A defect mediated lamellar to isotropic transition of amphiphile bilayers

TL;DR

This paper reports a novel isotropic phase in amphiphile bilayers formed by SDS and PTHC, driven by dislocation unbinding, representing a transition from lamellar to sponge-like phase influenced by water content and temperature.

Contribution

It introduces a new isotropic phase in an attraction-dominated amphiphile system and elucidates the transition mechanism involving dislocation unbinding.

Findings

Identification of a novel isotropic phase in SDS-PTHC bilayers.

Transition driven by unbinding of edge dislocation loops.

Isotropic phase analogous to sponge phase in steric repulsion systems.

Abstract

We report the observation of a novel isotropic phase of amphiphile bilayers in a mixed system consisting of the ionic surfactant, sodium docecylsulphate (SDS), and the organic salt p-toludine hydrochloride (PTHC). This system forms a collapsed lamellar ($L_\alpha$) phase over a wide range of water content, which transforms into an isotropic phase on heating. This transition is not observed in samples without excess water, where the $L_\alpha$ phase is stable at higher temperatures. Our observations indicate that the $L_\alpha$ - isotropic transition is driven by the unbinding of edge dislocation loops and that the isotropic phase in the present attraction-dominated system is the analogue of the sponge phase usually seen in amphiphile systems dominated by interbilayer steric repulsion.