DILUTE-SOLUTIONS OF HIGHLY ASYMMETRICAL ELECTROLYTES IN THE PRIMITIVEMODEL APPROXIMATION

The structure and thermodynamics of dilute solutions of a highly asymm etric electrolyte were studied in the primitive model approximation. S olutions of macroions and counterions in water with asymmetries in cha rge of 10:1 (10:2) and of 20:1 (20:2), and asymmetries in size of 10:2 and of 15:2 were considered. The calculations apply to the concentrat ion range where, an otherwise quite successful theory, the so-called h ypernetted chain approximation, does not provide convergent results. T he pair correlation functions and thermodynamic parameters were calcul ated using recently developed two-density theory and the cluster expan sion approach. The results of these two theories were compared with ne w Monte Carlo simulations for the same model solution. The two-density theory in the associated HNC approximation yields good agreement for both structure and thermodynamics in solutions with monovalent counter ions; i.e. for 10:1 and 20:1 electrolytes. The cluster expansion appro ach, as used in this paper, is only applicable for low asymmetry in ch arge and for dilute solutions of macroions and monovalent counterions. Among the versions of the two-density theory studied here, only the m ixed closure, i.e. associated MSA/HNC approximation, gives convergent solutions for systems of macroions and divalent counterions. The agree ment of this theory with the Monte Carlo results is only qualitative.