(324a) Origin and Nature of Charge Carriers in Nonpolar Solvents

Using either electrochemical impedance spectroscopy [1] or a commercial meter, we measured conductivity of various commercial surfactants in dodecane for surfactant concentrations between 1 and 100 mM. For Span85, AOT, Span80, OLOA and Span20, the conductivity was found to be proportional to surfactant concentration and the charge carriers are micelles of the surfactant. The proportionality constant (conductivity divided by the molar concentration of surfactant) increases exponentially with the size of the micelle. This is surprising because larger micelles should be less mobile than smaller ones. The explanation is that the fraction of micelles that bear a charge (which is always quite small compared to unity) increases exponentially with the size of the micelle. Bigger micelles mean that opposite charges in ion-pairs are further apart, leading to a lower attractive energy compared to kT and a greater degree of dissociation of the ion pairs.

These observations suggest that larger, wormlike micelles should be highly potent charging agents. To test this, we have synthesized a series of phospholipid-derived surfactants known to form wormlike micelles in nonpolar solvents. In this presentation we will present their concentration-dependent conductivity enhancement in nonpolars and compare it to the charge-fluctuation theory developed by Eicke and coworkers [2]. We have also synthesized a library of AOT-like surfactants with varying head and tail group chemistry and counterions. These materials have very different chemical composition but form micelles of similar size. Conductivity and impedance spectroscopy measurements on each reveal the relative importance of micelle size and chemical composition on their effectiveness as charging agents, and suggest mechanisms for the origin of charge formation in doped nonpolar liquids. These will also be discussed along with a consideration of the role of adventitious water in these systems.

[1] B.A. Yezer, A.S. Khair, P.J. Sides and D.C. Prieve, J. Colloid Interface Sci. (2014), dx.doi.org/10.1016/j.jcis.2014.08.052.

[2] Eicke H-F, Borkovec M, Das-Gupta B. Conductivity of Water-In-Oil Microemulsions: A Quantitative Charge Fluctuation Model. J Phys Chem. 1989;93:314-7.