Fluorine-19 Nuclear Magnetic Resonance Study of Atrazine in Humic

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Fluorine-19 Nuclear Magnetic Resonance Study of Atrazine in Humic and Sodium Dodecyl Sulfate Micelles Swollen by Polar and Nonpolar Solvents Yao-Ying Chien and William F. Bleam* Department of Soil Science, University of WisconsinsMadison, Madison, Wisconsin 53706-1299 Received March 3, 1997. In Final Form: July 23, 1997X Though sodium dodecyl sulfate (SDS) and humic micellar solutions both solubilize atrazine, the behavior of atrazine in humic micellar solutions differs markedly from SDS micellar solutions, in both the capacity to absorb solvents and the effect of these solvents on the chemical shift of atrazine within the solventswollen micelles. From this we conclude humic hydrophobic domains differ substantially from the SDS micellar core and the latter is not a suitable model for the former. Several lines of evidence, ranging from dimerization to partitioning between two-phase (solvent and micellar solution) systems, suggest atrazine forms strong, probably cooperative, hydrogen bonds when absorbed within humic hydrophobic domains. Polar hydrogen-bond donor or acceptor solvents can dissociate atrazine from humic molecules above a critical threshold, but nonpolar aprotic solvents seem unable to overcome the strong interactions that bind atrazine to humic molecules. Finally, humic micellar solutions exhibit a remarkable capacity to absorb organic solvents and most likely form stable microemulsions.

Introduction Humic Micelles. To explain the seemingly contradictory nature of humic substances, a hydrophilicity that allows them to absorb many times their dry weight in water combined with a hydrophobicity that permits them to efficiently absorb nonpolar organic molecules, Wershaw1,2 proposed the “membrane-micelle” model. The “membrane-micelle” model accounts for this dual nature by postulating “micellar” humic aggregates with hydrophilic exterior surfaces and predominantly hydrophobic interiors. Humic substances, according to the “membrane-micelle” model, absorb nonpolar organic compounds by partitioning them into the hydrophobic “micellar” core. Wershaw3 solubilized DDT (dichlorodiphenyltrichloroethane) in concentrated humic solutions, a finding confirmed by others4,5 with other nonpolar solutes, implicating the existence of micelle-like humic aggregates. Numerous studies4-13 demonstrate humic substances from diverse origins (soil, aquatic, and marine) exhibit surface activity. The surface activity and solubilizing behavior of humic substances parallel that of synthetic surfactants5,14 whose * To whom correspondence should be addressed: telephone, 806/ 262-9956; fax, 608/265-2595; e-mail, [email protected]. X Abstract published in Advance ACS Abstracts, September 15, 1997. (1) Wershaw, R. L. J. Contam. Hydrol. 1986, 1, 29-45. (2) Wershaw, R. L. Environ. Sci. Technol. 1993, 27, 814-816. (3) Wershaw, R. L.; Burcar, P. J.; Goldberg, M. C. Environ. Sci. Technol. 1969, 3, 271-273. (4) Shinozuka, N.; Lee, C. Mar. Chem. 1991, 33, 229-241. (5) Guetzloff, T. F.; Rice, J. A. Sci. Total Environ. 1994, 152, 31-35. (6) Tschapek, M.; Wasowski, C. Geochim. Cosmochim. Acta 1976, 40, 1343-1345. (7) Tschapek, M.; Scoppa, C. O.; Wasowski, C. Z. Pflanzenernaehr. Bodenk. 1978, 141, 203-207. (8) Chen, Y.; Schnitzer, M. Soil Sci. 1978, 125, 7-15. (9) Tschapek, M.; Wasowski, C.; Scoppa, C. O.; Torres Sanchez, R. M. Agrochimia 1980, 24, 30-38. (10) Tschapek, M.; Wasowski, C.; Torres Sanchez, R. M. Plant Soil 1981, 63, 261-272. (11) Hayano, S.; Shinozuka, N.; Hyakutake, M. J. Jpn. Oil Chem. Soc. 1982, 31, 357-362. (12) Hayase, K.; Tsubota, H. Geochim. Cosmochim. Acta 1983, 47, 947-952. (13) Yonebayashi, K.; Hattori, T. Sci. Total Environ. 1987, 62, 5564.

