Paramagnetic relaxation of atrazine solubilized by humic micellar solutions

This study examines a central tenet of the ''membrane-micelle'' model of humic substances-that humic molecules form ''micelle-like'' aggregates with hydrophobic interiors into which nonpolar organic compounds partition. We solubilized atrazine, labeled by a trifluoromethyl group on the ethylamino side chain, in aqueous solutions containing 10% humic acid by weight and observed the F-19 nuclear magnetic resonance (NMR) relaxation of atrazine induced by paramagnetic probes added to the humic solution. One paramagnetic probe, Gd.EDTA anion, remains in aqueous solution while the other, TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy), is sorbed by humic micelles. The hydrophilic anionic paramagnetic probe induces virtually no paramagnetic relaxation in atrazine solubilized by 10% aqueous humic acid solutions while the hydrophobic neutral paramagnetic probe causes rapid paramagnetic relaxation. These results show that atrazine solubilized by concentrated humic acid solutions occupies a domain accessible only to neutral hydrophobic molecules and, hence, confirms the existence of hydrophobic domains. We suggest that atrazine resides in the hydrophobic interior of humic acid micelles.