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Completely dissociated aqueous solutions of alkali halides are successfully modeled to about 500−600 K, for instance, by the ion-interaction equations. At higher temperatures, the ions associate to neutral molecules and clusters. In the more concentrated solutions, some “effective chemical continuum” treatment may be applied. Neither approach is, however, suitable for the intermediate region of near-critical temperatures and low solution concentrations, where the ions are only partially associated and thermodynamic properties are subject to rapid changes. A Helmholtz energy model coupled with chemical association was proposed for these systems and used for simultaneous correlation of experimental apparent molar volumes, apparent molar heat capacities, and association constants of dilute NaCl(aq) solutions at temperatures from 573 to 723 K and at concentrations where the solutions are partially associated. Conversion from a MacMillan−Mayer to a Lewis−Randall reference state was included in the calculations. Performance of the model for data description is good, suggesting its application for other associating electrolytes in a wide range of state conditions.

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