Charging Dynamics of Aerosol OT Inverse Micelles

Abstract

Aerosol OT (AOT) is a commonly used surfactant and charging agent in nonpolar liquids. Properties such as the conductivity of AOT suspensions in nonpolar liquids and the behavior of charged AOT inverse micelles at interfaces have been studied recently, but still little is known about the generation dynamics of charged AOT inverse micelles. In this article, the generation dynamics of charged AOT inverse micelles in dodecane are investigated with transient current measurements. At low applied voltages, the generation rate is sufficiently fast to maintain the equilibrium concentration of charged inverse micelles, such that the current scales proportionally with the applied voltage. However, above a threshold voltage the current becomes limited by the generation of charged inverse micelles. Al 2 O 3-coated electrodes are used to achieve these high-voltage current measurements while reducing surface generation currents. The dependency of the resulting generation-limited currents with the micelle concentration and the liquid volume is compatible with a bulk disproportionation mechanism. The measured currents are analyzed using a model based on drift, generation, and recombination of charged inverse micelles and the corresponding generation and recombination rates of charged AOT inverse micelles have been determined. INTRODUCTIONSurfactants are widely used for charging colloidal particles in reflective displays, 1−8 inkjet printing, 9 electrorheological fluids, 10 emulsion polymerization, 11,12 and dry cleaning. 13Over recent years, Anionic sodium bis (2-ethylhexyl) sulfosuccinate (Aerosol OT, AOT) with its double-tail and polar headgroup has attracted increasing attention in aqueous, 14,15 nonaqueous 16−19 and water-in-oil (w/o) microemulsions. 20−26 During the last decades, different methods have been used to investigate the properties of AOT inverse micelles in nonpolar solvents. Peri has combined ultracentrifugation, light scattering, and viscometric measurements of AOT in various solvents, and he has suggested that AOT inverse micelles are monodisperse and spherical.27 Ekwall et al. have found similar results by using light and X-ray scattering. 28 Their studies indicate that the aggregation number N g , the number of monomers per inverse micelle, for AOT is about 20−30. Using a photon correlation experiment, Zulauf and Eicke 29,30,29,30 have studied the aggregation number of AOT micelles over a wide range of concentrations and in different solvents.27,28 Kotlarchyk et al. have used small-angle neutron scattering (SANS) and found an aggregation number of 22 ± 2 monomers per micelle. This difference can be explained by the hygroscopic properties of AOT, which makes the micelles absorb ambient moisture, and increases the conductivity of the AOT solution. 34 The charge-bearing entities are either trace ions such as formed from the dissociation of water into H + and OH − , or even charged AOT molecules that have released a H + . When two micelles collide there is a chance that these charged entities are e...

Electrodynamics of Electronic Paper Based on Total Internal Reflection

“…Two kinds of boundary conditions for the negative inverse micelles are compared by numerical simulation: the boundary condition of adsorption and release [12] (where the inverse micelles stick to the surface when they touch it and are released when the field is zero) and the blocking boundary condition [30] (where the inverse micelles form a diffuse double layer or Debye layer with thickness λ DL = (εε 0 k B T/2q 2 n − ) ≈ 100 nm near the interface when the electric field is directed from the liquid to the solid). There is a subtle but important difference in the location of the negative charges in steady state between the two kinds of boundary conditions.…”

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

“…There are various experimental methodologies to study the charge formation and the EDL of dispersed and bulk solids in contact with nonpolar liquids; these include but are not limited to electrical conductivity measurements, ,,− electrophoretic mobility measurements, − transient current measurements, − or total internal reflection microscopy . Recently, Yezer et al , used electrical impedance spectroscopy (EIS) to study the EDL of mixtures of dodecane and the nonionic surfactants OLOA 11 000, Span 80, and Span 85.…”

Polarity-Dependent Adsorption of Inverse Micelles

“…13−15 Based on the measured capacitance of the Stern layer, the monovalent "core" of an adsorbed micelle appears to be situated even closer to the indium tin oxide (ITO) than the steric hindrance of the inverse micelle would allow, suggesting a different arrangement of the surfactant such as a hemi-micelle-like geometrical configuration. For example, in the case of AOT/ dodecane, 13 adsorption of charged inverse micelles on ITO is fast and bulk field screening is directly governed by Stern capacitances. In the case of OLOA11k (and OLOA 1200), 1 the adsorption of charged inverse micelles on ITO is much slower, such that the bulk electric field screening is dominated by the formation of a diffuse double layer.…”

“…To measure the properties of charged inverse micelles, there are two commonly used methods: transient current measurements − and impedance spectroscopy. , These methods allow the measurement of bulk properties such as the concentration of charged inverse micelles and the electrophoretic mobility (and related to this the hydrodynamic radius) and details on the charge generation mechanism ,, and on interface phenomena. ,− When a potential difference is applied across a mixture, the behavior of charged inverse micelles near the electrodes depends on the type of surfactant system and on the type of surface. ,− Bulk dynamics based on drift and diffusion predict the formation of a diffuse double layer, though, due to adsorption of inverse micelles onto the electrode, this can evolve into a Stern layer. − Based on the measured capacitance of the Stern layer, the monovalent “core” of an adsorbed micelle appears to be situated even closer to the indium tin oxide (ITO) than the steric hindrance of the inverse micelle would allow, suggesting a different arrangement of the surfactant such as a hemi-micelle-like geometrical configuration. For example, in the case of AOT/dodecane, adsorption of charged inverse micelles on ITO is fast and bulk field screening is directly governed by Stern capacitances. In the case of OLOA11k (and OLOA 1200), the adsorption of charged inverse micelles on ITO is much slower, such that the bulk electric field screening is dominated by the formation of a diffuse double layer.…”