Model for the Water−Amorphous Silica Interface: The Undissociated Surface

Abstract

The physical and chemical properties of the amorphous silica-water interface are of crucial importance for a fundamental understanding of electrochemical and electrokinetic phenomena, and for various applications including chromatography, sensors, metal ion extraction, and the construction of micro- and nanoscale devices. A model for the undissociated amorphous silica-water interface reported here is a step toward a practical microscopic model of this important system. We have extended the popular BKS and SPC/E models for bulk silica and water to describe the hydrated, hydroxylated amorphous silica surface. The parameters of our model were determined using ab initio quantum chemical studies on small fragments. Our model will be useful in empirical potential studies, and as a starting point for ab initio molecular dynamics calculations. At this stage, we present a model for the undissociated surface. Our calculated value for the heat of immersion, 0.3 J x m(-2), falls within the range of reported experimental values of 0.2-0.8 J x m(-2). We also study the perturbation of water properties near the silica-water interface. The disordered surface is characterized by regions that are hydrophilic and hydrophobic, depending on the statistical variations in silanol group density.