Determining Soil Water Structure, Density, and Viscosity via Infrared Spectroscopy

Pore water in soil often exhibits properties that significantly deviate from those of free water, such as abnormally high density and viscosity. These deviations fundamentally arise from water structural differences between pore water and free water. While the structure of capillary water only slightly differs from that of free water, adsorptive water typically exhibits significantly more ordered and rigid configurations. Here, infrared (IR) spectroscopy is innovatively employed to probe the diverse pore-water structures in soils, quantify their respective contributions to soil water retention, and develop IR-based methods for determining soil water density and viscosity. The IR spectra for two montmorillonites are recorded at nine relative humidity (RH) levels. Decomposing the spectra via multipeak Gaussian fitting identifies three types of water in soils: free-like water with relatively dynamic and transient structures; surface adsorptive water with highly ordered structures with enhanced hydrogen-bond strength; and hydrated water subject to strong ionic electric fields. The ratios of the decomposed spectral areas enable tracking the content evolution of these water types as RH changes. Furthermore, a density-wavenumber function and an activation energy-wavenumber function are developed to represent local water density and activation energy, respectively. Integration of these functions over the spectra allows for calculations of average soil water density and viscosity. The calculated results accord well with independently measured datasets, confirming their validity and offering a pathway for developing IR-based approaches to assess fundamental soil properties of density and viscosity distributions. The developed approaches provide bridges from molecular-scale IR spectral descriptors to continuum soil properties and behaviors for potential engineering practices.