Extraordinary Destabilization of Silica Nanocolloids by Filtration

After silica nanoparticles in solutions were filtered by a syringe filter with a much larger pore size than the particle diameter D_p, the filtrated effects on the rapid coagulation rate in 1 M KCl solution, the dynamic light scattering diameter, and the zeta potential at pH ∼ 6 were investigated by employing the particles of two different sizes: S particles (D_p ∼ 50 nm) of silica and latex and L particles (D_p ∼ 300 nm) of silica. It was found that the hydrodynamic diameters of silica particles became a little smaller and the absolute values of their zeta potentials decreased significantly by filtration, but that is not the case for latex particles. As for the rapid coagulation rate, the value of silica S particles increased more than 2 orders of magnitude by filtration, but no significant difference was found in the case of silica L and latex S particles. From these data, it was postulated that the gel-like layer was removed from the surface of silica S particles by filtration and the existence of the gel-like layer resulted into about 2 orders of magnitude reduction of the rapid coagulation rate. The extraordinary reduction of rapid coagulation of silica particles at D_p < 150 nm was successfully estimated by the revised Smoluchowski theory, which we call the Higashitani-Mori (HM) model. It was also found that the rapid coagulation rate of filtrated particles decreased slowly with a decreasing particle size at D_p < ca. 250 nm, which was also estimated properly by the HM model, neglecting the contribution of the redispersion of coagulated particles. Another finding in this study was that the gel-like layers were recovered with time even if they were removed by filtration, although the detailed mechanism of this recovering is not known at present and left as a future problem.