Aggregate structures formed via a bridging flocculation mechanism

A high molecular weight cationic polyelectrolyte has been used to flocculat e a colloidal dispersion of anionic polystyrene latex particles. The polyme r used had a high charge density and the flocculation occurred at a solutio n pH where both the polymer and the particles were fully charged. Under the se conditions, flocculation is expected to occur through a bridging floccul ation mechanism. Low angle laser light scattering has been used to follow t he flocculation process as a function of time; parameters of interest were the aggregate sizes, size distributions, and aggregate mass fractal dimensi ons. The light scattering measurements showed that the flocs formed had a m ass fractal character All the systems examined here were overdosed with res pect to the optimum flocculation concentration of polymer. Under these cond itions, decreasing the polymer concentration was seen to result in an incre ased flocculation efficiency. A secondary growth process was also observed whereby initially formed fractal aggregates can subsequently aggregate agai n. These larger aggregates are also expected to be mass fractals although t his cannot be determined from the light scattering measurements due to the superposition of Fraunhofer diffraction effects. This type of fractal-in-fr actal character is unusual. (C) 2000 Elsevier Science B.V. All rights reser ved.