(285h) Effect of Surfactant Structure on Self-Assembly and Charging Processes in Anhydrous Nonpolar Liquids.

Impedance spectroscopy has been used to determine the conductivity and concentration of charged inverse micelles of Span surfactants in dodecane.^{(JCIS, 449, 2-12, 2015)} In this technique, an oscillating voltage is applied between two parallel electrode surfaces with surfactant solution sandwiched in between. The conductivity of the charged micelles is derived from the electrical resistance arising from the transport of charge carriers across the bulk solution towards the electrode surface. The concentration of charge carriers in solution is then obtained from the electrical capacitance of the diffuse clouds of counterions accumulating next to either electrode surface. Despite previous success with this technique, severe charge adsorption on the electrode surface has been shown to obscure measurement of the double layer capacitance and hence the charge carrier concentration.^{(JCIS, 449, 2-12, 2015)} Charge adsorption also effectively limits the concentration range and catalog of surfactants available for impedance spectroscopy study. To expand the available concentration ranges of the impedance method, a passivating coating of plasma polymerized octafluorocyclobutane is fabricated and determined to successfully suppress charge adsorption. An equivalent circuit representing the surfactant solution and fluoropolymer coating is derived and used to model and interpret the impedance spectra. The circuit model captures most of the experimental charge transport processes with the exception of a small systematic deviation. To control and monitor the water content of solution during impedance experiments, a rigorous water elimination protocol involving nitrogen gas is developed and implemented. The conductivity and charge carrier trends with respect to surfactant concentration is then studied and rationalized with respect to surfactant structure and chemistry.