Thermodynamics of Partially Miscible Micelles and Liquid Crystals

Abstract

We have used the thermodynamic models we developed earlier to describe the mixing in aqueous solution of binary nonionic/nonionic and ionic/ionic surfactants with the same polar group and counterion. We have also derived general conditions for demixing of mixed micelles for the Redlich-Kister mixing model with one, two, and three parameters, mainly for the pseudo-phase separation model of mixed micellization. These models have been used to construct diagrams of total surfactant concentration versus the mole fraction of one surfactant in the mixture. These diagrams depict regions where one or two types of micellar pseudo-phases are present, as in mixtures of hydrocarbon surfactants with fluorocarbon surfactants. With the three-parameter model, there can be up to two two-phase regions, which were observed by Yedgar et al. (1974) for mixtures of Triton X-100 with a sphingo-lipid. The diagrams also depict lines of first mixed cmc’s, second mixed cmc’s, and of critical demicellization concentrations. We have used the same models when a mixed micellar pseudo-phase is at equilibrium with a mixed liquid crystalline phase, as in mixtures of nonionic surfactants with phospholipids and in sodium dodecylsulfate with sodium 4 - (1’ -heptylnonyl) benzenesulfonate.