Molecular Simulation of a Dichain Surfactant/Water/Carbon Dioxide System. 2. Self-Assembly and Aggregation Dynamics

We report on the aggregation dynamics of the dichain surfactant [(C7F15)(C7H15)CHSO4 -Na+] + water + carbon dioxide (solvent) system described in our preceding paper. Our results indicate that the aggregation process follows a two-step mechanism: hydration of anionic headgroups and Na+ counterions followed by aggregation of hydrated surfactant molecules. The kinetics of aggregation is consistent with the Smolouchowski theory of diffusion-limited aggregation, though on a much faster time scale compared to aggregation observed in aqueous surfactant systems. This is attributed to the fact that the intermolecular interactions responsible for aggregation in the system under investigation are electrostatic in nature and the carbon dioxide permits faster diffusion of surfactant molecules than water. The surfactant tails forming the corona of the aggregates provide steric stabilization of the aggregates thereby reducing the rate of aggregation.