A Thermodynamic Model for Electrolyte Solutions with Partial Ionization

A Helmholtz-energy-based model has been developed to describe the thermodynamic properties of single strong electrolytes (of the alkali-halide type) in water within a wide range of temperatures from 25 °C to near the critical point of the solvent. The new model contains three major contributions: (1) a discrete-solvent term to account for short-range interactions between uncharged particles using the Peng-Robinson equation of state, (2) a stepwise hydration equilibrium between cation and water, and (3) a charge-charge interaction term given by the explicit mean-spherical-approximation (MSA) expression. The final model contains only three adjustable parameters: ionization constant, solvated diameter of the cation and the Peng-Robinson short-range interaction parameter between cation and water. Only the dissociation constant was considered to change with temperature. The proposed model was applied to the representation of mean ionic activity coefficients, osmotic coefficients and volumetric properties of various 1-1, 2-1, 1-2 and 2-2 electrolytes in water over wide range of temperatures, pressures and salt molalities. The results indicated a good agreement between the experimental data and those calculated using the present model.