Solubilization and Phase Behavior of Microemulsions with Mixed

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Langmuir 1999, 15, 7973-7979

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Solubilization and Phase Behavior of Microemulsions with Mixed Anionic-Cationic Surfactants and Hexanol Xingfu Li,† Koichi Ueda,‡ and Hironobu Kunieda*,† Graduate School of Engineering, Yokohama National University, Tokiwadai 79-5, Hodogaya-ku, Yokohama 240-8501, Japan, and Basic Research Laboratory, Noevir Co., Ltd., Okada-cho 112-1, Youkaichi 527-0057, Japan Received December 22, 1998. In Final Form: July 26, 1999 The minimum surfactant (C + D + E) fraction in the system, Xb, to solubilize equal amounts of water and oil in a single microemulsion phase, in the aqueous NaBr (A)/dodecane (B)/sodium dodecyl sulfate (C)/dodecyltrimethylammonium bromide (D)/hexanol (E) systems, decreases with the change in the fraction Y of the cationic to anionic and cationic surfactants, D/(C + D), from 0 to 0.30 and from 1.00 to 0.70 at 25 °C, respectively. However, the three-phase region is not formed and the solubilization capacity becomes very low at Y between 0.40 and 0.60 due to the formation of liquid crystal. The monomeric hexanol solubility S1 in the micro-oil domain and the surfactant fractions C1 and C2 at the oil-water interface inside the microemulsion were estimated according to the surfactant distribution equations under the assumption that the cationic-anionic surfactants are located only at the interface. The decrease in Xb (or the increase in the solubilization) by mixing two surfactants is mainly attributed to two factors: the decrease in the S1 and the increase in the solubilization capacity of the mixed surfactant layer inside the microemulsion itself (or the decrease in C1 + C2). The latter factor may be directly related to the strong interactions between anionic and cationic amphiphile molecules. However, around Y ) 0.517 with equal molar mixture of C and D, the interaction is too strong and the liquid crystal is formed even in the dilute region.

Introduction Recently there has been a growing interest in research on the supramolecular surfactant assemblies that include micelles, microemulsions, liquid crystals, monolayers, vesicles, etc., with mixed surfactants containing one anionic and one cationic surfactant by Kaler,1-4 Lindman,5,6 Shah,7 Khan,8,9 Zhao,10 et al. because in such mixed systems there exist many unique physicochemical properties that arise from the strong electrostatic interactions between the oppositely charged headgroups. Since the mixture of anionic and cationic surfactants tends to form a crystalline precipitate,6 research involving the microemulsions using a combination of one anionic and one cationic surfactant had not appeared on the publications until Bourrel’s first research11,12 in 1984. Lindman and Jonsson5,6 investigated the phase behavior of microemulsions with catanionic surfactants9 composed of a cationic * To whom correspondence should be addressed. † Yokohama National University. ‡ Noevir Co., Ltd. (1) Kaler, E. W.; Murthy, A. K.; Rodriguez, B.; Zasadzinski, J. A. Science 1989, 245, 1371. (2) Kaler, E. W.; Herrington, K. L.; Murthy, A. K.; Zasadzinski, J. A. J. Phys. Chem. 1992, 96, 6698. (3) Herrington, K. L.; Kaler, E. W.; Miller, D. D.; Zasadzinski, J. A.; Chirulvolu, S. J. Phys. Chem. 1993, 97, 13792. (4) Yatcilla, M. T.; Herrington, K. L.; Brasher, L. L.; Kaler, E. W.; Chirulvolu, S. J.; et al. J. Phys. Chem. 1996, 100, 5874. (5) Marques, E.; Khan, A.; Miguel, M. G.; Lindman, B. J. Phys. Chem. 1993, 97, 4729. (6) Jonsson, B.; Jokela, A.; Khan, B.; Lindman, B.; Sadaghiani, A. Langmuir 1991, 7, 889. (7) Patist, V.; Chhabra, V.; Pagidipati, R.; Shah, R.; Shah, D. O. Langmuir 1997, 13, 432. (8) Jokela, P.; Jonsson, B.; Khan, A. J. Phys. Chem. 1987, 91, 3291. (9) Caria, A.; Khan, A. Langmuir 1996, 12, 6282. (10) Yu, Zhi-Jian; Zhang, Xingkang; Xu, Guangzhi; Zhao, Guo-Xi J. Phys. Chem. 1989, 93, 7441; 7446; 1990, 94, 3675. (11) Bourrel, M.; Schechter, R. S. Microemulsions and Related Systems; Academic Press: New York, 1988; Chapter 4, 5. (12) Bourrel, M.; Bernard, D.; Graciaa, A. Tenside Deterg. 1984, 21, 311.

