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Insight into the Intermolecular Interactions in [Bmim]BF4/[Amim]Cl-Ethanol-Water Mixtures by Near-Infrared Spectroscopy Bo Wu, Yumei Zhang, and Huaping Wang* State Key Laboratory for Modification of Chemical Fiber and Polymer Materials, Donghua UniVersity, Shanghai 201620, China ReceiVed: June 4, 2009; ReVised Manuscript ReceiVed: July 12, 2009
In this contribution, we examined the effect of ethanol on the structural organization of aqueous solution of ionic liquids (ILs) using the near-infrared (NIR) technique and two-dimensional (2D) correlation spectra. The ILs used here are 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF4) and 1-allyl-3-methylimidazolium chloride ([Amim]Cl). It was easily found, from the distinct change tendency of NIR spectra for their aqueous solution, that the added ethanol exerted a different effect on the solution structure of [Bmim]BF4 and [Amim]Cl. For [Amim]Cl/H2O, it was deduced that ethanol molecules prefer to compete with water by interacting with imidazolium C2-H rather than C4,5-H groups, accompanied by the formation of C2-H · · · O interactions with ethanol molecules, while ethanol molecules do not interact specifically with any imidazolium C-H groups for [Bmim]BF4. Furthermore, it was shown that the nonpolar tail of [Amim]Cl is more sensitive to the decrease of polarity or dielectric constant of solvents than its polar head, whereas the converse is true for [Bmim]BF4. However, for both ILs, ethanol molecules were capable of changing the interaction between cations and water, anions and water by introduction of C-H · · · O interactions with cations, as well as the strong hydrogen-bond interactions between the anions and the hydroxyls of the ethanol. This information is suggestive for recycling the hydrophilic ILs by distillation from their aqueous diluted solutions, as well as for purifying hygroscopic ILs. Introduction It is well accepted that pure imidazolium-based ionic liquids (ILs) can be described as polymeric hydrogen-bonded supramolecules, and in some cases when mixed with other molecular solvents (MS) they should be better regarded as nanostructured aggregates with polar and nonpolar regions rather than homogeneous solvents.1 To elucidate their heterogeneity, both experimental2-32 and theoretical works33-40 have shed light on the interactions between ILs and MS. However, it is worthy to note that most of these abovementioned works are limited to the simulations results, and/ or the measurement is only limited to the mixture of ILs and a small amount of MS. As mentioned above, ILs are highly ordered hydrogen bonded materials, and hydrogen bonding is most straightforwardly exhibited in IR or Raman spectra. Vibrational spectroscopy is a very powerful tool to explore the structural and dynamic properties of the mixtures of ILs and MS. Unfortunately, these studies are very limited,22-30 in particular, for research focusing on the interaction of ILs-MS in the MS-rich region. This may be due to lack of a suitable technique that has noninvasive and in situ capabilities without any pretreatment of samples. Near-infrared (NIR) spectrometry can offer a solution to this problem because of its noninvasiveness, nondestructiveness, as well as its sensitivity to the overtone and combination transitions of the C-H, O-H, and N-H groups. Hence, this technique has been used to probe intermolecular interactions in mixtures of ILs and water in our earlier paper,32 where insight into interactions between ILs and water over the whole concentration range was provided. As a continuation, the aim of this work is to investigate the effect of * Corresponding author. E-mail:
[email protected]. Tel.: +86-2167792957. Fax: +81-21-67792958.
