Effect of solvent on the stability of mixtures of sterically stabilized dispersions and free polymers

Abstract

The stability of mixtures of sterically stabilized colloidal particles and free polymer molecules in solution has been examined theoretically by assuming that the pair potential is the sum of contributions from (i) steric repulsion due to the adsorbed polymer layers, (ii) the attraction due to the exclusion of the free polymer molecules from the inter-particle space, and (iii) van der Waals attraction forces. Using the hard-sphere perturbation theory, the phase boundaries of the transition from a dilute, disordered (fluid) state to a concentrated, ordered (solid) state have been located. The systems chosen correspond to the situation in which the adsorbed polymer and the free polyner are chemically different and the molecular weight of the free polymer is much greater than that of the adsorbed polymer. The effect of varying the nature of the solvent from being good for both the adsorbed and free polymers to being good only for the adsorbed polymers is examined. If stable dispersions are desirable, it is advisable to have a solvent which is good for the adsorbed polymer but poor for the free polymer. The effect of other variables such as the molecular weight of the free polymer, the temperature, the thickness of the adsorbed layer and the particle size on the stability of the dispersion has also been examined. The limiting free polymer concentration, above which the disorder—order transition occurs, is found to decrease with increasing molecular weight of the free polymer, with increasing particle size, with increasing temperature, with thinner steric layers and with better solvents for the free polymer, in accordance with the trends observed experimentally. The case of pure steric stabilization is examined by setting the free polymer concentration equal to zero, and one concludes that destabilization is possible under better-than-theta conditions. It is also shown that for pure steric stabilization, the critical particle concentration above which the disorder—order transition occurs decreases with increasing particle size and increases with increasing thickness of the adsorbed layer, again as observed experimentally.