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.