Macroion-macroion correlations in presence of divalent counterions

Monte Carlo results for a primitive model of a micellar solution are presented. The macroions are assumed to carry 20 negative charges and the counterions are monovalent and/or divalent, The ratio of diameters of the ionic species, described as charged spheres, is 3.0 nm:0.4 nm (macroions:small ions). An important conclusion of the study is that by replacement of monovalent counterions with divalent ones it is possible to change the structure of the solution. In solutions with predominantly divalent counterions, the macroions approach each other and share the layer of counterions. This structure is rather stable; in our case the macroion clusters do not "dissolve" upon dilution from 0.02 to 0.01 mole dm(-3) and are not significantly affected by moderate addition of simple electrolyte. Thermodynamic considerations indicate an energy-entropy compensation in the process of dilution.