Bridging atomistic simulations and thermodynamic hydration models of aqueous electrolyte solutions

Abstract

Chemical thermodynamic models of solvent and solute activities predict the equilibrium behaviour of aqueous solutions. How-ever, these models are semi-empirical. They represent micro-scale ion and solvent behaviours that control the macroscopic properties using small numbers of parameters whose values are obtained by fitting to activities and other partial derivatives of the Gibbs energy measured for the bulk solutions. We have conducted atomistic simulations of aqueous electrolyte solutions (MgCl2 and CaCl2) to determine the parameters of aqueous thermodynamic hydration models. We have implemented a coopera-tive hydration model to categorize the water molecules in electrolyte solutions into different subpopulations. The value of the electrolyte-specific parameter, k, was determined from the ion-affected subpopulation with the lowest absolute value of the free energy of removing the water molecule. The other equilibrium constant parameter, K1, associated with the first degree of hydra-tion, was computed from the free energy of hydration of hydrated clusters in solution. The hydration number, h, was determined from a reorientation dynamic analysis of the water subpopulations compared to bulk-like behaviour. The computed values of these parameters were inserted in the Stokes & Robinson (Journal of Solution Chemistry 1973, 2, 173-191) and Balomenos (Fluid Phase Equilibria 2006, 243, 29-37) models, which were applied to evaluate the osmotic coefficients of MgCl2 solutions. Such an approach removes the dependence on the availability of experimental data and could lead to aqueous thermodynamic models capable of estimating the values of solute and solvent activities, thermal and volumetric properties for a wide range of composition and concentrations.