Determination of Charge Concentration in Surfactant-Doped Nonpolar Liquids
The technique of impedance spectroscopy was used in a thin cell (10 μm) to infer conductivity, permittivity and the differential double-layer capacitance of solutions of dodecane doped with oil soluble commercial surfactants. Solutions of OLOA 11000® [poly(isobutylene succinimide)] in dodecane were fit to an equivalent circuit model having four elements including a constant-phase element representing the double-layer capacitance. Using Gouy-Chapman theory for small zeta potentials and assuming univalent charge carriers, the double-layer capacitances were converted into charge carrier concentration. The impedance of dodecane doped with sorbitan trioleate (Span 85®), sorbitan monooleate (Span 80®) and sorbitan monolaurate (Span 20®) was also measured in a thin cell. Nyquist plots of all three surfactants showed the high frequency semicircle characteristic of parallel resistance and capacitance but often exhibited a second semicircle at low frequencies which was attributed to charge adsorption and desorption. The adsorption rate constants were independent of surfactant concentration while the desorption rate constants were proportional to the surfactant concentration. The surfactants AOT®, OLOA 11000®, Span 85®, Span 80®, and Span 20® all increased the conductivity of dodecane. Phase analysis light scattering was used to find the hydrodynamic diameter of the micelles formed by the surfactants. For each surfactant the measured conductivity increased linearly with concentration above the critical micelle concentration (CMC). The molar conductivity of the different surfactants increased exponentially with the estimated diameter of the polar core of the surfactants. An apparatus and a method for determining the zeta potential of planar solids in contact with nonpolar liquids are described and values of the zeta potential are provided. The motor rotated the disk-shaped sample at several thousand rpm. A glassy carbon electrode with a circular exposed end was near the sample at its axis and a second glassy carbon electrode was far from the sample. Rotating the sample generated streaming potential as measured by an electrometer. The zeta potential of ITO coated glass was shown to have a magnitude smaller than 25 mV in solutions of OLOA 11000® in dodecane. The measured zeta potential of ITO justifies the assumption of small zeta potentials made in the theory used to fit the impedance data.