Charge reversal seen in electrical double layer interaction of surfaces immersed in 2:1 calcium electrolyte

Abstract

Interactions between charged surfaces immersed in aqueous calcium solutions were measured using the surface force apparatus and the atomic force microscope. With the surface force apparatus, good agreement with previously reported measurements was found for mica surfaces in dilute solutions up to 0.1 M. However, at higher concentrations large discrepancies 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 indicated similar force-distance profiles for the interaction between silicon nitride and mica. With this technique concentrations as high as 5 M can be investigated, and owing to the small radius of curvature much higher pressures can be recorded. Results obtained by both methods confirm that the force is strongly attractive at very small surface separations, in agreement with the theoretical predictions based on calculations of ion correlations. Just outside of that interval the interaction is repulsive, and it can be quantitatively explained by taking into account the adsorption of hydrated ions onto the surface (sign reversal of the effective surface charge) and the layering of co- and counterions. At larger surface separations, the behavior indicates a balance between the double layer repulsion and the van der Waals attraction (the presence of a secondary minimum).