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capacity to solubilize hydrophobic organic compounds appears above the critical micelle concentration (cmc). Early studies15,16 of nonpolar compounds in humic solutions found little solubility enhancement “because of the relatively [small] organic environment and the normally more polar nature of the dissolved organic matter”.16 An equally plausible interpretation would first consider whether the humic solutions are sub- or supramicellar. In every case,15-19 the humic concentrations were a hundred- to a thousand-fold more dilute than representative humic cmc values, which range from 0.1% to 3%.4-13 Atrazine Conformers and Dimerization. Restricted rotation about the atrazine alkylamino side chains gives rise to four conformers (Figure 1) apparent in both 1H20 and 19F21 nuclear magnetic resonance spectra. The relative intensities of each conformer in infinitely dilute solutions reflect their intrinsic stabilities. Dimerization and complexation stabilize certain conformers more than others, leading to changes in the relative populations of each conformer.20,21 The two side-chain alkylamino nitrogens serve as hydrogen-bond donors while the ring imino nitrogen serves as a hydrogen-bond acceptor. Intermolecular hydrogenbond complexes between atrazine and donor or acceptor compounds further alter the relative conformer populations.22,23 Complexing agents with both hydrogen-bond donor and acceptor capacity, e.g., acetic acids and amides, (14) Kile, D. E.; Chiou, C. T. Environ. Sci. Technol. 1989, 23, 832838. (15) Chiou, C. T.; Brinton, T. I.; Kile, D. E.; Malcolm, R. L. Environ. Sci. Technol. 1986, 20, 502-508. (16) Kile, D. E.; Chiou, C. T. ACS Symp. Ser. 1989, No. 219, 131157. (17) Morra, M. J.; Corapcioglu, M. O.; von Wandruszka, R. M. A.; Marshall, D. B.; Topper, K. Soil Sci. Soc. Am. J. 1990, 54, 1283-1289. (18) Puchalski, M. M.; Morra, M. J.; von Wandruszka, R. Environ. Sci. Technol. 1992, 26, 1787-1792. (19) Engebretson, R. R.; von Wandruszka, R. Environ. Sci. Technol. 1994, 28, 1934-1941. (20) Welhouse, G. J.; Bleam, W. F. Environ. Sci. Technol. 1992, 26, 959-964. (21) Welhouse, G.; Barak, P.; Bleam, W. F. J. Phys. Chem. 1993, 97, 11583-11589. (22) Welhouse, G. J.; Bleam, W. F. Environ. Sci. Technol. 1993, 27, 500-505. (23) Welhouse, G. J.; Bleam, W. F. Environ. Sci. Technol. 1993, 27, 494-500.

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Figure 1. Atrazine adopts four conformations because partial double bonds between the alkylamino side chains and the triazine ring hinder free rotation. The circled Arabic numbers identify the four hydrogen-bond sites and the Roman numerals identify the four conformers.

