Yield stress and zeta potential of washed and highly spherical oxide dispersions - Critical zeta potential and Hamaker constant

The linear relationship between yield stress and the square of the zeta potential based on the yield stress–DLVO force model, was used to determine the zeta potential at the point of transition from flocculated to dispersed state of a range of oxide dispersions. The critical zeta potential at this point for washed α-Al2O3, TiO2 and ZrO2 dispersions was of magnitude 40, 49 and 52 mV respectively. For highly spherical silica and alumina dispersions, this value was 23 and 38 mV respectively. The square of the critical zeta potential is proportional to the Hamaker constant of the oxide in water when the van der Waals force is the only attractive force in play. Thus the critical zeta potential data obtained allowed the Hamaker constant ratio between the three oxide dispersions to be determined. This ratio between rutile TiO2/water and α-Al2O3/water was 1.50. In comparison, a similar value of 1.46 was obtained for the ratio calculated from Hamaker constant value determined via Lifshitz theory. The ratio between rutile TiO2/water and ZrO2/water is 0.90. Using the Hamaker constant of rutile TiO2/water of 61.2 zJ as the standard, the Hamaker constant determined by our method is 41 zJ for α-Al2O3, 68 zJ for ZrO2 and 13.6 zJ for silica.