Volume increase of silica glass due to water intercalation and silanol group formation

We present a theoretical study on the change of volume of silica glass due to the intercalation of molecular water and the formation of silanol groups. By a statistical representative set of density functional theory calculations, we obtained a volume increase per mole of molecular water in amorphous SiO 2 of 2.5 cm 3 /mol. For the reaction of water to silanol groups we found a volume increase of 8.7 cm 3 /mol. These results partially deviate from previous experimental and theoretical work concerning the mechanisms and the size of the volume change: according to our simulations, the volume change due to molecular water is not negligible. Furthermore, our results show that the exothermic dissolution of H 2 O into silanol pairs is not restricted to small rings of size three and four. We find an equal distribution over all ring sizes which we explain by the structural relaxation and the related energy gain of the entire amorphous neighbourhood. Most exothermic dissolution of H 2 O may happen at five-membered rings since they outnumber the three- and four-membered rings in amorphous SiO 2 .