Electrostatic charging of non-polar colloids by reverse micelles

TL;DR

This study investigates how non-polar colloids acquire charge in the presence of reverse micelles, revealing that the charge depends on particle size and micelle adsorption, supported by a thermodynamic model.

Contribution

It demonstrates that colloid charge in non-polar solvents is independent of micelle concentration and scales linearly with particle size, supported by a thermodynamic adsorption model.

Findings

Colloid charge is independent of micelle concentration.

Charge scales linearly with particle size.

A thermodynamic model explains charge generation via micelle adsorption.

Abstract

Colloids dispersed in a non-polar solvent become charged when reverse micelles are added. We study the charge of individual sterically-stabilized poly(methyl methacrylate) spheres dispersed in micellar solutions of the surfactants sodium bis(2-ethyl 1-hexyl) sulfosuccinate [AOT], zirconyl 2-ethyl hexanoate [Zr(Oct)$_{2}$], and a copolymer of poly(12-hydroxystearic acid)--poly(methyl methacrylate) [PHSA-PMMA]. Although the sign of the particle charge is positive for Zr(Oct)$_{2}$, negative for AOT, and essentially neutral for PHSA-PMMA the different micellar systems display a number of common features. In particular, we demonstrate that, over a wide range of concentrations, the colloid charge is independent of the number of micelles added and scales linearly with the colloid size. A simple thermodynamic model, in which the particle charge is generated by the competitive adsorption of both positive and negative micelles, is in good agreement with the experimental data.