(727a) Effect of Interparticle Interactions on Agglomeration and Sedimentation Rates of Colloidal Silica Microspheres

The sedimentation half-times t_s of monodisperse dispersions of 750, 505, and 350 nm silica spheres were measured in water, in ethanol, and in aqueous NaBr solutions of concentrations c_NaBr ranging from 50 to 1000 mM. In water and in ethanol, was about 8, 18, and 33 h for the 750, 505, and 350 nm particles, respectively. These values were the same as the ones predicted by the Stokes law, suggesting that the particles were monodisperse and remained so during sedimentation. t_s-values remained the same with increasing particle weight fraction up to 0.03, indicating no hydrodynamic interactions. Three regions of NaBr concentration with different settling behaviors were found for each size. In region I or at lower NaBr concentrations, t_s-values were the same as at no salt conditions, implying that there was no significant agglomeration before particles settled. In region II, t_s decreased with increasing NaBr concentration, suggesting that the agglomeration and sedimentation times of medium-size clusters were comparable. In region III, the t_s-values were quite close for all particles and independent of the NaBr concentration, indicating that the particles, in a short time, formed large clusters which settled rapidly. The zeta potentials of the particles in water or in NaBr solutions were measured and used to predict the corresponding Fuchs-Smoluchowski stability ratios, which were sensitive to the chosen Hamaker constant values and the NaBr concentrations. Two models, the Smoluchowski steady-state and a more general unsteady-state model, were presented for obtaining the agglomeration times t_an for forming 2^Nm-mers, and the net sedimentation time t_s^* for those clusters. In the model, the clusters were described by a fractal model with a fractal dimension d_f; diffusion-limited clusters with d_f=1.8, chosen to correspond to agglomeration with no subsequent internal rearrangements of the clusters, were compared to the coalescence-limit clusters, d_f=3.0, corresponding to spheres. The models provide some upper-bounds of t_an and t_s^*, Moreover, the effective sizes, density differences, and volume fractions of the clusters were obtained. The predicted trend of t_s^* was fairly consistent with the experimental data, and the predictions supported our inferences that the particles unagglomerated in Region I, formed medium-size clusters in Region II, and large clusters in Region III. (See paper by the same authors, in Langmuir, in press.)