Carbohydrates, Moisture, and Mineral Interfacial Interactions

230 pagesSwelling clays play a critical role in the trapping of fluids within soil and atmospheric nanoparticles. The hydrodynamics of cation-saturated smectite clay nanopores are well documented, however, two knowledge gaps exist. First, direct experimental probing of water entrapment in hydrated nanopore environments is largely lacking. Techniques commonly used to study smectite hydration are often proxy hydration measurements that do not explicitly monitor water molecules. Experimental probing of smectite hydration is frequently supplemented with modeling techniques for complete understanding. Second, little is known about how the presence and structure of common soil organic compounds impact smectite hydration. Organic compounds from root exudation or organic material decomposition have long been implicated in enhancing soil hydration, but detailed, quantitative analyses of the relationship between organic adsorption and clay moisture as well as experimental probing of water populations are missing. To address these knowledge gaps, three sets of studies have been completed and organized into the following three chapters: Chapter 1 will cover how the presence of simple carbohydrates regulates smectite nanopore water trapping; Chapter 2 will investigate the hydration properties of a smectite clay using analytical techniques that allow for the direct monitoring of hydrated mineral environments; and Chapter 3 will cover the influence of different carbohydrate structures on carbohydrate-smectite interactions and subsequent mineral hydration dynamics. The work detailed here utilized a unique subset of analytical methodologies and applied novel techniques to gain insights into the hydrated environments within montmorillonite, a prototypical smectite clay. Each study probed moisture-dependent nanopore size dynamics with relative humidity-controlled X-ray diffraction (XRD) analyses. In addition, moisture-dependent 23Na nuclear magnetic resonance spectroscopy (NMR), water adsorption-desorption isotherms, Fourier-transform infrared spectroscopy (FTIR), and thermogravimetric analysis coupled with mass spectrometry (TGA-MS) were implemented to investigate the multiple facets of montmorillonite hydration, from the direct measurement of water loss during dehydration to quantifying the hydration environments of Na ions within the mineral matrices.In Chapter 1, we demonstrated that, when smectite-type clays were enriched with adsorbed carbohydrates, there was re-structuring and expansion of the mineral nanopores as well as changes in total water content and water trapping. In Chapter 2, we probed smectite hydration beyond monitoring mineral interlayer expansion, deducing how mineral moisture conditions impacted sodium hydration environment and total mineral water population dynamics. Our TGA-MS and 23Na NMR data captured unique and dynamic behavior of the Na-associated and mineral-associated water populations not captured by XRD nanopore size dynamics as well as quantified the persistence of partially-hydrated mineral environments even at low moisture conditions. In Chapter 3, our research elucidated how carbohydrate structural differences facilitated the retention of water within organo-mineral samples. We found that the adsorption of all tested carbohydrate structures increased montmorillonite moisture retention, but polymer carbohydrates were discovered to induce greater moisture retention compared to monomer and dimer carbohydrates. Our Chapter 3 results furthered the comprehension of organo-mineral interactions, most notably by providing direct quantitative analysis of the carbohydrate structure-dependent moisture retention in montmorillonite matrices with FTIR and TGA-MS techniques. There is currently a shifting paradigm on how differences in organic matter structure influence the long-term retention of soil organic matter and soil hydration properties. The findings from this work shed light on the dependence of moisture retention in organo-mineral composites on the molecular structure of the organic compounds. The discoveries detailed here provide critical mechanistic understanding of the role of structure in facilitating the persistence of soil organic matter as well as the importance of the structure of organic inputs on dictating soil hydration and water retention.2023-09-1