Yield stress and contact forces in coagulated oxide dispersions - Role of electrostatic interactions

The adsorption of low molecular weight oxyanions onto oxide surfaces is known to Limit the distance of the closest approach between particles in dispersions of such oxides. As a consequence, the value of van der Waals interactions at the point of contact remains finite. By calculating the forces for conditions where the charge density of the absorbed ions is small, it has been demonstrated that the electrostatic repulsion, for pH values sufficiently far from the isoelectric point (i.e.p.), can become large enough to overcome the attraction, even at distances of closest approach. More significantly, this conclusion remains true at high electrolyte concentrations. It has also been shown that the current results can account for the apparent narrowness of the experimental shear stress vs. pH curves, recently reported in a number of studies for oxide dispersions. It has been predicted that, regardless of the electrolyte concentration, coagulation for such systems can only occur over a limited range of pH values around the i.e.p. The significance of these results for the problem of peptisation, in dispersions with high acid contents, has also been briefly discussed.