and an anionic surfactant in equimolar ratio with the small counterions removed. However, in the absence of cosurfactant such as medium-chain alcohol, the surfactant mixture tends to be precipitate or to form a lamellar liquid crystal instead of isotropic microemulsions due to the strong interaction of oppositely charged headgroups. Our group13-16 has investigated the solubilization and phase behavior of microemulsions with mixed anioniccationic surfactants and alcohol and found that the mixed surfactants are more beneficial to form w/o microemulsions with a low surfactant content than the single surfactant alone. Although the three-phase behavior in the systems containing anionic and cationic surfactants highly depends on the cationic surfactant fraction,14,15 the effect of added quaternary salt surfactant on the maximum solubilization has not been comprehensively understood yet.11,15 It is well-known that the solubilizing capacity of surfactant reaches its maximum when a surfactant or surfactant mixture is in a balanced state, in which the HLB becomes optimum for a given water-oil system.17-24 Since the middle-microemulsion phase near the threephase point, at which the microemulsion phase coexists with excess water and oil at the HLB temperature17,19,20,23,24 (13) Lin Enhui and Li Xingfu, Shiyoulianzhi 1991, (8), 26. (14) Li, Xingfu; Zhao, Guohu; Lin, Enhui J. Dispersion Sci. Techn. 1996, 17, 111. (15) Li, Xingfu; Lin, Enhui J. Colloid Interface Sci. 1996, 184, 20. (16) Li, Xingfu; Wang, Jianzhong; Wang, Jin J. Dispersion Sci. Technol. 1999, 20, 993. (17) Kunieda, H.; Nakano, A.; Akimaru, M. J. Colloid Interface Sci. 1995, 170, 78. (18) Kunieda, H.; Aoki, R. Langmuir 1996, 12, 5796. (19) Kunieda, H.; Ushio, N.; Nakano, A.; Miura, M. J. Colloid Interface Sci. 1993, 159, 37. (20) Kunieda, H.; Nakano, A.; Pes, M. A. Langmuir 1995, 11, 3302. (21) Pes, M. A.; Aramaki, K., Nakamura, N.; Kunieda, H. J. Colloid Interface Sci. 1996, 178, 666. (22) Kunieda, H.; Sato, Y. In Organized Solutions; Lindman, B., Friberg, S. E., Eds.; Marcel Dekker: New York, 1992; p 67. (23) Kunieda, H.; Yamagata, M. Langmuir 1993, 9, 3345. (24) Stubenrauch, C.; Paeplow, B.; Findenegg, G. H. Langmuir 1997, 13, 3652.

10.1021/la981748m CCC: $18.00 © 1999 American Chemical Society Published on Web 09/16/1999

