Ion-ion correlations in highly asymmetrical electrolytes

A study of model polyelectrolytes is reported based on both Monte Carlo and integral equation approaches. Solutions of macroions and counterions in water with asymmetries in charge of 20:1 and of 12:2, and asymmetries in size of 15:2 and of 10:1 are considered. The first results for highly asymmetrical electrolytes with divalent counterions are presented. The Monte Carlo results indicate strong clustering of divalent counterions around the macroion: the macroion-macroion pair correlation function is less structured than in the equivalent solution with monovalent counterions. The simulation results are used to assess the validity of the hypernetted chain (HNC) approximation and the recently developed two-density theory, based on Wertheim's formalism. The new theory yields better agreement with the simulation results for the 20:1 case than the conventional HNC approximation. Further, the theory provides good agreement with the Monte Carlo data for the range of concentrations where the ordinary HNC does not yield a convergent result. For divalent counterions (12:2 case), none of the theories presented is sufficiently accurate in the description of the structural and thermodynamic properties.