Phase separation in the H2O-SiO2 system

Abstract

Phase separation in the H2O-SiO2 system is examined in view of immiscibility in the alkali and alkaline earth silicates, critical parameters of which correlate with the charge and size of network modifier cations. Although the miscibility gaps of the H2O-SiO2 system have not been completely characterized, available data indicate a phase separation tendency greater than that of Li2O-SiO2, consistent with H+ being smaller than Li+. Extension of critical parameter correlations to H2O-SiO2 leads, however, to unrealistic predictions of critical composition due to neglect of cation/anion size asymmetry. To capture this effect, a new coulombic cell model is developed and combined with an asymmetric hard-sphere mixture model. The resulting equation of state predicts H2O-SiO2 critical parameters consistent with expected critical temperature and observed critical concentration. Suppression of the miscibility gap with pressure is explained as a consequence of silanol condensing into molecular H2O and increasing the background dielectric constant.

Keywords

• Silica glass
• Water
• Phase separation
• Equation of state
• Coulombic interaction
• Hard sphere mixtures