Direct Measurement of Relative Partial Molar Enthalpy of SiO2 in SiO2–M2O (M=Li, Na, K, Cs) Binary and SiO2–CaO–Al2O3 Ternary Melts

The relative partial molar enthalpies, ΔSiO2, of SiO2 in SiO2–M2O (M = Li, Na, K and Cs) binary and SiO2–CaO–Al2O3ternary melts were directly measured by drop-solution calorimetry at 1465 K and 1663 K. ΔSiO2 changes from exothermic to endothermic as silica content increases, confirming the tendency toward immisciblity seen from activity measurements. It is concluded that ΔSiO2 is negative due to acid-base reactions and charge-coupled substitutions when the melt is composed of fewer Q4 and more Q3 and Q2 species, but positive due to structural strain when the melt is composed of mostly Q4 species. The ΔSiO2 obtained by calorimetry is a useful measure of basicity, when comparing different alkali and alkaline earth oxides.