Ion-specific colloidal aggregation: population balance equations and potential of mean force

Recently reported colloidal aggregation data obtained for different monovalent salts (NaCl, NaNO(3), and NaSCN) and at high electrolyte concentrations are matched with the stochastic solutions of the master equation to obtain bond average lifetimes and bond formation probabilities. This was done for a cationic and an anionic system of similar particle size and absolute charge. Following the series Cl(-), NO(3)(-), SCN(-), the parameters obtained from the fitting procedure to the kinetic data suggest: (i) The existence of a potential of mean force (PMF) barrier and an increasing trend for it for both lattices. (ii) An increasing trend for the PMF at contact, for the cationic system, and a practically constant value for the anionic system. (iii) A decreasing trend for the depth of the secondary minimum. This complex behavior is in general supported by Monte Carlo simulations, which are implemented to obtain the PMF of a pair of colloidal particles immersed in the corresponding electrolyte solution. All these findings contrast the Derjaguin, Landau, Verwey, and Overbeek theory predictions.