Direct Force Measurements between Dissimilar Metal Oxides

An atomic force microscope has been used to measure the force of interaction between a Si02 glass sphere (-5 p m diameter) and a Ti02 crystal, in an aqueous medium over the pH range 3-9. The force-separation profiles obtained were compared with DLVO theory, which was found to adequately account for the experimental results up to separations of ca. 2 nm. At distances below this point repulsive interactions in excess of DLVO may be present under certain conditions. To complement the diffuse layer potential results extracted from the force-separation data for dissimilar materials, force-separation interactions were also measured between two Si02 glass spheres. For both sets of surfaces, modeling of the experimental force-separation curves was best achieved using constant-charge rather than constant-potential boundary conditions. In t r o d u c t i o n The development of the atomic force microscope1 (AFM) has made it possible to study directly the total force of interaction between a colloidal particle and, usually, a flat surface as a function of separation. AFM force measurements are normally made between a colloidal sphere glued to a cantilever spring and a flat surface which is driven toward and away from the