Rheological evidence of adsorbate-mediated short-range steric forces in concentrated dispersions

In order to examine quantitatively the effect of short-range forces on the state of dispersion in dense suspensions, the effect of a series of anionic adsorbates on the rheology of a model zirconia concentrated suspension has been studied in detail. The anionic additives include sulfate, phosphate, pyrophosphate and polyphosphates as well as simple organic acid anions such as lactate, malate and citrate. The adsorbates shifted the pH of maximum static yield stress and, equivalently, the pH of zero zeta potential (ζ), to lower pH values. The additives also lowered the magnitude of the maximum yield stress at ζ = 0. The results are all consistent with the adsorbates producing a steric barrier equivalent to the size of the adsorbed molecule along the attractive van der Waals interaction. Separately the polyphosphate adsorbates are shown to assume a flat orientation and do not appear to produce a thick electro-steric barrier to coagulation.