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 (c), to lower pH values. The additives also lowered the magnitude of the maximum yield stress at [ = 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.

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This paper examines the effects of particle concentration and size on the yield stress of ZrO, suspensions at a welldefined surface chemistry condition of the isoelectric point (IEP). At the IEP, the relationship between yield stress T~~~~ and particulate volume fraction & and mean particle size d was evaluated to be T~~, , = K +:o/dz.o. The difference in size distribution of the various ZrO, suspensions examined causes some degree of scatter in the data used to establish the T~_.,, &, and d relation. The use of particle concentration n, based on the fine size fraction instead of volume fraction & provided a better correlation, because the fine particles govern the properties of the flocculated network structure.

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