Chemical association in simple models of molecular and ionic fluids

A simple approximation scheme is given for the association constant of a family of models of associating particles introduced and considered by Cummings and Stell, Rasaiah et al., and Stell. The result correctly reduces to the law of mass action at low concentrations. For the ionic-species version, the theory requires accurate assessment of the cavity function of charged hard spheres, for which we develop a recipe. We show the close similarity (and some differences) between the above models and those considered by Wertheim, through a simple extensions of his models. We also note the equivalence of our approximation and Wertheim’s first-order thermodynamic perturbation theory.