Phase separations induced in aqueous colloidal suspensions by dissolved polymer

Abstract

Several experimental studies have demonstrated the tendency of non-adsorbing dissolved polymer to cause a phase separation in otherwise homogeneous colloidal suspensions. This separation occurs with hard-sphere systems as well as those stabilized either sterically or electrostatically. A restabilization at higher polymer concentrations has been observed in some electrostatically and sterically stabilized dispersions. A theory based on statistical mechanics presented here predicts such phase transitions in aqueous colloidal suspensions. The effective interaction potential between two colloidal particles comprises as combination of the volume restriction potential of Asakura and Oosawa and the classical electrostatic repulsion of Derjaguin. The free energy is calculated via second-order perturbation theory with an effective hard-sphere system serving as the reference state. A fluid–solid phase diagram emerges showing the volume fraction of particles in each phase as a function of osmotic pressure. The details of the phase behaviour depend on the polymer concentration and molecular weight, the ionic strength and the electrostatic potential and size of the particles. At high polymer concentrations, a semi-dilute theory of Muthukumar and Edwards is employed to account for the decreasing range of polymer-polymer and polymer-particle interactions with increasing concentration. This behaviour provides a mechanism for electrostatic restabilization at sufficient polymer concentrations. The results for the polymer concentration required for destabilization, c_F, agree well with experimental observations of Sperry, Hopfenberg and Thomas. Corresponding measurements of the densities of the resulting phases have not been reported. The predictions of restabilization also remain to be tested quantitatively, but the results conform qualitatively with experimental observation.