Theory of lamellar–lamellar phase transitions in pure and mixed surfactant solutions

There is a variety of experimental evidence for lamellar-to-lamellar phase transitions in aqueous solutions of single- and two-component surfactant systems. We develop here the statistical thermodynamics of alternative scenarios for lamellar–lamellar coexistence. In one case the transition involves phase separation driven by demixing of the two amphiphiles within each bilayer; a purely repulsive interaction between bilayers is sufficient, because of its dependence on bilayer composition. In the complementary case, a condensation-type phase change is involved, caused by the competition between attractive and repulsive interactions; the primary order parameter for this transition is the overall surfactant concentration.