Metal oxide surfaces separated by aqueous solutions of linear polyphosphates: DLVO and non-DLVO interaction forces

An atomic force microscope has been used to study the interaction between silica and titanium dioxide surfaces, in the form of both colloidal particles and flat surfaces, as a function of pH and in the presence of linear polyphosphates. In the absence of polyphosphate, the interaction force between silica and titanium dioxide in both symmetric (silica sphere-silica flat, titanium dioxide sphere-titanium dioxide flat) and asymmetric cases (silica sphere-titanium dioxide flat) was well described by DLVO theory. Independent streaming potential measurements of the titanium dioxide flat surfaces under the same conditions showed excellent agreement with the diffuse layer potentials derived from the force data. In the presence of polyphosphate the interaction force between silica and titanium dioxide was dominated at long range by electrostatic double layer forces and at short range by non-DLVO steric interactions. Both the repulsive electrostatic double layer interaction and the extent of the steric force increased with increasing polyphosphate concentration and n, where n is the number of phosphorus atoms in the molecule. Measurements of the steric layer thickness taken directly from the force data compared extremely well with predictions based on molecular modelling.