Application of the Mean Spherical Approximation to the Estimations of Single Ion Thermodynamic Quantities of Solvation for Monoatomic Monovalent Ions in Aqueous Solutions

The expression for the Gibbs energy of solvation for simple 1-1 electrolytes within the mean spherical approximation has been fitted to data for 20 alkali metal halides in water. In performing this fit, one needs to assume a radius for each ion, the Pauling values often having been chosen in the past. The MSA parameters appear as corrections to these radii and are different for cations and anions. Two sets of ionic radii were considered, the Pauling values from crystallographic data and values based on X-ray diffraction results obtained in aqueous solutions. Excellent fits between theory and experiment were obtained, the results with the latter set of radii being preferred. This analysis provides an extrathermodynamic route to single ion solvation free energies and yields results which are very close to the real free energy of solvation measured by Randles.