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. © 2012 American Chemical Society