Ionic association in model 2–2 electrolyte solutions

A detailed study has been made of the structural consequences of a charged soft sphere model for aqueous 2–2 electrolytes. The HNC approximation for this model gives structural features as well as thermodynamic functions that agree closely with the reported HNC results for a charged hard sphere model provided that R, the distance at which the pair potential u+−(r) is a minimum, is the same in both models. In both cases there is a strong enhancement of the like-charge correlation function g++(r) [equals g−−(r) in these models] in the range where r?2R. The trends in the HNC results are generally confirmed by Monte Carlo (MC) results for the same model, but the enhancement of g++ is greatly reduced compared to HNC. The MC results confirm earlier interpretation of the enhancement as due to larger than pairwise clusters even at concentrations below 0.005 M. Analysis of the discrepancy between the MC and HNC results leads to a revised form of the latter, BHNC, which incorporates a simple approximation for the sum of bridge diagrams and which gives g++(r) in much closer agreement with the Monte Carlo results.