O Microemulsion Investigated by

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Stepwise Process Forming AOT W/O Microemulsion Investigated by Dielectric Measurements R. Tanaka* and T. Shiromizu Department of Chemistry, Graduate School of Science, Osaka City University, Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan Received March 9, 2001. In Final Form: August 31, 2001 The dielectric constant, r, and the electrical conductivity, κ, were measured precisely for (AOT + r‚H2O) in c-hexane, heptane, octane, and decane as a function of the molar ratio, r, of water to AOT up to percolation onset at 298 K for 1 kHz to 30 MHz. The aggregation process depends strongly on the oil used and r. The unique change observed in r and κ was interpreted in terms of shape and size of the assemblies. In c-hexane, water is stabilized stored in spherical reversed micelles for a wide range of r. In heptane and octane, water is bound in unsymmetrical aggregates formed first in a region of small r, and then reversed micelles are formed from aggregates on addition of water. The behavior in decane is quite different from other systems. Our results suggest that large particles are formed from small aggregates directly in decane. The percolation transition occurs because of the coalescence of droplets with addition of water, leading to abrupt increase of r and κ. The value of r that induces percolation is in the order decane < octane < heptane , c-hexane. This order was related to the curvature energy of droplets, which is influenced strongly by the degree of oil penetration. Our speculation for the aggregation process is supported by theoretical and experimental results found in the literature.

Introduction The ternary mixtures composed of surfactant, water, and oil have attracted much interest since they form thermodynamically stable phases involving self-organized assemblies. One of the most commonly investigated surfactants is AOT (sodium bis(2-ethylhexyl) sulfosuccinate) which can form reversed micelles or W/O microemulsions under a wide range of conditions such as water content, temperature, oil, and concentration. Those parameters are responsible for the size and shape of micelles as well as interactions between microemulsion droplets. A well-known phenomenon called percolation transition has been clearly observed in electrical-conductivity studies. This transition occurs because of an increase in the temperature1-6 or on addition of water to a solution with a constant concentration of surfactant,7-11 followed by a steep increment in the electrical conductivity with several orders of magnitude. The dramatic change observed in W/O microemulsion occurs with relatively high concentration of surfactant and water.5,11 A generally accepted idea is that the percolation threshold corresponds to the formation of the first clusters of droplets coming in close contact. The percolation threshold depends strongly on the length of alkanes used for solvent as well as cosurfactants such as alcohol and * To whom correspondence should be addressed. (1) Bhattacharya, S.; Stokes, J. P.; Kim, M. W.; Huang, J. S. Phys. Rev. Lett. 1985, 55, 1884. (2) Hilfiker, R.; Eicke, H.-F.; Geiger, S.; Furler, G. J. Colloid Interface Sci. 1985, 105, 378. (3) Jada, A.; Lang, J.; Zana, R. J. Phys. Chem. 1989, 93, 10. (4) Mays, H.; Pochert, J.; Ilgenfritz, G. Langmuir 1995, 11, 4354. (5) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. J. Phys. Chem. 1995, 99, 8222. (6) Mays, H.; Ilgenfritz, G. J. Chem. Soc., Trans. 1996, 92, 3145. (7) Lagues, M.; Ober, R.; Tauprin, C. J. Phys. Lett. 1978, 39, 487. (8) Evans, D. F.; Mitchell, D. J.; Ninham, B. W. J. Phys. Chem. 1986, 90, 2817. (9) Kim, M. W.; Huang, J. S. Phys. Rev. A 1986, 34, 719. (10) Maitra, A.; Mathew, C.; Varshney, M. J. Phys. Chem. 1990, 94, 5290. (11) Manabe, M.; Ito, T.; Kawamura, H.; Kinugasa, T.; Sasaki, Y. Bull. Chem. Soc. Jpn. 1995, 68, 775.

