2020 Impact factor 1.890
Soft Matter and Biological Physics
Eur. Phys. J. E 3, 377-388

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

O. Dhez1 - S. König2 - D. Roux2 - F. Nallet2 - O. Diat1

1European Synchrotron Radiation Facility, F-38043 Grenoble, France
2 Centre de recherche Paul-Pascal, CNRS, Avenue du Docteur-Schweitzer, F-33600 Pessac, France
odiat@cea.fr.

Received 6 July 1999 and Received in final form 17 April 2000

Abstract
A lyotropic system, consisting of a lecithin (DMPC) and a non-ionic surfactant ( \ensuremath{\mathsf{C_{12}E_{5}}}) 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 \ensuremath{\mathsf{C_{12}E_{5}}}systems, the transition to the nematic phase at lower temperatures coincides with a micellar phase in the pure \ensuremath{\mathsf{C_{12}E_{5}}}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.

PACS
61.30.Jf Defects in liquid crystals - 64.70.Md Transitions in liquid crystals - 87.16.Dg Membranes, bilayers, and vesicles

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