SUMMARY
The Brownian coagulation(or flocculation) rates of nearly monodispersed particles in aqueous solutions whose diameter goes down to about 5 nm were measured by a low angle light scattering apparatus to clarify the features of the stability of ultrafine particles. It was found that the rapid coagulation rate obeies the modified Smoluchowski theory when the particles size is submicron. But the rate depends on the particle size, the kind of electrolytes, the pH of solvent and the excessive electrolyte concentration, when particles are sufficiently small. The reduction and variation of coagulation rate were explained by the shallow minimum of the inter- particle interaction which is attributed to the layer of water molecules and ions adsorbed on the particle surface.
The flocculation of colloidal particles which were comparable with polymers in size and neutralized by polymer flocculants was also investigated. The flocculation rate was found to increase proportionally with increasing size ratio of polymers to particles if the approximate size of polymers is less than the particle radius.