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United States Department of Agriculture

Agricultural Research Service

Title: Dielectric Spectra of Bound Water in Hydrated Smectite

Authors
item Logsdon, Sally
item Laird, David

Submitted to: Journal of Non Crystalline Solids
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 30, 2001
Publication Date: July 8, 2002
Citation: LOGSDON, S.D., LAIRD, D.A. DIELECTRIC SPECTRA OF BOUND WATER IN HYDRATED SMECTITE. JOURNAL OF NON CRYSTALLINE SOLIDS. 2002. v. 305. P. 243-246.

Interpretive Summary: Scientists need to know the water content of soils so they can measure how much water is used by crops, how much water infiltrates during rain or irrigation, and how water moves chemicals through soil. Many of the methods currently used to measure soil water content actually measure an electrical property of the soil called the dielectric, which is highly correlated with soil water content. Unfortunately, the water near the surfaces of clay and soil organic matter, called "bound water", has different dielectric properties than the rest of the water in the soil. Variable dielectric properties of soil water make it difficult to accurately infer soil water content and limits the usefulness of these measurement techniques for an important range of soils and soil conditions. In this study, we measured the dielectric properties of bound water, so that more accurate soil water content measurements can be made. This information will be valuable to scientists who need to measure soil water content and make recommendations concerning crop water use, infiltration, and leaching of agricultural chemicals through soils.

Technical Abstract: Water contained in larger pores is considered to have the properties of free water, but water associated with colloid surfaces is often called bound water. The objective of this study is to utilize dielectric spectra to elucidate properties of various forms of bound water in Ca-smectite. The < 2 micrometer fraction of Wyoming bentonite was Ca-saturated and prepared as oriented films. The loosely-stacked oriented films were equilibrated above saturated salt solutions to control relative humidity, then compressed with a hydraulic press, and finally packed into coaxial cells with the preferred orientation of the clay layers parallel with the cell axis. A vector network analyzer was used to indirectly determine the complex dielectric spectra. The gravimetric water contents ranged from 0.209 to 0.421 g/g for relative humidities (RH) ranging from 54 to 98%, and static electrical conductivity (EC) ranged from 0.001 to 0.037 S/m. Volumetric water contents ranged from 0.203 to 0.506 m**3 m**-3, and variable amounts of air were present in the different samples, ranging from 0.040 to 0.338 m**3 m**3. Increased water content increased the EC because of greater continuity of water and lower tortuosity. Both the real and imaginary component of the dielectric were about an order of magnitude smaller at 54% RH than at 98% RH.

Last Modified: 10/1/2014
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