Coexistences of lamellar phases in ternary surfactant solutions

We theoretically investigate the coexistences of lamellar phases both in binary and in ternary surfactant solutions. The previous free energy of a lamellar stack is extended to take into account the translational entropy of membrane segments. The obtained phase diagram for binary surfactant solutions (surfactant/water mixtures) shows a phase separation between two lamellar phases and also exhibits a critical point. For lamellar phases in ternary surfactant solutions (surfactant/surfactant/water mixtures), we explore the possible phase behaviors and show that the phase diagrams exhibit various three-phase regions as well as two-phase regions in which different lamellar phases coexist. We also find that finite surface tension suppresses the undulation fluctuations of membranes and leads to a wider three-phase and two-phase coexistence regions.