Water Dynamics and Its Role in Structural Hysteresis of Dissolved Organic Matter

Abstract

Knowledge of structural dynamics of dissolved organic matter (DOM) is of paramount importance for understanding DOM stability and role in the fate of solubilized organic and inorganic compounds (e.g., nutrients and pollutants), either in soils or aquatic systems. In this study, fast field cycling (FFC) (1)H NMR relaxometry was applied to elucidate structural dynamics of terrestrial DOM, represented by two structurally contrasting DOM models such as Suwanee River (SRFA) and Pahokee peat (PPFA) fulvic acids purchased by the International Humic Substance Society. Measurement of NMR relaxation rate of water protons in heating-cooling cycles revealed structural hysteresis in both fulvic acids. In particular, structural hysteresis was related to the delay in re-establishing water network around fulvic molecules as a result of temperature fluctuations. The experiments revealed that the structural temperature dependency and hysteresis were more pronounced in SRFA than in PPFA. This was attributed to the larger content of hydrogel-like structure in SRFA stabilized, at a larger extent, by H-bonds between carboxylic and phenolic groups. Moreover, results supported the view that terrestrial DOM consist of a hydrophobic rigid core surrounded by progressively assembling amphiphilic and polar molecules, which form an elastic structure that can mediate reactivity of the whole DOM.

Nuclear Magnetic Resonance with Fast Field-Cycling Setup: A Valid Tool for Soil Quality Investigation

“…Hysteresis appeared to be more pronounced when more hydrophilic dissolved organic matter was accounted for. The relaxometry results also supported the view that soil DOM consists of a hydrophobic rigid core surrounded by amphiphilic and polar molecules progressively assembled, and forming an elastic structure able to mediate the reactivity of the whole dissolved organic matter [109].…”

“…The temperature dependence of the aforementioned changes were attributed to the dynamic re-arrangement of the water clusters around DOM [113]. This latter hypothesis was verified by NMR relaxometry with fast field-cycling setup [109]. It came out that a delay (also referred to as hysteresis) in re-establishing water network around DOM occurs as a result of the temperature fluctuations.…”

“…The complexity and flexibility of DOM are considered to be responsible for its ability to enhance solubility of hydrophobic organic compounds, its capacity to decrease water surface tension, or its capability to trap and transport nutrients across the space [109].…”

“…The kosmotropic effect of the solid soil surface reduces with the distance, and so activates the 3D "jumps" described in Figure 13. The jump frequency depends on soil wettability which, in turn, is affected by chemical nature of natural organic matter (NOM) [84,109]. For this reason, the higher the wettability, the stronger the surface affinity of H 2 O molecules, thereby lowering the number of the possible 3D jumps.…”

Ultrasonication‐Assisted Synthesis of 3‐Substituted Indoles in Water Using Polymer Grafted ZnO Nanoparticles as Eco‐Friendly Catalyst

“…Reactions conducted in water are cheaper, safe, reduce environmental pollution and in many cases enhances selectivity ,. It has been noted that most of the organic reactions proceed sluggishly in water because of poor solubility of reactants, though, according to Breslow effect hydrophobic aggregation of organic molecules reduces the activation energy . To overcome this problem either surfactant are used or amphiphilic catalysts are used that can successfully create hydrophobic reaction environment in water …”