Forces between Colloidal Particles in Aqueous Solutions Containing Monovalent and Multivalent Ions

TL;DR

This paper reviews recent experimental techniques for measuring forces between colloidal particles in aqueous solutions with monovalent and multivalent ions, confirming the applicability of DLVO theory with some modifications at short distances.

Contribution

It provides a comparative overview of force measurement methods and discusses the validity of DLVO theory in complex ionic environments.

Findings

DLVO theory accurately describes force profiles with multivalent ions.

Force measurements show long-range repulsion at low salt concentrations.

Short-range non-DLVO interactions become significant with multivalent counterions.

Abstract

The present article provides an overview of the recent progress in the direct force measurements between individual pairs of colloidal particles in aqueous salt solutions. Results obtained by two different techniques are being highlighted, namely with the atomic force microscope (AFM) and optical tweezers. One finds that the classical theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO) represents an accurate description of the force profiles even in the presence of multivalent ions, typically down to distances of few nanometers. However, the corresponding Hamaker constants and diffuse layer potentials must be extracted from the force profiles. At low salt concentrations, double layer forces remain repulsive and may become long ranged. At short distances, additional short range non-DLVO interactions may become important. Such an interaction is particularly relevant in the presence of multivalent counterions.