ethanol on the interactions between ILs (1-butyl-3-methylimidazolium tetrafluoroborate, [Bmim]BF4, and 1-allyl-3-methylimidazolium chloride, [Amim]Cl) and water at the molecular level by NIR, which has never been reported. Furthermore, generalized two-dimensional (2D) correlation analysis, proposed by Noda in 1993,41 was employed to simplify the overlapped peaks of the complex spectra and discern the specific sequential order of the spectral intensity changes under the influence of ethanol content perturbation, hence providing some dynamic properties of the systems investigated. This information about the interactions between ILs and MS is of great importance for better understanding the structural organization of ILs in MS and for the recovery of hydrophilic ILs from their aqueous solution.42-44 Experimental Section Materials. The chlorobutane, allyl chloride, 1-methylimidazole, ethyl acetate, acetone, and NaBF4 are all purchased from Shanghai Chemical Reagents Company. They are of analytic grade and used as received. Doubly distilled water was used in all experiments. The details about the synthesis of [Amim]Cl and [Bmim]BF4, as well as the NIR measurements, were described elsewhere.32 Two-Dimensional Correlation Analysis. Spectra recorded in NIR experiments were divided into two wavelength ranges, 900-1300 and 1300-1800 nm, respectively. The generalized 2D correlation analysis was conducted with the 2D Shige software created by Shigeaki Morita (Kwansei-Gakuin University, Japan). Results and Discussion In our previous work, it was found that [Amim]Cl maintained its supermolecular structure to a great extent when the mole
10.1021/jp905252j CCC: $40.75 2009 American Chemical Society Published on Web 08/17/2009
[Bmim]BF4/[Amim]Cl-Ethanol-Water Mixtures
Figure 1. NIR spectra of the [Amim]Cl/H2O/ethanol mixture with mole fraction of [Amim]Cl ) 0.0734.
fraction of water was less than 0.4178. Above this concentration of water, the hydrogen-bonding network was broken, and [Amim]Cl almost dissociates into free ions in the case of xwater ) 0.9266. To explore the solvent effect on the structural organization of IL in water, we add ethanol to the aqueous solution of [Amim]Cl. To facilitate comparison, we maintained the same composition; i.e., the mole fraction of solvents (water + ethanol) ) 0.9266, and x[Amim]Cl ) 0.0734. In Figure 1, there are several remarkable features. First, it is shown from the inset that the intensity of the band at ca. 970 nm assigned to the O-H bond decreases with an increase of ethanol content, accompanied by a red shift. The red-shift phenomenon can also be observed at band ca. 1152 nm (alkyl C-H group), the intensity of which also decreases. This not only is due to the decrease of water content but also indicates that ethanol molecules are capable of changing the interaction between alkyl C-H and water, alkyl C-H and Cl-, as well as water and Cl-. In other words, ethanol molecules do participate in the interaction with the alkyl C-H group and anion of [Amim]Cl. Another interesting feature is the change of split peak at 1400-1530 nm with the addition of ethanol. This may be a consequence of the C-H · · · O interactions between imidazolium C-H groups and ethanol molecules. In other words, ethanolimidazolium C-H interactions destroy the complex cation · · · HOH · · · anion and thus reduces the dissociation degree of [Amim]Cl in water. Meanwhile, it is worthy to note in Figure 1 that no blue shift or red shift at band ca. 1628 nm occurred with increasing ethanol content but was accompanied by a decrease of peak intensity. On the contrary, the peak at ca. 1698 nm undergoes a red-shift and an enhancement of the peak strength. This indicates that there are no ethanol molecules competing with water in interaction with imidazolium C4,5-H groups. However, addition of ethanol is not favorable for water molecules to interact with C2-H because of the C2-H · · · O interactions with ethanol molecules and the strong hydrogen-bond interactions between Cl- and the hydroxyls of the ethanol. Simply speaking, the hydrogen-bond cooperative and geometric effect of water and ethanol may be the result of this unique behavior. This also corroborates our previous study that water prefers to interact with imidazolium C2-H groups rather than C4,5-H groups.32 The most remarkable feature is the isosbestic point at ca. 1735 nm, which implies that a complicated associated complex forms with the addition of the ethanol. This corroborates our previous
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Figure 2. NIR spectra of the [Bmim]BF4/H2O/ethanol mixture with mole fraction of [Bmim]BF4 ) 0.0734.
