THE LAMELLAR-TO-ISOTROPIC TRANSITION IN TERNARY AMPHIPHILIC SYSTEMS

We study the dependence of the phase behavior of ternary amphiphilic s ystems on composition and temperature. Our analysis is based on a curv ature elastic model of the amphiphile film with sufficiently large spo ntaneous curvature c(0) and saddle-splay modulus (k) over bar sufficie ntly negative that the stable phases are the lamellar phase and a drop let microemulsion. In addition to the curvature energy, we consider th e contributions to the free energy of the long-ranged van der Waals in teraction and of the undulation modes. We find that for bending rigidi ties of order k(B) T, the lamellar phase extends further and further i nto the water apex of the phase diagram as the phase inversion tempera ture is approached, in good agreement with experimental results.