A defect mediated lamellar to isotropic transition of amphiphile bilayers

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 ($L_α$) 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 $L_α$ phase is stable at higher temperatures. Our observations indicate that the $L_α$ - 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.

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