Solvation in high-temperature electrolyte solutions. II. Some formal results

Abstract

Our molecular-based formalism for infinitely dilute supercritical nonelectrolyte solutions is extended to electrolyte solutions by establishing rigorous connections between the microscopic behavior of the solvent around individual ionic species and their macroscopic solvation behavior. The formalism relies on the unambiguous splitting of the mixture’s properties into short-ranged (finite) and long-ranged (diverging) contributions, associated with the corresponding solvation and compressibility-driven phenomena, respectively. The salt (solute) and solvent’s residual chemical potentials are linked to the change of the local solvent’s environment around the infinitely dilute anion and cation, and the salt partial molar properties are interpreted in terms of the individual ion partial molar counterparts without introducing any extra-thermodynamic assumption. This is achieved with the use of Kusalik and Patey’s version of the Kirkwood–Buff fluctuation theory of mixtures. Moreover, the salt-and the individual ion-induced effects are connected to the solvent’s electrostriction around the ions, and to the coefficients of the Helmholtz free energy expansion for dilute mixtures. The ion-induced effects are also linked to well-defined excess solvation numbers which do not rely on any choice of solvation shell radius. Finally, some theoretical implications concerning the modeling of high-temperature aqueous-electrolytes solutions are discussed.