Effect of silica nanoparticle size on the stability of alumina/silica suspensions

The influence of the addition of silica particles (5, 15, 25, and 300 nm) on the zeta potential and viscosity of aqueous alumina slurries (250 nm particles) was investigated in a pH range where the surface charge was positive for alumina and negative for silica, For slurries formulated with the smaller silica particles, the isoelectric point shifted from pH 9.0 to pH approximate to3 (depending on the particle size of the silica) with increasing volume fraction of silica particles. At pH 9, the original isoelectric point for the alumina alone, these mixed slurries had a shear-rate-independent, low viscosity (Newtonian behavior). Both of these results show that the smaller (less than or equal to 25 nm) silica particles adsorb to the surface of alumina. The fraction of silica adsorbed to the alumina surface was dependent on the size of the silica particles, and was consistent with surface coverage calculations based on the effect of surface curvature on the limits of dense random parking. The larger silica particles (300 nm) could not physically cover the surface of the alumina particles, and simply formed a mixed, attractive particle network that exhibited a much higher viscosity with non-Newtonian (viz,, shear rate thinning) behavior.