Yield stress-zeta potential relationship of oxide dispersions with adsorbed polyacrylate — Steric effect and zeta potential at the flocculated-dispersed transition state

The yield stress-DLVO force relationship is obeyed by α-Al2O3 and alumina-coated TiO2 dispersions with adsorbed polyacrylate only if the yield stress and its corresponding zeta potential data were collected in the positively charged region. In this region, the underlying surface positive charge density of the particles exceeds the negative charge density of the polyacrylate. At this state the adsorbed polyelectrolyte lies flat on the particle surface forming a steric layer of fixed thickness at a given polymer concentration. In the negative charge region, the steric layer thickness is not constant and hence yield stress-DLVO relationship is not obeyed. The (critical) zeta potential at the flocculated-dispersed transition state decreases with increasing polymer concentration. This result reflects a decreasing van der Waals force as the steric layer increases in thickness. A steric layer ensured that the surface or zeta potential is sufficiently low in the flocculated regime for the DLVO theory to remain valid. The ratio of the critical zeta potential square between alumina-coated TiO2 and α-Al2O3 is an indication of their Hamaker constants ratio in water. The effect of alumina coating on the value of this ratio is presented and discussed.

With adsorbed polyacrylate and under the condition of the net positive particle surface charge, the yield stress decreases linearly with square of zeta potential as predicted by the yield stress-DLVO model. The model breakdowns when the charges of the adsorbed polyacrylate exceed the particle surface positive charges. This is explained in terms of a non-constant steric layer thickness.Figure optionsDownload as PowerPoint slide