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Séminaire Sajal Kumar Ghosh

Counterion Influence on the Phase Behaviour of Ionic Amphiphiles

Sajal Kumar Ghosh

Raman Research Institute
Bangalore, India

The talk will discuss the influence of strongly bound counterions on the
self-assembled structures formed by some ionic surfactants. The cationic
surfactants used in these studies form cylindrical micelles over a wide
range of water content, and the addition of certain counterions is found
to transform these micelles into mesh-like aggregates. At high water
content these aggregates form a random mesh phase, which is a lamellar
phase with no long-range trans-membrane correlations of the in-plane

At low water content they, however, lock into a three dimensional
structure. The structure and stability of these mesh phases are found to depend crucially on factors such as the length of the hydrocarbon chain of the surfactant, its hydrophilic part, and the nature of the surfactant counterion. The phase diagrams of these systems have been determined using polarising microscopy and x-ray diffraction, and the mesh phases are always found to intervene between the hexagonal and lamellar phases. Interestingly, in the case of an anionic surfactant the mesh phases are not formed in the presence of strongly bound counterions, but a new transition sequence is found between the hexagonal and lamellar phases, which seems to involve a gradual change in the micellar morphology from cylindrical to planar.

The influence of salts on the coexistence of two lamellar phases in a cationic surfactant-water system has also been studied. All the salts studied are found to behave in a quantitatively similar fashion irrespective of the nature of the counterion, in striking disagreement with some recent
theoretical predictions.

The talk will also present the influence of a strongly bound counterion
on the structure of cationic surfactant-DNA complexes. The competition between the counterion and DNA to bind to the micelle is found
to lead to the formation of some novel structures of these complexes.