Lamellar-to-nematic phase transition in a lipid-surfactant mixture

A lyotropic system, consisting of a lecithin (DMPC) and a non-ionic surfact ant (C12E5) in water was studied. The system exhibits a lamellar-to-nematic phase transition. The nematic phase appears as the temperature is decrease d and only exists in a very limited temperature and concentration range, fo r specific lipid-to-surfactant ratios. While a lamellar phase is found at h igher temperatures in both mixed and pure C12E5 systems, the transition to the nematic phase at lower temperatures coincides with a micellar phase in the pure C12E5 system. The transition appears to be driven by the strong te mperature dependence of the surfactant film spontaneous curvature. The stru ctural properties of the lamellar phase close to the lamellar-to-nematic bo undary have been studied by polarised light microscopy and small-angle neut ron and X-ray scattering experiments. The signature of a helical defect wit h Burgers vector of magnitude 2 is apparent in our data, close to the lamel lar-to-nematic phase transition. The proliferation of screw dislocations in the lamellar phase might be a plausible mechanism for driving this transit ion.