Past exam of the chemistry course of the year 4 of the natural sciences course of the University of Cambridge / 2022 / Paper 2 / Section D / 25 / d / i / B / Solution

Primary-minimum aggregation occurs when particles can cross the repulsive barrier and reach near contact. Its deep attractive well usually makes escape slow on experimental timescales, giving effectively irreversible coagulation. Higher electrolyte concentration screens the repulsion and can eliminate the barrier. The critical coagulation concentration is the electrolyte concentration at which rapid, approximately diffusion-limited coagulation sets in under the specified conditions; in an idealized DLVO description it is associated with loss of the protective barrier. Ion valence and surface boundary conditions strongly influence this threshold.