Atomistic Simulations of Micellization of Sodium Hexyl, Heptyl, Octyl, and Nonyl Sulfates

Molecular dynamics simulations have been used to study the micellization behavior of atomistic models for sodium alkyl sulfates in explicit water. A major finding of the present work is the observation of a strong dependence of free surfactant concentration on overall surfactant concentration, that has not been reported previously and that is key to comparing simulation results for the critical micelle concentration (CMC) to experimental data. The CMC and aggregate size distributions were obtained for alkyl tail lengths from six to nine at temperatures from 268 to 363 K, from 400 ns simulations covering a number of surfactant and water model combinations. The free surfactant concentration is much lower than the critical micelle concentration for strongly micellizing systems at the relatively high concentrations accessible by simulations. Thus, counterion association must be accounted for in determining the CMC from the raw simulation data. Simulation results are in qualitative agreement with experimental trends for aggregate size and CMC as functions of alkyl tail length and temperature.