POLYELECTROLYTE SOLUTIONS PLUS MULTIVALENT SALTS = PHASE-SEPARATION

Aqueous solutions of highly charged flexible polyelectrolytes phase separate in the presence of multivalent counterions. We present a theoretical model which explains this behavior in terms of electrostatic bridging between monomers via the condensed counterions. The <<Polymer Reference Interaction Site Model>> with the Debye-Huckel closure (equivalent to the <<Random Phase Approximation>>) is sufficient to understand the crucial role of the valency of the counterions, to predict a demixion above a given valency and a redissolution of the precipitate in excess of added salt. The model successfully reproduces the different experimental phase diagrams of polystyrene sulfonate suspensions with different added electrolytes.