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and/or HLB composition,18 has a bicontinuous structure, surfactant and cosurfactant molecules are distributed in microwater and oil domains, and at the interface (surfactant layer) inside the microemulsion. In the previous studies, we investigated the effect of mixed nonionic surfactants17,19,20,23 and added salt18 on the maximum solubilization in the three-phase microemulsion systems. When the total surfactant concentration is used to evaluate the solubilizing capacity of mixed surfactants, the distribution of each surfactant in excess oil and at interface must be considered. For the mixture of ionic surfactant and cosurfactant such as a mediumchain alcohol, for example, a large amount of alcohol is dissolved in oil and only a part of alcohol is adsorbed at the micro oil-water interface inside the microemulsion phase. Hence, we have to know the net amount of total surfactant at the interface to evaluate the solubilizing power. A quantitative method20-22 has been developed to calculate the net composition at the microinterface in the balanced microemulsions by determining the three-phase plane in phase diagrams. However, this method has not been applied to the catanionic systems yet. In this context, the solubilization and phase behavior of SDS-DTAB-hexanol-dodecane-NaBr aqueous systems were investigated at constant pressure and temperature. The three-phase behavior of microemulsions was studied as a function of the cationic surfactant fraction in order to understand the relationships between the maximum solubilization of microemulsions and surfactant distribution of mixed surfactants at the water-oil interface in the microemulsion phase. Experimental Section Materials. Sodium dodecyl sulfate (SDS) was obtained from Sigma Chemical Co. with purity greater than 99%. Dodecyltrimethylammonium bromide (DTAB) at G.R., extra-pure grade n-dodecane (C12), 1-hexanol, and sodium bromide (NaBr) were used as received from Tokyo Kasei Co. Water was distilled and deionized. Phase Diagram Determination. Samples were prepared by first making stock solutions of either ionic surfactant or sodium bromide at the desired concentration in deionized water. The stock solutions were equilibrated at room temperature, and then samples were prepared by vortex mixing the stock solutions at the desired ratio. Samples were equilibrated for at least 1 week in a thermostatic bath. Except for vortex mixing and gravity filtration, the solutions were not subjected to any type of mechanical agitation. All subsequent experiments were performed at 25.0 °C. Phase equilibria were determined by visual observation and between crossed polarizer films to determine the number of phases and the presence of any lamellar or hexagonal liquid crystal phases. Gas Chromatography. The monomeric solubility of hexanol in excess oil phase was determined by means of gas chromatography (Yokogawa-Hewlett-Packard Co., HP-5890, FID as detector) and column chromatography (J & W Co., NB-1, 0.25 µm × 60 m). Helium was used as a carrier gas (2.5 psi flow speed). Temperature was increased from 100 to 250 °C (15 °C/ min) and kept for 5 min at the start temperature and 2 min at the final temperature. The measurement was repeated three times for each sample, and the experimental error is in the range of 0.3%.

Results Ternary Phase Diagrams for Dodecane-NaBr Solution (1 wt %)-SDS-DTAB-Hexanol Systems. The ternary phase diagrams for dodecane-NaBr solution (1 wt %)-SDS-DTAB-hexanol systems at 25 °C and at different DTAB weight fractions, Y, are shown in Figure 1. In this case an equal mass fraction of oil to brine is maintained (Row ) 0.50), and the weight fractions (Y) of

