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Densely Stacked Multilamellar and Oligovesicular Vesicles, Bilayer Cylinders, and Tubes Joining with Vesicles of a Salt-Free Catanionic Extractant and Surfactant System Zaiwu Yuan,†,§ Zhilei Yin,‡ Sixiu Sun,‡ and Jingcheng Hao*,†,‡ State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China, Key Laboratory of Colloid and Interface Chemistry (Shandong UniVersity), Ministry of Education, Jinan 250100, China, and Graduate School of the Chinese Academy of Sciences, Beijing 100080, China ReceiVed: September 11, 2007; In Final Form: NoVember 5, 2007
In the phase diagram of an excellent extractant of rare earth metal ions, di(2-ethylhexyl) phosphate (HDEHP, commercial name P204), mixing with a cationic trimethyltetradecylammonium hydroxide (TTAOH) in water, a birefringent LR phase was found, which consists of densely stacked multilamellar vesicles. The densely stacked multilamellar vesicles are remarkably deformed, as observed by means of cryotransmission electron microscopy (cryo-TEM). Further, self-assembled structuressoligovesicular vesicles, bilayer cylinders, and tubes joining with vesiclesswere also observed. The self-assembled phase is transparent, anisotropic, and highly viscous, possessing elastic properties determined by rheological measurements. This is the first time that birefringent LR phase with remarkably deformed amphiphilic bilayer membranes has been constructed through combining a hydrophobic organic extractant having double chains with a water-soluble surfactant having a single chain, which may direct primarily toward acquiring an understanding of the mechanism of salt-free catanionic vesicles and secondarily to determine if vesicle-extraction technology utilizing extractants is possible.
Introduction As one of the soft matters, amphiphilic bilayer membranes are very important in cytology, which represent simple model systems for biological membranes.1,2 Since Kaler reported the spontaneous formation of cationic and anionic (catanionic) single-tailed surfactant vesicles in aqueous solution,3 a lot of works have been focused on bilayer membranes of catanionic surfactant systems.4 In general, when a cationic surfactant solution and an anionic one are simply mixed, the strong reduction in area per head group resulting from ion pairing induces the formation of molecular bilayers at low concentrations. At the right mixing ratios, vesicles may be established spontaneously and are thermodynamically stable species.5-7 The cationic-anionic surfactant systems can produce a precipitate when the stoichiometry between the cationic and anionic surfactants is exactly 1.8 The bilayer membranes in solution can be theoretically described by the harmonic approximation to the bending free energy,9 which determines the properties including shape, size, and thermodynamic stability of various amphiphilic bilayers; i.e., among all the interfacial free energy terms the curvature elasticity mainly determines the energy of amphiphilic bilayers.9
(
) ( )
1 1 1 1 2 fc ) κ + + κj 2 R1 R2 R0 R1R2
(1)
where fc is the curvature free energy per unit area of the membrane, R1 and R2 are the principle radii of the curvature of * Corresponding author. Fax: +86-531-88366074. E-mail: jhao@ sdu.edu.cn. † Lanzhou Institute of Chemical Physics. ‡ Shandong University. § Graduate School of the Chinese Academy of Sciences.