conclusion that ions tend to associate in the presence of ethanol. Meanwhile, the blue shift of the band at ca. 1788 nm with ethanol content, concomitantly with a decrease of peak strength, evidently suggests the formation of large water-ion-ethanol clusters in a mixture of water-[Amim]Cl-ethanol. To investigate the effects of cation and anion on the interactions between ILs and MS, we also investigated the effect of ethanol on interaction between water and [Bmim]BF4. Comparison of Figure 2 (including inset) with the spectral for the [Amim]Cl/H2O/ethanol mixture (Figure 1) clearly shows some similar features. For example, the trend of band at ca. 970 nm also decreases with an increase of ethanol content, accompanied by a red shift. The red-shift phenomenon can be also observed at band ca. 1180 nm, even though its intensity almost undergoes no change. This indicates that ethanol molecules prefer to interact with water and BF4- rather than with the alkyl C-H group. In other words, ethanol molecules are unfavorable around the alkyl chain of [Bmim]BF4. This difference is a result of their geometry difference according to which Cl- is planar with the imidazolium ring, while BF4- is positioned on top of the imidazolium ring.32 Apart from these similarities, the peak at band ca. 1628 nm also suffers from no appreciable changes except for the decreased intensity with increasing ethanol content. Likewise, a new peak at ca. 1700 nm (first overtone of the aromatic C2-H groups) appears, but it does not undergo a red shift or blue shift. This indicates that ethanol molecules do not interact specifically with any imidazolium C2-H or C4,5-H groups. This difference between two systems studied may also result from the geometry difference of [Bmim]BF4 and [Amim]Cl, but more importantly, it corroborates our speculation that water molecules prefer to interact with BF4- which does not interact specifically with aromatic C-H groups, whereas the converse is true for Cl-.32 Finally, in Figures 1 and 2, one sees clearly an isosbestic point at ca. 1735 nm, which implies that a complicated associated complex forms with the addition of the ethanol. This corroborates our previous conclusion that ions tend to associate in the presence of ethanol. We can speculate that there must exist equilibrium among free ions, ion pairs, and higher aggregates in a mixture of ILs/H2O/ethanol. The exact composition of various species needs further research, and this work is in progress. The above discussion is based on the information exhibited via one-dimensional NIR spectra, and their most important
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Wu et al.
Figure 3. Two-dimensional synchronous (a) and asynchronous (b) correlation spectra of the [Amim]Cl/H2O/ethanol mixture in the region of 900-1300 nm.
Figure 4. Two-dimensional synchronous (a) and asynchronous (b) correlation spectra of the [Amim]Cl/H2O/ethanol mixture in the region of 1300-1800 nm.
drawback is that it has no capability to show the dynamic process of ILs dissolved in MS because we are not able to identify the overlapped bands as well as the sequence of spectral intensity changes with the concentration of MS, which can be solved with the help of 2D NIR. The white and gray areas in the 2D-IR correlation spectra, respectively, represent positive and negative correlations. The environment of the ILs and water can be reflected through changes of overtone and combination of the C-H group and O-H group, respectively. To elucidate the effect of water and ethanol on the structural organization of ILs, we address our discussion in the range of 900-1300 and 1300-1800 nm. Figure 3 shows two-dimensional correlation spectra of [Amim]Cl/H2O/ethanol solutions in the region of 900-1300 nm. Two autopeaks can be observed at 970 and 1152 nm in a synchronous spectrum. As stated above, the autopeak manifests that the corresponding peak changes greatly with chemical environment variations. Hence, the autopeaks at 970 and 1152 nm, corresponding, respectively, to the O-H group of water and the C-H group of the alkyl chain indicate that both bands are susceptible to ethanol effect, as expected. What is more, there are two positive cross peaks at Ψ(970, 1152) and Ψ(1152, 970), suggesting that intensity variations of the two peaks at 970 and 1152 nm with ethanol content are in the same direction. The asynchronous spectrum shown in Figure 3b is somewhat noisy, implying that water is not homogenously around the alkyl chain of ILs, and different small aggregates of water molecules are found. Figure 3b also shows four major negative cross peaks, Ψ(924, 970), Ψ(924, 1152), Ψ(1062, 970), and Ψ(1062, 1152). It is noticeable that the bands at 924 and 1062 nm are not observed in one-dimensional NIR, whereby we speculate there are three different water O-H involved conformations in this [Amim]Cl/H2O/ethanol solution, represented by peaks appearing at 924, 970, and 1062 nm, respectively. According to Noda’s rule,41 the sequence of the related changes of the O-H bonds is 1062 (O-H)1 > 970 (O-H)2 > 924 (O-H)3. Because 1062 nm is between 970 and 1152 nm, we can assign (O-H)1 to the hydrogen bonds between alkyl C-H and H2O/ethanol. (O-H)2 is formed between water-water aggregates, and (O-H)3 is formed between water and ethanol. Therefore, the sequence of the band intensity changes with the water/ethanol ratio changes can be discerned as follows: 1152 nm (alkyl C-H group) > 1062 (O-H)1 > 970 (O-H)2 > 924 (O-H)3. This indicates water to be present in three different states around the alkyl C-H group of [Amim]Cl, which is sensitive to change of ethanol content. This is consistent with our above interpretation that ethanol molecules are capable of changing the interaction between alkyl C-H and water.