form much stronger complexes with atrazine than complexes employing only one hydrogen-bond site, e.g., ethanol and acetone.20-23 Conformers I, II, and III (Figure 1) appear in the atrazine 19F NMR spectra when it is dissolved in an organic solvent (Figure 2a). These three conformers appear as four resonances (Figure 2a), representing chemical exchange through intermolecular hydrogen bonding at active sites 1-4. The relative abundance of each conformer determines the intensity of each resonance. Conformer IV, which does not dimerize, does not appear because its abundance is negligible.21 The chemical shift position of each resonance represent perturbations of the -CF3 fluorines through hydrogen bonding interactions. Each resonance undergoes chemical exchange between a state where the active site is not hydrogen bonded and a state where a hydrogen bond exists at that site. Hydrogen-bonding perturbations result in a larger change in the chemical shift of resonance IIe relative to IIIi because site 3 is closer to the -CF3 label than site 4; see Figure 5 in ref 21. Conformer I appears as two resonances, Ie and Ii, because sites 1 and 2 form hydrogen bonds independently of one another21 and, as with IIe and IIIi, the relative magnitude of their influence on the -CF3 chemical shift differs because of proximity. Conformer I interacting through site 1, resonance Ii, results in weaker perturbations of the label than when conformer I interacts through site 2, resonance Ie; see Figure 5 in ref 21. NMR Studies of Micellar Solutions. Numerous NMR studies reveal the properties of micellar solutions24-33 and the behavior of organic solutes associated with or solubilized by micelles.34-40 The dynamics of micellar (24) Wennerstro¨m, H.; Lindman, B.; So¨derman, O.; Drakenberg, T.; Rosenholm, J. B. J. Am. Chem. Soc. 1979, 101, 6860-6864. (25) Yablonskii, O. P.; Shapiro, Y. E.; Konovalova, V. P. Colloid J. (Engl. Transl.) 1984, 46, 610-612. (26) Soderman, O. Colloid Polym. Sci 1987, 265, 76-82. (27) Chachaty, C. Prog. Nucl. Magn. Reson. Spectrosc. 1987, 19, 183222. (28) Gao, Z. S.; Kwak, J. C. T.; Wasylishen, R. E. J. Phys. Chem. 1989, 93, 2190-2192. (29) Gao, Z.; Kwak, J. C. T.; Labonte, R.; Marangoni, D. G.; Wasylishen, R. E. Colloids Surf. 1990, 45, 269-281. (30) Wasylishen, R. E.; Kwak, J. C. T.; Gao, Z. S.; Verpoorte, E.; Macdonald, J. B.; Dickson, R. M. Can. J. Chem. 1991, 69, 822-833. (31) Bratt, P. J.; Gillies, D. G.; Krebber, A. M. L.; Sutcliffe, L. H. Magn. Reson. Chem. 1992, 30, 1000-1011. (32) Fujiwara, H.; Kanzaki, K.; Kano, T.; Kimura, A.; Tanaka, K.; Da, Y. Z. J. Chem. Soc., Chem. Commun. 1992, 736-737. (33) Chung, J. J.; Kang, J. B.; Lee, K. H.; Seo, B. I. Bull. Korean Chem. Soc. 1994, 15, 198-204.

