7176
Langmuir 1999, 15, 7176-7179
Phase Behavior and Water/Oil Microemulsion Formation of r-Sulfonate Methyl Ester Derived from Palm Stearin/ 1-Hexanol/Hydrocarbon/Water Systems Wen-Huei Lim* Advanced Oleochemical Technology Centre, Palm Oil Research Institute of Malaysia (PORIM), 43650 Bandar Baru Bangi, Selangor, Malaysia Received March 18, 1999. In Final Form: June 23, 1999 The formation of a water/oil (w/o) microemulsion in the isotropic region of sodium salt R-sulfonate methyl ester derived from palm stearin (R-SMEPS)/1-hexanol/alkane/water systems was investigated at 30.0 ( 0.1 °C. The formation of clear isotropic microemulsion phases in quaternary mixtures of surfactant, cosurfactant, oil, and water was mapped in phase composition diagrams. This region is associated with electrical conductivity due to the water connectivity in the system. The effect on the percolation threshold in the isotropic region by linear hydrocarbons of different chain lengths was also observed by electric conductivity measurements. The percolation threshold of the w/o microemulsion was found to decrease with an increase in the hydrocarbon chain length. A similar trend also occurred as the weight ratio of (R-SMEPS + 1-hexanol):alkane increased.
Introduction Water-in-oil (w/o) microemulsions are isotropic solutions of low viscosity and are thermodynamically stable.1 They are formed when suitable compositions of surfactant, cosurfactant, water, and hydrocarbon are mixed together. In an isotropic solution, the surfactants are aggregated in a polar solvent with their polar heads directed toward the core of the aggregates. Their hydrophobic tails are then directed outward, shielding the inner polar core from the nonpolar medium. Many studies on ionic and nonionic surfactant systems have been conducted on the formation of micelles and microemulsions. Isotropic solutions, containing low molecular weight aliphatic or aromatic hydrocarbon oil components, have been extensively studied, and there exists a large amount of data on their composition, properties, and structures.2-5 The formation of isotropic solutions in the presence of alkanols has also been intensively studied.6-14 * To whom correspondence should be addressed. Phone: 038255708, ext. 139. Fax: 03-8256197. E-mail:
[email protected]. (1) Danielsson, I.; Lindman, B. Colloids Surf. 1981, 3, 391. (2) Bansal, V. K.; Shah, D. O.; O’Connell, J. P. J. Colloid Interface Sci. 1980, 75, 462. (3) Robbins, M. L.; Bock, J. J. Colloid Interface Sci. 1988, 124, 462. (4) Robbins, M. L.; Bock, J. J. Colloid Interface Sci. 1988, 124, 486. (5) Robbins, M. L.; Bock, J.; Huang, J. S. J. Colloid Interface Sci. 1988, 126, 114. (6) Bisal, S. R.; Bhattacharya, P. K.; Moulik, S. P. J. Phys. Chem. 1990, 94, 350. (7) Lindemuth, P. M.; Duke, J. R.; Blum, F. D.; Venable, R. L. J. Colloid Interface Sci. 1990, 135, 539. (8) Clausse, M.; Zradba, A.; Nicolas-Morgantini, L. In Microemulsion Systems; (H. L. Rosano and M. Clausse, Eds.), p 63. Dekker: New York, 1987. (9) Murry, B. S.; Drummond, C. J.; Griesser, F.; White, L. R. J. Phys. Chem. 1990, 94, 6804. (10) Das, M. L.; Bhattacharya, P. K.; Moulik, S. P. Colloid Surf. 1990, 49, 247. (11) Sjo¨blom, E.; Jo¨nsson, B.; Jonsson, A.; Stenius, P.; Saris, P.; O ¨ ldberg, L. J. Phys. Chem. 1986, 90, 119. (12) Natoli, J.; Benton, W. J.; Miller, C. A. J. Disp. Sci. Technol. 1986, 7, 215. (13) Baker, R. C.; Florence, A. T.; Tadros, Th. F.; Wood, R. M. J. Colloid Interface Sci. 1984, 100, 311. (14) Ray, S.; Moulik, S. P. J. Colloid Interface Sci. 1995, 173, 28. (15) Nilsson, P. G.; Lindman, B. J. Phys. Chem. 1982, 86, 271.
