Interactions and Transport of Charged Species in Bulk and at Interfaces
Charge transport by inverse micelles in non-polar mediaKristiaan Neyts
Coauthor(s) : Manoj Prasad, Masoumeh Karvar, Filip Beunis, Filip Strubbe
LCP group, ELIS Department, Ghent University, 9000 Gent, Belgium
In non-polar liquids surfactant molecules form inverse micelles. These are important in applications, such as electronic inks or liquid toners. Most inverse micelles are uncharged, but there are also charged inverse micelles (CIMs) which have charge +e or -e. In an electric field, these CIMs drift in the same or in the opposite direction of the electric field vector. When a DC voltage is applied over the electrodes the CIMs accumulate near the electrode with the opposite polarity. We have observed two kinds of behaviors for the CIMs near the electrodes. Some CIMs stay in the bulk and because of their Brownian motion, they form a diffuse double layer near the electrode . Other CIMs remain in a very thin Helmholtz layer which has a thickness in the order of a few nm . Inverse micelles in the bulk can become charged when two neutral micelles collide and form a positive and a negative CIM .
The above statements are based on series of transient current measurements that have been carried out on non-polar media, doped with surfactant (OLOA 11K, AOT and others) placed between parallel electrodes. The current that is measured in the external circuit is an integral of the current densities that are present in the volume. If the concentration of CIMs is sufficiently large, an inhomogeneous distribution of CIMs can generate an electric field which is comparable to the applied electric field. When a DC voltage is applied the CIMS will move and the electric field in the bulk is partly screened due to the resulting charge distribution. The interplay between applied field and charge leads to complicated transient currents [4, 5]. In addition to the transient current related to the CIMs that were initially present, there is a current that is due to the continuous generation of CIMs in the bulk of the layer. This generation current comes to a halt when the field in the bulk of the device is screened. For OLOA 11K in dodecane, we observed a reduction of the current when the field is screened by a diffuse double layer of CIMs, which means the CIMs indeed remain in the bulk and are subjected to Brownian motion. For high voltages, electrohydrodynamic turbulence is observed . For OLOA 11K the generation of new CIMs in the bulk is relatively slow and increases proportionally with the square of the surfactant concentration. For AOT and some other surfactants the density of CIMs is much larger and the generation and recombination of new CIMs is a more rapid process. For AOT the transient current can often be described well by a bulk material with a homogeneous conductivity .
In this contribution we will present experimental measurements and numerical simulations of transient current measurements. The link between the current measurements and the properties of the inverse micelles will be explained. From this we can conclude that electrical measurements are a good tool to determine many properties of charged inverse micelles.
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