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 th e stability and flocculation of dispersed particles the adsorption of stati stic copolymers of diallyl-dimethyl-ammonium-chloride (DADMAC) and of N-met hyI-N-vinylacetamide (NMVA) and of the corresponding homopolymers was inves tigated on particles of colloidal silica, quartz powder Sikron and polystyr ene latex. The adsorbed amounts and the hydrodynamic thicknesses of the pol ymer layers and furthermore the surface charge densities and the electropho retic mobilities of the bare and covered particles have been measured. On s ilica and latex the adsorbed amounts and the layer thicknesses increase wit h ionic strength and pH (only silica), with decreasing chain charge density of the polycations and at high electrolyte concentration with the molar ma ss of the polymers. The zero point of charge (zpc), and the shape of the ze ta potential-pH plots vary with the coverage of the surface by the polycati ons. The zpc-values are independent of the ionic strength. Zeta potentials, diffusion coefficients and flocculation rates measured in dependence of th e adsorbed amount of the different charged copolymers at various salt conce ntration and pH demonstrate fast flocculation by van der Waals attraction a t small zeta potentials because of screening of the repulsive surface charg es at high ionic strength and by adsorbed polycations. Additional with high molecular polycations slow mosaic flocculation is observed at low PE conce ntrations at still higher surface charge and slow bridging flocculation at approach of plateau adsorption. Stabilization is accomplished at low salt c oncentration at full coverage by electrostatic repulsion because of charge reversal. At large ionic strength high molar mass PEs are necessary to stab ilize the particles sterically by osmotic repulsion of the long tails. For Sikron particles optimum flocculation is obtained at about 30% of plateau a dsorption of poly(diallyl-dimethyl-ammonium-chloride) (PDADMAC) and the siz e of the flocs increase with molar mass. (C) 1999 Elsevier Science B.V. All rights reserved.