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'' aggrega tes with hydrophobic interiors into which nonpolar organic compounds p artition. We solubilized atrazine, labeled by a trifluoromethyl group on the ethylamino side chain, in aqueous solutions containing 10% humi c acid by weight and observed the F-19 nuclear magnetic resonance (NMR ) relaxation of atrazine induced by paramagnetic probes added to the h umic solution. One paramagnetic probe, Gd.EDTA anion, remains in aqueo us solution while the other, TEMPO (2,2,6,6-tetramethyl-1-piperidinylo xy), is sorbed by humic micelles. The hydrophilic anionic paramagnetic probe induces virtually no paramagnetic relaxation in atrazine solubi lized by 10% aqueous humic acid solutions while the hydrophobic neutra l paramagnetic probe causes rapid paramagnetic relaxation. These resul ts show that atrazine solubilized by concentrated humic acid solutions occupies a domain accessible only to neutral hydrophobic molecules an d, hence, confirms the existence of hydrophobic domains. We suggest th at atrazine resides in the hydrophobic interior of humic acid micelles .