(68c) Wong-Sandler Mixing Rules Applied to Aqueous Single-Electrolyte Solutions

An electrolyte G-excess model and a cubic equation of state (Peng-Robinson P-R) have been combined through the use of the Wong-Sandler W-S mixing rules to represent ionic properties of single strong electrolytes in water. To insure a better representation of ionic properties at 25°C a new semi-continuum solution model was developed for the excess Gibbs energy which incorporates three contributions: (1) a ion-ion interaction contribution via the use of the explicit approximation to the mean spherical aproximation MSA, (2) a contribution for short-range forces represented by a simplified Margules-type expression, and (3) solvation effects of the ionic species using the primitive hydration model of Born. The solution model assumes a complete dissociation of the electrolyte in solution. In the P-R framework the solution containing ions and water is treated as a binary mixture with both the cation and the anion forming the solute. For each electrolyte the present approach involves five parameters: a salt-water interaction parameter in the Margules equation, hydrated ionic diameters in the Born and MSA models, a binary interaction parameter in the P-R equation of state, and the P-R attraction parameter of the electrolyte. For a given electrolyte system, first the G-excess model was used to reproduce experimental mean-ionic activity coefficients at 25 °C by adjusting its parameters. The P-R parameters were then determined at the same temperature using the W-S mixing rules by equating the G-excess values obtained from the equation of state with that of the solution model. The predictive capabilities of the W-S mixing rules were tested at higher temperatures using the same parameters obtained at 25 °C (except for the P-R attraction and repulsion parameters of the salt) in the reproducibility of observed mean-ionic activity coefficients. This approach was applied to both symmetric (1-1: NaCl, 2-2:MgSO4) and asymmetric electrolytes (1-2: K2SO4, 2-1: CaCl2). The results indicated a good accuracy over a wide salt molality (from the infinite dilution region to the saturation point) and temperature ranges.