Electrostatic Interactions of Colloidal Particles in Nonpolar Solvents: Role of Surface Chemistry and Charge Control Agents

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This paper studies the interactions of colloidal particles in nonpolar solvents. Even though ionization in nonpolar solvents is not possible because of the high energy cost under environments with low dielectric polarizability, it is possible to charge colloidal particles in nonpolar solvents with the use of certain charge control agents.

Generally, the electrostatic interaction between the colloidal particles depends on 1) the surface potential, and 2) the screening length of the solvent. In a polar solvent, the screening length can be easily determined through the concentration and valence of the ionic species and the associated conductivity. However, this is difficult in nonpolar solvents because the ionic species are not very well characterized. In addition, the surface potential is difficult to be determined because it is difficult to measure accurately the electrophoretic mobility of particles, since it is too low. Then there are debates about the validity of using the DLVO theory to predict the interaction forces between the colloidal particles in nonpolar solvents.

In this paper, they studied the forces between polystyrene (PS) and poly(methyl methacrylate) PMMS particles in hexadecane and using aerosol-OT as charge control agents, which are surfactants with sulfonate groups on the polar heads and two branched hydrocarbon tails. They observed that the concentration and composition of the charge control agents affect the surface potential and screening length.

A schematic of the sample cell and optical microscopy setup is shown below

Nonpolar 1.png

where a holographic optical tweezer is used to manipulate the particles. This setup allows for the determination of the hydrodynamic interactions, the solvent screening length, the particle surface potential, and the particle hydrodynamic radius in a single experiment. At large separations between the colloidal particles, the diffusion coefficient approaches the Stokes-Einstein values. While at smaller separations, the hydrodynamic and electrostatic interactions become evident.

They observed that the hydrodynamic radii increase with increasing charge control agents concentration, this could be due to the adsorption of reverse micelles onto the particle surface. Electrostatic forces cause the particles to drift away form each other at smaller separations. The dependence of the interactions of two isolated PMMA particles on the concentration of the charge control agents (AOT) is shown below.

Nonpolar 2.png

The main achievement in this paper is that they demonstrated that long-range electrostatic interactions between colloidal particles in nonpolar solvents can be tuned through surface functionalization and soluble charge control agents.