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 (
)
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.
PACS
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