The 2D synchronous and asynchronous spectra of [Amim]Cl/ H2O/ethanol solutions in the region of 1300-1800 nm are displayed in Figure 4. In the synchronous spectrum (Figure 4a), two strong autopeaks are found at 1404 and 1530 nm, as well as one weak autopeak at 1698 nm, implying both bands at ca. 1404 and 1530 nm are susceptible to solvent effect. The two bands may be as a result of the peak splitting at ca. 1450 nm, accompanied, respectively, by the formation of ion clusters and hydrated free ions. The positive cross peaks at Ψ(1404, 1530), Ψ(1530, 1404), Ψ(1530, 1650), and Ψ(1650, 1530) and negative cross peaks at Ψ(1404, 1698), Ψ(1698, 1404), Ψ(1530, 1698), and Ψ(1698, 1530) demonstrate that intensity variations of the three peaks at 1404, 1530, and 1650 nm take place in the same direction (both increase or decrease) under the perturbation of ethanol, while intensities of the two peaks at 1404 (or 1530) nm and 1698 nm change in opposite directions (one increases while the other one decreases). The asynchronous spectrum in Figure 4b shows nine major negative cross peaks, Ψ(1404, 1422), Ψ(,1404, 1488), Ψ(1404, 1506), Ψ(1422, 1698), Ψ(1506, 1698), Ψ(1530, 1422), Ψ(1530, 1488), Ψ(1650, 1422), and Ψ(1650, 1506). Again, based on Noda’s rule,41 the band at 1404 nm comes from ion clusters different from that of bands at 1422, 1488, and 1506 nm; meanwhile, the band at 1698 nm due to the C2-H group is different from bands at 1422 and 1506 nm; additionally, the band at 1530 nm comes from hydrated ions different from that of the bands at 1422 and 1488 nm; and finally, the band at 1650 nm due to C4,5-H is different from bands at 1422 and 1506 nm. No correlation peak at 1422 and 1506 nm is observed, and therefore, we could deduce that bands at 1422, 1488, and 1506 nm may originate from three different classes of ion pairs. The sequence of the band intensity changes with the water/ethanol ratio. These changes can be discerned as follows: 1698 nm (C2-H) > 1422, 1488, 1506 nm (ion pairs) > 1404 nm (ion clusters), 1530 nm (hydrated ions), 1650 nm (C4,5-H). On the basis of this sequence, it is expected that the network structures or ionic clusters of ILs would have higher C-H vibrational frequencies than that of the ionic pair of ILs in vibration spectroscopy, which is consistent with the literature.27 Likewise, the 2D correlation analysis shown in Figure 5 was also conducted for [Bmim]BF4/H2O/ethanol mixtures in the 900-1300 nm region. Compared to Figure 3a, the most remarkable difference is that only one autopeak (1180 nm corresponding to alkyl C-H group) is observed in Figure 5a. This indicates that only the intensity of the peak for the alkyl C-H group changes evidently with mole fraction of ethanol. However, from the inset in Figure 2, one can see that the intensity of peaks for the alkyl C-H group and water both changed with addition of ethanol. The reason is not clear. We
[Bmim]BF4/[Amim]Cl-Ethanol-Water Mixtures
Figure 5. Two-dimensional synchronous (a) and asynchronous (b) correlation spectra of the [Bmim]BF4/H2O/ethanol mixture in the region of 900-1300 nm.