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the cationic to anionic and cationic surfactants are fixed at 0, 0.10, and 0.20, respectively. Single- (I), two- (II), and three-phase (III) regions appear and are duly noted in Figure 1. “LR-present” represents a region containing a lamellar liquid crystal, and “Solid present” indicates a multiphase region containing solid crystals of surfactants. The birefringent LR phase intrudes an isotropic microemulsion domain in a dilute region and is surrounded by the single-phase microemulsion designated as region I. The three-phase region III touches the microemulsion phase at a so-called three-phase point indicated by the point R in Figure 1a. As a small amount of cationic surfactant is added into the systems, the LR-present and microemulsion regions are shifted to a lower hexanol content region at Y ) 0.10 and 0.20, as shown in parts b and c of Figure 1. This means that the surfactant becomes less hydrophilic by replacing a part of SDS with DTAB. The strong interaction between the oppositely charged headgroups of the surfactants causes the reduction of the hydrophilicity. The hexanol and ionic surfactant fractions in the system at the three-phase points are 0.232, 0.018; 0.120, 0.020; and 0.056, 0.014 at Y ) 0, 0.10, and 0.20, respectively. This implies that the maximum solubilization increases when the single phase moves toward the W/O apex upon increasing the cationic surfactant fraction. In Figure 1, the three-phase regions do not orient toward the W/O apex. It means that the mixing fractions of each amphiphile to form the three-phase body are changed with varying the total amphiphilic fraction in the system. As mentioned before, the distribution of oil-soluble cosurfactant between the bulk oil and the oil-water interface causes distortion of the three-phase body. In other words, if the oil-soluble cosurfactant, hexanol, was not soluble in the bulk oil phase, the three-phase region should orient toward the W/O apex. Since the weight fraction of hexanol in ionic surfactants + hexanol is always too high at 1 wt % NaBr, it is difficult to analyze the three-phase behavior accurately. For this reason, the detailed three-phase behavior is investigated at 2 and 3 wt % NaBr in the following sections. Phase Diagram for Dodecane-NaBr Solution (2 and 3 wt %)-SDS-DTAB-Hexanol Systems. In the absence of external fields and at constant pressure, pseudoquinary systems brine (A)-oil (B)-anionic surfactant (C)-cationic surfactant (D)-alcohol (E) have five independent thermodynamic variables,15,17,25 namely, the temperature and four composition variables at constant salinity. We found it convenient to introduce the weight fraction of the oil in the mixture of oil and brine, Row ) B/(A + B), that of both amphiphiles in the mixture, X ) (C + D + E)/(A + B + C + D + E), that of the cationic in the mixture of the two surfactants, Y ) D/(C + D), and the hexanol weight fraction, W1 ) E/(C + D + E). Since a salt mainly dissolves in water, the salt/water weight ratio is approximately kept constant in each phase of the three-phase region. The aqueous salt solution is considered to be pseudo one component at fixed temperature and salinity.18,25 Although the mixing of SDS and DTAB forms the additional NaBr, this does not increase the degree of freedom of these mixed systems. To represent the phase behavior in two-dimensional space, one has to dispense with two of those variables by, e.g., keeping them constant. We can study the hexanol weight fraction W1, for example, as a function of total surfactant concentration X, at constant Row and temperature, with an increase in the cationic surfactant fraction Y. (25) Kahlweit M.; Strey, R. J. Phys. Chem. 1987, 91, 1553; 1988, 92, 1557.

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Figure 1. Ternary phase diagrams for the systems dodecane-brine-SDS-DTAB-hexanol at different DTAB weight fractions: (a) Y ) 0, (b) Y ) 0.10, (c) Y ) 0.20; T ) 25 °C. The weight ratio of 1 wt % NaBr aqueous solution to dodecane is kept constant. I, II, and III indicate one-, two-, and three-phase regions, respectively. “LR-present” represents the region including a lamellar liquid crystal, and “Solid present” indicates a multiphase region containing the solid crystal of mixed surfactants. The apex, W/O is a point at which 50 wt % brine and 50 wt % dodecane are present.

The W1-X phase diagrams are shown in Figures 2 and 3 at 25 °C and Row ) 0.50. The w/o-type microemulsion phase coexists with excess water at high W1, whereas the o/w-type microemulsion phase coexists with excess oil at low W1. Upon addition of a sufficient quantity of amphiphile (surfactant plus alcohol), a single-phase microemulsion region (I) is formed. The amphiphilic fraction Xb at the three-phase point, as marked in Figures 2a and 3c, indicates the minimum concentration of amphiphile required to dissolve equal mass fractions of oil and brine in a single-phase microemulsion at each W1b. “LR-present” has the same meaning as appeared in Figure 1. The maximum solubilization, as can be seen from Figures 2 and 3, increases (or Xb decreases) when the three-phase bodies moves downward with an increase in Y from 0 to 0.30 in the SDS-rich systems (Figures 2a and

