SURFACE-CHARGE RELAXATION DURING COAGULATION

Transient deviations from the equilibrium surface charge density during the interaction of colloidal particles have been studied. Such deviations cause the process of particle encounter to become a non-first-order Markov process, complicating the analysis of colloid stability. Two methods are presented to calculate a modified colloid stability ratio, taking such deviations into account in an approximate way. These methods differ by their estimates of the time scale of the Brownian encounter and its dependence on the height of the energy barrier. Despite these differences, both methods show that double-layer dynamics can have major consequences for the stability ratio. However, the resulting dependences of the rate of slow coagulation on the particle radius are different for the two methods. This indicates that double-layer dynamics could explain the experimentally found insensitivity of the stability ratio to the particle size, provided the time scale of the encounter is strongly dependent on the height of the energy barrier. However, this proviso is unlikely to be satisfied. A simple statistical analysis indicates that the time scale of any individual encounter slightly decreases with growing barrier height. © 1993 by Academic Press, Inc.