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solutions involve processes operating on different time scales ranging from milliseconds to nanoseconds.41 The lifetime of a typical micelle is measured in milliseconds,41 but individual molecules exchange between solution and the micelle on the microsecond time scale.41 A micelle with a millisecond lifetime is a relatively permanent feature when solute chemical exchange occurs on the microsecond time scale. Adding water-immiscible organic solvents to aqueous micellar solutions may cause the micelles to swell.34,36 The core within solvent-swollen micelles should differ from unswollen micelles if the properties of the solvent differ substantially from the hydrophobic portion of the surfactant comprising the micelle. The previously mentioned studies reporting parallels between synthetic and humic micellar solutions encouraged us to design NMR studies of atrazine solubilized in aqueous micellar solutions of sodium dodecyl sulfate (SDS) and humic acid. This study examines the behavior of atrazine in micelles swollen by selected organic solvents. The solvents chosen for this study include a simple alkane (n-octane), two solvents capable of π-π interactions (benzene and 2,5-dimethyl-2,4-hexadiene), and two solvents capable of hydrogen-bond interactions (1-octanol and 2-octanone). The simple alkane (n-octane) serves as reference solvent to contrast the difference between the π-π and hydrogen-bond solvents. Materials and Methods NMR Instrumentation and Experiments. We collected all 19F NMR spectra at the National Magnetic Resonance Facility at Madison (NMRFAM) on a Bruker AM-400 (9.6 T) spectrometer operating at 376.48 MHz. Following a single 5.0 µs pulse, we collected 8K data points without proton decoupling and zerofilled to 16K before Fourier transformation. Unless otherwise noted, all spectra represent 256 scans separated by 1 s relaxationdelay intervals. A standard Bruker heating coil maintained a constant 300 K temperature. A coaxial NMR tube held both our sealed external 19F reference-lock solution, 0.3% v/v (trifluoromethyl)benzene in 99.9 atom % DMSO-d6, and the aqueous micellar solution. Source of Atrazine and Other Compounds. The CF3atrazine (2-chloro-4-(trifluoroethylamino)-6-(isopropylamino)-striazine) was used as supplied by Ciba Corp.; though the purity was verified by thin-layer chromatography. Sodium n-dodecyl (lauryl) sulfate (SDS) was recrystallized twice from ethyl acetate and washed with ethyl ether in a Soxhlet extractor as pointed out by Fendler37 to remove dodecanol and homologes. All other chemicals used in this study were reagent-grade and used as supplied. The water used throughout was deionized distilled water. Extraction and Preparation of Soil Humic Acids. We extracted the humic acid from an alluvial soil (Sparta sand: mesic, uncoated Typic Quartzipsamment) of the lower Wisconsin River Valley near Arena, WI. The extraction method followed the procedure recommended by the International Humic Substances Society.42 Prepare a 10:1 solution-to-solids suspension in 0.1 M NaOH under a N2 atmosphere and shake intermittently for 4 h. Acidify the supernatant to pH 1.0 with 6 M HCl while constantly (34) Mukerjee, P.; Cardinal, J. R.; Desai, N. R. In Micellization, Solubilization, and Microemulsions; Mittel, K. L., Ed.; Plenum Press: New York, 1977; Vol. 1, pp 241-261. (35) Mukerjee, P.; Cardinal, J. R. J. Phys. Chem. 1978, 82, 16201627. (36) Mukerjee, P. Pure Appl. Chem. 1980, 52, 1317-1321. (37) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975. (38) Pyter, R. A.; Ramachandran, C.; Mukerjee, P. J. Phys. Chem. 1982, 86, 3206-3210. (39) Lindman, B. Prog. Colloid Polym. Sci. 1984, 69, 39-47. (40) Mukerjee, P.; Ko, J.-S. J. Phys. Chem. 1992, 96, 6090-6094. (41) Nusselder, J. J. H.; Engberts, J. B. F. N.; Boelens, R.; Kaptein, R. Recl. Trav. Chim. Pays-Bas 1988, 107, 105-107. (42) IHSS In International Humic Substances Society Symposium; Denver, CO, 1982.

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Figure 2. Fluorine-19 NMR spectra of CF3-atrazine: (a) carbon tetrachloride solution containing 7 mM atrazine; (b) 1 mol/kg aqueous SDS micellar solution containing 14 mM atrazine; (c) 1 mol/kg aqueous SDS micellar solution containing 14 mM atrazine and 310 mM 2,5-dimethyl-2,4-hexadiene; (d) 10% (w/w) aqueous humic micellar solution containing 7 mM atrazine; (e) 10% (w/w) aqueous humic micellar solution containing 7 mM atrazine and 360 mM 1-octanol. stirring and allow to stand for 12-16 h. Acidification precipitates the humic acid fraction. Centrifuge to separate the precipitated humic acids. Redissolve the humic acid fraction with a minimal quantity of 0.1 M KOH, purge the headspace of the centrifuge tube with N2, and centrifuge at 7000g for 3 h to remove the mineral fraction. Transfer the humic acid solution to a plastic bottle and acidify with a 0.1 M HCl/0.3 M HF solution to dissolve silicate minerals. Repeat 0.1 M HCl/0.3 M HF washing until the ash content is less than 1%. Rather than using silver chloride to detect residual chloride ions, we continued dialysis (Spectra Por CE, molecular weight cutoff 100) until the conductivity of the external solution reached ≈4 µS, yielding a humic acid ash content of 0.65% by weight. We stored the humic acid fraction as the freeze-dried solid. Preparation of Humic Acid Solutions and Sorption of Atrazine. We dissolved humic acid in alkaline solutions, adjusting the pH to 11.8 with 1 M NaOH. Atrazine solubility in humic micellar solutions is very sensitive to pH, decreasing quickly as the pH approaches 7. Aqueous solutions containing 10% humic acid by weight dissolved sufficient atrazine to collect high-resolution 19F NMR spectra. Atrazine is a solid at 300 K and supplied as a fine powder. We mixed the desired amount of solid atrazine with freeze-dried