the structures, R0 is the spontaneous radius of the curvature, κ is the curvature modulus, and κj is the saddle-splay modulus. In addition, entropic and van der Waals attraction effects are taken into account as the higher order corrections to the energy when the Debye length is smaller in comparison with the interlamellar distance. However, at low salt concentration or in salt-free catanionic surfactant systems, i.e., H+ and OH- as counterions and thus forming water by the combination of the counterions, the long-range Coulomb interactions can strongly modify the local properties of the membranes.10 Typical examples are the formation of nanodisks and regular hollow icosahedra in the salt-free catanionics (the so-called “true” catanionics) aqueous solution reported by Zemb et al.11,12 They tested a general mechanism to explain how the ratios of cationic and anionic surfactants in the absence of added salt control the nanodisks to punctured planes; during crystallization, excess (nonstoichiometric) surfactant molecules accumulate on edges or pores rather than being incorporated into crystalline bilayers.13 Recently, our group focused on the zero-charged vesicle phase of salt-free catanionic surfactant mixtures (i.e., H+ and OH- as counterions to form H2O at equimolar mixtures14) including a densely packed onion phase14c and charged vesicles formed through M2+ (Zn2+, Ca2+, Ba2+, Mg2+, etc.)-ligand coordination, where M2+ is the central ion.15 Although these multilamellar vesicles with zero-charged or charged bilayer membranes in our previous systems15,16 have almost the same rheological properties, i.e., viscoelastic solutions, which were determined by rheogram measurements as a function of the angular frequency, the functional applications of salt-free zero-charged catanionic surfactant mixtures such as for enhancing the solubility of fullerenes16 and the charged vesicle-assisting synthesis of nanoparticles in aqueous solution15b are interesting topics of surfactant sciences. Herein, a new salt-
10.1021/jp077292b CCC: $40.75 © 2008 American Chemical Society Published on Web 01/15/2008
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free catanionic system of excellent extractant and surfactant mixtures in solution was investigated. The detailed phase diagram of salt-free catanionic mixtures was determined; the birefringent LR phase contains densely stacked multilamellar and oligovesicular vesicles, bilayer cylinders, and tubes joining with vesicles, which were determined by cryotransmission electron microscopic (cryo-TEM) observations. The macroproperties such as the rheology were studied for the birefringent LR phase. These results may provide the possibility for the extraction of rare earth metal ions by vesicle aqueous phase composed of excellent extractants as an extraction technology. Experimental Section Chemicals. Tetradecyltrimethylammonium bromide (TTABr) was purchased from Sigma-Aldrich Chemical Company. Di(2ethylhexyl) phosphate (HDEHP) was purchased from Shanghai Chemical Company (China). Both were directly used without further purification. Ion exchanger III was obtained from Merck Co. Ltd., and all the other reagents were of analytical grade. Water was triply distilled. Preparation of Trimethyltetradecylammonium Hydroxide (TTAOH). TTAOH stock solution was prepared from TTABr aqueous solution (140 mmol‚L-1) using a strong base anion exchanger (Ion exchanger III, Merck) at 40 °C. Bromide ions could not be detected by AgNO3 in the TTAOH stock solution with excess HNO3 (Ag+ + Br- f AgBrV, yellow precipitates), so the ion exchange with hydroxide was >99%. The total concentration of the stock TTAOH solution was determined by acid-base titration with 0.10 mol‚L-1 HCl to be 122.1 mmol‚L-1. The critical micelle concentration (cmc) of TTAOH was determined to be 1.8 mmol‚L-1. Phase Diagram. On the basis of observations collected on more than 150 samples with cTTAOH < 100 mmol‚L-1 after equilibration for 10 weeks, the phase diagram of TTAOH/ HDEHP/H2O at 25.0 ( 0.1 °C was established. Phase boundaries were delineated based on visual observations, which remained unchanged over an extended period of time, and were also carefully determined by conductivity measurements. Conductivity Measurements. The conductivity measurements were performed on a DDSJ-308A (China) “conductivity meter” at 25.0 ( 0.1 °C. Two-phase samples were stirred during the conductivity measurements. Cryo-TEM Images. Carbon film grids with a hole size between 1 and 12 µm were used for specimen preparation. A drop of the sample solution was put on the untreated coated TEM grid (copper grid, 3.02 mm, 200 mesh). Most of the liquid was removed with blotting paper, leaving a thin film stretched over the holes. The specimens were instantly shock frozen by plunging them into liquid ethane in a temperature-controlled freezing unit (Zeiss, Oberkochen, Germany). After freezing the specimens, the remaining ethane was removed using blotting paper. The specimens were inserted into a cryo-transfer holder (Zeiss, Oberkochen, Germany) and transferred to a Zeiss CEM 902, equipped with a cryo stage. Examinations were carried out at a constant temperature of 90 K. The TEM was operated at an accelerating voltage of 80 kV. Zero-loss filtered images (∆E ) 0 eV) were taken under low dose conditions. Rheological Measurements. The rheological properties of the LR phase samples were measured using a dynamic shear rheometer with a concentric cylinder measurement cell (Rheostress RS75 HAAKE). The slit between the inner cylinder and the outer cylinder was 3 mm. Samples were placed in the temperature-controlled measurement vessel and allowed to equilibrate to the required temperature (25 °C) for 5 min prior
Figure 1. Phase diagram of TTAOH/HDEHP/H2O system at T ) 25.0 ( 0.1 °C. The phase at the dilute region is denoted by U1. The phase denoted by U2 is at near equimolar ratio of TTAOH and HDEHP, which is very narrow and sensitive. With a small amount of HDEHP, the phase could be transferred into a two-phase microemulsion (ME)/L1.