With improved understanding of isotropic solutions, knowledge on their microstructures has become important. Instrumentation such as that for measuring NMR selfdiffusion coefficients,15-17 small-angle neutron, X-ray scattering, light scattering,18-20 and electrical conductivity21-27 is often used to study the structure of droplets. In oil-surfactant mixtures, the conductivity can change substantially with the addition of water. The conductivity is initially very low in an oil-surfactant mixture but increases as water is added. This is due to the hydration of the surfactant molecules.25,28 With further addition of water, the conductivity can increase substantially because of percolation. But if even more water is added, closed structuresswater droplets surrounded by surfactant monolayerssare formed and a maximum conductivity is reached. Various attempts have been made to understand the association structure in surfactant solution by electrical conductivity,6,29-31 and several theoretical quantitative treatments of the percolation phenomenon in microemul(16) Stilbs, P.; Lindman, B. Prog. Colloid Polym. Sci. 1984, 69, 39. (17) Chen, S. J.; Evans, D. F.; Ninham, B. W. J. Phys. Chem. 1984, 88, 1631. (18) Sjo¨blom, E.; Friberg, S. J. Colloid Interface Sci. 1978, 67, 16. (19) Magazu´, S.; Majolino, D.; Maisano, G.; Mallamace, F.; Micali, N. Phys. Rev. A 1989, 40, 2643. (20) Lombardo, D.; Mallamace, F.; Majolino, D.; Micali, N. Prog. Colloid Polym Sci. 1992, 89, 82. (21) Mathew, C.; Patanjali, P. K.; Nabi, A.; Maitra, A. Colloids Surf. A 1988, 30, 253. (22) Almgren, M.; van Stam, J.; Swarup, S.; Lofroth, J.-E. Langmuir 1986, 2, 432. (23) Jada, A.; Lang, J.; Zana, R. J. Phys. Chem. 1989, 93, 10. (24) Peyrelasse, P.; Boned, C., Phys. Rev. A 1990, 41, 938. (25) Baker, R. C.; Florence, A. T.; Ottewill, R. H.; Tadros, Th. F. J. Colloid Interface Sci. 1984, 100, 332. (26) Chittofrati, A.; Visca, M.; Kallay, N. Colloids Surf. A 1993, 74, 251. (27) Chittofrati, A.; Sanguineti, A.; Visca, M.; Kallay, N. Colloids Surf. A 1992, 63, 219. (28) Zulauf, M.; Eicke, H.-F. J. Phys. Chem. 1979, 83, 480. (29) Lague¨s, M.;Sauterey, C. J. Phys. Chem. 1980, 84, 3503. (30) Clarkson, M. T. Phys. Rev. A 1988, 37, 2079. (31) Venable, R. L.; Fang, J. J. Colloid Interface Sci. 1987, 116, 269. (32) Granqvist, C. G.; Hunberi, O. Phys. Rev. B 1976, 18, 1554. (33) Bernasconi, J.; Wiesmann, H. J. Phys. Rev. B 1976, 13, 1131. (34) Gu, G. X.; Wang, W. Q.; Yan, H., J. Colloid Interface Sci. 1996, 178, 358.
