POLYELECTROLYTE INDUCED AGGREGATION OF LATEX-PARTICLES - INFLUENCE OF THE STRUCTURAL RELAXATION OF ADSORBED MACROMOLECULES ON THE COLLOID AGGREGATION MODE

An experimental procedure allowing reproducible mixing of colloids and polymers in dilute suspension was used to study perikinetic aggregation resulting from polymer adsorption onto colloids. Changes of the temporal variation of mean characteristics of the colloid suspension (N0 and N2, zeroth and second moments of the aggregate size distribution) and of the shape of the colloid size distribution (cn(t) versus n, the number of elementary particles constituting an aggregate), were related to the slow structural relaxation of adsorbed macromolecules. At the time of initial contact of polymer and colloid, the adsorbed macromolecules had characteristics of solution macromolecules. Then, this coil-like conformation developed into a more flattened form. The parameter followed in the study was the relative polymer/latex concentration, which governed the initial surface coverage θ(0) and the rate of the temporal evolution of θ(t) toward equilibrium. At low polymer dosage, the destabilizing process at short times was interparticle bridging which developed with features of reaction-limited aggregation (RLA). After a given flocculation period depending on the polymer dosage, diffusion-limited aggregation (DLA) was found to describe the observed phenomenon. At higher polymer concentrations, "instantaneous" flocculation occurred when the polymer was continuously supplied to the stirred suspension. The aggregates formed during this RLA process thus became sterically protected against further agglomeration by the polymer layer on the surface and the suspension remained unchanged for days. © 1991.