Static and dynamic properties of the water/amorphous silica interface: a model for the undissociated surface

Abstract

Amorphous silica–water interfaces are found ubiquitously in nanoscale devices, including devices fabricated from silica as well as from silicon that acquire a surface oxide layer. The surface silanol groups serve as hydrogen-bonding sites for a variety of chemical species, and their reactivity enables convenient chemical modification, making silica surfaces strategic in bio-sensing applications. 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, 0.6Jm−2, falls within the range of reported experimental values of 0.2–0.8Jm−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. We report non-equilibrium molecular dynamics simulations of Poiseuille flow of water near an amorphous silica surface.