434537 Electrochemical Impedance Spectroscopy of Doped Nonpolar Liquids II: Adsorption of Charge Carriers

Tuesday, November 10, 2015: 9:15 AM
Ballroom E (Salt Palace Convention Center)
Benjamin Yezer, Aditya S. Khair, Paul J. Sides and Dennis C. Prieve, Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA

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


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.

[1]. B.A. Yezer, A.S. Khair, P.J. Sides and D.C. Prieve, “Use of electrochemical impedance spectroscopy to determine double-layer capacitance in doped nonpolar liquids,” J. Colloid Interf. Sci. 2015 449, 2-12 (doi:10.1016/j.jcis.2014.08.052).

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