(587c) Equilibrium Micellization of An Atomistic Model for Sodium Alkyl Sulfates

Surfactants are used for a wide range of industrial and biological applications. Composed of a hydrophilic head group and a hydrophobic tail group, they self-assemble into various structures in aqueous solutions. In particular, sodium alkyl sulfates are frequently used as model surfactants. We have analyzed a recent united atom model developed for this class of surfactants (M.Sammalkorpi et al., J.Phys. Chem. B, 2007, 11722-11733). The model has been used to simulate sodium dodecyl sulfate, but equilibrium properties could not be obtained for this strongly micellizing system. In this presentation, we describe calculations for shorter surfactants in this class, specifically sodium hexyl sulfate and sodium octyl sulfate, for which sufficiently long simulations near the critical micellar concentration can approach equilibrium. For sodium hexyl sulfate, we have obtained data from 200 ns simulations with varying salt concentrations of sodium and calcium chloride. We observe an increase of aggregate size with salt concentration, especially for calcium chloride. The apparent cmc of the system seems to underpredict the experimental cmc, but the trend of decreasing cmc with increasing salt concentration is captured. For sodium octyl sulfate, we have conducted simulations of hundreds of nanoseconds and have studied the evolution of surfactant aggregates over time. We have been able to observe the temperature dependence of these properties for the models as well as the dependence of the apparent cmc on overall surfactant concentration.