AN ANALYTICAL STUDY OF THE EFFECTS OF ASSOCIATION IN A 2-2 ELECTROLYTE SOLUTION .1. ASSOCIATIVE MEAN SPHERICAL APPROXIMATION

An analytical theory of the effects of association in a 2-2 electrolyte solution, modelled by the restricted primitive model, is presented. Our approach is based on Wertheim's integral equation theory for associating fluids, reformulated to describe the effects of association in fluids with spherically symmetric interactions. We consider the simplest two-density version, coinciding with the corresponding orientationally averaged version of Wertheim's theory. An analytical solution of the associative mean spherical approximation (AMSA) is obtained, the latter being the two-density analogue of the regular MSA. It is demonstrated that the AMSA substantially extends the range of applicability of the MSA. The AMSA results for thermodynamic properties are as good as or slightly worse than the hypernetted chain results. A similar statement holds for the pair correlation function of unlike charged ions in the region of the low (0.0001M) and intermediate (0.005M) concentrations. Predictions of the present version of AMSA for the pair correlation function of like charged ions are poor. Possible ways of improvement of the AMSA are briefly discussed.