Dielectric Properties of Bound Water: Application to Porous Media Surface Area and Grain Moisture Determination

The dielectric properties of molecular layers of water bound to solid surfaces or to macromolecules are substantially different than those of free water. Consequently, determination of moisture content from dielectric properties of porous mixtures, such as soils and rocks having moderate to substantial surface area and cereal grains, is affected by the amount and energy state of bound water. We use earth materials and seed or grain products as specific examples having high economic and environmental relevance. A physical model that considers modified properties of water near solid surfaces is presented. The thermo-dielectric sensitivity of TDR-measured bulk dielectric permittivity (eb) to bound water reflects interactions with the porous medium surface area, and permits model inversion to infer specific surface area (As) from measured response to thermal perturbation. Measured eb(T) using TDR along with independently measured mass water contents and bulk densities are used in the model to calculate As for soils and sand-clay mixtures having a range in As and wetness. Results are in agreement with As of the mixtures measured using ethylene glycol monoethyl ether (EGME) monolayer adsorption. Estimation of bound water within grains or legumes is a critical component towards predicting water content from dielectric permittivity measurements. Application of the physical model to this problem is also discussed and compared to permittivity measurements in starch and grain samples. INTRODUCTION Time domain reflectometry (TDR) is commonly used to infer liquid water content and bulk electrical conductivity of soils and other porous media. Estimation of soil water content (?) from measured eb using TDR travel time analysis is based on the dominance of the high dielectric permittivity of liquid water relative to that of other common soil constituents. Bound water, water that is closely adjacent to solid surfaces, is subject to a number of forces that hinder its response to an imposed electromagnetic field resulting in lower eb than free liquid water. Water closest to solid surfaces is held most tightly, and has lowest measured eb. Successive molecular layers of water have increasingly higher eb until that of free water is reached at about 3 molecular layers from the solid (Bockris et al. 1966, Sposito and Prost 1982). Water incorporated into starches may also be considered as bound in the sense that it is rotationally constrained in response to an alternating electromagnetic field imposed by TDR. In this paper we discuss development and preliminary applications of a physical model to describe the relationships between bound and free water contents of porous media and food grains, the measured responses in eb using TDR under ambient or imposed temperature (T) changes, and specific surface areas and grain moisture contents. t (x,T) ' 4 p r 3 ?0 (T)exp a x T