A Study of Doped Nonpolar Liquids Using Electrochemical Impedance Spectroscopy

The electrical response of nonpolar media to an AC field is described and its conductivity, Debye length, and permittivity are determined.  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.  Developments in this field require definitive measurements of electrical properties of the bulk fluid to determine mechanisms for the formation and stabilization of charges.  Electrochemical impedance spectroscopy (EIS) was applied to a cell comprising parallel plate indium tin oxide electrodes separated by 10 μm of dodecane doped with a commercial surfactant OLOA 11000.  At high frequency the charging of the electrodes was observed as a maximum in the out of phase, or imaginary, impedance response.  Fitting a simple equivalent circuit of a resistor and capacitor in parallel to the high frequency response determined the electrical conductivity and permittivity of the fluid.  At low frequency the charging of the electrical double layers at either electrode was observed as a minimum in the imaginary part of the impedance response and was fit with a 5 element equivalent circuit model to determine the Debye length of the fluid.  The number of charge carriers in solution inferred from the Debye length measurement increased linearly with surfactant concentration.  For example, the conductivity of 10% OLOA in dodecane was 1.7 ± 0.1 nS/cm and the dielectric constant was 2.3 ± 0.1.  The diffusion coefficient of charge carriers calculated from the measured Debye length and conductivity was 25 ± 3.6 μm²s^-1.  A numerical solution of the Poisson-Nernst-Planck model that predicted the motion of charge carriers and current response inside the cell was in agreement with experimental observations at high and moderate frequencies.  At frequencies below 10 Hertz, the real part of the cell impedance increased as frequency decreased, which might indicate unequal size of charge carriers.