ION-ION CORRELATIONS IN HIGHLY ASYMMETRICAL ELECTROLYTES

A study of model polyelectrolytes is reported based on both Monte Carl o and integral equation approaches. Solutions of macroions and counter ions in water with asymmetries in charge of 20:1 and of 12:2, and asym metries in size of 15:2 and of 10:1 are considered. The first results for highly asymmetrical electrolytes with divalent counterions are pre sented. The Monte Carlo results indicate strong clustering of divalent counterions around the macroion: the macroion-macroion pair correlati on function is less structured than in the equivalent solution with mo novalent counterions. The simulation results are used to assess the va lidity of the hypernetted chain (HNC) approximation and the recently d eveloped two-density theory, based on Wertheim's formalism. The new th eory yields better agreement with the simulation results for the 20:1 case than the conventional HNC approximation. Further, the theory prov ides good agreement with the Monte Carlo data for the range of concent rations where the ordinary HNC does not yield a convergent result. For divalent counterions (12:2 case), none of the theories presented is s ufficiently accurate in the description of the structural and thermody namic properties.