Ion–solvent interactions and the activity coefficients of real electrolyte solutions

A thermodynamic argument is developed which gives the activity coefficient of a real electrolyte solution in terms of a formula involving integrals over the radial distribution functions of the several ionic species, together with additional terms which take account of ion–solvent interactions. The ion–solvent interaction contributions can be calculated from a knowledge of the molal volume of the solution. Both required quantities are observable and easily accessible. This permits an evaluation of the validity of the restricted primitive model and of Debye–Hückel theory for real electrolyte solutions.