humic acid before dissolving in alkaline water. Solubilizing atrazine in these humic acid solutions typically required stirring overnight after an hour of sonification. Our aqueous humic micellar solutions contained 7 mM atrazine, confirmed by comparing the integrated NMR signal to our external reference. We were able to ensure uniform atrazine concentrations by using the same sealed external-reference solution throughout all our experiments. We prepared 1 mol/kg SDS solution containing 14 mM atrazine by mixing the desired amount of SDS and atrazine powder with 10 mL of deionized distilled water. Sonifying 30 min dissolved both SDS and atrazine powder. We measured and transferred the organic solvents using a 10 µL syringe. The organic solvents and aqueous micellar solutions were prepared and mixed directly in 5 mm NMR tubes.

Results Atrazine in Aqueous SDS and Humic Micellar Solutions. Figure 2 displays representative 19F NMR spectra of CF3-atrazine in aqueous humic and SDS micellar solutions and in carbon tetrachloride. All three conformers form when SDS micelles solubilize atrazine

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Chien and Bleam Table 1. Gelation and Phase Separation Summary for Organic Solvents in 1 mol/kg SDS, and 10% Humic Acids

organic solvents 2-octanone 1-octanol benzene 2,5-dimethyl-2,4-hexadiene n-octane

1 mol/kg SDS

10% humic acidsa

phase gelation separation

phase separation

110b 82 600 310 270

> 2100c > 1700c 1000 620 1090

a 10% humic acid is 10 g of humic acid in 100 mL of H O. b All 2 concentrations are in mM. c No phase separation observed at the listed concentration.

Figure 3. 19F NMR chemical shift CF3-atrazine in aqueous SDS and humic micellar solutions. The chemical shift of the most intense resonance (SDS solutions on left-hand vertical axis and humic acid solutions on right-hand axis) is plotted against the atrazine content of the respective micellar solutions.

(Figure 2b), though the relative populations differ from that observed in carbon tetrachloride solutions (Figure 2a). The chemical shift for the most abundant conformer changes as the atrazine content in the SDS micellar solution increases (Figure 3), as observed in carbon tetrachloride solutions,21 demonstrating the existence of a significant atrazine dimer population in SDS micelles. Adding the organic solvents mentioned earlier to SDS micellar solutions shifted the position of each atrazine conformer peak but left the relative conformer populations virtually unchanged (Figure 2c). The behavior of atrazine in humic micellar solutions (Figure 2d) differs markedly from SDS solutions (Figure 2b) in that only one conformer appears in the NMR spectra. Qualitatively, this is our first clue that the hydrophobic domain within SDS micelles is not chemically equivalent to the hydrophobic domains within humic micelles. Atrazine in Solvent-Swollen SDS and Humic Micellar Solutions: Gelation and Phase Separation. We now turn our attention to the visual appearance of solvent-swollen SDS and humic micellar solutions. Comments regarding gelation are based on qualitative observations. We made no quantitative measurements of gelation. Aqueous SDS micellar solutions gel when the octanol content exceeds 82 mM or when the octanone content exceeds 110 mM (Figure 5). The effect of octanol and octanone on SDS micellar solutions differs from the effect of the aprotic nonpolar solvents (benzene, hexadiene, and octane) because the polar solvents cause SDS micellar solutions to gel prior to phase separation. The data points in Figure 5 stop where gelation prevented us from collecting good quality NMR spectra: 82 mM in octanolswollen micellar solutions and 400 mM in octanoneswollen micellar solutions. Gelation typically results from a change in micellar geometry.39 Table 1 summarizes our observations on gelation and phase separation for organic solvents in 1 mol/kg SDS, and 10% humic acids. Hydrogen-bonding solvents induce a change in SDS micellar geometry that causes gelation at solvent concentrations as low as 82110 mM. The aprotic nonpolar solvents swell SDS micelles without changing micellar geometry, creating stable microemulsions that do not gel at solvent contents as high as 270-600 mM. Humic micellar solutions, in contrast, do not gel even when octanol or octanone concentrations reach 1700-2100 mM. This suggests that the long-chain hydrogen-bonding solvents (2-octanone and 1-octanol) do not induce changes in humic micellar geometry.

