On the Flocculation and Agglomeration of Ceria Dispersion

The ceria dispersions displayed a maximum in the yield stress at the isoelectric point (pI) at pH similar to 7. At pH below pI, the flocculated dispersions at solids concentration of 30 wt% and above produced a smooth and homogeneous appearance. However, at pH above pI, phase separation was observed to occur quite quickly forming a clear layer of liquid supernatant. Upon examination, relatively large discrete aggregates had formed. This behavior can be explained by the surface chemical model proposed by Nabavi et al.([1]) According to Nabavi et al., nitrate anion, normally an indifferent electrolyte, is both adsorbed and bounded covalently to the surface of the ceria particles at pH below pI. A steric layer is formed preventing particle agglomeration. Such agglomeration was also prevented at high pH with the adsorption of pyrophosphate additive added at 1dwb% concentration. The model proposed by Nabavi et al also explained the agglomeration of ceria particles at pH>pI. Both the covalently bounded and adsorbed nitrates were displaced by the hydroxyl group forming this surface -OH group that on the interacting particles condenses to form -Ce-O-Ce- bridging bond and caused particle agglomeration. The yield stress-DLVO force model was obeyed by the ceria dispersion at pH below pI. A critical zeta potential of 56mV was obtained and this gives an estimate value for its Hamaker constant in water of 82 zJ.