Host-Guest Interactions between 2,4-Dichlorophenol and Humic Substances As Evaluated by ¹H NMR Relaxation and Diffusion Ordered Spectroscopy

Abstract

1H NMR measurements of spin-lattice (T1) and spin-spin (T2) relaxation times and diffusion ordered spectroscopy (DOSY) were applied to investigate the association of 2,4-dichlorophenol (DCP) with a soil fulvic (FA-VICO) and humic acid (HA(-)-VICO), and a lignite humic acid (HA-LIG). The 1H T1 and T2 values of DCP were found to decrease with increasing humic concentration, indicating reduction in molecular mobility due to formation of noncovalent interactions. The increased shortening of relaxation times observed upon addition of HA suggested more extensive association of DCP with HA than with FA. The extent of binding was inferred from diffusion coefficients (D) which showed slower diffusion for bound DCP. At 1 mg mL(-1) DCP was completely bound by 4.1 and 5.8 mg mL(-1) of HA-VICO and HA-LIG, respectively, while full DCP association was not observed even up to 20 mg mL(-1) of FA. This was reflected by association constants (Ka): 3.1 +/- 0.3 M(-1) for FA-DCP, and 15.5 +/- 3.1 M(-1) and 11.0 +/- 1.2 M(-1) for HA-VICO and HA-LIG DCP complexes, respectively. The stronger binding to HA is attributed to their larger hydrophobic character enabling formation of stable hydrophobic domains to which DCP becomes associated in host-guest complexes. DCP complexation within humic hydrophobic domains was confirmed by upfield chemical shifts and signal line broadenings observed in 1H NMR spectra. Similar chemical shift variations for the three DCP aromatic protons further indicated pi-pi interactions, rather than H-bonding, as the main driving force for noncovalent association between DCP and dissolved humic substances. Relaxation and diffusion 1H NMR techniques provide rapid and accurate measurements of binding constants and thermodynamic parameters for host-guest complexes between environmental contaminants and natural organic matter.