Cation and Water Content Effects on Dipole Rotation Activation Energy of Smectites

In soil science, single frequency permittivity measurements are used to determine soil water content, and single frequency bulk electrical conductivity is used to determine soil salinity. The accuracy of these measurements may be influenced by complex interactions between frequency, temperature, and water that is tightly bound to day surfaces. The purpose of this study is to determine the effect of temperature, saturating cation, water content, smectite properties, and frequency on electrical properties of humidified clays by analyzing three different activation energies for dipole rotations, which are calculated from the temperature dependence of electrical properties. Four reference smectites saturated with K, Na, Ca, and Mg and equilibrated at relative humidities ranging from 56 to 99% were investigated over a frequency range from 3 × 10 5 to 1 x 10 9 Hz. Two of the three activation energies were found to decrease slightly as water content increased. Higher activation energies were found for smectites saturated with Mg and K and lower values for smectites saturated with Na and Ca. Trends for type of clay were variable but appeared to be influenced by both the total water content and the distribution of water between the interlayers and the external surfaces of the smectite quasi-crystals. Changes in quasi-crystal orientation induced by thermal cycling were also found to influence the activation energies. The results indicate complex frequency and temperature dependent interactions impact electrical properties of the clays. We conclude that no simple equation will correct for temperature and clay content effects on single frequency measurements of permittivity or bulk electrical conductivity.

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