ADSORPTION OF WATER ON SILICA HYDROXYLS - DFT CALCULATIONS

We re-examine two hypothetical mechanisms proposed in the literature f or the coordination of H2O on the isolated silica OH group by DFT calc ulations of related molecular models. The computed energies suggest th at H2O stabilizes on the isolated OH group by means of two hydrogen bo nds, acting as a proton acceptor towards the silica OH group and as a proton donor towards a nearby SiOSi bridge. The alternative structure, with H2O acting exclusively as a proton acceptor towards the isolated OH group, is found to be less stable by similar to 8 kJ/mol. The calc ulations suggest no significant difference in mechanism between the ad sorptions on the isolated and geminal OH groups of silica. On a gemina l OH group H2O can form an extra hydrogen bond not only with the nearb y SiOSi bridge, but also with the complementary geminal OH group.