Charge reversal of colloidal particles

TL;DR

This paper develops a theory explaining how colloidal particles can undergo charge reversal in the presence of multivalent counterions, highlighting the complex interplay of solvation energies and salt concentrations.

Contribution

It introduces a new theoretical framework for charge renormalization considering multivalent ions, differing from previous models by accounting for non-monotonic effective charge behavior.

Findings

Effective charge can exceed the bare charge, causing charge reversal.

Multivalent salt concentration affects charge reversal non-monotonically.

Monovalent salt generally hinders charge reversal at higher concentrations.

Abstract

A theory is presented for the effective charge of colloidal particles in suspensions containing multivalent counterions. It is shown that if colloids are sufficiently strongly charged, the number of condensed multivalent counterion can exceed the bare colloidal charge leading to charge reversal. Charge renormalization in suspensions with multivalent counterions depends on a subtle interplay between the solvation energies of the multivalent counterions in the bulk and near the colloidal surface. We find that the effective charge is {\it not} a monotonically decreasing function of the multivalent salt concentration. Furthermore, contrary to the previous theories, it is found that except at very low concentrations, monovalent salt hinders the charge reversal. This conclusion is in agreement with the recent experiments and simulations.