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

TL;DR

This study investigates how different monovalent salts influence colloidal aggregation by analyzing kinetic data and modeling the potential of mean force, revealing complex behaviors that challenge classical DLVO theory predictions.

Contribution

The paper introduces a combined experimental and simulation approach to characterize ion-specific effects on colloidal aggregation beyond classical theories.

Findings

PMF barrier increases with certain salts

Contact PMF varies between cationic and anionic systems

Secondary minimum depth decreases with specific ions

Abstract

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 latices. 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.