HETEROFLOCCULATION BY ASYMMETRIC POLYMER BRIDGING

Heteroflocculation by polymer bridging can lead to behavior that is qu alitatively different from homoflocculation, Heteroflocculation betwee n two different particles can occur, even when the (freely dissolved) polymer does not adsorb on one of the two particles, The polymer can f irst adsorb on the surface of one particle, which modifies the configu rations and thus the entropy of the polymer, and subsequently on the s urface of the other particle, thus causing bridging. This type of asym metric polymer bridging does not lead to steric stability at excess po lymer concentrations, This behavior was observed for the heterofloccul ation of clay particles with pulp fibers induced by polyethylene oxide (PEG). An additional interesting feature of this system is that heter oflocculation is only transient, apparently due to rearrangement and f lattening of the PEO molecules adsorbed on the clay surface, which cau ses the clay particles to leave the fibers. The rearrangement time inc reases strongly with the fractional coverage of PEO on clay, Thus, in principle, transient heteroflocculation can be used to measure the rea rrangement time as a function of polymer coverage on the surface. (C) 1996 Academic Press, Inc.