nonionic surfactant. The electrical conductivity during the percolation has been attributed to either “hopping” of surfactant anions between droplets within clusters,9,10,12-14 or transfer of counterions through water channels opening in clusters.3,15 However, the matter has not been settled yet. It should be reasonable to consider that those droplets are formed from smaller size assemblies, namely, reversed micelles and AOT molecules to which small amounts of water might be bound. Therefore, we focused on the aggregation process of AOT that progressed by adding water into the binary solutions of (AOT + alkane), and carried out precise measurements of electrical conductivity κ and dielectric constant r for the frequencies from 1 kHz to 30 MHz at temperature of 298 K. Investigated solvents were c-hexane, heptane, octane, and decane. We have previously reported dielectric properties for (AOT + H2O + c-hexane).16-18 However, we found that the resonance method used in an earlier work induced erroneous results particularly in a region where the electrical conductivity becomes significant. Therefore, a reexamination was carried out. Experimental Section Materials. AOT (Aldlich Chemical Co., purity > 98%) was purified as follows. After 100 g of AOT was dissolved in 200 cm3 of benzene, 60 cm3 of water was added. It was stirred for 1 h at a temperature of 273 K and then put in a refrigerator for 2 days. To the supernatant of the solution 30 cm3 of water was added, and after stirring for 1 h it stayed for 2 days in the refrigerator. (12) Grest, G. S.; Webman, I.; Safran, S. A.; Bug, L. R. Phys. Rev. A 1986, 33, 2842. (13) Ponton, A.; Bose, T. K.; Delbos, G. J. Chem. Phys. 1991, 94, 6879. (14) Arcoleo, V.; Goffredi, M.; Turco Liveri, V. J. Solution Chem. 1995, 24, 1135. (15) Feldman, Y.; Kozlovich, N.; Nir, I.; Garti, N.; Archipov, V.; Idiyatullin, Z.; Zuev, Y.; Fedotov, V. J. Phys. Chem. 1996, 100, 3745. (16) Tanaka, R.; Okazaki, K.; Tsuzuki, H. Chem. Lett. 1995, 1131. (17) Tanaka, R.; Tsuzuki, H.; Okazaki, K.; Kinoshita, T. Fluid Phase Equilib. 1996, 123, 131. (18) Tanaka, R.; Okazaki, K. J. Colloid Interface Sci. 1996, 184, 601.

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This process was repeated. The supernatant was dried under reduced pressure at a temperature around 338 K. The drying process was repeated several times by adding 25 cm3 of benzene to AOT. Finally, AOT was dried under reduced pressure for a week. According to the determination with the Karl Fisher method, the mole fraction of water in AOT was 0.01-0.04. No impurity was detected by a HPLC test. Cyclohexane, heptane, and octane were fractionally distilled, and the purity tested by GLC was better than 99.9% in each case. Decane (Tokyo Kasei Kogyo Co. Ltd., purity > 99%) was used as supplied. Using an osmosis membrane (Millipore Co., Milli-Q Labo), water was purified. Experimental Technique. The measurements of electrical capacitance C and conductance G were carried out by using LCR meters (Hewlett-Packard): the model HP-4284A was used for 1 kHz to 1 MHz with imprecision of (0.001 pF for C and (0.03 µs for G, respectively. The electric terminals of the dielectric cell consist of coaxial three-cylinders made of nickel plates and they are fixed tightly in a glass vessel. The whole cell was wrapped with aluminum film and grounded. The capacitance of the cell was about 180 pF under vacuum. The cell (85 cm3) is connected in series with a pump for circulation and also a mixing vessel from which liquid is introduced with a gastight hypodermic syringe. The cell was immersed in an oil bath and the temperature was controlled at 298.15 K with instability of (0.002 K. This apparatus permitted us the successive addition of components so that the concentration could be varied precisely. For the frequencies from 1 to 30 MHz, model HP-4285A was used to detect dielectric relaxation expected to occur for relatively large r by using a simple two-terminal cell with capacitance of 1 pF in a vacuum. The measurements were made at a room temperature of (298.0 ( 0.3) K. The detail of the experimental technique is described in previous reports.17

Tanaka and Shiromizu

Figure 1. Cole-Cole plots for (AOT + r‚H2O + c-hexane) at m ) 0.5 mol kg-1 and 298 K.