3a), and with a decrease in Y from 1.00 to 0.70 in the DTAB-rich systems (Figure 3c), respectively. The threephase regions are not parallel to the horizontal axis and the W1 to form the three-phase region increases with decreasing X in Figures 2a and 3a,c. This distortion18,21 of the three-phase body clearly suggests that the oil-soluble cosurfactant, hexanol, tends to dissolve in the excess oil phase in a dilute region. As the surfactant fraction Y approaches 0.517, corresponding to the equal-molar mixture of SDS and DTAB, the three-phase body is not formed at Y ) 0.40-0.60, as shown in Figures 2b and 3b. In these phase diagrams, the liquid crystal is present even in a dilute region. It is known that the cationic-anionic mixture with an equal molar ratio tends to form vesicles or lamellar liquid crystals, instead of micelles in water, because the cross sectional

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Figure 2. Vertical sections through the pseudo-ternary-phase prism for dodecane-brine-SDS-DTAB-hexanol systems with varying DTAB weight fraction Y at equal mass fractions of 2 wt % NaBr solution and dodecane (Row ) 0.50) and 25 °C: (a) Y ) 0, 0.10, 0.20, 0.30; (b) Y ) 0.40, 0.517, respectively.

area of the polar headgroups of surfactants in aggregates decreases due to the strong interaction between the oppositely charged polar groups.2,4,5,26 The compositions Xb and W1b at each three-phase point are plotted as a function of Y, and the result is shown in Figure 4. Both Xb and W1b decrease sharply upon increasing the cationic surfactant fraction Y from 0 to 0.35 and decreasing Y from 1.00 to 0.70, respectively. W1b reaches a minimum at the lowest hexanol content whereas Xb arrives at a maximum with the least solubilization at Y ) 0.517. It is clear from Figure 4 that since the hydrophilicity of the mixed ionic surfactants reaches the lowest at Y ) 0.517, the required content of hexanol to obtain the balanced microemulsion also becomes minimum. However, the solubilization of water and oil does not reach its maximum at this Y because the LR phase is formed even in a very dilute region. Discussion Surfactant Fractions inside the Bicontinuous Microemulsion. As mentioned before, to understand the (26) Bergstrom, M.; Eriksson, J. C. Langmuir 1998, 14, 3754.

Figure 3. Vertical sections through the pseudo-ternary-phase prism for dodecane-brine-SDS-DTAB-hexanol systems with varying DTAB weight fraction Y at equal weights of 3 wt % NaBr solution and dodecane (Row ) 0.50) and 25 °C: (a) Y ) 0, 0.10, 0.20, 0.30; (b) Y ) 0.40, 0.517, 0.60; (c) Y ) 0.70, 0.90, 1.00, respectively.

net solubilizing capacity of the mixed surfactant or its phase behavior, one has to know the distribution of hexanol between the bulk oil and the interface (surfactant layer) inside the microemulsion. According to the phase rule, there is no degree of freedom left in the three-phase region because the 5 intensive variables (T, P, salinity, Row, and

Microemulsions with Mixed Surfactants

Figure 4. Xb and W1b values obtained from the three-phase points (a) in Figure 2a,b and Figure 3a-c as a function of DTAB weight fraction Y for dodecane-NaBr aqueous-SDS-DTABhexanol systems at T ) 25 °C and Row ) 0.50: Xb (2) and W1b ([) for the systems containing 2% NaBr solution; Xb (9) and W1b (×) for the systems containing 3% NaBr solution.

Figure 5. Schematic representation of the bicontinuous microemulsion structure in the systems containing SDS, DTAB, and hexanol.

