Modelling of the thermodynamic properties of ionic solutions using a stepwise solvation-equilibrium model

The stepwise solvation-equilibrium model of Stokes and Robinson is used for a description of departures from ideality in ionic solutions. It is shown how to construct a thermodynamically consistent model including solvation effects. Simple expressions are derived for the mean ion solvation number. The model is applied to strong electrolyte solutions (pure water + salt and mixed aqueous solvent + salt ) by taking the mean spherical approximation (MSA) for the long-range contribution to the Gibbs energy and a local composition model, the nonrandom two-liquid (NRTL) model, for the short-range contribution.