Kinetic aspects of polymer bridging: Equilibrium flocculation and nonequilibrium flocculation

Abstract

We carried out a detailed study of the flocculation of polystyrene latices by poly (ethylene oxide) with a specially designed single particle optical sizer. With this instrument the aggregate size distribution of a flocculating system can be determined very accurately. We found clear evidence for two types of flocculation mechanisms. In the first, which we denote as equilibrium flocculation, the time of reconformation is small with respect to polymer attachment and particle collisions. Consequently, the adsorbed layer is relaxed before two particles meet. In this case the flocculation process obeys normal second-order kinetics. On the other hand, in nonequilibrium flocculation the adsorbed polymer is still extended when particles collide. The flocculation is now more efficient and the overall process follows quite different kinetics. The time needed for the initially adsorbed polymer to flatten within twice the double layer thickness was found to be, for long polymers and low salt content, of the order of several seconds. We discuss the conditions under which these two mechanisms occur as a function of polymer molecular weight, salt concentration and particle concentration. In some cases both equilibrium and nonequilibrium flocculation take place simultaneously.