Noncovalent Interactions between Acenaphthenone and Dissolved Fulvic Acid As Determined by ¹³C NMR <i>T</i>₁ Relaxation Measurements

Abstract

Non-covalent interactions between 13C-labeled acenaphthenone (13C-labeled in the carbonyl position) and Suwannee River fulvic acid in a methanol/D2O solvent have been examined using 13C NMR T1 relaxation measurements. The influence of solvents upon the non-covalent interactions were assessed by examining acenaphthenone in pure solvents of varying solvation capacity (chloroform, methanol, methanol/D2O). Interactions with fulvic acid were examined as a function of acenaphthenone and fulvic acid concentrations, fulvic acid counter-cation (H+ or Na+), and pH. In the presence of fulvic acid in a methanol/D2O solvent, three non-covalent interactions were identified: a weak sorption interaction between acenaphthenone and fulvic acid, an enhanced solubilization of acenaphthenone by fulvic acid, and an interaction between just the solvent and acenaphthenone. The enhanced solubilization is hypothesized to arise from fulvic acid forming hydrophobic regions that are predominantly solvated with methanol and have excluded water. Acenaphthenone in these hydrophobic regions displays similar behavior to when it is dissolved in pure methanol. The ability of fulvic acid to form hydrophobic regions was found to be dependent upon the identity of the fulvic acid counter-cation and upon pH.