Defect-mediated lamellar–isotropic transition of amphiphile bilayers

Abstract

We report the observation of an isotropic phase of amphiphile bilayers, characterized by small average inter-bilayer spacing and short-range positional correlations. These bilayers form a weakly swollen lamellar (L_α) phase over a wide range of water content, which transforms into an isotropic (L_i) phase on heating. This transition is not observed in samples without excess water, where the L_α phase is stable at higher temperatures. Small-angle X-ray scattering and ionic conductivity studies indicate that the L_α–L_i transition is driven by the unbinding of edge dislocation loops, which has been shown to destroy the quasi-long-range positional order of the lamellar phase. Observation of similar behavior in other bilayer forming systems dominated by attractive inter-bilayer interactions suggests that this isotropic phase is generic to such systems.