We propose a direct method to determine absorption anisotropy of colloidal quantum rods. In this method, the rods are aligned in solution by using an alternating electric field and we measure simultaneously the resulting average change in absorption. We show that a frequency window for the electric field exists in which the change in absorbance as a function of field strength can be analyzed in terms of the quantum-rod dipole moment and the absorption coefficient for light that is polarized parallel or perpendicular to the long axis of the rod. The approach is verified by measuring the absorbance change of CdSe rods at 400 nm as a function of field strength, where we demonstrate excellent agreement between experiment and theory. This enables us to propose improved values for the CdSe quantum-rod extinction coefficient. Next, we analyze CdSe/CdS dot-in-rods and find that the absorption of the first exciton transition is fully anisotropic, with a vanishing absorption coefficient for light that is polarized perpendicularly to the long axis of the rods.

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Transient current measurements are used to characterize a wide variety of charge carriers in nonpolar liquids. The transient current method allows us to obtain both the concentration and mobility of charge carriers and therefore also the hydrodynamic radius using Stokes' law. In this article, five different surfactants in dodecane are investigated: OLOA11K, Solsperse13940, Span80, Span85, and AOT. We show that different types of currents are observed depending on the size of the inverse micelles. For large inverse micelles such as for OLOA11K, Solsperse13940, and Span80, the measurement of the transient current is straightforward because of the low steady-state current level. However, for small inverse micelles such as AOT and Span85, the current from the generation of charges is much larger such that high voltages, a small distance between the electrodes, and dielectric coatings on the electrodes are required to measure the signal related to the initially present charged inverse micelles. The estimated hydrodynamic radii of AOT and Span85, the two smallest inverse micelles, are in good agreement with the values reported in the literature. The comparison of the transient currents with simulations indicates that the dynamics of the charge transport are well-understood.

Surfactant molecules in a non-polar liquid form charged and uncharged inverse micelles. When a potential difference is applied over the mixture, the charged inverse micelles drift towards the electrode with the opposite polarity. The motion of charges is associated with a transient current, which can be measured in an external circuit. In this paper, transient currents and steady state charge densities are described analytically in different ranges of parameter values (applied voltage, charge density, device thickness, mobility,...). The generation of additional charged inverse micelles and the electrophoretic motion of colloidal particles in the mixture is modelled and measured experimentally.

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We have measured the electrophoretic mobility of single, optically trapped colloidal particles, while gradually depleting the co-ions and counterions in the liquid around the particle by applying a dc voltage. This is achieved in a nonpolar liquid, where charged reverse micelles act as co-ions and counterions. By increasing the dc voltage, the mobility first increases when the concentrations of co-ions and counterions near the particle start to decrease. At sufficiently high dc voltage (around 2 V), the mobility reaches a saturation value when the co-ions and counterions are fully separated. The increase in mobility is larger when the equilibrium ionic strength is higher. The dependence of the experimental data on the equilibrium ionic strength and on the applied voltage is in good agreement with the standard theory of electrophoretic retardation, assuming that the bare particle charge remains constant. This method is useful for studying the electrophoretic retardation effect and charging mechanisms for nonpolar colloids, and it sheds light on previously unexplained particle acceleration in electronic ink devices.

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...