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Nanostructure of Open Water-Channel Reversed Micelles. I. 1H NMR Spectroscopy and Molecular Modeling Taleb H. Ibrahim* Chemical Engineering Department, American University of Sharjah, P.O. Box 26666, Sharjah, U.A.E.
Ronald D. Neuman Department of Chemical Engineering, Auburn University, Auburn, Alabama 36849-5127 Received December 20, 2003. In Final Form: February 10, 2004 A recently proposed model for the rodlike reversed micelles of nickel(II) bis(2-ethylhexyl)phosphate is examined in greater detail using 1H NMR spectroscopy and molecular modeling. 1H NMR spectra show that the solubilized water molecules are situated in a different environment compared with the water molecules in classical (AOT) reversed micelles. Geometry optimization and molecular dynamics simulation clearly indicate that the water molecules are not located in the interior core of the reversed micelles, but instead the water molecules exist in compartments or channels in the surface of these rodlike reversed micelles, thereby confirming the open water-channel model of reversed micelles. Molecular modeling was also employed to examine the effects of surfactant molecular structure, cosurfactant, solvent aromaticity, and temperature on the nanostructure of the reversed micellar aggregates. It is significant that molecular modeling provides an interpretation of the nanostructure of reversed micellar aggregates that is consistent with a variety of known experimental observations reported in the liquid/liquid extraction literature. These findings show that the structure of reversed micelles is much richer at the nanoscale level than previously recognized.
Introduction The term reversed micelle describes a micellar aggregate of surfactant or amphipathic molecules wherein the polar headgroups are oriented toward the interior of the reversed micelle and the hydrocarbon tails are extended toward a bulk nonaqueous solvent. Often reversed micelles have been compared to microreactors whose size can be changed by increasing or decreasing the amount of solubilized water in the inner core of the micellar aggregates.1 This structural feature of reversed micelles has been utilized in a variety of applications, for example, the production of nanoparticles of semiconductor,2 metallic,3 or magnetic4 materials and the separation of amino acids and proteins.5 An improved knowledge of the nanostructure of reversed micellar aggregates is essential for continued advances in understanding their equilibrium and dynamic properties as well as the development of future technological applications of reversed micelles. It is almost universally accepted that water molecules are solubilized in the polar core of reversed micelles. The example most cited is 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.6 Importantly, there is now evidence * To whom correspondence should be addressed. Phone: (971)6-515-2460. Fax: (971)-6-515-2979. E-mail:
[email protected]. (1) Eicke, H-. F.; Kvita, P. In Reverse Micelles; Luisi P. L., Straub, B. E., Eds.; Plenum Press: New York, 1984; p 21. (2) Qi, L.; Ma, J.; Cheng, H.; Zhao, C. Z. Colloids Surf. 1996, 111, 195. (3) Lisiecki, I.; Lixon, P.; Pileni, M. P. Prog. Colloid Polym. Sci. 1991, 84, 4, 342. (4) Gobe, M.; Kon-No, K.; Kandori, K.; Kitahara, A. J. Colloid Interface Sci. 1983, 93, 293. (5) Dekker, M.; Hilhorst, R.; Laane, C. Anal. Biochem. 1983, 178, 217. (6) Pileni, M. P. Structure and Reactivity in Reverse Micelles; Elsevier: New York, 1989.
that some surfactants, such as the acidic organophosphorus reagents employed in liquid/liquid extraction,7,8 do not allow this idealized compartmentalization of solubilized water and that other organizational variations are possible for the nanostructure of hydrated reversed micelles. The state of aggregation of metal salts of bis(2ethylhexyl) phosphoric acid (HDEHP) in nonaqueous media has been a matter of debate over the years. This situation is still not completely resolved, not only for the multivalent metal salts of HDEHP, but also for the monovalent alkali-metal salts such as NaDEHP.9-14 Various investigators have reported that the divalent transition-metal [e.g., Ni(II) and Co(II)] salts of HDEHP form polymeric species in nonpolar (or apolar) solvents.15-19 The evidence cited usually for the formation of polymers has been the very high solution viscosity.17 However, it is equally plausible to interpret this same solution behavior as support for the formation of quasi-one-dimensional or rodlike reversed micelles.20 Indeed, Neuman and co(7) 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. (8) Neuman, R. D.; Ibrahim, T. H. Langmuir 1999, 15, 10. (9) Eicke, H.-F.; Christen, H. J. Colloid Interface Sci. 1974, 46, 417. (10) Eicke, H.-F.; Arnold, V. J. Colloid Interface Sci. 1974, 46, 102. (11) Yu, Z. J.; Neuman, R. D. Langmuir 1994, 10, 2553. (12) Yu, Z. J.; Neuman, R. D. In Dynamic Properties of Interfaces and Association Structures; Pillai V., Shah D. O., Eds.; AOCS Press: Champaign, IL, 1996; p 166. (13) Feng, K. I.; Schelly, Z. A. J. Phys. Chem. 1996, 99, 17207. (14) Feng, K. I.; Schelly, Z. A. J. Phys. Chem. 1996, 99, 17212. (15) Brisk, M. L.; McManamey, W. J. J. Appl. Chem. 1969, 19, 103. (16) Sato, T.; Nakamura, T. J. Inorg. Nucl. Chem. 1972, 34, 3721. (17) Kolarik, Z.; Grimm, R. J. Inorg. Nucl. Chem. 1976, 38, 1721. (18) Thiyagarajan, P.; Diamond, H.; Danesi, P. R.; Horwitz, E. P. Inorg. Chem. 1987, 26, 4209. (19) Kunzmann, M. W.; Kolarik, Z. In Solvent Extraction 1990; Sekine, T., Ed.; Elsevier: New York, 1990; p 207.
