Properties of polyelectrolyte-modified clays: Influence of the particle concentration on the degree of modification

Adsorption of polycation as well as the modification with oppositely charge d polyelectrolytes is a useful tool for surface modification. As shown prev iously, a strong enhancement of the attainable specific cationic surface ch arge was observed, e.g., with poly(diallyl-dimethylammonium chloride) (PDAD MAC) as the polycation and poly(maleic acid-co-cr-methylstyrene) (P(MS-alph a-MeSty) as the polyanion, at a ratio of anionic to cationic charges of n-/ n+ = 0.6...0.7. However, because the surface charge seemed to be strongly a ffected by the conditions of modification, the influence of particle concen tration was investigated in detail. One gram of clay was suspended in diffe rent volumes of water so that the particle concentration was varied from 1 to 100 g/L. These suspensions were treated under the same conditions. Far c haracterizing the surface charge of clay polyelectrolyte titration, dye ads orption, or microelectrophoresis was used. The comparison between the resul ts of polyelectrolyte titration and the carbon content in the residual solu tion allowed prediction of the existence of a polyelectrolyte complex and c alculation of the stoichiometry. It was found for a constant concentration of PDADMAC per 1 gram of clay that the surface charge of modified clays cha nged from negative (1 g/L) to strong positive, depending on the particle co ncentration. The cause was assumed to be the differences in the probability of interaction of particles, combined with the changes in the stoichiometr y of the polyelectrolyte complex built in the solution, whereas the effect of other factors (conductivity, time of adsorption) was rather low. A stron g cationic surface modification was obtained for medium and high particle c oncentrations only. (C) 2000 John Wiley & Sons, Inc.