  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.
