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Langmuir 1999, 15, 10-12
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 (1H NMR) spectroscopy and molecular simulation. 1H 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 waterchannel model” of reversed micelles. This finding has significant implications regarding the physicochemical properties and technological applications of reversed micelles.
Reversed micelles are thermodynamically stable association 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 fundamental and practical points of view. For example, reversed micelles have numerous applications in drug transport,1 semiconductor,2 metallic,3 and magnetic4 nanoparticle processing, catalysis,5 enzyme-mediated synthesis,6 artificial 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(2ethylhexyl) sulfosuccinate (AOT), which is the classical * To whom correspondence should be addressed (e-mail address:
[email protected]). (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 Macromolecular 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.
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 spherical14 or cylindrical9 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 1H NMR results obtained for the Ni(DEHP)2, AOT, and Ni(AOT)2 reversed micellar systems at selected Wo values, where Wo is the number of solubilized water molecules per amphipathic molecule. 1 H NMR spectra of the Ni(DEHP)2/n-heptane/water (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.
10.1021/la980732t CCC: $18.00 © 1999 American Chemical Society Published on Web 12/11/1998
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Langmuir, Vol. 15, No. 1, 1999 11
Table 1. Chemical Shifts of 1H NMR Spectra for the Metal Salt/n-Heptane/Water System as a Function of Wo chemical shift (ppm)b metal
salta
Ni(DEHP)2
AOT
Ni(AOT)2
Wo
CH3
CH2
H2O
2 4 6 8 2 4 6 8 2 4 6 7
4.907 4.981 4.951 4.959 0.849 0.682 0.679 0.661 1.159 1.140 1.142 1.111
5.302 5.367 5.346 5.348 1.249 1.082 1.077 1.059 1.565 1.546 1.528 1.515
7.828 8.092 8.054 8.042 4.040 4.082 4.169 4.241 4.700 4.551 4.513 4.520
a Ni(DEHP) , n-heptane, and water were the same as previously 2 described.16 AOT (Fluka) was purified following the procedure developed by Maitra and Eicke.17 Ni(AOT)2 was prepared in a similar manner as Ni(DEHP)2. b 1H NMR spectra were recorded on a Bruker NMR spectrometer at 250 MHz. NMR samples were prepared from stock solutions of Ni(DEHP)2, AOT, or Ni(AOT)2 in n-heptane (0.24 g of solute per g of solvent) dried with molecular sieves (Linde 4A) to which measured amounts of water were added to yield various Wo values. External tetramethylsilane (TMS) was employed as a reference standard.
system show three identifiable peaks at 4.907-4.981 (CH3), 5.302-5.367 (CH2), and 7.828-8.092 ppm (H2O) as shown in Table 1. When compared with the 1H NMR spectra for HDEHP and n-heptane, not only is the OCH2 peak for Ni(DEHP)2 seen to be unresolved but the CH3 and CH2 peaks for n-heptane overlap those for Ni(DEHP)2 and are shifted downfield (higher proton chemical shift) as compared to the CH3 (0.787-0.903 ppm) and CH2 (1.214-1.452 ppm) peaks for HDEHP due to the paramagnetic effect. Furthermore, the H2O peak for the Ni(DEHP)2 reversed micellar system is shifted further downfield, not only than that for water dissolved in n-heptane (4.319 ppm), but also the primary H2O peak for Ni(AOT)2 reversed micelles (Table 1). This chemical shift behavior is a clear indication that the water molecules in the two reversed micellar systems are in different environments. If the water molecules were in the same environment, one would expect the chemical shifts to be comparable because the molecular structures are similar and the micellar sizes (hydrodynamic radii from QELS measurements) are approximately the same. In the former case, the Ni(DEHP)2 complexes are organized in a manner such that the paramagnetic moments introduced by the unpaired electrons of the nickel atoms do not cancel out. Also, the water molecules are located in an environment where the paramagnetic effect is able to affect the water molecules, both directly and by “reflection” from other atoms and/or molecules (since they too are affected by the field created by the unpaired electrons). The net effect is that the opposing field is “enhanced”, and therefore a large chemical shift for the water protons is observed. However, in the latter case, the Ni(AOT)2 complexes are oriented such that the paramagnetic moments almost cancel out as can be seen when the chemical shifts are compared with those for AOT reversed micelles (Table 1). It thus can be concluded that the water molecules are likely located “inside” Ni(AOT)2 reversed micelles in a manner similar to those in AOT reversed micelles, whereas the water molecules in Ni(DEHP)2 reversed micelles seem to be situated “outside” in the channels between the hydrocarbon tails of the Ni(DEHP)2 complexes, which is consistent with CPK space-filling molecular models. Molecular simulation shows that Ni(DEHP)2‚2H2O complexes can exist as quasi-one-dimensional (rodlike)
Figure 1. Optimized structures of (a and b) Ni(DEHP)2‚2H2O reversed micelles and (c and d) Ni(DEHP)2‚2H2O reversed micelles with solubilized water. Color key: carbon (white), hydrogen (blue), oxygen (red), nickel (green), phosphorus (yellow) and hydrogen bonds (yellow dotted lines). HyperChem and SYBYL molecular modeling programs were employed to obtain energy minimization for Ni(DEHP)2‚2H2O aggregates with and without added water molecules. The MM+ force field, ZINDO/1 (HyperChem) and Tripos force field (SYBYL) were applied in vacuo using the conjugate gradient method for optimization. This was followed by further optimization in n-heptane using the Tripos force field of SYBYL (SILVERWARE of the Tailor solvent algorithm was used for solvation).
aggregates via hydrogen bond, van der Waals, and electrostatic forces. One of the most stable aggregates (based on energy minimization) consists of five complexes (parts a and b of Figure 1), which is in agreement with the VPO results of Neuman and co-workers17 that the mean aggregation number is about 5.2. Close examination of the optimized structure indicates that the octahedral Ni(DEHP)2‚2H2O complexes are organized in a slip pattern with a mean distance of 0.54 nm between the nickel atoms. Furthermore, the coordinated water molecules form an organized network of intra- and intercomplex hydrogen bonds (yellow dotted lines, see Figure 1a) with the oxygen atoms of the oxyalkyl groups and the acidic oxygens of other complexes. The n-heptane molecules are not displayed for clarity reasons. It is also evident that two channels form along the axis of the aggregate between the hydrocarbon tails of the Ni(DEHP)2‚2H2O complexes. On the basis of the hydrogen-bonding nature of the association of the Ni(DEHP)2‚2H2O complexes, it can be concluded that the aggregates are reversed micelles. Parts c and d of Figure 1 show the optimized structure of five Ni(DEHP)2‚2H2O complexes with 30 added water molecules (Wo ) 8). It can be clearly seen that the (solubilized) water molecules are located in the channels present between the hydrocarbon tails of the nickel(II) salt of HDEHP. This hydrogen-bonded network of water molecules bridges the complexes and provides an additional attractive force which reduces the distance between nickel atoms to 0.50 nm, thereby enhancing the stability of the reversed micellar aggregates. In conclusion, molecular modeling directly confirms the open water-channel model, where water molecules can be (17) Maitra, A. N.; Eicke, H. F. J. Phys. Chem. 1981, 85, 2687.
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localized in compartments on the outside (surface) of reversed micelles. Thus, micellar structures in apolar media are more diverse than previously believed possible. These unique findings raise many questions such as whether other structural variations are viable and how they may mediate the physicochemical properties and dynamics of association nanostructures.
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Acknowledgment. Financial support provided by the Office of Basic Energy Sciences, Division of Chemical Sciences, Department of Energy, is gratefully acknowledged. LA980732T
