Diffusion Coefficients in a Lamellar Lyotropic Phase: Evidence for Defects Connecting the Surfactant Structure

We measure diffusion coefficients in the lamellar phase of the nonionic binary system ${\mathrm{C}}_{12}{\mathrm{EO}}_6/{\mathrm{H}}_2\mathrm{O}$ using fluorescence recovery after photobleaching. The diffusion coefficient across the lamellae shows an abrupt increase upon approaching the lamellar-isotropic phase transition. We interpret this feature in terms of defects connecting the surfactant structure. An estimation of the defect density and of the variation in defect energy close to the transition is given in terms of a simple model.