Solubilization and Phase Behavior of Microemulsions with Mixed Anionic−Cationic Surfactants and Hexanol

The minimum surfactant (C + D + E) fraction in the system, X b, 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 S 1 in the micro-oil domain and the surfactant fractions C 1 and C 2 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 X b (or the increase in the solubilization) by mixing two surfactants is mainly attributed to two factors: the decrease in the S 1 and the increase in the solubilization capacity of the mixed surfactant layer inside the microemulsion itself (or the decrease in C 1 + C 2 ). 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.