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 structure.

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.