Influence of alkali mixed effect on the mixing enthalpy in 0.75B2O3–0.25[xNa2O–(1 − x)K2O] glass system

The mixed alkali effect was investigated in the glass system 0.75B2O3–0.25[xNa2O–(1 − x)K2O] through thermodynamic properties. The calorimetric measurements were performed in HF solution calorimetry at 298 K. The mixing enthalpy values show non-linear behaviour upon substitution of one alkali ion by another. This thermodynamic non-ideality is caused by the slight variations of distance between metallic cations, the macromolecular structure being unchanged. It can be explained, at least qualitatively, using electrolyte theory based on the Coulombic interactions of charged species originally developed by Debye and Hückel.