Charging Mechanisms of Surfactant Doped Nonpolar Liquids

Impedance spectroscopy has been used in the literature to elucidate the charging mechanisms of inverses micelles in surfactant doped dodecane solutions. However, the existing literature has been limited in surfactant scope due to instrumental complications and surfactant purity issues. Charge adsorption on the electrode surface may obscure measurement of the double layer capacitance and hence the ionic strength.(JCIS, 449, 2-12, 2015) This charge adsorption phenomenon effectively limits the concentration range and catalog of surfactants available to the impedance spectroscopy method.(JCIS, 469, 325-337, 2016) 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 is determined to capture most of the experiment charge transfer processes with the exception of a small instrumental artifact at low frequencies. Analysis of the impedance spectra for Span-20, Span-80, and Span-85 in dodecane demonstrates dependency of charged micelle size on surfactant HLB number.

Charge formation in nonpolar liquids of commercial surfactants have been extensively studied in the literature; however, these surfactants are often of low purity and lack systematic changes in chemical structure. A flexible synthetic technique is developed and used to produce high-purity AOT analog surfactants with a systematic change in counterion chemistry. The charging properties of these AOT analogs are studied using impedance spectroscopy and compared to bulk micelle properties obtained from small angle neutron scattering (SANS) and dynamic light scattering

(DLS). Measured conductivity and ionic strength values for analog surfactants were an order of magnitude than those of sodium AOT despite forming micelles of similar size. This increased affinity for charged micelle formation cannot be modeled by commonly used disproportionation or charge fluctuation theories for micelle formation in nonpolar media and reveals new relationships between counterion chemistry and micelle charging not seen in commercial surfactants.

Following successful impedance spectroscopy characterization of AOT analogs, we synthesize additional surfactants of high purity and incrementally altered head groups, counterions and hydrocarbon tail chemistries. Then using electrochemical impedance spectroscopy, we identify specific chemical structures that promote charging in nonpolar liquids. These trends are contrasted with dynamic light scattering studies and highlight the pitfalls of inferring electrical properties from scattering results. Impedance spectroscopy experiments reveal that ion-pair dissociation is the chemical property essential to prediction of reverse micelle charging in nonpolar liquids for both ionic and nonionic surfactants.

Finally, we measure the zeta potential of mica and fluoropolymer surfaces immersed in solutions of synthesized surfactant using ZetaspinTM. Such experiments aimed to deduce relationships between surfactant chemistry and surface charging in nonpolar liquids. The charge of mica surfaces was found to be positive in sign and highly dependent on the presence of water (from the ambient air) as well as contact time between the surface and surfactant solution. The sign and magnitude of charging of fluoropolymer surfaces was dependent on surfactant structure.