Results Dielectric relaxation observed in AOT W/O microemulsion has been reported by many authors. It was measured with a relatively large amount of water involving a region of percolation threshold. According to the literature, the dielectric dispersion becomes significant at a frequency higher than 0.1-1 MHz.13,19-25 It is well known that the W/O microemulsion systems show a dielectric relaxation resulting from so-called interfacial polarization.26 To detect the interfacial polarization that is expected to occur in the present systems, the dielectric dispersion was measured for the solution of (AOT + r‚H2O) with c-hexane at m ) 0.5 mol kg-1, where r represents the ratio of the amount of water to that of AOT, and m represents the molality of AOT in alkane. Figure 1 shows the Cole-Cole plots for different amounts of water. The dielectric dispersion asymptotically diminishes with decreasing amounts of water and becomes fairly small for r < 60. In each case, the relaxation frequency was determined to be 100 MHz. The present result is very similar to a typical pattern of interfacial polarization reported by Hanai for a W/O microemulsion.27 Since the solution turns turbid for r > 80, the observed dielectric dispersion may be attributed to the interfacial polarization. Thus, the effect due to the interfacial polarization is not significant, if (19) Peyrelasse, J.; Boned, C. J. Phys. Chem. 1985, 89, 370. (20) van Dijk, M. A.; Casteleijn, G.; Joosten, J. G. H.; Levine, Y. K. J. Chem. Phys. 1986, 85, 626. (21) Cametti, C.; Codastefano, P.; Di Biasio, A.; Tartaglia, P.; Chen, S. H. Phys. Rev. A 1989, 40, 1962. (22) Robertus, C.; Joosten, J. G. H.; Levine, Y. K. J. Chem. Phys. 1990, 93, 7293. (23) Feldman, Y.; Kozlovich, N.; Nir, I.; Garti, N. Phys. Rev. E 1995, 51, 478. (24) D’Angelo, M.; Fioretto, D.; Onori, G.; Palmieri, L.; Santucci, A. Phys. Rev. E 1996, 54, 993. (25) Bordi, F.; Cametti, C.; Copdastefano, P.; Sciortino, F.; Tartaglia, P.; Rouch, J. Prog. Collid Polym. Sci. 1997, 105, 298. (26) Hanai, T. Emulsion Science; Academic Press: New York, 1968. (27) Hanai, T. Kolloidzeitschrift 1961, 177, 57.

Figure 2. Dielectric constants for (AOT + r‚H2O + c-hexane) measured at m ) 0.5 mol kg-1 for frequencies from 1 kHz to 1 MHz at 298 K.

any, in the transparent solutions with small r investigated in the present work. Figures 2 and 3 show the experimental results of r and κ, respectively, measured for the c-hexane system at m ) 0.3 mol kg-1 for 1 kHz to 1 MHz. A large deviation that is revealed in r at 1 kHz is supposed to be due to the electrode polarization. The electrode polarization is caused by accumulated charges on the electrode at a low frequency. Therefore, it implies the existence of charge carriers that migrate in the media. Figure 4 shows the difference in dielectric constants, ∆r, between the results measured at 1 kHz and 10 kHz for the octane system, along with the curve of κ at 1 kHz, where the values for the ordinate are scaled arbitrarily for comparison. The curve of ∆r resembles that of κ very closely despite its complexity. A strong correlation between ∆r and κ described above was seen in all the systems, corresponding to the abnormal r at 1 kHz. This allows us to consider that the large deviation in r at 1 kHz is certainly attributed to the electrode polarization, whereas dielectric dispersion was observed even for small r at frequencies higher than 1 kHz as is shown in Figures 2 and 3. This dispersion becomes larger with increasing r. For the limiting value at low frequency we adopted the results of κ at 1 kHz and the those of r at 10 kHz, respectively. In Figure 5 three sets of r measured in c-hexane at different molalities of AOT are shown. Although the

Forming AOT W/O Microemulsion

Figure 3. Electrical conductivities for (AOT + r‚H2O + c-hexane) measured at m ) 0.5 mol kg-1 for frequencies from 1 kHz to 1 MHz at 298 K.

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Figure 5. The dielectric constants for (AOT + r‚H2O + c-hexane) at 298 K. The molalities m of AOT in c-hexane are 4, 0.1 mol kg-1; O, 0.3 mol kg-1; 0, 0.5 mol kg-1.

