Influence of volume-restriction effect on phase separation phenomena in steric dispersions

Abstract

Steric systems such as paints, inks, milk and other dairy products, and biocolloids such as tissue and blood cells and protein solutions form a significant class of practically and theoretically important dispersions. Stability and phase separation phenomena in steric systems are complex due, in part, to the role of the polymeric macromolecules which are used to impart steric stability (or, instability). In this study, phase separation phenomena in selected steric systems are examined using a statistical mechanical formalism. Specifically, the volume-restriction effect in steric systems, which arises from the depletion of free polymer from the interparticle region due to the loss of configurational entropy, is investigated. The exclusion of the polymer molecules from the region of interparticle separation leads to an attractive force which is described by an interparticle pair potential first developed by Asakura and Oosawa ( J. Chem. Phys. 22, 1255 (1954); J. Polym. Sci. 33, 183 (1958)). In the case of aqueous systems in which electrolytes are present, a suitable electrostatic interaction is added to the Asakura-Oosawa potential for the volume-restriction interaction. With or without electrostatic and van der Waals interactions. the volume-restriction interaction potential is used in combination with perturbation theory (one statistical mechanical technique) to obtain osmotic pressures and free energies in fluid-like dispersions. Results for the solid phase are obtained using another statistical mechanical technique, a cell model technique developed by Lennard-Jones and Devonshire ( Proc. R. Soc. London Ser. A 163, 53 (1937); 165, 1 (1938)). These results, in turn, are used to construct phase diagrams for nonaqueous and aqueous dispersions in terms of polymer concentration. These results are compared with selected experimental and other theoretical results available in the literature.