A Molecular Dynamics Study of Sodium Dodecyl Sulfate (SDS) at the Silica-WATER Interface: pH Effect
Naga Rajesh Tummala, Dimitrios Argyris, and Alberto Striolo. School of Chemical Biological and Materials Engineering, The University of Oklahoma, Norman, OK 73019
Molecular dynamics simulations will be reported for the adsorption of sodium dodecyl sulfate (SDS) at the silicon dioxide-water interface. The surface hydrophobicity was varied by changing the number of hydroxyl groups on the surface. Three different number densities of hydroxyl groups were chosen that correspond to (1) pH approximately equal to the isoelectric point of the silica surface, (2) pH less than the isoelectric point (acidic solution), and (3) pH greater than isoelectric point. We will discuss how the surface properties affect the molecular adsorption of SDS on the three different surfaces by presenting results for density profiles, planar radial distribution functions, and mean square displacement of both water and SDS molecules. The results will be compared to those of pure water at the SiO2 surface in the absence of SDS. This comparison will allow us to understand whether or not the equilibrium surfactant adsorption is influenced by hydrogen bonding within the water layers and/or by the local dielectric constant. Because pH effects manage to alter the nature of SiO2 surface, spanning from hydrophobic to hydrophilic macroscopic properties, our results will yield a better understanding on the competitive adsorption of water and amphiphilic compounds on charged surfaces, a phenomenon very important for industrial applications ranging from detergency to oil recovery.