(497a) Electromigration and Adsorption of Charge Carriers in Doped Nonpolar Liquids

Surfactants are added to nonpolar media to increase the electrical conductivity and control particle dispersion in a number of industrial applications. Steric stabilization of dispersions is typically assumed in nonpolar media; electrostatic effects are less well characterized. Electrochemical Impedance Spectroscopy (EIS) was used to analyze the electrical and physicochemical properties of dodecane containing up to 10 wt% of different surfactant ‘dopants’. Commercial surfactants with the trade names OLOA 11000, Span 20, Span 80, Span 85, and Aerosol-OT (AOT) were studied. The nonpolar liquid solution was held between two parallel glass slides, coated with indium tin oxide (ITO) and held at a distance of 10 µm. Fitting the impedance of each solution to a 4-element equivalent circuit model provided the conductivity, dielectric constant, and Debye length. The conductivity of each surfactant solution was proportional to surfactant concentration. The Debye length of each surfactant solution was inversely proportional to the square root of the concentration. The number of charge carriers was calculated from the conductivity and Debye length measurements and found to be proportional to the number of surfactant molecules. Differences in the number of charge carriers per surfactant molecule between similar surfactants Span 20, Span 80, and Span 85 show that the structure of the nonpolar surfactant tail affects the number of charge carriers. The Nyquist impedance plots of AOT, Span 20, Span 80, and Span 85 solutions were qualitatively different from that of OLOA 11000. These differences suggest charge-carrier adsorption at the electrodes. More direct evidence of charge carrier adsorption was obtained using a Quartz Crystal Microbalance. Adsorption of surfactant and charge carriers may be an important mechanism for the acquisition of charge on solid surfaces in nonpolar media.