10.1021/la036422e CCC: $27.50 © 2004 American Chemical Society Published on Web 03/11/2004
Nanostructure of Open Water-Channel Reversed Micelles
workers21-23 presented evidence obtained using a variety of scattering (SANS and QELS), spectroscopic (fluorescence and FTIR), and analytical techniques that the aggregates which form in the nonpolar phase by the nickel salt of HDEHP during liquid/liquid (L/L) extraction are cylindrical-type reversed micelles containing between five and six water molecules per DEHP anion. More recently, Steytler and co-workers24 also concluded that divalent metal salts of HDEHP in apolar medium (cyclohexane) form short rodlike reversed micelles based on SANS measurements. Furthermore, although these latter investigators found rodlike reversed micelles in the presence of water, it was suggested that spherical Ni(DEHP)2 aggregates exist under dry conditions. The question of the actual location of water molecules in rodlike reversed micellar aggregates has been even less studied in a direct manner. Kotlarchyk et al.25 employed SANS to obtain direct size and shape measurements of the aqueous core of w/o microemulsions. These findings often have been extrapolated to hydrated or swollen reversed micelles. In the case of Ni(DEHP)2 reversed micelles, it has been proposed that any solubilized water molecules are localized in the inner core of spherical26,27 or cylindrical21-23 reversed micellar aggregates. Steytler and co-workers,24 although not specifically addressing the existence of an aqueous core, commented that the hydration approaches that of the hexaqua nickel ion in water. However, it is to be emphasized that the conventional picture of reversed micelles is not consistent with what is known about the extent and selectivity of nickel extraction by HDEHP on the basis of an analysis of the liquid/liquid extraction literature. Therefore, in a marked departure from traditional concepts, Neuman et al.7 hypothesized that solubilized water molecules might exist in open water channels which are in contact with the nonaqueous solvent rather than in an inner core (or closed water channel) of rodlike Ni(DEHP)2 reversed micelles. Subsequently, Neuman and Ibrahim8 were able to provide initial confirmation on the basis of a preliminary molecular modeling study of what was designated as the “open waterchannel” model, namely, the solubilized water molecules are localized in compartments on the outside (surface) of rodlike reversed micelles. In this paper we examine in greater detail the proposed open water-channel model with particular emphasis on the reversed micelles of nickel(II) bis(2-ethylhexyl)phosphate. The techniques of 1H NMR spectroscopy and molecular modeling were employed to investigate the nature of the water environment in reversed micelles. Whereas an aliphatic hydrocarbon (n-heptane) was used in our preliminary investigation, the expanded study reported herein includes both apolar (n-heptane and benzene) and polar (chloroform) nonaqueous solvents. In addition, molecular modeling was employed to examine (20) Yu, Z. J.; Ibrahim, T. H.; Neuman, R. D. Solvent Extr. Ion Exch. 1998, 16 (6), 1437. (21) Park, S. J.; Neuman, R. D. In Proceedings of the International Solvent Extraction Conference ISEC 90, Part A; Elsevier: New York, 1992; p 201. (22) Neuman, R. D.; Park, S. J. J. Colloid Interface Sci. 1992, 152, 41. (23) Neuman, R. D.; Park, S. J.; Zhou, N-. F.; Shah, P. In Proceedings of the International Solvent Extraction Conference ISEC 93; Elsevier: New York, 1993; Vol. 3, p 1689. (24) Steytler, D. C.; Jenta, T. R.; Robinson, B. H.; Eastoe, J.; Heenan, R. K. Langmuir 1996, 12, 1483. (25) Kotlarchyk, M.; Chen, S.-H.; Huang, J. S. J. Phys. Chem. 1982, 86 (17), 3273. (26) Stoyanov, E. S.; Mikhailov, V. A. Koord. Khim. 1990, 16, 1537. (27) Stoyanov, E. S. In Solvent Extraction in Process Industries; Logsdail, D. H., Slater, M. J., Eds.; Elsevier: New York, 1993; p 1720.