Y) are fixed for the six-component-containing systems. The composition and structure of the microemulsion phase along the midst curve in the three-phase body are also fixed and equal to the single phase at the three-phase point in Figures 2 and 3.11,20-21 Only the total volume of microemulsion increases as the hexanol weight fraction is decreased and finally reaches the maximum at the optimum hexanol weight fraction.11 The schematic structure of the bicontinuous middle-phase microemulsion phase for the mixed-surfactant system near the threephase point is shown in Figure 5. The surfactant molecules are adsorbed and form a surfactant layer at the interface between oil and water microdomains. Using the following assumptions, we can analyze the three-phase behavior to evaluate the cosurfactant distribution in the microemulsion phase. The monomeric solubility of each surfactant in the excess water phase forming a three-phase body is extremely low and negligible,18 the monomeric solubility of hexanol in the excess water is also very low and can be neglected, especially in the presence of ionic surfactant and inorganic

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salt. Hence, the excess water phase can be regarded as a pure brine. It is assumed that the water microdomain within the microemulsion is the same as the excess water phase, as shown in Figure 5. Each ionic surfactant is also almost insoluble in the excess oil phase in a monomeric state. It is also assumed that the excess oil phase is the same as the micro-oil domain inside the microemulsion.18-20 Under the above assumptions, the particular threephase triangle including the microemulsion at the threephase point R, should be a plane in the composition tetrahedron of a pseudo-four-component brine/mixed surfactants/hexanol/oil system at constant temperature and pressure, in which the salinity and the surfactant mixing fraction are fixed. Then, the particular triangle becomes a curve in the midst of the three-phase region11,20,22 in Figures 2 and 3. According to the composition relations in the three-phase triangle in the tetrahedron space, we obtain20,22

W1 ) S1s -

S1(1 - S1s) (1 - S1)

Row(1/X - 1)

(1)

where Row is the weight fraction of oil in the mixture of water and oil and is fixed at 0.5 in the present study. S1 is the monomeric solubility of hexanol in the micro-oil domain and is equal to the hexanol fraction in the excess oil phase. S1s represents the hexanol weight fraction in total amphiphile mixture (C + D + E) at the water-oil interface inside the microemulsion phase. X is the weight fraction of total amphiphile (C + D + E) in the system. It is verified that eq 1 holds in many ionic and nonionic surfactant systems.19-21,23,24 Since the critical micelle concentration (cmc) in the anionic-cationic mixture system is much lower than that in the previous single surfactant systems, the assumptions would be more valid in the present system. The hexanol weight fraction W1 along the midpoint line of the three-phase regions in Figures 2a and 3a,c is plotted against 1/X - 1, and the result conforms to the linear relationships shown in Figure 6. The S1 and S1s can be estimated from the intercept and the slope of those straight lines according to the surfactant distribution equation (eq 1). The S1 values in the excess oil phase were also measured by GC technique at the points P along the midpoint lines in the three-phase regions marked in Figures 2 and 3. Both columns of values are in a good agreement, as shown in Table 1. Therefore, our assumption is approximately valid in the present SDS-DTAB systems. Since surfactant molecules at the interface (surfactant layer) are directly related to the solubilization of water and oil in the microemulsion, the surfactant concentration at the microinterface has to be obtained. By use of the mass balance equations,18,20 the following relations hold for two-surfactant and alcohol systems

C1 ) XbW1b -

Row(1 - Xb)S1 (1 - S1)

(2)

and

C2 ) Xb(1 - W1b)

(3)

where C1 and C2 are the weight fractions of lipophilic hexanol and hydrophilic surfactants (SDS + DTAB) in the system at the water-oil interface within the singlephase microemulsion. One can compare the net solubilizing capacity of surfactants by using the weight fraction

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Table 1. Xb, W1b, S1, S1s, C1, and C2 for Dodecane-NaBr aqueous-SDS-DTAB-Hexanol Systems at 25 °C Y

Xb

W1b

S1 by eq 1

0 0.1 0.2 0.3 0.4 0.517

0.136 0.091 0.055 0.032 0.160 0.240

0.800 0.750 0.650 0.500 0.125 0.080

0.196 0.115 0.061 0.025

0. 0.1 0.2 0.3 0.4 0.517 0.6 0.7 0.9 1

0.103 0.067 0.037 0.023 0.160 0.240 0.160 0.032 0.092 0.116

0.746 0.700 0.650 0.500 0.125 0.080 0.100 0.300 0.415 0.485

0.129 0.077 0.029 0.020

0.011 0.032 0.051

C 1 + C2

C1/(C1 + C2)

