Sodium Dodecyl Sulfate at Water-Hydrophobic Interfaces: A Simulation Study

Using molecular dynamics simulations, we have studied the water vapor and water oil (decane) interfaces of aqueous solutions of sodium dodecyl sulfate (SDS). The water vapor interface is often used as a model for water oil (hydrophobic) interfaces, yet we observe that the behavior of amphiphilic DS- ions at these two interfaces is very different. Specifically, on a water vapor interface, SDS forms aggregates at low coverages, while it is homogeneously distributed on the water oil interface. Two decane parametrizations resulted in dramatically different conformations: decane parametrized based on a GROMOS force field "froze", while decane parametrized with a TraPPE force field remained liquid at 300 K. The calculated effective second-order susceptibilities and nonlinear sum frequency scattering intensities of DS- ions at the "frozen" decane-water agree well with experimental data of DS- ions at the hexadecane droplet water interface. This suggests that the orientation of longer alkane molecules is predominantly parallel to the interface and that, at low coverages, DS- ions follow the orientation of oil molecules.