Charge Stabilization in Nonpolar Solvents

Original Entry by Michelle Borkin, AP225 Fall 2009

Overview

M. F. Hsu, E. R. Dufresne, and D. A. Weitz, Langmuir 21, 4881-4887 (2005).

Keywords

Colloidal Dispersion, interaction potential, Reverse Micelle, surface potential, Debye length

Summary

This paper investigates the use of surfactants to control charges is nonpolar solvents ($\epsilon \approx 2$ versus for an aqueous solution $\epsilon \approx 80$) where the electrostatic charge barrier is much greater than kT. Understanding nonpolar solvents and colloid interactions are important for industrial and commercial applications such as electrophoretic ink or the stabilization of soot particles in oil. Surfactants play the key role in creating in such solutions the charge-stabilizing aggregates. The research presented in this paper focuses on nanometer sized reverse micelles in nonpolar solvents and investigates the electrokinetics and thermodynamic properties to explain how the micelles effect charge. As opposed to simple salt ions, these large reverse micelles have low ionization energies and charge surfaces by stabilizing counterions. They find very strong surface potentials (2.0-4.4 kT) and Debye screening lengths (0.2-1.4 $\mu m$) that strongly depend on the concentration of reverse micelles in the system.

Soft Matter

Figure 1: Charge stabilization of a nonpolar suspension before (a) and after (b) adding reverse micelles (AOT solution above cmc). The field of view is 135 x 108 $\mu m ^2$.

For the experiments, the reverse micelles were created using aerosol-OT (AOT) which forms nanometer sized reversed micelles (contains ~30 surfactant molecules) above its 1mM in dodecane critical micelle concentration (cmc). The colloid particles in the system are PMMA particles with PHSA grafted to their surface for steric stabilization (radius = 780 nm). The solution is contained in cells between glass plates thus the model and results presented are based on 2D descriptions and analysis. As shown in Figure 1, when AOT (i.e. the reverse micelles) is added to the solution (b) the colloidal particles evenly disperse due to the electrostatic forces.

They observe a strong dependence on AOT concentration in controlling the interactions, specifically by controlling the range of interaction among the particles. As shown in Figure 2 (b), is "soft" and long ranged - the interparticle repulsion is greater than the thermal energy for 5x the particle radius and the interactions become "stiffer" and short ranged as the micelle concentration is increased. However, the surface potential barley changes with micelle concentration. Other striking observations include that the measured zeta potential is comporable to those measured in water with highly charged particles (the Debye screening lengths are also much larger than those measured in such a solution).

Finally, to further investigate screening lengths the ionic strength of conductivity was measured. The measured conductivity ($\sigma$) as a function of AOT concentration is shown in Figure 3 (a). The conductivity values span more than two orders of magnitude and the fraction of ionized micelles is independent of concentration. The derived inverse screening lengths are also plotted in Figure 3 (b). Also, instead of following a standard charge exchange description as found in weak electrolytes, a neutral micelle reversibly exchanges charge through collision $A + A \rightleftharpoons A^{+} + A^{-}$ leading to an ionization fraction independent of concentration (schematic in Figure 3).

Figure 3: Charge stabilization of a nonpolar suspension before (a) and after (b) adding reverse micelles (AOT solution above cmc). The field of view is 135 x 108 $\mu m ^2$.
Figure 2: (a) Measured pair potentials (u(r)/kT) versus varying cell thicknesses (h) with screen Coulomb potentials fitted (see inset table for parameters). This shows how interparticle potentials strongly depend on the cell thickness. (b) Interaction potentials at different AOT concentrations (squares, circles, diamonds, and triangles represent 3.1, 13, 50, and 200mM of AOT). Screen Coulomb potentials are fitted with parameters inset.