considerably large when the final temperature is low (20 °C), as is shown in Figure 6a. The size distribution of droplets in Figure 6a is also much larger than that in Figure 6b. It is also suggested that the coalescence of water droplets takes place when the temperature change is slow. As described in the former section, the electroconductivity curve has a maximum in the latter case (Figure 6a). Therefore, it is considered that water droplets are considerably coalesced when the system slowly passes through the two-phase region and the final emulsion droplet size becomes larger, as is shown in Figure 6a. Change in Self-Organizing Structure during Spontaneous Formation. The hydrophile-lipophile property of the polyoxyethylene-type nonionic surfactant is changed from hydrophilic to lipophilic with increasing temperature. The spontaneous curvature of the surfactant self-organizing structure is also changed from convex to concave toward water. Figure 7 shows the schematic change in shape of self-organizing structures during spontaneous formation of highly concentrated emulsions. In a single Wm region, oil-swollen micelles are present and no excess
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Figure 6. VEM pictures of highly concentrated emulsions. The composition is arrow B in Figure 1. The final temperature is 20 °C (a) and 50 °C (b). The bar indicates 10 µm.
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is decreased although the curvature of the surfactant molecules is convex toward water and the system is still water-continuous. In the lamellar liquid crystal, most of the water is trapped in the bilayer network and the electroconductivity is much less than that in the vesicle systems. The minima in electroconductivity data in Figure 4 are comparable to those measured in a stationary state (Figure 3) though the latter is slightly lower than the former. Therefore, the water-swollen lamellar liquid crystal spontaneously forms by simple temperature change. When the curvature of the surfactant layer becomes flat and vesicles are merged to the lamellar liquid crystal, most of the water is spontaneously taken up in the surfactant layers. However, with the increase in temperature, the bilayer becomes flexible and the phase transition from lamellar liquid crystal to L3 phase occurs. When the system enters the single L3 phase, the curvature of the surfactant molecular layer is considered to be slightly concave toward water, as is shown in Figure 7.24 It means that water is trapped in the flexible surfactant bilayers and micro-water domains are formed in this phase. However, the electroconductivity is considerably increased in the L3 phase region. Since surfactant bilayers are more flexible in the L3 phase, the micro-water domain may be quickly connected to and disconnected from each other. In fact, as mentioned in the previous section, the electroconductivity in the L3 phase is much higher than that of the W/O emulsion in the Om + W region. When the single L3 phase is changed to the L3 + W region, the excess water phase is separated from the L3 phase due to the coalescence of water droplets. If the temperature change is rapid, the coalescence of water droplets is not progressed and macroscopic phase separation does not occur in the two-phase region. Therefore, it is very important to change temperature quickly in order to form fine concentrated emulsions. In the W/O emulsion of the Om + W region, surfactant molecules form reverse micelles and they are also adsorbed at the interface of water droplets, as is shown in Figure 7. Conclusions
Figure 7. Schematic change in spontaneous curvature of surfactant layers in the process of spontaneous formation of gel-emulsions.
oil phase is separated. When vesicles are formed, some water is trapped in vesicles and the vesicular size is much larger than micelles. Therefore, the electroconductivity
Highly concentrated W/O-type emulsions or gel-emulsions are spontaneously formed from microemulsions or oil-swollen aqueous micellar solutions with a quick change in temperature in a water/tetraoxyethylene dodecyl ether/ decane system. The surfactant phase behavior is determined, and it is found that the self-organizing structure is changed from micelle to reverse micelle via the vesicle, the lamellar liquid crystal, and the surfactant-continuous L3 phase during the temperature change. The electroconductivity data and the VEM pictures clearly show the change in self-organizing structures. Emulsions in the L3 + W region are extremely unstable, and the complete phase separation takes place within a short time. When the temperature change is slow, the water droplet size in the resulting highly concentrated emulsion is large because the coalescence of water droplets occurs in the L3 + W region. Consequently, it is very important to change temperature quickly in order to produce fine emulsions. LA950752K
