Lamellar-to-nematic phase transition in a lipid-surfactant mixture
1European Synchrotron Radiation Facility, F-38043 Grenoble,
2 Centre de recherche Paul-Pascal, CNRS, Avenue du Docteur-Schweitzer, F-33600 Pessac, France
Received 6 July 1999 and Received in final form 17 April 2000
A lyotropic system, consisting of a lecithin (DMPC) and a non-ionic surfactant ( ) in water was studied. The system exhibits a lamellar-to-nematic phase transition. The nematic phase appears as the temperature is decreased and only exists in a very limited temperature and concentration range, for specific lipid-to-surfactant ratios. While a lamellar phase is found at higher temperatures in both mixed and pure systems, the transition to the nematic phase at lower temperatures coincides with a micellar phase in the pure system. The transition appears to be driven by the strong temperature dependence of the surfactant film spontaneous curvature. The structural properties of the lamellar phase close to the lamellar-to-nematic boundary have been studied by polarised light microscopy and small-angle neutron and X-ray scattering experiments. The signature of a helical defect with Burgers vector of magnitude 2 is apparent in our data, close to the lamellar-to-nematic phase transition. The proliferation of screw dislocations in the lamellar phase might be a plausible mechanism for driving this transition.
61.30.Jf Defects in liquid crystals - 64.70.Md Transitions in liquid crystals - 87.16.Dg Membranes, bilayers, and vesicles
Copyright EDP Sciences, Società Italiana di Fisica, Springer-Verlag