Thermodynamic mixing properties of sodium silicate liquids and implications for liquid–liquid immiscibility

Enthalpies of solution have been measured for synthetic glasses in the SiO2–Na2O system from 0 to 50 mol% Na2O. The positive enthalpies of mixing determined for the compositional range between 0 and 33.3 mol% Na2O, combined with entropies for the same compositions based on ideal mixing involving Q4 and Q3 species, allowed us to calculate a liquid–liquid solvus in good agreement with the experimental data of Haller et al. [Haller, W., Blackburn, D.H., Simmons, J.H., 1974. Miscibility gaps in alkali–silicate binaries—data and thermodynamic interpretation. J. Am. Ceram. Soc. 57, 120–126]. This demonstrates a high degree of compatibility among the data for entropy, enthalpy, and phase equilibrium and also confirms the requirement of positive enthalpies of mixing for liquid immiscibility in the SiO2–Na2O system.