Application of the modified BET model to concentrated salt solutions with relatively high water activities: Predicting solubility phase diagrams of NaCl + H2O, NaCl + LiCl + H2O, and NaCl + CaCl2 + H2O

Solubility phase diagrams of mixtures of salts and water are of interest in various application fields and can be calculated using the modified BET model. In comparison with other thermodynamic models, the modified BET model requires only two salt-specific model parameters and has shown to possess a predictive capability. In this work, the modified BET equations are extended in order to calculate solubility phase diagrams of concentrated salt solutions with relatively high water activities within the range of undersaturation. As an example, BET parameters of NaCl are determined and applied to calculate solubility phase diagrams of NaCl + H2O, NaCl + LiCl + H2O, and NaCl + CaCl2 + H2O within the temperature range of around 250-500 K. In the ternary systems, the best agreement with solubility data from literature is obtained using constant BET parameters of NaCl and an additional temperature-dependent regular solution parameter to account for salt-salt interaction.