(320b) Electrochemical Impedance Spectroscopy of Doped Nonpolar Liquids II: Adsorption of Charge Carriers

Electrochemical impedance spectroscopy of doped nonpolar
liquids II: adsorption of charge carriers

AIChE Annual Meeting 2015, Salt Lake City,
UT

Session T3010: Electrokinetics in Non-Polar Media

Benjamin
A. Yezer, Aditya S. Khair,
Paul J. Sides and Dennis C. Prieve

Center
for Complex Fluids Engineering

Department
of Chemical Engineering

Carnegie
Mellon University

Pittsburgh,
PA 15213

Abstract

Surfactants are added to nonpolar media to increase the charge
concentration and charge solid surfaces. Electrochemical impedance spectroscopy
has been used to determine the permittivity, charge carrier concentration, and
charge carrier size of solutions of dodecane doped
with different surfactants.[1]
The spectra showed the frequency response of various phenomena within the cell,
such as the geometric charging, conductance, and double layer charging.   The impedance of dodecane
doped with OLOA 11000 [poly(isobutylene succinimide)],
Span 80 (sorbitan monooleate),
and Span 20 (sorbitan monolaurate)
was measured as a function of excitation by a 10 mV amplitude sinusoidal
voltage applied across a parallel plate cell with a 10 micron spacing.  The tested solutions varied in concentration
from 1 mM to 100 mM and the
frequency range was 10 mHz
to 100 kHz.  Nyquist
plots of all three surfactants showed the high frequency semicircle
characteristic of parallel resistance and capacitance. The plots of Span 80 and
Span 20 in dodecane also exhibited a second
semicircle at low frequencies.  The
electrical conductivity of each surfactant was proportional to surfactant
concentration for concentrations above 10 mM.  The low frequency behavior was attributed to
charge adsorption and desorption. Fitting the data to models for charge
migration, differential capacitance, and adsorption allowed extraction of both
charge concentration and two kinetic parameters that characterize the rate of
adsorption and desorption.  Above 10 mM the ratio of charge carriers per surfactant molecule was
17 ppm for OLOA 11000, 23 ppm for Span 20, and 3 ppm for Span 80.  Larger micelles had an exponentially higher
ratio of charge carriers per molecule of surfactant.  The adsorption rate constants were
independent of surfactant concentration while the desorption rate constant was
proportional to the surfactant concentration when the concentration was above
the critical micelle concentration (CMC). 
This dependence indicated that uncharged micelles participated in the desorption of charge.