Phase behavior of aqueous polyion-surfactant ion complex salts: A theoretical analysis

A simple thermodynamic theory is presented for water swollen complex salts formed by ionic surfactant and oppositely charged polyions. The description takes into account, on approximate level, free energy contributions from attractive and repulsive polyion-mediated interactions between the micelles, the mixing of micelles, polyion chains and water, and the hydrophobic effect. Explicit expressions for the chemical potentials of water, polyion, and surfactant ion are derived and used to calculate phase diagrams at various degrees of polymerization and linear charge density of the polyion and for surfactants with 12 and 16 carbons in the tail group. In all calculations the aggregation number is optimized and the phase structure (disordered or fcc) is determined. The effect of varying the charge densities of spheres interacting with a cross-linked polyion network is also investigated. Results from theory are compared with experimental and Monte Carlo simulation data reported in the literature.