A separate solvent phase appears in aqueous SDS micellar systems when the hexadiene content reaches 310 mM and when the benzene content reaches 600 mM. Phase separation in aqueous humic micellar systems is delayed until the added aprotic nonpolar solvent reaches 620, 1000, and 1090 mM for hexadiene, benzene, and octane, respectively. Polar solvents (1-octanol and 2-octanone) do not form separate phases even at concentration as high as 1700-2100 mM. Atrazine in Solvent-Swollen SDS and Humic Micellar Solutions: Dimerization and Complexation. Further differences in the behavior of atrazine in humic and SDS micellar solutions appear when observing atrazine NMR spectra in response to added organic solvent. Multiple atrazine conformers never appear in the NMR spectra of humic micelles swollen by the aprotic nonpolar solvents (octane, benzene, and hexadiene) though they are always present in swollen SDS micellar solutions, regardless of solvent polarity. Polar solvents capable of hydrogen-bonding interactions (octanol and octanone) can stabilize atrazine dimers in humic micellar solutions (Figure 2e), but only above a critical threshold. The contrasting behavior of atrazine in humic micelles swollen by polar and nonpolar solvents, when combined with the absence of dimers in unswollen humic micelles, suggests the hydrophobic domain at the core of humic micelles possesses a significant hydrogen bonding capacity. Aprotic nonpolar solvents (octane, benzene, and hexadiene) and polar solvents (octanol and octanone) induce changes in the chemical shift of each atrazine conformer when added to aqueous SDS micellar solutions (Figures 4 and 5). Presumably this occurs because these waterimmiscible solvents swell the SDS micelles thereby altering the solvent environment of the atrazine within the micelle. Both nonpolar and polar solvents (Figures 4 and 5) induce upfield change of the chemical shifts in atrazine as the solvent content in SDS micellar solutions increases. Solvent-induced change of the chemical shifts of the most abundant conformer range from 3 to 13 Hz over the solvent contents used in this study. Our primary interest is not interpreting the effect of different solvents on the atrazine NMR spectra in aqueous SDS micellar solutions. Rather, we wish to compare and contrast the effects of these solvents on SDS and humic micellar solutions. From such comparisons we hope to learn whether atrazine displays similar behavior in solvent-swollen SDS and humic micelles. Figure 6 shows the chemical shifts induced by added solvents in humic micellar solutions. Results from solvent-swollen humic (Figure 6) and SDS micellar solutions (Figures 4 and 5) demonstrate yet again the profound difference between hydrophobic domains within SDS and humic micelles. Regardless of the quantity of nonpolar solvent swelling humic micelles, only one conformer appears in all NMR

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Figure 4. Fluorine-19 NMR chemical shift of 14 mM CF3atrazine conformers solubilized in 1 mol/kg aqueous SDS micellar solutions plotted against the aprotic solvent (n-octane, benzene, and 2,5-dimethyl-2,4-hexadiene) content. Phase separation in aqueous SDS micellar solutions occurs at about 600 mM for benzene and 310 mM for 2,5-dimethyl-2,4-hexadiene.

