The influence of Na+, Ca2+, Ba2+, and La3+ on the ζ potential and the yield stress of calcite dispersions

The effect of Na+, Ca2+, Ba2+, and La3+ on the potential of calcite was investigated. Variations in C potential lead to a changed aggregation behavior under shear of the concentrated calcite dispersions though the changes in C potential were moderate. This change in aggregation was characterized by measuring the yield stress. Adding salt influences the electrostatic interactions between the particles and leads to a change in the shear-induced aggregation. The shear-induced structure formed at low shear rates was assumed to be partially or completely destroyed when the shear was increased. The disintegrated structure was partly reconstructed upon lowering the shear rate. This reduced relaxation was not clearly observed for the TiO2 dispersions investigated by J. Gustafsson (unpublished data). The salts added were chosen to investigate the influence of cationic charge on the colloidal stability under flow. For Na+ a pure electrostatic screening was observed as expected. The two divalent cations, Ca2+ and Ba2+, showed a similar adsorption behavior. Up to a certain concentration they caused an increased C potential, as a result of specific adsorption, after which it decreased. Particularly for Na+ and Ca2+ there is a striking correlation between the C potential and the yield stress. Overall the effect of these four ions on the C potential fit well into the general behavior, with the monovalent sodium showing the weakest and the trivalent lanthanum the strongest effect. (C) 2001 Academic Press.