Monte Carlo Simulations of Polyelectrolytes at Charged Micelles. 3. Effects of Surfactant Tail Length

Abstract

The complexation of a charged micelle and an oppositely charged polyelectrolyte was studied by the use of a simple model system. The size of the micelle was varied to correspond to the change in surfactant tail length of real cationic surfactants, and the surface charge density of the micelle was kept constant. Structural data of the micelle−polyelectrolyte complex and thermodynamic quantities of the complexation as a function of the micellar aggregation number were obtained by the use of the Monte Carlo simulation technique and thermodynamic integration. The ratio of the critical aggregation concentration, cac, and the critical micellization concentration, cmc, was calculated, cac being the lowest surfactant concentration at which the surfactants self-assemble in the presence of polyelectrolyte. The cac was found to be ca. 15 (for smaller micelles formed by C 8 TA + surfactants) to ca. 75 (for larger micelles formed by C 12 TA + surfactants) times smaller than the corresponding cmc values. This decrease in the cac/cmc ratio at increasing aggregation numbers agrees well with experimental data from solutions of alkyltrimethylammonium bromide and charged polysaccharides.