  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.
