Role of surface chemistry in primary and secondary coagulation and heterocoagulation

Abstract

Stability ratios were calculated from a modified Fuchs' equation using the double-layer interaction computed by solving the nonlinear Poisson-Boltzmann equation subject to boundary conditions which consider dissociation equilibria. Stability ratios calculated for identical particles with weakly acidic surfaces show less sensitivity to ionic strength than with strongly acidic surfaces and the same density of ionizable sites. Oppositely charged sols may also be stable to heterocoagulation over a narrow interval of salt concentration, provided the ratio of surface charge densities is sufficiently different from -1. Stability arises from the double-layer repulsion which occurs between oppositely charged surfaces separated by less than one Debye length. Particles may coagulate in the secondary minimum without overcoming the barrier which opposes coagulation in the primary minimum. When the secondary minimum is deep compared to kT, an analysis was presented which indicated that coagulation in the secondary minimum is initially rapid. After a relaxation time, which increases with the depth of the secondary minimum, the total rate of coagulation in the primary and secondary minima slows to the steady-state value predicted by Fuchs' theory. Consideration of variations in the surface potential with the salt concentration or the particle-particle separation did not resolve the disparity with respect to particle size between predicted stability ratios and those measured by Ottewill and Shaw [ Disc. Faraday Soc. 42,154 (1966)]. Likewise, consideration of the secondary minimum did not resolve the disparity. However, considering a Gaussian-like distribution of charge densities among particles of a sol may lead to agreement between predicted and observed stability ratios.