J. Phys. Chem. B, Vol. 113, No. 36, 2009 12335 [Bmim]BF4 due to ethanol-BF4- interation, while the ethanol molecules are planar with the imidazolium ring of [Amim]Cl. In other words, the polar head of [Bmim]BF4 is more sensitive to the decrease of polarity or dielectric constant of solvents, which is associated with the ion solvation. From the above discussion, a conclusion may be drawn that, irrespective of anions and length of alkyl chain, the addition of ethanol weakens the dissociation of ILs in water. As a result, some big ionic clusters of ILs are coming into being, and even the network structure of ILs regains to a great extent. The process can be roughly described as follows: With the continuous increase of ethanol amount, the anion and cation of ILs surrounded by water molecules release due to the ion solvation of ethanol, accompanied by the appearance of solvated ionic pairs in water/ethanol solution. The evolution of ionic pairs into ionic clusters and then into network structures to some extent can be clearly detected and visualized in the asynchronous spectrum. Conclusions
Figure 6. Two-dimensional synchronous (a) and asynchronous (b) correlation spectra of the [Bmim]BF4/H2O/ethanol mixture in the region of 1300-1800 nm.
are continuing to investigate the effect of MS in ILs at higher concentrations, and this will be reported elsewhere. Another small difference is that the asynchronous spectrum shown in Figure 5b is not as complicated as that of Figure 3b, indicating that ethanol causes a more significant effect on the nonpolar domain structure of [Amim]Cl/H2O compared with [Bmim]BF4/H2O. In other words, the nonpolar tail of [Amim]Cl is more sensitive to the change of polarity or dielectric constant of solvents because the allyl group is more hydrophobic than the butyl group, and BF4- is more hydrophobic than Cl-. Looking into more details, only two major negative cross peaks, Ψ(960, 1164) and Ψ(1152, 1180), were observed in Figure 5b. Undoubtedly, the band located at 960 nm comes from water species, implying only one molecular state of water is present as also observed by Cammarata.2 This also indirectly reflects that water does not prefer to be around the alkyl chain of [Bmim]BF4. Furthermore, the bands located at 1152 and 1164 nm, which are not readily observable in the one-dimensional spectra due to overlap with the band at 1180 nm, are revealed via asynchronous correlation map. No correlation peak at Ψ(1164, 1180) is observed, which indicates that these two bands correspond to the same alkyl C-H group. It can be concluded that the alkyl C-H group of [Bmim]BF4 has two conformations, and the O-H bond of water has only one state in the nonpolar domain of [Bmim]BF4. Meanwhile, the alkyl C-H group changes prior to the O-H bond. The 2D synchronous and asynchronous spectra of [Bmim]BF4/ H2O/ethanol solutions in the region of 1300-1800 nm are illustrated in Figure 6. At a glance, they are more complicated than that of [Amim]Cl/H2O/ethanol solutions, but they are almost the same if one looks into more detail. On this basis, we can surmise that (i) the presence state of water molecules in the polar domain is identical in both ILs; (ii) ethanol molecules prefer to lie above the imidazolium ring of
In conclusion, the effect of ethanol on the structural organization of aqueous solutions of [Bmim]BF4 and [Amim]Cl was investigated using one-dimensional NIR spectroscopy and 2D correlation analysis. The distinct changing tendency between NIR spectra of [Bmim]BF4 and [Amim]Cl with the increase of ethanol concentration suggested that the added ethanol exerted a different effect on the solution structure of both ILs. It was found that ethanol molecules preferred to compete with water in interacting with imidazolium C2-H rather than C4,5-H groups of [Amim]Cl, accompanied by the formation of C2-H · · · O interactions with ethanol molecules, as well as the strong hydrogen-bond interactions between the Cl- and the hydroxyls of the ethanol, while ethanol molecules do not interact specifically with any imidazolium C2-H or C4,5-H groups of [Bmim]BF4. Furthermore, the nonpolar tail of [Amim]Cl is more sensitive to the decrease of polarity or dielectric constant of solvents than its polar head, whereas the converse is true for [Bmim]BF4. However, irrespective of [Bmim]BF4 or [Amim]Cl, the common observation is that ethanol molecules are capable of breaking the complexes cation · · · HOH · · · anion via the interaction between cation and water and anion and water. These physical insights based on experimental work need to be further theoretically investigated, so our following work will be devoted to DFT calculations in both systems. Nevertheless, the information obtained in this contribution is suggestive for recycling hydrophilic ILs from their aqueous diluted solutions, as well as for purifying hygroscopic ILs. For example, addition of ethanol can break the strong interaction between water and ILs, implying that ethanol added may be capable of entraining water in advance in the course of distillation when water is rich; meanwhile, when water amount is low, water would not be solvated by ILs due to the destruction of the strong interaction between water and ILs. Acknowledgment. We are thankful for financial support from the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (707026), Shanghai Science and Technology Commission (08XD14005), Shanghai Municipal Education Commission DAWN Project (08GG11), and Shanghai Leading Academic Discipline Project (B603). References and Notes (1) Dupont, J. J. Braz. Chem. Soc. 2004, 15, 341–350.
