²H NMR Study of Dynamics of Benzene-d₆ Interacting with Humic and Fulvic Acids

Abstract

Samples of three humic acids and one fulvic acid with 1% loading of benzene-d(6) in sealed glass tubes have been studied with solid-state deuterium quadrupole-echo nuclear magnetic resonance spectroscopy. Calculated spectra combining three motional models, two isotropic models and a third more restricted small-angle wobble (SAW) motional model, are fit to the experimental spectra. One isotropic motion (ISO(v)) is assigned to vaporous benzene-d(6) due to the small line width, short T(1), and the loss of this component by about -25 °C when the temperature is lowered. The remaining two motional components, ISO(s) and SAW, are sorbed by the humic or fulvic acid. Benzene-d(6) slowly interacts with the humic substances, progressively filling SAW sites as ISO(s) motion diminishes. Both the sorption and increase in percentage of SAW motion are for the most part complete within 200 days but continue to a lesser extent over a period of a few years. For the SAW motion there are at least two and most likely a series of T(1) values, indicating more than one adsorption environment. Enthalpies of sorption, obtained from application of the van't Hoff equation to the percentages of the different motional models derived from a series of variable temperature spectra, are comparable in magnitude to the enthalpy of vaporization of benzene. In Leonardite humic acid, ΔH and ΔS for the ISO(s) to SAW transition change from positive to negative values with age, implying a transition in the driving force from an entropic effect associated with expansion and deformation in the molecular structure of the humic substance to accommodate benzene-d(6) to an enthalpic effect of strong benzene-d(6)-humic substance interactions. In contrast, at advanced ages, Suwannee River humic and fulvic acids have small positive or near zero ΔH and positive ΔS for the ISO(s) to SAW transition.

Development of an equilibrium headspace gas chromatographic method for the measurement of noncovalent association and partitioning of n-alkylbenzenes at infinite dilution in fulvic acid pseudophase

“…Eq. (14) shows that the measurements of these parameters by eHSGC do not require solute concentrations. Fig.…”

“…Techniques such as nuclear magnetic resonance (NMR), electron spin resonance (ESR) with probes at mM concentration and atomic force microscopy (AFM) and transmission electron microscopy (TEM) images revealed micelle-like pseudophase aggregate due to self-association of supramolecular structure containing large hydrophobic domains clusters connected via H-bonds [14][15][16].…”

“…These results may not be used to calculate the noncovalent association constants of monomer solute-monomer FA and monomer solute self-associated FA n at infinite dilution of the solute in aqueous media. Also, results obtained by NMR used mM concentration of benzene, which could not discern small changes in the signal from weak noncovalent interactions between benzene and the humics at very low concentration of solute [14]. Other studies used organic solvents to dissolve solutes or aqueous buffers as bulk media to obtain partition coefficients [18].…”