10.1021/la990323c CCC: $18.00 © 1999 American Chemical Society Published on Web 08/20/1999
R-SMEPS/1-Hexanol/Hydrocarbon/Water Systems
Langmuir, Vol. 15, No. 21, 1999 7177
sions have been proposed.32-34 But the equations have been unsatisfactory in modeling the structures of the systems. In this paper, the effects on w/o microemulsion systems by hydrocarbons of different chain lengths, R-SMEPS, 1-hexanol, and water were determined. A physicochemical study using electrical conductivity measurements was done to detect the percolation threshold of the microemulsion region as affected by the different hydrocarbons in the system. Experimental Section Materials. The sodium salt of R-sulfonate methyl ester derived from palm stearin (R-SMEPS) (94.4% active ingredient) was used as supplied by Chemithon Corp., and deionized distilled water was used as the solvent. A cosurfactant, 1-hexanol (C6H13OH) (>98% purity), was used as received from Merck. Hydrocarbons n-octane (C8H18), n-decane (C10H22), n-dodecane (C12H26), ntetradecane (C14H30), and n-hexadecane (C16H34) (all of 99% purity), from Sigma, were used as received. Method. (a) Phase Diagrams. The phase behavior of (RSMEPS:C6H13OH)/H2O/CnH2n+2 systems, where n ) even numbers from 8 to 16, was determined by titrating samples (mixtures of R-SMEPS, 1-hexanol, and hydrocarbon) with water. The samples were vortexed to mix thoroughly, sealed in screw-top vials, and kept in a 30.0 ( 0.1 °C water bath. To ensure that equilibrium was attained, the samples were heated, mixed, and allowed to reequilibrate two or three times. The phase behavior was determined by visual observation between crossed polarized sheets for formation of optical birefringence and turbid or opaque solutions. (b) Conductivity Measurement. W/O microemulsions were prepared in the weight ratios of 90:10, 30:70, 50:50, and 70:30 (R-SMEPS:C6H13OH):CnH2n+2 at suitable water contents. The samples were vortexed, sealed in screw-top vials, and left to equilibrate at 30.0 ( 0.1 °C in a water bath. Further samples were prepared by dilution to maintain the weight ratios of the mixtures isotropic. The sample with the lowest water concentration was measured for conductivity first. It was then diluted with water and measured again. Further dilutions and measurements were made to cover a wider range of water concentration in the isotropic phase. The other samples were similarly measured. The measurements were taken with a conductivity meter (Model Crison 2202) at constant frequency using a glass-dipping cell with platinum electrodes. The measurements were taken under a thermostatic 30.0 ( 0.1 °C. The cell constant was determined using a standard KCl solution.
Results and Discussion Phase Behavior. The w/o microemulsion regions were determined by titrating samples with different linear hydrocarbon chain lengths incorporated into the 9:11 weight ratio of R-SMEPS:C6H13OH at 30.0 ( 0.1 °C (Figure 1). The microemulsion regions decreased in area with an increase in hydrocarbon chain length from C8 to C16 in the R-SMEPS/C6H13OH/H2O/CnH2n+2 systems. In the specific cases of R-SMEPS/C6H13OH/H2O/C8H18 and R-SMEPS/ C6H13OH/H2O/C10H22, a two-phase region (clear liquid in the upper layer and liquid crystal in the lower portion) was observed in the center of the isotropic region. This two-phase region increased with a decrease in hydrocarbon chain length from C10 to C8. The maximum solubility of water in the isotropic regions occurred at a 7:3 weight ratio of (R-SMEPS-C6H13OH): CnH2n+2. Along this line, most of the water was solubilized by incorporating n-decane into the system. Therefore, n-decane is the most efficient coagent among the linear hydrocarbons of chain lengths C8-C16 in the solubilization of water and 1-hexanol in R-SMEPS. The solubility of water along this line progressively reduces from 55.5 to 51.5 wt % as the hydrocarbon chain length increases
Figure 1. Pseudoternary phase diagram of R-SMEPS/1hexanol/linear aliphatic alkane chain/water systems at 30.0 ( 0.1 °C. Mass ratio of R-SMEPS:1-hexanol is 45:55. W: 100% water. S: 100% R-SMEPS combined with 1-hexanol. H: 100%.