Figure 5. Fluorine-19 NMR chemical shift of 14 mM CF3atrazine conformers solubilized in 1 mol/kg aqueous SDS micellar solutions plotted against the protic solvent (1-octanol and 2-octanone) content.

spectra. Even when the quantity of added solvent exceeded the solubilizing capacity of the humic acid and an organic phase separates from the humic micellar solution, atrazine did not migrate into the organic phase but remained associated with the swollen-humic micelles. The only solvents capable of inducing the appearance of all four atrazine conformers were the polar solvents capable of hydrogen bonding. Even with polar solvents, the solvent content had to exceed a critical threshold before the additional conformers appear in the NMR spectra. Discussion Synthetic Micelles as Models for Humic Hydrophobic Domains. While synthetic surfactants and humic substances exhibit superficial similarities, e.g., surface activity and the capacity to solubilize neutral organic compounds, closer examination clearly demonstrates profound differences between humic acids and surfactants with alkyl chains. First, atrazine dimerization survives in SDS micellar solutions but is absent in humic micellar solutions. This is not to say dimers do not form in humic micellar solutions, rather intermolecular interactions skew speciation to the point where dimerization is no longer apparent. Second, phase separation into an organic phase and a swollen aqueous micellar phase occurs at high solvent contents and polar solvents (at least the ones we used) but never induced gelation in humic solutions. Finally, both the direction and magnitude of solventinduced atrazine chemical shifts differ in SDS and humic micellar solutions. Hydrogen Bonding within Humic Hydrophobic Domains. Several lines of evidence indicate a capacity for intermolecular hydrogen-bonding interactions within the humic micellar core. First, and most obvious, atrazine dimerization seems to be absent in humic micellar

Figure 6. Fluorine-19 NMR chemical shift of 7 mM CF3atrazine solubilized in 10% aqueous humic micellar solutions plotted against the solvent (octane, benzene, 2,5-dimethyl-2,4hexadiene, 1-octanol, and 2-octanone) content.

solutions (Figures 2d and 3), though we interpret this as a skewed equilibrium where strong, probably cooperative, hydrogen-bond complexes dominate.21-23 Second, the solvent-induced atrazine chemical shift change in humic micelles (Figure 6) is much less than in solvent-swollen SDS micelles (Figures 4 and 5). Third, even upon phase separation (i.e., an organic phase and a solvent-swollen humic micellar phase) a single atrazine resonance persists

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if the solvent is aprotic. Atrazine does not migrate from the solvent-swollen humic micellar phase into the separated organic phase indicating a strong interaction that skews partitioning in favor of the humic micellar phase. Finally, additional atrazine conformers appear in humic micellar solutions only when swollen by polar solvents (Figure 2e). Polar hydrogen-bond donor or acceptor solvents dissociate atrazine from humic molecules above a critical threshold, but aprotic nonpolar solvents seem unable to overcome the strong interactions that bind atrazine to humic molecules. Microemulsions in Aqueous Humic Micellar Systems. Microemulsions are thermodynamically stable dispersions of one immiscible liquid in another.43,44 The key to this thermodyamic stability is the stabilization of nanometer radii of the dispersed droplets by a surfactant. Not all surfactants have the capacity to form microemul(43) Robbins, M. L. In Micellization, Solubilization, and Microemulsions; Mittel, K. L., Ed.; Plenum Press: New York, 1977; Vol. 1, pp 713-754. (44) Friberg, S.; Buraczewska, I. In Micellization, Solubilization, and Microemulsions; Mittel, K. L., Ed.; Plenum Press: New York, 1977; Vol. 1, pp 791-799.

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sions and may require the addition of another component to further lower the water-hydrocarbon surface tension.44 The extraordinary capacity of humic micellar solutions to absorb certain organic solvents and resist phase separation indicates humic micellar solutions may form stable microemulsions. However, evidence for microemulsification must await careful delineation of the phase diagrams in three-component aqueous-humic acidsolvent systems. Registry Numbers Provided by the Authors. Atrazine, 1912-24-9; (trifluoromethyl)benzene, 98-08-8; DMSOd6, 2206-27-1; 1-octanol, 111-87-5; 2-octanone, 111-13-7; benzene, 71-43-2; 2,5-dimethyl-2,4-hexadiene, 764-13-6; n-octane, 111-65-9. Acknowledgment. The authors gratefully acknowledge the assistance from NMRFAM staff and Ciba Corp. for providing both radiolabeled and fluorine-labeled atrazine. This research was funded by the National Research Initiative-Competitive Grants Program of the USDA (Project No. 94-37107-0357). LA970232K