Systems Containing 2 wt % NaBr 0.075 0.0028 0.0271 0.260 0.0093 0.0228 0.200 0.0049 0.0193 0.190 0.0036 0.0160

0.0299 0.0321 0.0241 0.0196

0.094 0.289 0.203 0.183

Systems Containing 3 wt % NaBr 0.260 0.0098 0.0260 0.275 0.0079 0.0201 0.440 0.0099 0.0131 0.110 0.0015 0.0114

0.0358 0.0280 0.0229 0.0129

0.273 0.283 0.430 0.115

0.120 0.071

0.0266 0.0771 0.0922

0.157 0.302 0.352

0.010 0.030 0.050

S1s

0.160 0.310 0.350

Figure 6. Hexanol weight fraction W1 values as a function of 1/X - 1 obtained from the midpoints of the three-phase bodies in Figures 2a,b and 3a-c for the systems containing 2 wt % NaBr solution (a) and 3 wt % NaBr solution (b).

of total surfactants (C + D + E) in the mixture, C1 + C2, in the surfactant monolayer at the water-oil interface inside the microemulsion phase. Using eqs 2 and 3, C1 and C2 are calculated for the mixed surfactant systems and are shown together with Xb, W1b, S1, S1s, etc., in Table 1. S1s in eq 1 should be equal to C1/(C1 + C2) in eqs 2 and 3. Both columns of values are in fairly good agreement although there is some experimental error. Note that eqs

C1

C2

0.0042 0.0233 0.0325

0.0224 0.0538 0.0597

S1 by GC

0.102

1-3 cannot apply to a very dilute region near the cmc, where the distribution of surfactants and hexanol in water has to be taken into account. Solubilization Capacity of Mixed Surfactants. As shown in Table 1, the hexanol solubility in oil, S1, dramatically decreases as the cationic surfactant fraction, Y, approaches 0.517 from both cationic and anionic surfactant-rich sides. The weight fraction of total amphiphile in the system at the three-phase point, Xb, also decreases when Y approaches 0.517 except the Y between 0.4 and 0.6, as shown in Figure 4 and Table 1. Therefore, the solubilizing capacity of the mixed surfactant increases mainly due to the reduction of the monomeric solubility of hexanol in oil. With increasing salinity from 2 to 3 wt % NaBr, Xb also decreases due to the decrease in the hexanol solubility in oil, S1, as shown in Table 1 and illustrated in Figure 4. Therefore, the net solubilization capacity of surfactant layer at the oil-water interface inside the microemulsion also increases at high salinity in the present range of salinity. With cationic and anionic surfactants mixed or salinity increased, the dissociation of polar headgroups is suppressed and hexanol molecules become more soluble in the surfactant layer of the microemulsion. This may be the reason S1 decreases. However, this is not only one reason for the increase in the solubilization, because C1 + C2, the weight fraction of cationic-anionic surfactant and hexanol at the oil-water interface inside the microemulsion, also decreases when Y approaches 0.517 except for the case at Y ) 0 in 2 wt % NaBr system. The lower the C1 + C2, the more efficient the interface inside the microemulsion. In other words, the solubilizing power per surfactant molecule itself increases by mixing cationic and anionic surfactants. The surfactant layer, by mixing the anionic-cationic surfactants, becomes more rigid due to the strong interaction of oppositely charged polar heads resulting in a decrease in an effective cross sectional area of the headgroup. It is known that a self-organized structure is changed from micelle to vesicle in the binary waterDTAB + SDS system when Y approaches 0.517.26 It means that the cross sectional area is considerably reduced. Hence, the surfactant molecular layer becomes rigid and the surfactant curvature tends to be reduced. In fact, a similar phenomenon is observed in mixed nonionic surfactant systems although the origin of the interactions is different.27,28 This may be the reason the solubilization (27) Nakamura, N.; Yamaguchi, Y.; Hakansson, B.; Olsson, U.; Tagawa, T.; Kunieda, H. J. Dispersion Sci. Technol. 1999, 20, 535. (28) Kunieda, H.; et al. Submitted.