to the measurements. An oscillatory amplitude testing was used to measure the magnitude of the elastic modulus (G′), viscous modulus (G′′), and phase angle (δ) of the system. Results and Discussion Considering the most widely studied and used acidic organophosphorus extractant for rare earths, bis(2-ethylhexyl) phosphate, generally known as di(2-ethylhexyl) phosphate (D2EHPA, HDEHP, DEHPA, EHPA),17,18 and marketed variously as Hostarex PA 216, DP-8R, and P-204, i.e., the important applications in the hydrometallurgical industry due to the short phase-separation time, very high recovery rate, the easy recycle, and, of course, the prevalent composites of its sodium salt (NaDEHP) for microemulsion formation, we worked on the phase diagram of the salt-free ternary extractant and surfactant system HDEHP/TTAOH/H2O. On the basis of observations collected on more than 150 samples with cTTAOH < 100 mmol‚L-1 after equilibration for 10 weeks, the phase diagram of the system HDEHP/TTAOH/H2O at 25.0 ( 0.1 °C was established (Figure 1). Phase boundaries were delineated based on visual observations of sample solutions, which remained unchanged over an extended period of time, and were also carefully determined by conductivity measurements. A viscous and anisotropic phase with densely packed, large in size, and remarkably deformed multilamellar and oligovesicular vesicles was observed by cryo-TEM observations, which is the first time the vesicle phase is constructed through combining the hydrophobic organic extractant having double chains with surfactant. The observations of the remarkably deformed vesicle phase of the new extractant and surfactant mixtures may prove valuable and stimulating to fellow specialists, not only because “true” catanionic surfactant systems do not seem to be exhaustively investigated yet, but also because the potential applications of extraction technology should be attempted. The salt-free catanionic mixed solutions were prepared by dissolving HDEHP slowly into TTAOH micelle solution. HDEHP is a monacid with pKa ) 3.57,17c consisting of two short hydrocarbon chains bonded to the phosphate headgroup and almost does not dissolve in water. In the micelle solution of cationic TTAOH prepared by anion exchange from the commercial bromide form TTABr at 40.0 ( 1.0 °C, HDEHP can well dissolve due to acid-basic reaction (Figure 2). From the phase diagram (Figure 1), for 100 mmol‚L-1 TTAOH micelle solution mixing with HDEHP, one could distinguish the single transparent, low-viscosity solution, i.e., the L1 phase (spherical micelle phase) at cHDEHP < 58.0
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Figure 2. Schematic reaction between TTAOH and HDEHP. The saltfree catanionic mixtures form due to the counterions H+ and OH-.
Figure 3. Cutting ternary phase diagram of TTAOH/HDEHP/H2O system at cTTAOH ) 100 mmol‚L-1. Conductivity as a function of HDEHP concentration was inserted.
