Macroion-macroion correlations in the presence of divalent counterions. Effects of a simple electrolyte

The Monte Carlo and integral equation results are presented for a primitive model of a micellar solution. The macroions and small ions are modeled as charged hard spheres embedded in a continuous dielectric. The ratio of diam eters of the macroions and small ions (counterions and co-ions) is 3.0 nm:0 .4 nm. The macroions are charged with 20 negative charges, the co-ions have one negative charge, and the counterions may carry one or two positive cha rges. In a previous study of solutions containing macroions and counterions (B. Hribar and V. Vlachy, J. Phys. Chem. B, 1997, 101, 3457-3459) it was f ound that in solutions with monovalent counterions (-20:+1) the macroions w ere distributed at large distances from each other, while in solutions with divalent counterions (-20:+2) the macroions shared the layer of counterion s. In the present paper the effects of addition of a simple electrolyte to these solutions are explored. For systems with monovalent counterions, addi tion of a low-molecular weight electrolyte causes the macroions to approach each other; the first peak of the pair-distribution function is shifted to ward smaller distances. On the other hand, the solutions with divalent coun terions seem to be less affected by addition of a simple electrolyte. The r esults for the excess internal energy, and osmotic coefficient for two diff erent macroion concentrations are presented in addition to the structural i nformation.