Direct Force Measurements between TiO₂ Surfaces

An atomic force microscope (AFM) has been used to measure the force of interaction between a rutile titanium dioxide colloid and a single macroscopic rutile crystal in aqueous solution. The effect of pH and electrolyte concentration on the force has been investigated, ζ potentials were derived from electrophoretic mobility measurements on the rutile colloid as a function of pH and electrolyte concentration. Experimental decay lengths for the repulsive electrical double layer interaction are in good agreement with the theoretical Debye lengths at <10⁻² M electrolyte. Measurements at the isoelectric point, i.e. pH = 5.6 of the TiO₂, could be fitted with a nonretarded Hamaker constant of 6 ± 2 10⁻²⁰ J. This value agrees well with the van der Waals interaction calculated within the framework of the Lifshitz theory. In the calculation we used the Ninham–Parsegian representation for the dielectric susceptibility function and have utilized refractive index versus wavelength data to characterize the van der Waals interaction in rutile systems. A nonretarded Hamaker constant of 7 ± 1 × 10⁻²⁰ J was calculated for two rutile surfaces interacting across water.