(301b) 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.