Dielectric Constant. The fact that the presence of water is necessary for the AOT aggregation has been elucidated.11,28-30 In each case the dielectric constant starts to increase on addition of 1 mol of water per AOT molecule. Dielectric constants reflect the magnitude of apparent dipole moment, µ, of interacting polar solutes. The

calculation of µ is available for obtaining information about the symmetry of aggregates.31 A steep increment associated with the first step of forming aggregates is revealed in the systems with straight-chain alkane, and its slope increases in the order of heptane < octane < decane. The present results indicate that aggregates first formed in straight-chain alkanes with the help of water have significant magnitude of dipole moment; therefore, we may conclude that the aggregates in decane are most unsymmetrical and consequently, highly strained. Accordingly, it is reasonable to assume that the polar groups of aggregates first formed in straight-chain alkanes are, more or less, exposed to the media, whereas the firstformed aggregates in c-hexane are nearly symmetrical. The assemblies with a large aggregation number tend to take a symmetrical shape. The present results imply that the size of the first-formed aggregates is rather small. We have found that the aggregation of polyoxyethylene glycol monodecyl ether, C10En, is enhanced in a medium of longer chain alkane by measuring apparent molar heat capacity.32 Those aggregates take an extended hank-like form according to an analysis with a small angle neutron scattering,33 while in c-hexane, C10En does not form aggregates without the existence of water. These results strongly suggested that the topology of the solvent molecule is responsible for the shape of assembly as well as the driving force on association. In the present cases, the straight-chain alkanes interact attractively with the tails of AOT, while no specific interaction acts between AOT and c-hexane. In general, assemblies are composed of smaller elements when their concentration reaches a high enough value to associate. This condition is attained with increasing amount of water in the present cases. Thus, the aggregation is progressed with water as well as oil. A plateau appears at r ≈ 7 in heptane, at r ≈ 8 in octane, and at r ≈ 9 in decane. This shows that another kind of aggregate is formed with increasing amounts of water and they have essentially no dipole moment, namely, they are nearly symmetrical. We call these assemblies “reversed micelles” in this report. The small minimum, which is observed in

(28) Eicke, H. F.; Christen, H. Hev. Chim. Acta 1978, 61, 2258. (29) Kotlarshyk, M.; Huang, J. S.; Chen, S.-H. J. Phys. Chem. 1985, 89, 4382. (30) Tanaka, R.; Adachi, M. Netsu Sokutei 1991, 18, 138.

(31) Stokes, R. H.; Marsh, K. N. J. Chem. Thermodyn. 1976, 8, 709. (32) Tanaka, R.; Saito, A. J. Colloid Interface Sci. 1990, 134, 82. (33) Ravey, J. C.; Buzier, M.; Picot, C. J. Colloid Interface Sci. 1984, 97, 9.

Figure 4. The difference in dielectric constants ∆r, r(1 kHz) - r(10 kHz), and the conductivity κ at 1 kHz observed for (AOT + r‚H2O + octane) at m ) 0.3 mol kg-1. The values for ordinate are scaled arbitrary for comparison. 0, κ; b, ∆r.

change in r with increasing r is not clear at m ) 0.1 mol kg-1 the characteristic change appears clearly for m > 0.3 mol kg-1. Therefore, we decided to compare all the systems, measuring at m ) 0.3 mol kg-1. The molality of solution was adjusted within deviation of (0.003 mol kg-1. The results in dielectric constant at 10 kHz and conductivity at 1 kHz are illustrated in Figures 6a, 6b, and 7, respectively, along with enlargement for small r. Discussion

