Polymer-induced phase separations in nonaqueous colloidal suspensions

Abstract

A theory based on statistical mechanics is developed to predict the destabilization or flocculation of nonaqueous colloidal suspensions by nonadsorbing polymer. The effective interaction potential between two colloidal particles is modeled via the volume restriction potential of Asakura and Oosawa. The free energy is calculated using second-order perturbation theory with the hard sphere system serving as the reference. A fluid—solid (as opposed to a fluid—fluid) phase separation emerges, indicating that the colloid-rich phase exhibits long range order. Phase diagrams are calculated which show the volume fraction of colloidal particles in each phase as a function of polymer concentration, polymer molecular weight, and colloidal particle size. The results agree well with the experimental observations of deHek and Vrij. The prediction of a fluid—solid phase separation remains to be verified experimentally, however, indicating the need for measurements of the colloid volume fraction in and structure of each phase.