Coagulation rate of silica dispersions investigated by single-particle optical sizing

Using a single-particle optical sizer, the kinetics of the rapid coagulation of monodisperse silica particles of radius 265 nm in KCl and CaCl2 solutions was studied. The rate constant k(11) of the process is 1.7 x 10(-12) cm(3) s(-1), which is three times smaller than the Smoluchowski value for the overall rate constant. This discrepancy is explained by the retardation effect of viscous interactions and the existence of long-range attractive forces between particles. For suspensions with an initial particle concentration between 2.5 x 10(8) and 2.4 x 10(9) cm(-3), the k(11) values are independent of the pH value (in the range 7-10) and of the particle number concentration. The ratio between the coagulation concentrations of monovalent and bivalent counterions is 50, under both perikinetic and orthokinetic conditions, which indicates that this silica suspension is electrostatically stabilized. This is in contrast to literature data for very small SiO2 particles. We propose that the so-called ''structural'' forces are much less important for large silica particles.