mmol‚L-1. After the L1 phase, L1/LR phase was observed at the range of cHDEHP ) 58.0-74.0 mmol‚L-1; the upper LR phase was birefringent and bluish. Between cHDEHP ) 74.0 and 100.0 mmol‚L-1, a very stable and viscoelastic single LR phase was found; the LR phase was birefringent and slightly bluish. A typical LR-phase sample with polarizers was inserted in Figure 1, which represents the oily streaks, indicating the lamellar structures. When cHDEHP reached 100.0 mmol‚L-1, i.e., TTAOH and HDEHP were exactly equimolar, the solution suddenly lost its viscoelastic properties (the phase is denoted by U2). Finally, with a little addition of HDEHP the single phase (U2 phase) separated into ME/L1 phase (ME represents microemulsion). A cutting ternary phase diagram of the system TTAOH/HDEHP/ H2O at 25 °C was shown in Figure 3, in which the conductivity data were inserted. The concentration of TTAOH was kept at 100 mmol‚L-1, and the ratio of the TTAOH and HDEHP was changed. One can observe that the conductivity quickly decreased due to the neutralization of OH- of TTAOH by H+ of HDEHP, and the conductivity reached a very low value at the LR phase region. The microstructures of the birefringent LR phase were determined via cryogenic transmission electron microscopy (cryo-TEM) observations. Typical pictures for a LR phase sample of 90 mmol‚L-1 TTAOH/80 mmol‚L-1 HDEHP are given in Figure 4, where one sees that the birefringent LR phase consists of vesicles. The vesicles are remarkably deformed (meaning largely soft vesicles) and densely stacked multilamellar vesicles; the size of the vesicles is polydispersed. The inter-
Figure 4. Cryo-TEM images for a typical LR-phase sample: 90 mmol‚L-1 TTAOH/80 mmol‚L-1 HDEHP.
lamellar spacing between two adjacent bilayers or the interval of two adjacent vesicles is around 12 nm. Observed in more detail for the cryo-TEM photographs (Figure 5) of the birefringent LR phase, interesting structures could be distinguishable. These structures should form among apposed vesicles: (1) Oligovesicular vesicles located in the lower left corner of Figure 5c can be seen. (2) Tubes (jamming the tube arrays in the end by black arrows in Figure 5a-c) by aggregation in string between apposed bilayers, Siegel et al.19 proposed that the pairs of inverted micellar structures could also form structures known as line defects (LDs) that rapidly elongate and align into inverted hexagonal (Hll) phase-tube arrays. Herein the typical tubes joining with multilamellar vesicles were observed (Figure 5d) clearly, indicating that the structures known as LDs could form due to the fusion of vesicles. (3) Bilayer cylinders with hemispherical and end caps were observed (indicated by white arrows); some cylinders are straight but some have a crescent shape (indicated by white arrowheads).20 The reason to form different aggregates in a single phase could be interesting, which should relate to a spontaneous curvature, R0, and a positive Gaussian curvature modulus, kh.7b
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Figure 5. Novel structures observed from cryo-TEM images of the typical LR-phase sample. Image d shows clearly tube arrays joining with vesicles.
A great number of vesicle membranes have large-scale planar regions (zero curvature) where the bilayers consist of two completely symmetric monolayers.5,21 The size of the vesicles presents a high polydispersity ranging from several tens of nanometers to more than several micrometers; moreover, zeroand nonzero-curvature regions coexist in the membranes. The polydispersity both in the size and in the radius of membrane curvature as well as the multilamellar property are associated with a very small bending elastic constant κ.9,22 However, the mechanism of the small bending elastic constant has not been well understood on the molecular scale at present. We speculate that the small bending elastic constant is related to the mixing of TTAOH and HDEHP, which have one long hydrocarbon chain and two short chains, respectively. The spacing distances between bilayers are very small, leading to highly compact multilamellae. Therefore, the tubes should be constructed due to the fusion of two adjacent vesicles. A similar observation of densely stacked multilamellar vesicles occurs in the TTAOH/ OA (oleic acid) aqueous solution, only with a less separate onion phase.14c Another feature of these multilamellar vesicles is that the outer bilayers are usually very flat and smooth, while the inner ones show somewhat thermal fluctuations. This could probably be attributed to the larger curvature free energy of the inner layers than that of the outer ones and the smaller spacing distance between the outer bilayers than that of the inner ones. The
smaller spacing, the greater magnitude of van der Waals attraction between bilayers, and the undulation effect of membranes would possibly be suppressed by the considerably tighter and ordered packing due to relatively stronger van der Waals attraction to some extent. Interestingly, from the cryoTEM image (Figure 5), a number of giant oligovesicular vesicles can also be clearly seen. In the absence of excess salt, ionic strength is relatively smaller and the Debye screening length is relatively larger (4 mmol‚mL-1 and 3.5 nm, respectively, at the current concentration). Thus the Coulomb interaction between membranes is not well screened and the electrostatic repulsion should be considered. From the cryo-TEM images, however, the spacing distance between the bilayers of multilamellar vesicles as well as the bilayers of two adjacent vesicles are slightly larger in comparison with twice the Debye screening length (9-12 nm vs 7 nm). Therefore, the electrostatic repulsion is very weak between two adjacent multilamellar vesicles. On the other hand, due to being a short-range force, the van der Waals attraction between mutilamellar vesicles is also very weak. This well explains the weakness of the elastic behavior of the novel structures. From the cryo-TEM images, the morphology of the outer membranes is quite different from that of the inner ones in each vesicle. For outer membranes, it presents a very compact lamellar structure, but for inner ones, the interlamellar spacing is relatively loose. Probably, more extra positive TTA+ ions locate
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Figure 6. Model of densely stacked multilamellar vesicles with both the outer membranes and the inner ones. Because of the relatively weak interattraction, the large deformation of vesicles against each other occurs under a low imposed stress. A rough sketch of a deformed bilayer membrane segment, with coexisting zero- and nonzero-curvature regions, is also given.
