Characterization of noncovalent interactions between 6-propionyl-2-dimethylaminonaphthalene (PRODAN) and dissolved fulvic and humic acids

Noncovalent interactions between the fluorescent probe 6-propionyl-2-dimethylaminonaphthalene (PRODAN) and dissolved Norman Landfill leachate fulvic acid, Suwannee River fulvic acid, Suwannee River humic acid, and Leonardite humic acid were examined as a function of pH, fulvic and humic acid (FA and HA) concentration, and solvent polarity using steady-state fluorescence spectroscopy. Static quenching processes, as indicated by linear Stern–Volmer plots and high Kd values, were positively correlated with the % aromaticity of the FA and HAs, as well as with solution pH. Results illustrate that for FA molecules with relatively low % aromaticity values, solvophobic interactions between PRODAN and FA are the primary interaction mode. For HA molecules with higher % aromaticity, PRODAN engages in both solvophobic interactions and π−π interactions, in particular electron donor–acceptor interactions, via condensed aromatic, electron-accepting moieties inherent within HA molecules. Experiments modifying solvent polarity demonstrated that protonation of carboxylic acid functional groups at low pH ( 4) increased the hydrophobicity of the dissolved FA and HA molecules, thereby enhancing noncovalent interactions with PRODAN through increased solvophobic forces.