PRIMITIVE MODEL FOR HIGHLY ASYMMETRIC ELECTROLYTES - ASSOCIATIVE MEANSPHERICAL APPROXIMATION

We study the primitive model of highly asymmetric electrolytes consist ing of a mixture of m + n sticky charged hard spheres, where m of thes e components represent polyions and n represent counterions. It is ass umed that the species representing polyions are highly charged and lar ge in size, while the counterions are small in size and have at most t wo charges. A two-density version of the mean spherical approximation (associative MSA) explicitly accounting for the effects of the asymmet ry is discussed and solved analytically. The approximation is based on the recently developed multidensity integral equation theory for high ly asymmetric electrolyte solutions [Kalyuzhnyi et al., Chem. Phys. Le tt. 215 (1993) 518; J. Chem. Phys. 102 (1995) 5770]. The analytic solu tion and the thermodynamic excess functions are given in terms of a si ngle screening parameter Tr. The excess entropy has the same remarkabl y simple form Delta S-MSA = (Gamma(T))(3)/3 pi + sticky terms as found in previous work.