Novel Structural Model of Reversed Micelles: The Open Water-Channel Model Ronald D. Neuman* and Taleb H. Ibrahim Department of Chemical Engineering, Auburn University, Auburn, Alabama 36849-5127 Received June 23, 1998 According to the classical model of reversed micelles, any water molecules present are solubilized in the micellar core. Here we report unique findings on the nanostructure of reversed micellar aggregates obtained by proton magnetic resonance ( 1 H NMR) spectroscopy and molecular simulation. 1 H NMR measurements of the chemical shift of water protons indicate that solubilized water can exist in different environments in reversed micellar systems. The molecular modeling shows that water molecules can be localized in channels within the surface of some rodlike micellar aggregates, thereby confirming the “open water- channel model” of reversed micelles. This finding has significant implications regarding the physicochemical properties and technological applications of reversed micelles. Reversed micelles are thermodynamically stable as- sociation nanostructures of amphipathic molecules where the polar (or ionic) headgroups occupy the interior of the micellar aggregates and the hydrophobic hydrocarbon tails extend into the bulk nonaqueous or apolar solvent. The structure of reversed micelles and the solubilization of polar compounds are of great interest from both funda- mental and practical points of view. For example, reversed micelles have numerous applications in drug transport, 1 semiconductor, 2 metallic, 3 and magnetic 4 nanoparticle processing, catalysis, 5 enzyme-mediated synthesis, 6 ar- tificial photosynthesis, 7 and liquid-liquid extraction. 8-10 Water molecules play an important role in the structure and function of reversed micelles. It is generally accepted that water is solubilized in the polar core of reversed micelles and that the size of reversed micelles increases with an increase in the amount of water present. 11 In this paper, however, we report new findings which show that this conventional view of the compartmentalization of water molecules in reversed micelles is not a universal paradigm. An alternative structural model, namely, the “open water-channel” model, 12 has been recently proposed for the reversed micelles which form during the liquid-liquid extraction of nickel(II) ions by bis(2-ethylhexyl)phosphoric acid (HDEHP). HDEHP is a common extractant with a molecular structure similar to that of sodium bis(2- ethylhexyl) sulfosuccinate (AOT), which is the classical surfactant used often in studies of the structure and properties of reversed micelles. 13 In the case of AOT, water molecules are solubilized inside its spherical reversed micelles. 11 This simple picture also has been assumed to hold for the reversed micelles of the nickel(II) salt of HDEHP, namely, Ni(DEHP) 2 , where solubilized water has been proposed to exist in the inner core of presumed spherical 14 or cylindrical 9 reversed micelles. However, upon careful examination, this interpretation is not fully consistent with what is known about the extent and selectivity of metal ion extraction by HDEHP. Thus, Neuman et al. 12 hypothesized that the solubilized water molecules exist in “open” water channels which are in contact with the nonpolar solvent rather than in a “closed” water channel in the polar core of rodlike micellar aggregates. In the study presented herein NMR spectroscopy was used to investigate the nature of the water environment in reversed micellar aggregates to test the proposed open water-channel model. In addition, molecular modeling techniques were employed to examine the interactions, orientation, and location of solubilized water molecules. 1 H NMR spectra of Ni(DEHP) 2 , AOT, and nickel(II) bis- (2-ethylhexyl) sulfosuccinate (Ni(AOT) 2 ) reversed micelles in n-heptane were obtained as a function of water content. AOT was selected for comparison because, as indicated earlier, its structure is similar to that of HDEHP and its reversed micelles are known to solubilize water molecules in the polar core of the micellar aggregates. Since the unpaired electrons on the nickel atom of Ni(DEHP) 2 will create a magnetic field which opposes the applied field, thereby causing higher proton chemical shifts, the nickel- (II) salt of AOT, namely, Ni(AOT) 2 , was also examined to account for the paramagnetic effect. 15 Table 1 summarizes the 1 H NMR results obtained for the Ni(DEHP) 2 , AOT, and Ni(AOT) 2 reversed micellar systems at selected W o values, where W o is the number of solubilized water molecules per amphipathic molecule. 1 H NMR spectra of the Ni(DEHP) 2 /n-heptane/water * To whom correspondence should be addressed (e-mail ad- dress: rdneuman@eng.auburn.edu). (1) Speiser, P. In Reverse Micelles; Luisi, P. L., Straub, B. E., Eds.; Plenum Press: New York, 1984; p 339. (2) Qi, L.; Ma, J.; Cheng, H.; Zhao, C. Z. Colloid Surf. 1996, 111, 195. (3) Lisiecki, I.; Lixon, P.; Pileni, M. P. Prog. Colloid Polym. Sci. 1991, 84, 342. (4) Gobe, M.; Kon-No, K.; Kandori, K.; Kitahara, A. J. Colloid Interface Sci. 1983, 93, 293. (5) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macro- molecular Systems; Academic Press: New York, 1975. (6) Visser, A. J. W. G.; Fendler, J. H. J. Phys. Chem. 1982, 86, 947. (7) Hilhorst, R.; Laane, C.; Veeger, C. Proc. Natl. Acad. Sci. U.S.A. 1982, 79, 3927. (8) Osseo-Asare, K. Sep. Sci. Technol. 1988, 23 (12 &13), 1269. (9) Neuman, R. D.; Park, S. J. J. Colloid Interface Sci. 1992, 152, 41. (10) Ono, T.; Goto, M.; Nakashio, F.; Hatton, T. A. Biotechnol. Prog. 1996, 12, 793. (11) Eicke, H. F.; Kvita, P. In Reverse Micelles; Luisi, P. L., Straub, B. E., Eds.; Plenum Press: New York, 1984; p 21. (12) Neuman, R. D.; Yu, Z. J.; Ibrahim, T. In Value Adding Through Solvent Extraction; Shallcross, D. C., Paimin, R., Prvcic, L. M., Eds.; The University of Melbourne: Parkville, 1996; p 135. (13) Pileni, M. P. Structure and Reactivity in Reverse Micelles; Elsevier: New York, 1989. (14) Stoyanov, E. S. In Solvent Extraction in Process Industries; Logsdail, D. H., Slater, M. J., Eds.; Elsevier: New York, 1993; p 1720. (15) Drago, S. R. Physical Methods in Chemistry; W. B. Saunders: Philadelphia, 1977. (16) Yu, Z. J.; Ibrahim, T. H.; Neuman, R. D. Solvent Extr. Ion Exch. 1998, 16 (6), 1437. 10 Langmuir 1999, 15, 10-12 10.1021/la980732t CCC: $18.00 © 1999 American Chemical Society Published on Web 12/11/1998