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DIVISION S-2—SOIL CHEMISTRY

Low Frequency Impedance Behavior of Montmorillonite Suspensions

Polarization Mechanisms in the Low Frequency Domain

^a Dep. of Plants, Soils and Biometeorology, Utah State University, Logan, UT 84322
^b Northeast Foundation Utilities Professor, Dep. Civil and Environmental Engineering, Univ. of Connecticut, 261 Glenbrook Road, Unit 2037, Storrs, CT 06269-2037
^c Chemistry and Biochemistry Dep., Utah State University, Logan, UT 84322
^d Physics Dep., Utah State University, Logan, UT 84322

^* Corresponding author (LDUD{at}MENDEL.USU.EDU)

Large changes in permittivity have been observed as the frequency of an electromagnetic (EM) field applied to systems containing phases of contrasting permittivity is changed. Two mechanisms, polarization of a diffuse double layer (DDL) and polarization of the charge imbalance created by contact of two phases of different permittivity (the Maxwell-Wagner [MW] effect), are responsible for the frequency dependence of dielectric properties. To use the frequency dependence of dielectric properties to determine soil geometrical and electrochemical properties, the two mechanisms must be quantified. Three models of the frequency dependent dielectric properties, based on terms representing polarization of the electrical double layer that develops at the electrode surface, polarization of the DDL and the MW effect, were used to investigate the dielectric spectrum of montmorillonite suspensions. Dielectric spectra of suspensions of three particle-size separates (r > 1.0 µm, 1.0 µm > r > 0.2 µm, 0.2 µm > r) of homoionic (Na⁺ or Ca²⁺) were measured at a suspension density of 5.0 g of clay in 50 mL of water. Impedance plane plots suggested the contribution of three relaxation processes to the spectra. While all three models reproduced the data, they gave different interpretations of the data. Two models attributed relaxation in the kHz range to electrode polarization, relaxation at approximately 10 kHz to DDL polarization and relaxation at 1 MHz to MW polarization. The third model assigned MW polarization to the relaxation at 10 kHz and DDL polarization to the relaxation at 1 MHz.

Abbreviations: dc, direct current • DDL, diffuse double layer • DM, Debye's relaxation model • ECM, equivalent-circuit model • EM, electromagnetic • MM, mixing model • MW, Maxwell-Wagner • TDR, time domain reflectometry

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