Reverse Micelles Enable Strong Electrostatic Interactions Between Colloidal Particles in Nonpolar Solvents
Charge Stabilization in Nonpolar Solvents
M.F. Hsu, E. R. Dufresne, and D. A. Weitz
Langmuir 21 (2005) 4881-4887
wiki entry by Emily Russell, Fall 2010
The article can be found here.
Note: the citation on Eric Dufresne's page on the wiki is for the earlier arXiv version of this paper; this wiki entry is on the final version published in Langmuir. A brief glance shows the version to be similar, although the Langmuir paper has more content.
Overview and Comments
This paper discusses the crucial role of surfactants in allowing charge effects to be introduced into nonpolar solvents. The small dielectric constant of a nonpolar solvent means that it takes a good deal of energy to separate ions, so that salts would not be expected to dissociate and charge effects would be predicted to be negligible. Added surfactants, however, form reverse micelles, the cores of which can become charged; this allows colloidal particles to also obtain a charge. Because screening lengths in the low-dielectric solvents can be quite long compared to those in water, once a charge is stabilized, the effects can be significant. The authors point out that these charge interactions in nonpolar solvents have wide applications in electrophoretic inks (such as the Amazon Kindle uses).
The experiments in this paper were performed on sterically stabilized colloidal PMMA particles, of radius 780 nm, in dodecane. Aerosol-OT (AOT) was added in varying quantities as the surfactant. Significant care was taken to minimize the introduction of water into the system; preparations were performed in a dry glovebox.
The primary experiments were imaging and analysis of quasi-two-dimensional systems; the suspension was loaded into a thin, wedge-shaped sample cell, and bright-field images were taken in a region of the wedge where the colloidal particles were confined to move mainly in-plane. Particle location using the standard Crocker and Grier algorithms allowed calculation of the radial distribution function g(r); this was then inverted, using some slightly clever algorithms, to obtain the pair potential u(r). Experiments were done at several concentrations of the surfactant.
In pure dodecane, without surfactant, the particles aggregated; upon addition of AOT above the critical micellar concentration (CMC), the particles dispersed, indicating a long-range (non-steric) repulsive potential (Fig. 1 and 2). This potential was well described by a screened Coulomb potential, as predicted by the DLVO theory for charged colloids in solution. The charge on the particles was found to be roughly independent of surfactant concentration (so long as the concentration was above the CMC), around 300 electron charges. The screening length of the potential decreased with increasing AOT concentration (Fig. 3); screening lengths measured from the g(r) compared well with those calculated from DLVO theory using independent conductivity measurements. The origin of the screening is the presence of charged surfactant micelles in the solution, which act as counterions. Because of the low dielectric constant, screening lengths of greater than one micron were seen, much greater than those usually seen in aqueous solutions.