Figure 2. Solubility of water in the w/o microemulsion region at 70:30 mass ratio of R-SMEPS:1-hexanol to aliphatic alkane chain length.
to C16 (Figure 2). This phenomenon is due to the compatibility of the alkyl chain length of the surfactant with the alkanol and hydrocarbon in the system.2,35,36 A comparison of the areas of the five isotropic regions shows clearly that the largest was that which incorporated n-decane into the R-SMEPS/C6H13OH/H2O system (Figure 3). Conductivity Behavior. Figure 4 shows the changes in electrical conductivity of the w/o microemulsion region of different weight ratios of (R-SMEPS:C6H13OH):C8H18 as the water contents were increased. All the lines show an increase in conductivity with water content. However, there were sudden increases along the lines of 50:50 and 30:70 weight ratios. At 50:50, the sudden increase occurred at 40 wt % water. The percolation threshold decreased to 20 wt % water as the weight ratio decreased to 30:70. A similar trend was also observed in the R-SMEPS/ C6H13OH/C10H22/H2O system (Figure 5). All the weight ratios liness90:10, 70:30, 50:50, and 30:70 of (R-SMEPS: (35) Bansal, V. K.; Chinnaswamy, K.; Ramachandran, C.; Shah, D. O. J. Colloid Interface Sci. 1979, 72, 524. (36) Lemaire, B.; Bothorel, P.; and Roux, D. J Phys. Chem. 1983, 87, 1023.
7178
Langmuir, Vol. 15, No. 21, 1999
Figure 3. Percentage area of w/o microemulsion region in the ternary phase diagram against the length of the aliphatic alkane chain.
Figure 4. Changes in conductivity with weight fraction of water. ([) 10:90, (9) 30:70, (2) 50:50, and (b) 70:30 (R-SMEPS: 1-hexanol):octane mixed systems at 30.0 ( 0.1 °C. Open square indicates two-phase region.
C6H13OH):C10H22sshowed an increase in conductivity with water. Three liness70:30, 50:50, and 30:70sexperienced sudden increases in conductivity at 40, 28, and 15 wt % water, respectively. In the R-SMEPS/C6H13OH/C12H26/H2O system, the conductivity was higher than those of systems with shorter hydrocarbon chains, e.g., C8 and C10. Figure 6 shows the conductivities of weight ratios 10:90, 30:70, 50:50, and 70:30 of (R-SMEPS:C6H13OH):C12H26. The percolation thresholds, determined through the curves of weight ratios 30:70, 50:50, and 70:30, were 37, 20, and 11 wt %, respectively. In the isotropic solution with R-SMEPS, C6H13OH, C14H30, and H2O, the conductivities were also high (Figure 7). In this system, only the 50:50 and 30:70 weight ratios exhibited percolations16 and 9 wt %, respectively. In the R-SMEPS/C6H13OH/C16H34/H2O system, an increase in conductivity on addition of water was observed (Figure 8). The high electrical conductivity (37) Paul, S.; Bisal, S. R.; Moulik, S. P. J. Phys. Chem. 1992, 96, 896. (38) Jada, A.; Lang, J.; Zana, R.; Makhloufi, R.; Hirsh, E.; Candu, S. J. J. Phys. Chem. 1990, 94, 387.
Lim
Figure 5. Changes in conductivity with weight fraction of water. ([) 10:90, (9) 30:70, (2) 50:50, and (b) 70:30 (R-SMEPS: 1-hexanol):decane mixed systems at 30.0 ( 0.1 °C. Open square indicates two-phase region.
Figure 6. Changes in conductivity with weight fraction of water. ([) 10:90, (9) 30:70, (2) 50:50, and (b) 70:30 (R-SMEPS: 1-hexanol):dodecane mixed systems at 30.0 ( 0.1 °C.
hindered the percolation threshold as is shown by the curves, except for weight ratios 50:50 and 30:70 which had percolation thresholds of 12 and 7 wt %. The decrease in electrical conductivity at different weight ratios of (R-SMEPS:C6H13OH):CnH2n+2 at a fixed amount of water was due to a decrease in the amount of Na+ ions in the sample. Thus, when the surfactants began to aggregate, an increase in the conductivity was clearly observed in the samples with a small quantity of Na+ ions. This result resembles percolation of surfactant aggregation in the isotropic region where the microwater droplets in the solution quickly cluster to form an open structure for the efficient transport of Na+ ions (counterions of R-SMEPS) by transient fusion and mass exchange.6,37-40 The fraction of micellar headgroups neutralized with ions depends only on the water:R-SMEPS molar ratio and increases as this ratio decreases. The micellar surface potential decreases with the amount of (39) Mukhopadhyay, L.; Bhattacharya, P. K.; Moulik, S. P. Colloids Surf. 1990, 50, 295. (40) Ray, S.; Bisal, S. R.; Moulik, S. P. J. Chem. Soc., Faraday Trans. 1993, 89, 3277.
