EFFECT OF PARTICLE-SIZE ON COAGULATION RATE OF ULTRAFINE COLLOIDAL PARTICLES

The Brownian coagulation rates of nearly monodispersed particles in aqueous solutions with diameters down to about 5 nm were measured with a low-angle light-scattering apparatus to clarify the features of the stability of ultrafine particles. It is found that the rapid coagulation rate obeys the modified Smoluchowski theory when particles are greater than about 0.1 μm in diameter. But the rate reduces abruptly with decreasing particle size when particles are sufficiently small, say smaller than 90 nm in diameter. Two possible mechanisms were considered for this reduction in coagulation rate: (1) particles are deflocculated because of the shallow primary minimum of the interparticle potential which is attributed to the layer of water molecules and ions adsorbed on the particle surface, and (2) the colliding velocity of particles is reduced considerably because of the existence of a layer of high viscosity around the particle surface. © 1991.