POLYELECTROLYTE SOLUTIONS PLUS MULTIVALENT SALTS = PHASE-SEPARATION

Aqueous solutions of highly charged flexible polyelectrolytes phase se parate in the presence of multivalent counterions. We present a theore tical model which explains this behavior in terms of electrostatic bri dging between monomers via the condensed counterions. The <<Polymer Re ference Interaction Site Model>> with the Debye-Huckel closure (equiva lent to the <<Random Phase Approximation>>) is sufficient to understan d the crucial role of the valency of the counterions, to predict a dem ixion 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.