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r after reaching plateau, implies that the reversed micelles are turning more spherical by taking more water in the pool. Then, the dielectric constant in straight-chain alkanes begins to increase again at r ≈ 15 followed by a steep increment because the domain of water in droplets grows larger creating “free water”. We may call the particles for r > 15 “droplets” from the viewpoint that they have free water. Hauser et al.34 studied the property of water in AOT reversed micelles formed in isooctane by means of DSC, ESR, and NMR, and concluded that 13 mol of water is bound to an AOT molecule. Their hydration number coincides with our findings, since the first free water may appear actually at r < 15. The very sharp increment in r for r > 50 in heptane, for r > 45 in octane, and for r > 25 in decane, is consistent with the onsets of steep increment found in conductivity, and corresponds to so-called percolation transition. The abrupt increments in r can be attributed to a structural change in water domain that is turning bulky because bulk water has a high dielectric constant of about 80. The present results suggest that droplets interact, coming close together and then forming interconnected water pools at the onset of percolation. On the other hand, the slope is moderate in the c-hexane system and no steep increment is seen up to r ) 60 (see Figure 6a). This means that the reversed micelles formed in c-hexane are nearly spherical and their size is considerably small for a wide range of r, in contrast to those formed in straight-chain alkanes. Electrical Conductivity. The behavior in conductivity is very unique as illustrated in Figure 7. A steep increment starts with the addition of water containing more than 1.5 mol per AOT molecule in straight-chain alkanes. This coincides with the observation of dielectric constants. The increments of κ reveal that the first formation of aggregates is initiated, and in parallel some charge carriers are created. This is due to the hydration of polar heads of AOT, although the change carriers are unknown. It is very interesting to see that the slope in κ, resulting from the first aggregation process, is nearly the same in straight-chain alkanes, whereas in c-hexane, the increment of κ with increasing amounts of water is much slower because charge carriers are bound inside of aggregates. This occurs because the polar heads of aggregates are shielded with the tails of AOT. A bending of κ occurs in straight-chain alkanes at relatively small r as indicated with an arrow for each system: r ) 4 in decane, r ) 6 in octane and heptane. Although it is not clearly visible in c-hexane, an inflection appears at around r ) 6.5. Those abrupt changes permit us to consider that initially formed small aggregates are combined into larger assemblies (reversed micelle) reaching a critical concentration. Since the initial aggregates in decane are highly strained, they tend to combine into reversed micelles with small amounts of water, while the transition to reversed micelles is gradual in c-hexane because the initial aggregates are rather symmetrical. By adding water, the reversed micelles with nearly the same size are successively formed in the initial stage. Since the amount of charge carriers supplied by small aggregates decreases in this process of forming reversed micelles, the increment in conductivity with increasing r is depressed. On the other hand, the aspects in decane for r > 4 are quite different from other systems. A new contribution to the electrical conductivity appears in decane. The ag(34) Hauser, H.; Haering, G.; Pande, A.; Luisi, P. L. J. Phys. Chem. 1989, 93, 7869.

Tanaka and Shiromizu

Figure 6. (a) The dielectric constants for (AOT + r‚H2O + alkane) at m ) 0.3 mol kg-1 and 298 K. b, c-hexane; 0, heptane; 4, octane; ], decane. (b) The dielectric constants of (AOT + r‚H2O + alkane) for the region of small r observed at m ) 0.3 mol kg-1 and 298 K. b, c-hexane; 0, heptane; 4, octane; ], decane.

gregation number, as well as the size of micelles, is possibly much larger in decane than those formed in other solvents, as will be discussed later. Eicke and co-workers35 proposed a charge fluctuation model for the conductivity of W/O microemulsion. Hall36 later reported an improved model. According to their theory, the droplets consisting of water pool are charged due to migration of charges among droplets. The conductivity induced by the charged droplets depends on waterdroplet radius and the specific conductivity changes passing through a maximum. This mechanism may account for the characteristic change of conductivity in decane that increases and then decreases sharply with increasing r, prior to the onset of percolation. We suppose that reversed micelles formed in decane have a considerably large size. The positive contribution to κ that is attributed to the formation of charged particles balances with the negative contribution resulting from the decreasing number of small aggregates. The successive increment of κ in decane found (35) Eicke, H.-F.; Borkovec, M.; Das-Gupta, B. J. Phys. Chem. 1989, 93, 314. (36) Hall, D. G. J. Phys. Chem. 1990, 94, 429.

Forming AOT W/O Microemulsion

Figure 7. Electrical conductivities for (AOT + r‚H2O + alkane) at m ) 0.3 mol kg-1 and 298 K. b, c-hexane; 0, heptane; 4, octane; ], decane.

