Models Having the Thermodynamic Properties of Aqueous Alkaline Earth Halides and NaCl-MgCl2 Mixtures

A model studied earlier in relation to aqueous 1-1 electrolytes has been applied to the interpretation of the excess free energies of aqueous solutions of the alkaline earth halides up to I M ionic strength. Several variations on the model could be fitted to the data. In the one which seems most consistent with the other observations, the cospheres (hydration layers) on the M(++) ions were two water molecules thick rather than one as for the alkali metal, tetraalkylammonium, and halide ions. The overlap of cospheres of an M(++), M(++) pair was found to make a small attractive contribution to the interionic force, while the overlap of cospheres of an M(++), X(-) pair makes a small positive contribution which is almost the same for X(-) = Cl(-), Br(-), or I(-). Thus, ionic hydration shells which are not penetrated by other ions are not required to account for the observed excess free energies. The model is also fitted to NaCl-MgCl(2) mixtures to see whether the new cosphere-overlap parameter which must be determined is consistent with the others. The calculation also shows how the thermodynamic behavior of the mixtures is consistent with the mixture limiting law for unsymmetrical mixtures. The singularities in the mixing coefficients g(0) and w(0) at infinite dilution dominate their concentration dependence only up to ionic strengths of about 0.01 M in models which fit the data.