Flow electrification in nonaqueous colloidal suspensions, studied with video microscopy

Charge and Electrical Double Layer Formation in a Nonpolar Solvent Using a Nonionic Surfactant

In this work, we study the charge formation and the characteristics of the electrical double layer in a nonpolar medium using electrical impedance spectroscopy. To stabilize the free ionic species, a nonionic surfactant is added to the system. The conductivity and permittivity of the medium are obtained from high- to medium-frequency impedance data. Based on the correlation between (viscosity-adjusted) conductivity and surfactant concentration, we conclude that charge formation occurs due to a disproportionation mechanism. We accordingly estimate the concentration of the charge carriers in the sample and the Debye length of the diffuse double layer. The capacitance of the electrical double layer can be extracted from the low-frequency impedance data. We use this data to calculate the electrode distance of an equivalent parallel-plate capacitor. It is found that this distance is on the order of magnitude of Angstroms, indicating that the measured electrical double-layer capacitance is in fact the Stern layer capacitance.

Particle charging and charge screening in nonpolar dispersions with nonionic surfactants

The electrostatic stabilization of colloidal dispersions is usually considered the domain of polar media only because of the high energetic cost associated with introducing electric charge in nonpolar environments. Nevertheless, some surfactants referred to as "charge control agents" are known to raise the conductivity of liquids with low electric permittivity and to mediate charge stabilization of nonpolar dispersions. Here we study an example of the particularly counterintuitive charging and electrostatic interaction of colloidal particles in a nonpolar solvent caused by nonionic surfactants. PMMA particles in hexane solutions of nonionic sorbitan oleate (Span) surfactants are found to exhibit a field-dependent electrophoretic mobility. Extrapolation to zero field strength yields evidence for large electrostatic surface potentials that decay with increasing surfactant concentration in a fashion reminiscent of electrostatic screening caused by salt in aqueous solutions. The amount of surface charge and screening ions in the nonpolar bulk is further characterized via measurements of the particles' pair interaction energy. The latter is obtained by liquid structure analysis of quasi-2-dimensional equilibrium particle configurations studied with digital video microscopy. In contrast to the behavior reported for systems with ionic surfactants, we observe particle charging and a screened Coulomb type interaction both above and below the surfactant's critical micelle concentration.

Electric charging in nonpolar liquids because of nonionizable surfactants

Nonpolar liquids do not easily accommodate electric charges, but it is known that surfactant additives can raise the conductivity and lead to electric charging of immersed solid surfaces. Here, we study the rarely considered conductivity effects induced by surfactant molecules without ionizable groups. Precision conductometry, light scattering, and Karl Fischer titration of sorbitan oleate solutions in hexane reveal a distinctly electrostatic action of the nonionic surfactant. The conductivity in dilute hexane solutions of sorbitan trioleate (Span 85) exhibits two regimes of linear scaling with surfactant concentration and a transition around the critical micelle concentration (cmc). Both regimes can be described with a statistical model of equilibrium charge fluctuations. The behavior observed above the cmc has a direct analogy in systems of ionic surfactants and can be explained by charge disproportionation of reverse micelles. The observed conductivity of Span 85 solutions below the cmc, however, represents a qualitative departure from the behavior reported for ionic surfactants. In the studied surfactant systems, the availability of ionic species may stem from a complexation of the surfactant with ionizable impurities; nonetheless, the ionization process appears to be limited entirely by the surfactant and not by the level of impurity. We therefore propose that nonionizable surfactants can offer a new and robust way of controlling the conductivity in nonpolar liquids.

Electrostatic charging of nonpolar colloids by reverse micelles

Colloids dispersed in a nonpolar 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 particle potential is a constant, independent of the number of micelles added and independent of 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.