In the interest of understanding the role of clays in silica-clay gelation, the interfacial properties of the basal plane and edge surface of both mica and talc have been studied using atomic force microscope (AFM) colloidal probe technique. The interaction forces between a colloidal silica probe and both the basal plane and edge surface of clays were measured in 1 mM KCl solutions of various pH values. The clay edge surfaces were prepared by a microtome cutting technique, while the basal planes were obtained by cleavage. The challenge and the reproducibility of this technique to prepare clay edge surfaces for colloidal force measurement will be discussed.

Direct force measurements show different charge properties of the basal plane and edge surface for both mica and talc. For the basal planes, the long-range forces are all repulsive at pH 6-10 and the force-distance curves are overlapped to some extent. This is in agreement with the common knowledge that the surface charge of basal planes is pH independent in this pH range. In the case of edge surfaces, the forces are repulsive at pH 10 and attractive at pH 6 indicating that the charge of the edge surface is pH dependent. The different trends of the measured interaction forces for basal and edge surfaces are attributed to different surface charge mechanisms on these two types of surfaces.