Evidence of Electrostatic Attraction between Equally Charged Macroions Induced by Divalent Counterions

Monte Carlo results are reported for solutions of macroions and counterions with asymmetries in charge of 20:1 and of 20:2 and asymmetry in size of 15:2. The ions are represented as charged hard spheres moving in a continuous dielectric. The results for divalent model electrolyte differ qualitatively from those for monovalent counterions. The macroion−counterion pair distribution function (pdf) indicates a very high accumulation of divalent ions about a macroion. This in turn yields an attraction between the like charged macroions; the location of the first peak in the macroion−macroion pdf suggests that there are one or two layers of counterions between the two macroions.