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, ΔH̄_SiO2, of SiO₂ in SiO₂-M₂O (M = Li, Na, K and Cs) binary and SiO₂-CaO-Al₂O₃ ternary melts were directly measured by drop-solution calorimetry at 1465 K and 1663 K. ΔH̄_SiO2 changes from exothermic to endothermic as silica content increases, confirming the tendency toward immisciblity seen from activity measurements. It is concluded that ΔH̄_SiO2 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 ΔH̄_SiO2 obtained by calorimetry is a useful measure of basicity, when comparing different alkali and alkaline earth oxides.