CHARGE REVERSAL SEEN IN ELECTRICAL DOUBLE-LAYER INTERACTION OF SURFACES IMMERSED IN 2-1 CALCIUM ELECTROLYTE

Interactions between charged surfaces immersed in aqueous calcium solu tions were measured using the surface force apparatus and the atomic f orce microscope. With the surface force apparatus, good agreement with previously reported measurements was found for mica surfaces in dilut e solutions up to 0.1 M. However, at higher concentrations large discr epancies were observed. Compared to the earlier work, the strength of the force was lower by two or three orders of magnitude and the range was diminished. Experiments using the atomic force microscope indicate d similar force-distance profiles for the interaction between silicon nitride and mica. With this technique concentrations as high as 5 M ca n be investigated, and owing to the small radius of curvature much hig her pressures can be recorded. Results obtained by both methods confir m that the force is strongly attractive at very small surface separati ons, in agreement with the theoretical predictions based on calculatio ns of ion correlations. Just outside of that interval the interaction is repulsive, and it can be quantitatively explained by taking into ac count the adsorption of hydrated ions onto the surface (sip reversal o f the effective surface charge) and the layering of co- and counterion s. At larger surface separations, the behavior indicates a balance bet ween the double layer repulsion and the van der Waals attraction (the presence of a secondary minimum).