Polyelectrolyte - Surfactant Interactions in Dilute Suspensions. A Study Based on Monte Carlo simulations and Mean-Field Calcultations

Sammanfattning: The complexation of macroions or charged micelles and oppositely charged polyelectrolytes was studied by the use of Monte-Carlo simulations and mean field calculations. The Monte-Carlo simulations were performed on a simple model system with the emphasis on the electrostatic interaction, (i) polyelectrolyte rigidity, (ii) polyelectrolyte linear charge density, (iii) surfactant tail length, and (iv) the polyelectrolyte concentration. Structural data for all simulations and thermodynamic quantities of the complexation for (i) to (iii) was obtained from the Monte Carlo simulation technique and thermodynamic integration. The ratio of the critical aggregation concentration, cac, and the critical micellization concentration, cmc, was calculated for (i) to (iii), cac being the lowest surfactant concentration at which the surfactants self-assemble. The largest reduction of the cmc occurred for the highest polyelectrolyte flexibility, largest linear charge density, and longest surfactant tail. The complexation is caused by the strong attractive electrostatic interaction between the micelle and the polyelectrolyte and is manifested by a decrease of the electrostatic energy due to small micelle-polyelectrolyte charge separations and an increase of the entropy due to the release of the counterions of the macroions. The simulations at various polyelectrolyte concentrations (iv) showed that for flexible and highly charged polyelectrolytes a tight complex, containing one macroion and one polyelectrolyte was formed at all polyelectrolyte concentrations considered. However, for flexible but lower charged polyelectrolytes, a looser macroion-polyelectrolyte complex was established, and the complex involved two polyelectrolytes when two or more polyelectrolytes were available. For the most rigid polyelectrolyte no complex was formed at all. For the mean-field calculations the polyelectrolyte linear charge density, polyelectrolyte concentration and polyelectrolyte backbone hydrophobicity, and the concentration of salt were varied. Structural data and information about the aggregation number, cmc's and cac's were obtained. The results showed that the solvency of the polymer backbone affects the structure of the aggregate.