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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The adsorption of the trivalent citrate anion on zirconia has been studied as a function of additive concentration and solution pH. The effects of citrate adsorption on the surface properties of the zirconia were monitored by using adsorption isotherms, FTIR spectroscopy and electrokinetic measurements. Microscopic information about the adsorbed species and its surface conformation were obtained with a n atomic force microscope (AFM). The force-distance profiles were obtained at AFM scan rates equivalent to those expected for Brownian collision rates between colloidal particles. For a given concentration of added adsorbate, the adsorbed amount of citrate was seen to increase with decreasing solution pH. This was attributed to increased Coulombic repulsions between t h e citrate and t h e surface at higher pH. The effective pHiep after adsorption was seen, from t h e electrokinetic data, to move towards lower pH values as t h e adsorbed amount increased. A limiting value of pH 3-3.2 was observed at an added citrate concentration of mol d ~n -~.AFM force-distance measurements demonstrated the presence of an ejectrosteric barrier to flocculation in the presence of adsorbed citrate. A short-range steric barrier of ca. 10 A per surface was seen under conditions where citrate was adsorbed, this barrier was in addition to a n y long-range electrostatic component of t h e overall force profile. Best fits to these long-range electrostatic parts of the data were used to calculate a diffuse-layer interaction potential. Comparison of these potentials with t h e electrokinetic data showed good agreement.

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