ARS | Publication request: Cation and Water Content Effects on Dipole Rotation Activation Energies of Smectites

Submitted to: Soil Science Society Of America Journal
Publication Acceptance Date: April 7, 2004
Publication Date: August 23, 2004
Citation: Logsdon, S.D., Laird, D.A. 2004. Cation And Water Content Effects On Dipole Rotation Activation Energies Of Smectites. Soil Science Society Of America Journal. 68:1586-1591.

Interpretive Summary: Electrical properties can be used to characterize materials. When an electrical field is applied to an object, one side is positively charged and one side is negatively charged. If water is present in the object, the water molecules line up with their positive end pointed to the negative side and their negative end pointed to the positive side. If the electrical field charges reverse side, the water molecule also rotates. These molecular oscillations are related to the dielectric properties. The applied electrical field also causes charges to flow to the electrodes at the side, which creates electrical conductivity. Both dielectric properties and electrical conductivity can be measured as a function of frequency (how fast the electrical field switches sides). This study measured these properties on soil clays at different temperatures, which allowed us to calculate activation energies (how difficult it is for molecules to rotate or generate electrical current). The soil clays responded differently than most other materials that have been measured, which suggests that the properties of the soil clays change as the temperature is changed. This information will be useful for scientists who study water in the soil in relation to crop water uptake and environmental contamination.

Technical Abstract: Frequency-dependent spectra of electrical properties are used to characterize materials. In soils the emphasis has been on single frequency permittivity to determine water content and bulk d.c. soil electrical conductivity to determine soil salinity. When measured at different temperatures, the complex dielectric and electrical conductivity spectra can also be used to determine activation energies for dipole rotation. The purpose of this study is to determine the effect of saturating cation, water content, and smectite properties on dipole rotation activation energies. The activation energy calculated from electrical conductivity was due to the greatest energy barrier for charge transfer between electrodes. We calculated the activation energy due to dipole rotation two ways, one of which varied with temperature. Potassium-saturated smectites had high activation energies (means of 17 to 30 kJ mol**-1) because of low water content, and Mg-saturated smectites had high activation energies (means of 21 to 30 kJ mol**-1) because of high hydration energy. Mean activation energies for Ca- and Na- saturated smectites ranged from 11 to 20 kJ mol**-1. The activation energies for dipole rotation would be expected to remain constant as a function of temperature or to increase as a function of temperature; however, for descending temperatures, 45% of our samples showed increasing activation energy as the temperature decreased. A possible mechanism could be increased entropy due to decreasing aniosotropy of smectite quasi-crystals in the packed coaxial cell.