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 (Lα) phase over a wide range of water content, which transforms into an isotropic (Li) phase on heating. This transition is not observed in samples without excess water, where the Lα phase is stable at higher temperatures. Small-angle X-ray scattering and ionic conductivity studies indicate that the Lα–Li 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.