Direct Measurement of Relative Partial Molar Enthalpy of SiO ₂ in SiO ₂ –M ₂ O (M=Li, Na, K, Cs) Binary and SiO ₂ –CaO–Al ₂ O ₃ Ternary Melts

Abstract

The relative partial molar enthalpies, Δ H̄ SiO2 , of SiO 2 in SiO 2 –M 2 O (M = Li, Na, K and Cs) binary and SiO 2 –CaO–Al 2 O 3 ternary melts were directly measured by drop‐solution calorimetry at 1465 K and 1663 K. Δ H̄ SiO2 changes from exothermic to endothermic as silica content increases, confirming the tendency toward immisciblity seen from activity measurements. It is concluded that Δ H̄ SiO2 is negative due to acid‐base reactions and charge‐coupled substitutions when the melt is composed of fewer Q 4 and more Q 3 and Q 2 species, but positive due to structural strain when the melt is composed of mostly Q 4 species. The Δ H̄ SiO2 obtained by calorimetry is a useful measure of basicity, when comparing different alkali and alkaline earth oxides.