Depletion flocculation of nonaqueous dispersions containing binary mixtures of nonadsorbing polymers. Evidence for Nonequilibrium effects

Depletion flocculation, induced in dispersions of hydrophobic silica particles in nonpolar media by the presence of binary mixtures of nonadsorbing polymers, has been investigated both experimentally and theoretically. The binary mixtures used were either similar polymers having different molecular weights or mixtures of chemically different polymers. To this end various monodisperse samples of polystyrene and polydimethylsiloxane homopolymers were used. The order of mixing of the components was found to affect crucially the minimum value of the (total) volume fraction of polymer required for flocculation to be observed. If the particles are added to a solution of the premixed polymers, then the experimental results indicate that the larger polymer dominates the interaction. On the other hand, intuition would suggest that the smaller molecules should dominate, in the sense that they ought to partially ''fill up'' any depletion layer formed by the larger molecules. Indeed, theoretical modeling studies, presented here, do in fact predict that the smaller polymer should dominate the depletion interaction, under equilibrium conditions. Nonequilibrium effects (namely, an inhomogeneous distribution of the large polymer) have, therefore, been used to explain the discordant nature of the experimental and theoretical findings.