Stability of colloidal silica, sikron and polystyrene latex influenced by the adsorption of polycations of different charge density

In order to describe the influence of cationic polyelectrolytes (PEs) on the stability and flocculation of dispersed particles the adsorption of statistic copolymers of diallyl-dimethyl-ammonium-chloride (DADMAC) and of N-methyI-N-vinylacetamide (NMVA) and of the corresponding homopolymers was investigated on particles of colloidal silica, quartz powder Sikron and polystyrene latex. The adsorbed amounts and the hydrodynamic thicknesses of the polymer layers and furthermore the surface charge densities and the electrophoretic mobilities of the bare and covered particles have been measured. On silica and latex the adsorbed amounts and the layer thicknesses increase with ionic strength and pH (only silica), with decreasing chain charge density of the polycations and at high electrolyte concentration with the molar mass of the polymers. The zero point of charge (zpc), and the shape of the zeta potential-pH plots vary with the coverage of the surface by the polycations. The zpc-values are independent of the ionic strength. Zeta potentials, diffusion coefficients and flocculation rates measured in dependence of the adsorbed amount of the different charged copolymers at various salt concentration and pH demonstrate fast flocculation by van der Waals attraction at small zeta potentials because of screening of the repulsive surface charges at high ionic strength and by adsorbed polycations. Additional with high molecular polycations slow mosaic flocculation is observed at low PE concentrations at still higher surface charge and slow bridging flocculation at approach of plateau adsorption. Stabilization is accomplished at low salt concentration at full coverage by electrostatic repulsion because of charge reversal. At large ionic strength high molar mass PEs are necessary to stabilize the particles sterically by osmotic repulsion of the long tails. For Sikron particles optimum flocculation is obtained at about 30% of plateau adsorption of poly(diallyl-dimethyl-ammonium-chloride) (PDADMAC) and the size of the flocs increase with molar mass. (C) 1999 Elsevier Science B.V. All rights reserved.