Thermodynamic Analysis of Liquidus Prediction Differences Between Redlich–Kister and MQMQA Descriptions for the NaCl–MgCl₂ System

Accurate prediction of melting behavior is essential for defining the operating window of molten chloride salts. This study compares the adopted Redlich–Kister (RK) and Modified Quasichemical Model in the Quadruplet Approximation (MQMQA) liquid-phase descriptions for NaCl–MgCl₂ using common pure-liquid reference Gibbs energies and solid-phase descriptions. Five previously measured DSC onset temperatures and 25 literature mixed-salt liquidus points were evaluated separately without parameter fitting or empirical temperature correction. Relative to the five DSC onset temperatures, the mean absolute deviations were 28.21 °C for MQMQA and 56.48 °C for RK. Agreement with the literature liquidus data varied among datasets, with neither description consistently yielding smaller deviations. Calculated minimum-liquidus temperatures were 459.22 °C for MQMQA and 487.11 °C for RK, both above the selected reported eutectic central values of 430–451 °C. At the five DSC conditions, MQMQA predicted liquid Gibbs free energies 0.74–0.79 kJ/mol salt lower, primarily through a more negative mixing contribution. These results characterize differences between the adopted descriptions and their agreement with specific datasets rather than establish the general superiority of either model formalism.