R-SMEPS/1-Hexanol/Hydrocarbon/Water Systems
Figure 7. Changes in conductivity with weight fraction of water. ([) 10:90, (9) 30:70, (2) 50:50, and (b) 70:30 (R-SMEPS: 1-hexanol):tetradecane mixed systems at 30.0 ( 0.1 °C.
Langmuir, Vol. 15, No. 21, 1999 7179
solution. Changes in the electrical conductivity were noted at the 70:30 weight ratio of (R-SMEPS:C6H13OH):CnH2n+2, as the surfactant aggregates were subject to more hydrocarbons of different chain lengths. Along this line, an increase in conductivity with hydrocarbon chain length on addition of water was observed. As the weight ratio of (R-SMEPS:C6H13OH):CnH2n+2 decreased further to 50:50, the percolation thresholds of the lines became more pronounced. The impact of different hydrocarbon chain lengths in the microemulsion region was significant. The percolation threshold shifted to the left (that is, became lower) as the hydrocarbon chain length increased in the isotropic region. A similar trend was also exhibited at a weight ratio of 10:90 of (R-SMEPS:C6H13OH):CnH2n+2. As observed earlier, the electrical conductivity decreased for all hydrocarbon chain lengths as the content of hydrocarbon increased, and the percolation thresholds of the systems were easily identified from the sharp increases in electrical conductivity. This phenomenon can be explained by hydration of the surfactant species in the aqueous solution and electrophoretic movement of the monodispersed water globules in a continuous oil medium.18,22,25,28,41 The increase in conductivity was caused by the increasingly stronger network interaction among the species brought about by hydration of the surfactants. The sharp rise in conductance above the percolation threshold with an increase in water content is due to effect the transfer of ions through the oil phase by a special transport mechanism.23,42,43 Conclusions
Figure 8. Changes in conductivity with weight fraction of water. ([) 10:90, (9) 30:70, (2) 50:50, and (b) 70:30 (R-SMEPS: 1-hexanol):hexadecane mixed systems at 30.0 ( 0.1 °C.
alcohol for each water:R-SMEPS ratio. These results are in agreement with the measurement of conductivity that increased with the degree of micellar ionization. Of all the lines studied in the R-SMEPS/C6H13OH/ CnH2n+2/H2O system, only the hydrocarbon molecules in the 90:10 weight ratio of (R-SMEPS:C6H13OH):CnH2n+2 exhibited an almost unchanged electrical conductivity with addition of water. In other words, with a low content of hydrocarbon in solution, the electrical conductivity was not affected by surfactant aggregation. Thus, it is believed that the percolation threshold along this line should be the same for different hydrocarbon chain lengths in the
The w/o microemulsion region formed in systems containing sodium salt R-sulfonate methyl ester derived from palm stearin (R-SMEPS), 1-hexanol, alkane, and water decreased with an increase in hydrocarbon chain length from C8 to C16. A two-phase region was observed at the center of the w/o microemulsions consisting of C8 and C10. Nevertheless, the R-SMEPS/1-hexanol/decane/ water system was found to be the most compatible among the mixtures based on its large isotropic area and maximum water solubility in the region. The percolation thresholds of the w/o microemulsions in these systems were found to decrease with an increase in hydrocarbon chain length. Acknowledgment. I extend my gratitude to the Director General of Palm Oil Research Institute of Malaysia for his support and permission to publish this paper. LA990323C (41) Peyrelesse, J.; McClean, V. E. R.; Boned, C.; Sheppard, R. J.; Clausse, M. J. Phys. D., Appl. Phys. 1978, 11, L117. (42) Hilfiler, R.; Eicke, H. F.; Geiger, S.; Furlen, G. J. Colloid Interface Sci. 1985, 105, 378. (43) Dutkiewicz, E.; Robinson, B. H. J. Electroanal. Chem. 1988, 251, 11.
