Modeling the atomic structure of high water content silica glasses

Silica-water liquid mixtures become completely miscible above the critical point. The resulting temperature–pressure quenched hydrous glasses, containing 8 and 12 wt.% H2O, have been modeled using Empirical Potential Structural Refinement based on high energy X-ray diffraction data to give representative atomic arrangements. For these compositions, winding chains and rings of SiO4 tetrahedra, surrounding clusters of water, are found to preserve the local atomic arrangement of silica as well as the intermediate range order, indicated by the persistence of the first sharp diffraction peak. Our models show that ∼7 % of oxygen atoms within the silicate network are replaced with hydroxyl molecules. These hydroxyls form silanol groups that effectively act as non-bridging oxygens to depolymerize the silicate network. Despite this, the ring size distribution remains very similar in the hydrous glasses to that of dry SiO2 glass. The resulting structures are found to be heterogenous over the atomic to nanometer length-scale, with the effective void space due to the cavities increasing by 8–21 % compared to dry silica. Trapped water molecules within the silicate network form moderately strong hydrogen bonds with the silanols in isolated cavities and pockets at a concentration of 8 wt.% water. At 12 wt.% water in SiO2, more water–water hydrogen bonds form within the cavities, along with water molecules residing in non-bonded interstitial locations, although the number of water molecules bonded to silanols within the silica network remains unchanged.