for 4 < r < 8 can be explained by the positive contribution resulting from charged particles that overcome the negative one since the created assemblies are large enough to have effective charge. From the observation of dielectric constant, it is suggested that the reversed micelles formed in decane are still unsymmetrical in a region of 4 < r < 9. On the other hand, the size of particles formed in c-hexane is relatively small and increases slowly and successively. Further addition of water results in a decrease of κ in all cases: the value of κ begins to decrease at r ≈ 17 in c-hexane, at r ≈ 15 in heptane and octane, and at r ≈ 8 in decane, respectively. Small aggregates are consumed to form larger assemblies by adding water so that this process provides a negative contribution on κ in c-hexane, heptane, and octane, while in decane, the particles lose their specific conductivity, since they are growing and passing through a critical size. The latter process also results in a decrease of κ. Consequently, an abrupt decrease of κ occurs definitely in decane by this mechanism. It is very interesting to see the behavior of ∆r illustrated in Figure 4 representing electrode polarization for the case of octane. The change of κ with r in the region of plateau is emphasized in the curve of ∆r, and there appears a shoulder at r ≈ 14. This small peak in ∆r, that is also observed in the system of heptane at r ≈ 14, might be attributed to the conductivity caused by charged particles as mentioned by Eicke et al.35 and Hall.36 Another possibility of charge carriers is the surface charges proposed by Schwarz.44 His model was applied for the dielectric property of (AOT + water + dodecane) system by Peyrelasse and Boned.19 It is expected that the surface charges of a reversed micelle result in a conductivity change accompanying a maximum with increasing (37) Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1981, 77, 601. (38) Gruen, D. W. R.; Haydon, D. A. Pure Appl. Chem. 1980, 52, 1229. (39) Gruen, D. W. R. Chem. Phys. Lipid 1982, 30, 105. (40) Sager, W. F. C.; Blokhuis, E. M. Prog. Colloid Polym. Sci. 1998, 110, 258. (41) Blokhuis, E. M.; Sager, W. F. C. J. Chem. Phys. 1999, 110, 3148. (42) Svergun, D. I.; Konarev, P. V.; Volkov, V. V.; Koch, M. H.; Sager, W. F. C.; Smeets, J.; Blokhuis, E. M. J. Chem. Phys. 2000, 113, 1651. (43) Johannsson, R.; Almgren, M. Langmuir 1993, 9, 2879. (44) Schwarz, G. J. Phys. Chem. 1962, 66, 2636.

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r, as was correlated with the dynamic viscosity by those authors. The surface charge model is also acceptable to explain the present results of conductivity, as well as charged particle model. Manabe et al.11 also found a unique change of the conductivity for the system in dodecane with increasing amounts of water. It is interesting to see that a sharp decrease in κ prior to the percolation transition, that is observed in the present systems, does not appear, while their measurements were carried out below m < 0.3 mol kg-1. The particles formed from small aggregates in dodecane must be highly strained. Therefore, we suppose that the droplets in dodecane coalesce with small amounts of water, leading to percolation without a decrease in κ. The oil penetration is of interest because it has a strong influence on the shape and properties of reversed micellar assemblies. Mitchell and co-workers8,37 argued that the microstructure in microemulsions is set by a curvature arising from a balance between repulsive headgroup forces and opposing forces due to oil uptake in surfactant hydrocarbon tails, together with overriding constraint set by geometric packing. In this point of view, the oil penetration is an important term for determining the droplet topology. Their discussion was based on the reports38,39 describing that a short-chain alkane, such as hexane, penetrates strongly but a long-chain alkane, such as hexadecane, does not absorb into the surfactant tail region. Sager and the co-workers40,41 theoretically analyzed the aggregation process of W/O microemulsion droplets using the curvature energy concept that is represented in terms of rigidity constants and spontaneous curvature. They have demonstrated that aggregation takes place in the direction of vanishing spontaneous curvature, leading to structural change from droplets to cylinders or rodlike aggregates, rather than to bi-continuous phase. They also carried out a careful study of a small-angle X-ray scattering and proved that a transition occurs from spherical droplets to cylindrical or rodlike aggregates in (AOT + H2O + isooctane) induced by temperature increment.42 As expected, they found that the size distribution of the larger particles shifts significantly depending on the water amount; however, there always exist distinct fractions of small particles corresponding to reversed micelles. Their observation is accounted for as follows, and is consistent with our results. During the inter-droplet interactions some molecular groups consisting of AOT and water, either reversed micelles or small aggregates, are expelled because the surface area of droplets decreases on coalescence. Alexandridis et al.5 calculated thermodynamic parameters associated with droplet clustering during percolation and the determined standard enthalpy of clustering was positive. Our model, described above, may also explain their results in terms of expeling of aggregates, while they proposed a rather complicated mechanism. The unique dependence of oil found in the dielectric properties, as AOT molecules aggregate and eventually attain percolation transition, is a strong piece of evidence that the reversed micelles as well as droplets interact with straight-chain alkanes used for solvent. Jada et al.3 have shown that the rate constant ke for droplet collision associated with the exchange of material must reach a specific value in order to cause percolation transition, irrespective of surfactant, oil, temperature and volume fraction of water. They also mentioned that a strong correlation exists between ke and inter-droplet interactions with surfactant and oil chain length. Johannsson and Almgren43 concluded that long chain solvents stabilize the droplet pair in an encounter and increase the prob-

