On the computer simulation of a hydrophobic vitreous silica surface

The experimental evidence that the surface of pure vitreous silica can be hydrophobic imposes strong limitations on possible atomic configurations at that surface. This is due primarily to the fact that the electric field of the partially ionic ${\mathrm{SiO}}_2$ can have very strong interactions with adsorbed polar molecules and with water in particular. The simulations reported here indicate that a surface structure consisting of a random net of almost regular corner-sharing ${\mathrm{SiO}}_4$ tetrahedra with a low concentration of defects such as nonbridging oxygen atoms is capable of producing hydrophobicity. It is shown that the defects as well as distortion of the ${\mathrm{SiO}}_4$ tetrahedra as measured by their dipole and quadrupole moments give rise to hydrophilic adsorption sites on the surface. Computer simulation of such a random net at a surface runs into a general problem typical of computer simulations of amorphous solids: at temperatures near to but above the glass transition temperature, the time scale of the molecular dynamics is many orders of magnitude less than the experimental structural relaxation times of the material. A solution to this problem was obtained here by imposing a constraint on the molecular dynamics simulation that directs the chain of simulated configurations toward one without nonbridging oxygens. This is demonstrated by showing that the binding energies of a water molecule over the surface of this solid are smaller than the energy liquefaction, which is taken here as the criterion for hydrophobicity.