Primitive model for highly asymmetric electrolytes. Associative mean spherical approximation

We study the primitive model of highly asymmetric electrolytes consiting of a mixture of m + n sticky charged hard spheres, where m of these components represent polyions and n represent counterions. It is assumed that the species representing polyions are highly charged and large in size, while the counterions are small in size and have at most two charges. A two-density version of the mean spherical approximation (associative MSA) explicitly accounting for the effects of the asymmetry is discussed and solved analytically. The approximation is based on the recently developed multidensity integral equation theory for highly asymmetric electrolyte solutions [Kalyuzhnyi et al., Chem. Phys. Lett. 215 (1993) 518; J. Chem. Phys. 102 (1995) 5770]. The analytic solution and the thermodynamic excess functions are given in terms of a single screening parameter ΓT. The excess entropy has the same remarkably simple form. ΔSMSA = (ΓT)3/3π + sticky terms as found in previous work.