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increases with changing Y from Y ) 0 to 0.35 and from Y ) 1.00 to 0.70, respectively. However, the interaction is too strong to form isotropic microemulsion in the vicinity of the equal molar mixture, Y ) 0.517. Then, the lamellar liquid crystal, instead of microemulsion, is formed in a dilute region around Y ) 0.517. Consequently, there are two factors to increase the solubilizing power of ionic surfactant-cosurfactant mixture by mixing cationic and anionic surfactants. The distribution of the hexanol at the surfactant layer increases and the efficiency of solubilization by the surfactant layer also increases upon the mixing. HLB Composition of Different Surfactants at Interface. The hydrophile-lipophile property of surfactant is just balanced in the three-phase region.18,20,21,23 In the most of previous studies, the apparent mixing fraction of cosurfactant, W1b, is used to evaluate the HLB composition or optimum mixing ratio of ionic surfactant and cosurfactant. However, since W1b includes the monomeric solubility of hexanol in oil, it does not directly reflect the optimum mixing fraction of cosurfactant at the micro water-oil interface inside the microemulsion phase. To evaluate the HLB composition accurately, one has to use the hexanol weight fraction S1s or C1/(C1 + C2). There is a very interesting tendency toward the change in S1s as a function of Y, as shown in Table 1. The S1s increases at first and then decreases with increasing Y from Y ) 0 to 0.517 and, on the other hand, decreases monotonically with decreasing Y from Y ) 1.0 to 0.517. It is reasonable that the S1s decreases by mixing the two surfactants because the hydrophilicity of the ionic surfactant decreases due to the strong interactions between the polar headgroups. Generally, the S1s should be monotonically decreased from both Y ) 0 and 1.0 to Y ) 0.517 if only Y changes. However, S1s cannot reach the highest value around Y ) 0. The S1s below the expected level at Y ) 0-0.1 is due to the dissolution of too much hexanol into the oil phase. Around Y ) 0, the monomeric solubility of hexanol in oil, S1, dramatically decreases with the increase in Y or upon

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addition of cationic surfactant. It is known that more hydrophilic surfactant is needed to form a three-phase body when polar oil is used instead of saturated hydrocarbon.18,23 Around Y ) 0, the large S1 means the oil is not simple dodecane but a dodecane-hexanol mixture. Therefore, the rather hydrophilic surfactant has to be used to adjust the HLB of mixed surfactants. This is the reason the S1s is low at Y ) 0 for 2 wt % NaBr system and at Y ) 0 and 0.10 for 3 wt % NaBr systems. Conclusions In aqueous NaBr/sodium dodecyl sulfate/dodecyltrimethylammonium bromide/ hexanol/dodecane systems, the minimum weight fraction of total surfactants (surfactants + hexanol) to solubilize equal mass fractions of water and oil in a single phase, Xb, decreases with an increase in Y from 0 to 0.35 and with a decrease in Y from 1.00 to 0.70, respectively. However, Xb increases around Y ) 0.517 (with equal molar mixture of cationic and anionic surfactants) and the three-phase regions are not formed in these mixed systems at Y ) 0.40-0.60. The hexanol solubility in oil phase and the surfactant fraction at interface were estimated according to the surfactant distribution equations applied to the threephase behavior. The decrease in Xb, or the increase in solubilization, upon mixing cationic and anionic surfactants, is mainly attributed to the decrease in both the hexanol solubility S1 in the oil phase and the surfactant fraction C1 + C2 at the oil-water interface, resulting from the strong interactions between the anionic and cationic amphiphile molecules at the interface inside the microemulsion. However, the interactions between two surfactants are too strong to form a flexible surfactant layer around Y ) 0.517; a lamellar liquid crystal, instead of an isotropic microemulsion, is formed in the dilute region, and consequently, the solubilization capacity decreases accordingly. LA981748M