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the effects of amphiphile molecular structure and temperature on the nanostructure of this important class of reversed micelles. Experimental Section Materials. HDEHP (Morton Thiokol) was purified as described elsewhere.28 AOT (Fluka AG) was purified following the procedure of Maitra and Eicke.29 The n-heptane (Aldrich, HPLC grade) and n-hexane (Phillips Chemical Co., 99 mol % pure grade) were double distilled and dried with molecular sieves (Linde 4A). Deuterated chloroform (Aldrich) was dried with molecular sieves. Nickel chloride (Johnson Matthey, ultrahigh purity grade) was used as received. High-purity water from a Millipore reverse osmosis/super-Q system was subsequently double distilled with the first distillation being from alkaline permanganate. Methods. The preparation of nickel(II) bis(2-ethylhexyl)phosphate was as follows: HDEHP and NiCl2 in a 2:1 molar ratio were dissolved in n-hexane and water (3:1 volume ratio), respectively. The two solutions were mixed together; the aqueous phase was maintained at pH 5.7-6.0 by adding NaOH solution; the organic phase was separated after equilibration, washed copiously with water, and dried with molecular sieves; and then the n-hexane solvent was removed by using a rotary evaporator. The solid Ni(DEHP)2 was further dried under high vacuum. NaDEHP was effectively removed from the Ni(DEHP)2/n-hexane solution by washing with water since atomic absorption analysis showed that the sodium content of the Ni(DEHP)2 was only 35 ppm. Nickel(II) bis(2-ethylhexyl)sulfosuccinate (Ni(AOT)2) was prepared from the acid form of AOT in a similar manner as that described for Ni(DEHP)2. Stock solutions of Ni(DEHP)2, Ni(AOT)2, and AOT were prepared at a ratio of 0.24 g of solute to 1.0 g of solvent. This concentration was employed in earlier studies of the Ni(DEHP)2 reversed micellar system,20 and it also yields high-quality NMR spectra. Molecular sieves were added to the stock solutions to remove any water present prior to the preparation of Ni(DEHP)2, Ni(AOT)2, and AOT reversed micellar systems of well-defined water content. 1H NMR spectra were obtained at ambient temperature with a Bruker 250 MHz Fourier transform (FT-NMR) spectrometer. Tetramethylsilane (TMS) was used as an external standard. Dynamic light-scattering measurements were performed using the apparatus and procedures described elsewhere.30 The mean diffusion coefficent (D h ) of the reversed micellar aggregates was obtained from the measured autocorrelation function of the stock h was n-heptane solutions by the method of cumulants,31 and D converted into the mean apparent hydrodynamic radius (R h h) using the Stokes-Einstein relationship. HyperChem (Hypercube, Waterloo, Ontario) and Sybyl (Tripos, St. Louis, MO) software were used for molecular modeling and molecular dynamics (MD) simulation of reversed micellar aggregates, respectively. Geometry optimization yields the most stable system (set of Cartesian coordinates) based on minimization of potential energy, whereas MD simulation provides a more realistic picture of aggregate nanostructure and motions of component (amphiphile, solvent, and water) molecules. The initial optimization of reversed micellar aggregates in vacuo was performed with the HyperChem simulation program. First, a single amphiphile or metal-amphiphile complex such as Ni(DEHP)2, with or without coordinated water molecules, was optimized using the MM+ force field and the ZINDO/1 method by applying the conjugate gradient method. Next, the optimized complex was used to build up a micellar aggregate that consisted of several (ranging from two to seven) metal-amphiphile complexes. Aggregate optimization then was performed using the MM+ force field of HyperChem software followed by the Tripos force field of Sybyl software. The aggregate was then solvated (as a droplet with no boundary conditions) with, for example, n-heptane by applying SILVERWARE of the Taylor solvent algorithm. The solvated aggregate was then optimized (28) Gaonkar, A. G.; Neuman, R. D. Sep. Purif. Methods 1984, 13, 141. (29) Maitra, A. N.; Eicke, H. F. J. Phys. Chem. 1981, 85, 2687. (30) Yu., Z. J.; Neuman, R. D. Langmuir 1992, 8, 2074. (31) Koppel, D. E. J. Chem. Phys. 1972, 57, 4814.
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Table 1. Chemical Shifts (ppm) of 1H NMR Spectra for the Metal Salt/n-Heptane/Water System as a Function of W0 metal salt
W0
CH3
CH2
H2O
H2O
Ni(DEHP)2
0 1 2 4 6 8 10 0 1 2 4 6 8 10 0 1 2 4 6 7
5.288 5.207 5.207 4.981 4.951 4.959 4.808 0.906 0.783 0.849 0.682 0.679 0.661 0.890 1.202 1.181 1.159 1.140 1.142 1.111
5.690 5.594 5.302 5.367 5.346 5.348 5.189 1.316 1.184 1.249 1.082 1.077 1.059 1.291 1.599 1.583 1.565 1.546 1.528 1.515
6.520 8.249 7.828 8.092 8.054 8.042 8.008 4.355 4.231 4.040 4.082 4.169 4.241 4.295 4.525 4.606 4.700 4.551 4.513 4.520
4.203 4.281 4.016 4.053a 4.189a 4.315a 4.800 4.800 4.850 4.850 4.840 4.885
AOT
Ni(AOT)2
a
Another small water peak appears at ∼4.2 ppm.
by the conjugate gradient method using the Tripos force field. The interactions of solubilized water (10-60 molecules) were investigated by adding water molecules 10 at a time close (