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ability of fusion. Therefore, the rate of material exchange between droplets will increase. Their speculation may be explained in terms of penetration and the bridge of long chain alkanes laid across droplets. Mays and Ilgenfritz6 have investigated cluster dynamics employing the time-resolved luminescence technique for AOT W/O microemulsion. They concluded that the mechanism for percolation changes from activation-controlled rate limiting to diffusion-controlled cluster aggregation. They found a very interesting fact, that the activation energy Ea of inter-cluster exchange increases proportionally with oil chain length, and the activation energy in a mixed oil with a different chain length of alkane is expressed by an ideal formula: Ea ) X1Ea,1 + X2Ea,2, where X1 and X2 are the mole fractions of mixed oil forming the chain length of a oil. We interpret their results as follows. Since a longer chain alkane penetrates deeper into the monolayer of droplets, the packing effect gives rise to a higher activation energy due to dispersion force making the outer shell of droplets more rigid. Consequently, the droplets favor smaller curvature that can be achieved on coalescence. This process is enhanced in a straight-chain alkane with larger length. The oil penetration competes with the potential energy of the oil molecules stabilized in bulk. One can see a nice linearity between the molar enthalpy of vaporization and the chain length for straight-chain alkanes, but those for branched alkanes deviate from this linearity. This fact is the evidence that straight-chain alkanes in a liquid state are stabilized forming hunk-like aggregates, as is also inferred from their structure in a solid state as determined by X-ray analysis. Therefore, a long-chain alkane such as undecane prefers staying in bulk rather than migrating into a micellar core, therefore causing phase separation6 before reaching percolation as the temperature is raised. Conclusion The aggregation process of AOT on addition of water depends strongly on the used solvent and progresses stepwise with the amount of water in straight-chain alkanes. The aggregation starts at r ≈ 1.5, forming the first aggregates with a small aggregation number. Its shape is unsymmetrical in a longer chain hydrocarbon because it is strongly strained by interactions with solvent molecules and, therefore, has a dipole moment. However,

Tanaka and Shiromizu

it is rather symmetrical in c-hexane. Since the polar groups of aggregates formed in straight-chain alkanes are exposed to media, they supply charge carriers inducing the electrical conductivity. The secondary formed aggregates (reversed micelles) are nearly symmetrical assemblies, except in decane. A straight-chain alkane with a longer length penetrates deeper into the tails of reversed micelles. Because those alkanes make the monolayer rigid due to the dispersion force, a small curvature for the outer core of micelles is favored. The positive contribution to the dielectric constant and conductivity brought about by unsymmetrical small aggregates, balances the negative contribution yielded by the formation of micelles. Thus, a plateau appears in dielectric constants, and a bending appears in conductivity for systems with heptane and octane. On the other hand, the particles of large size are charged by thermal fluctuation, or accompanied with surface charges. This mechanism occurs definitely in decane. On further addition of water, the conductivity begins to decrease resulting from the decreasing number of the first-formed aggregates, and the growth of reject particles in a long-chain alkane. In c-hexane the increasing number of the reversed micelles depresses the increment of the dielectric constant and conductivity, revealing smooth changes in κ and r with r. The growth of micelles, that begins for r > 15 in all cases, contributes positively to the dielectric constant because of its structural change in water. As the particles reach critical size, a dropletdroplet interaction, namely, the onset of percolation occurs. In c-hexane, however, water is stabilized while stored in small droplets so that the percolation transition is delayed. A number of studies have been carried out to understand the structure and physicochemical properties involving the aggregation process for AOT W/O microemulsion. However, they are not necessarily in agreement with each other. We propose a possible mechanism for forming assemblies of AOT with water in oil. Precise and systematic work, especially for the analysis of particle size and its distribution for small r, is required to clarify so many questions for this class of systems. Acknowledgment. We thank Dr. T. Hanai and Dr. T. Fujii for helpful discussions. LA010368P