Extension of the Wilson model to electrolyte solutions

The Wilson model for non-electrolytes has been extended to model the activity coefficients of electrolytes in aqueous solutions. The excess Gibbs energy of an aqueous electrolyte solution is expressed as a sum of contributions of a long-range and a short-range excess Gibbs energy term. The contribution of the long-range excess Gibbs energy is represented by the Pitzer–Debye–Hückel model. A new expression based on the local composition concept has been developed to account for the contribution of the short-range excess Gibbs energy. The existence of three types of local cells with a central cation, anion or solvent molecule is assumed. Due to the strong like-ion repulsion it is assumed that only counterions can be found in the immediate neighborhood of a central ion. The main difference between this model and the extensions of the NRTL model to electrolytes available in the literature is the assumption that the short-range energy parameter between species in a local cell has an enthalpic rather than Gibbs energy nature. For systems containing only molecular components the new model simplifies to the three-parameter version of the Wilson model. The model has been applied to several single electrolyte systems and it has been shown that it can represent the mean ionic activity coefficients with good accuracy.