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Figure 8. Elastic modulus G′, viscous modulus G′′, and phase angle (1.0 Hz) for 90 mmol‚L-1 TTAOH/80 mmol‚L-1 HDEHP as a function of imposed stress. The yield stress is defined as the crossover point, and the value is about 5.0 Pa. T ) 25.0 ( 0.1 °C.
only be attributed to relatively weak interaction between vesicles. Comparing the rheological properties in Figure 8 with those of the zero-charged or charged bilayer membrane systems reported in our previous measurements,15,16 one can see the different rheological properties for the new salt-free catanionic system of the excellent extractant and surfactant mixtures, which probably have potentially functional applications such as the design of templates for polymerization, mesoporous silicates, etc. Conclusions Figure 7. Shear stress (σ) and apparent viscosity (η) with increasing shear rate (γ˘ ) for a LR-phase sample of 90 mmol‚L-1 TTAOH/80 mmol‚L-1 HDEHP aqueous solution.
at the inner membranes, which leads to relative higher repulsion and a larger spacing distance. A typical structural model based on the analysis of experimental data is shown in Figure 6. Rheological properties provide useful information about the microstructure change. The rheogram, as shown in Figure 7, is a finite stress (yield stress) required to achieve flow; the apparent viscosity decreases with increasing shear rate. Thus the rheology curve accords with the Herschel-Bulkley model, indicating shear thinning solution properties. Oscillatory amplitude tests were carried out, in which G′ is the elastic modulus that represents the ability of a deformed solution to “snap back” to its original geometry, G′′ is the viscous modulus that represents the tendency of a material to flow under an applied stress, and δ, the phase shift or phase angle, is determined by the equation tan(δ) ) G′′/G′. As δ varies from 0 to 90°, the rheological property transits from solid to liquid. The transition state between liquid and solid is called viscoelasticity. An amplitude sweep experiment was carried out for 90 mmol‚L-1 TTAOH/80 mmol‚L-1 HDEHP, as shown in Figure 8. At an oscillation frequency of 1.0 Hz, the elastic modulus G′ (with typical value of 25 Pa) always exceeded the viscous modulus G′′ (with typical values of 0.8-2 Pa) and the phase angle was close to zero independent of imposed stress below the yield stress, indicating that the system behaved as elastic solids. Although the solidlike behavior dominates the liquidlike one, this system did not exhibit macroscopic gel behavior as expected because of the small elastic modulus G′. The yield stress σy was 5.0 Pa, indicating that the system began to flow at a low imposed stress with the phase angle increasing to 90° rapidly. Due to the extremely low yield stress, the system even began to flow under its own gravity. It is strange that although the multilamellar vesicles are highly stacked, both the elastic modulus and the yield stress are rather small. This can
