MOLECULAR-DYNAMICS STUDY OF PROTON BINDING TO SILICA SURFACES

Molecular statics calculations on gas-phase and solvated clusters and on gas-phase and solvated slabs representing aqueous species and surfa ces were applied to investigate acid/base reactions on silica surfaces . Our gas-phase approach, which was previously applied to goethite, pr edicts a surface pK(a) of 8.5 for the reaction >SiOH-->>SiO- + H+ whic h is in good agreement with estimates based on potentiometric titratio n. However, the model gives an unrealistically large pK(a) for the rea ction >SiOH2+ --> >SiOH + H+. The model dependence of this result was checked by using two different types of interaction potentials, one ba sed on quantum mechanical calculations on H4SiO4 clusters, and another empirical model fitted to the structure and elastic properties of Lu- quartz. Because these models gave similar results, we hypothesize that the failure of the gas-phase models is due to intrinsic solvation eff ects not accounted for by our previously developed correlations. We te sted this idea by carrying out energy minimization calculations on gas -phase clusters with one hydration shell as well as molecular dynamics simulations on fully-solvated H5SiO4+ and a fully solvated (0001) sur face of beta-quartz. Though we are unable to establish a quantitative measure of the pK(a) of SiOH2 groups, the solvated systems do indicate that SiOH groups do not protonate in any of our solvated models. (C) 1998 Academic Press.