Electrostatic attraction and phase separation in solutions of like-charged colloidal particles

Model systems of charged spherical macroions with either monovalent, divalent, or trivalent counterions interacting solely through hard-core and Coulomb forces have been investigated by Monte Carlo simulations. Although the direct macroion-macroion interaction is purely repulsive, we find for sufficiently strong electrostatic coupling an effective attraction that separates the solution into two fluid phases. The attraction is short ranged and arises from correlations among counterions localized near different macroions. This mechanism for attraction differs from that attraction inferred to operate in solutions of charged latex particles presently challenging the classical DLVO theory.