Proton Correlation Times and Segmental Mobility of Humic Acids in Two Solvents

Molecular dynamics of humic acid in solution are rarely investigated. Thus, the objective of this study is to determine proton correlation time and segmental mobility of humic acids in solution. Proton spin-lattice relaxation time (T1) was determined for humic acids in DMSO-d6 and 0.5 M NaOD at NMR frequencies of 300 MHz and 600 MHz; the correlation times were then estimated for aliphatic protons, protons associated with oxygen-containing functionalities, and aromatic protons. Results suggest that proton correlation times of humic acids in solvents were in the order of 10−10 seconds, which were near the T1 minimum at 600 MHz. Carbohydrate was identified as the largest and the most immobile component in humic acids, whereras aliphatic and aromatic fragments were relatively smaller and more mobile. The relaxation measurement is a useful approach for investigating the segmental mobility of humic acids in aqueous solution.