MOLECULAR-DYNAMICS STUDIES OF SODIUM OCTANOATE AND WATER - THE LIQUID-CRYSTAL MESOPHASE WITH 2-DIMENSIONAL HEXAGONAL SYMMETRY

Molecular dynamics calculations have been used to investigate structural and dynamical aspects of the two-dimensional hexagonal mesophase of sodium octanoate and water. To model this liquid-crystalline "middle soap" phase, comprised of rod-shaped amphiphillic aggregates, we have employed a periodically replicated molecular dynamics cell with hexagonal boundary conditions. The cell contained a single cylindrical aggregate of 90 octanoate anions, an equivalent number of sodium cations, and 838 water molecules. A phase-space trajectory has been used to determine the structure of the hydrocarbon core region, the distribution of counterions with respect to the carboxylate head groups, and the extent of water penetration into the core. The findings from the present calculation are contrasted with analogous results for a quasi-spherical micelle that forms in the isotropic phase at lower concentrations of sodium octanoate in water.