A new salt-free catanionic extractant and surfactant system was studied, in which Coulomb interaction is not well screened between colloids. A very stable and weak viscous and elastic aqueous solution composed of multilamellar and oligovesicular vesicles, bilayer cylinders, and tubes joining with vesicles were determined by cryo-TEM images. Different from the system studied by Zemb and Dubois,11,12 where extra charges focus either on the edges of nanodisks or on the microholes of icosahedra, we conclude that, in the current system, more extra charge carried by excess cationic surfactants locates at the inner membranes of multilamellar vesicles. Besides, these inner membranes show, more or less, thermal fluctuations because of the relatively large spacing between the membranes. We expect that the fascinating structure obtained in the current study can be of theoretical and practical interest and could be expected to have potential application in vesicle-phase extraction technology. Acknowledgment. This paper was financially supported by the NSFC (Grant 20625307). We are grateful to Professor Dr. Heinz Hoffmann for discussions and Dr. Markus Drechsler for cryo-TEM measurements at Bayreuth Universita¨t, Germany. References and Notes (1) Vesicles; Rosoff, M., Ed.; Surfactant Science Series 62; Marcel Dekker Inc.: New York, 1996. (2) Handbook of Biological Physics; Lipowsky, R., Sackmann, E., Eds.; Elsevier: Amsterdam, 1995; Vol. 1. (3) Kaler, E. W.; Murthy, A. K.; Rodriguez, B. E.; Zasadzinski, J. A. N. Science 1989, 245, 1371-1374. (4) Hao, J.; Hoffmann, H. Curr. Opin. Colloid Inter. Sci. 2004, 9, 279293. (5) Safran, S. A.; Pincus, P.; Andelman, D. Science 1990, 248, 354356. (6) Kaler, E. W.; Herrington, K. L.; Murthy, A.; Zasadzinski, J. A. N. J. Phys. Chem. 1992, 96, 6698-6707. (7) (a) Sackman, E.; Lipowsky, R. Handbook of biological physics; Hoff, A. J., Ed.; North-Holland: Amsterdam, 1995; Vol. 1B. (b) Jung, H. T.; Lee, S. Y.; Kaler, E. W.; Coldren, B.; Zasadziski, J. A. N. Proc. Natl.
Self-Assembled Structures Joining with Vesicles Acad. Sci. U.S.A. 2002, 99, 15318-15322. (c) Nieh, M. P.; Harroun, T. A.; Raghunathan, V. A.; Glinka, C. J.; Katsaras, J. Biophys. J. 2004, 86, 2615-2629. (8) Horbaschek, K.; Hoffmann, H.; Hao, J. J. Phys. Chem. B 2000, 104, 2781-2784. (9) Jung, H. T.; Coldren, B.; Zasadzinski, J. A. N.; Iampietro, D. J.; Kaler, E. W. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 1353-1357. (10) Hyde, S.; Andersson, S.; Larsson, K.; Landh, T.; Lidin, S.; Ninham, B. W. The language of shape; Elsevier: New York, 1997. (11) Zemb, Th.; Dubois, M.; Deme´, B.; Gulik-Krzywicki, T. Science 1999, 283, 816-819. (12) Dubois, M.; Deme´, B.; Gulik-Krzywicki, T.; Dedieu, J. C.; Vautrin, C; De´sert, S.; Perez, E.; Zemb, Th. Nature 2001, 411, 672-675. (13) Dubois, M.; Lizunov, V; Meister, A; Gulik-Krzywicki, T.; Verbavatz, J. M.; Perez, E.; Zimmerberg, J.; Zemb, Th. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 15082-15087. (14) (a) Hao, J.; Liu, W.; Xu, X.; Zheng, L. Langmuir 2003, 19, 1063510640. (b) Hao, J.; Hoffmann, H.; Horbaschek, K. Langmuir 2001, 17, 4151-4160. (c) Song, A.; Dong S.; Jia, X.; Hao, J.; Liu, W.; Liu, T. Angew. Chem., Int. Ed. 2005, 44, 4018. (d) Li, X.; Dong, S.; Jia, X.; Song, A.; Hao, J. Chem.sEur. J. 2007, http://dx.doi.org/10.1002/chem.200700778. (e) Hao, J.; Li, H.; Liu, W.; Hirsch, A. Chem. Comun. 2004, 5, 602-603. (f) Yuan, Z.; Hao, J.; Hoffmann, H. J. Colloid Interface Sci. 2006, 302, 673-681. (g) Hao, J.; Yuan, Z.; Liu, W.; Hoffmann, H. J. Phys. Chem. B 2004, 108, 19163-19168.
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