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 s ilica and titanium dioxide surfaces, in the form of both colloidal particle s and flat surfaces, as a function of pH and in the presence of linear poly phosphates. In the absence of polyphosphate, the interaction force between silica and titanium dioxide in both symmetric (silica sphere-silica flat, t itanium dioxide sphere-titanium dioxide flat) and asymmetric cases (silica sphere-titanium dioxide flat) was well described by DLVO theory. Independen t streaming potential measurements of the titanium dioxide flat surfaces un der the same conditions showed excellent agreement with the diffuse layer p otentials derived from the force data. In the presence of polyphosphate the interaction force between silica and titanium dioxide was dominated at lon g range by electrostatic double layer forces and at short range by non-DLVO steric interactions. Both the repulsive electrostatic double layer interac tion and the extent of the steric force increased with increasing polyphosp hate concentration and n, where n is the number of phosphorus atoms in the molecule. Measurements of the steric layer thickness taken directly from th e force data compared extremely well with predictions based on molecular mo delling.