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(2) Cammarata, L.; Kazarian, S. G.; Salter, P. A.; Welton, T. Phys. Chem. Chem. Phys. 2001, 3, 5192–5200. (3) Ko¨ddermann, T.; Wertz, C.; Heintz, A.; Ludwig, R. Angew. Chem., Int. Ed. 2006, 45, 3697–3702. (4) Bowers, J.; Butts, C. P.; Martin, P. J. Langmuir 2004, 20, 2191– 2198. (5) Malham, I. B.; Letellier, P.; Turmine, M. J. Phys. Chem. B 2006, 110, 14212–14214. (6) Dorbritz, S.; Ruth, W.; Kragl, U. AdV. Synth. Catal. 2005, 347, 1273–1279. (7) Vanyur, R.; Biczdk, L. Colloids Surf. A: Physico Chem. Eng. Aspects. 2007, 299, 93–100. (8) Belsic, M.; Marques, M. H. V.; Plechkova, N.; Seddon, K. R. Green Chem. 2007, 9, 481–490. (9) Schro¨der, U.; Wadhawan, J. D.; Compton, R. G.; Marken, F.; Suarez, P. A. Z.; Consorti, C. S.; de Souza, R. F.; Dupont, J. New. J. Chem. 2000, 24, 1009–1015. (10) Zhao, Y.; Gao, S. J.; Wang, J. J. J. Phys. Chem. B 2008, 112, 2031– 2039. (11) Wang, J. J.; Wang, H. Y.; Zhang, S. L. J. Phys. Chem. B 2007, 111, 6181–6188. (12) Zhang, H. C.; Liang, H. J.; Wang, J. J. Z. Phys. Chem. 2007, 221, 1061–1074. (13) Wang, H. Y.; Wang, J. J.; Zhang, S. B.; Xuan, X. P. J. Phys. Chem. B 2008, 112, 16682–16689. (14) Zhu, X.; Wang, Y.; Li, H. R. AIChE J. 2009, 55, 198–205. (15) Mele, A.; Tran, C. D.; Lacerda, S. H. D. Angew. Chem., Int. Ed. 2003, 42, 4364–4366. (16) Singh, T.; Kumar, A. J. Phys. Chem. B 2007, 111, 7843–7851. (17) Rollet, A. L.; Porion, P.; Vaultier, M.; Billard, I.; Deschamps, M.; Bessada, C.; Jouvensal, L. J. Phys. Chem. B 2007, 111, 11888–11891. (18) Rivera-Rubero, S.; Baldelli, S. J. Am. Chem. Soc. 2004, 126, 11788– 11789. (19) Baldelli, S. J. Phys. Chem. B 2003, 107, 6148–6152. (20) Katayanagi, H.; Shimozaki, K.; Nishikawa, H.; Miki, K. J. Phys. Chem. B 2004, 108, 19451–19457. (21) Miki, K.; Westh, P.; Nishikawa, K.; Koga, Y. J. Phys. Chem. B 2005, 109, 9014–9019. (22) Chang, H. C.; Jiang, J. C.; Liou, Y. C. J. Chem. Phys. 2008, 129, 044506. (23) Chang, H. C.; Jiang, J. C.; Chang, C. Y. J. Phys. Chem. B 2008, 112, 4351–4356.
Wu et al. (24) Jeon, Y.; Sung, J.; Seo, C. J. Phys. Chem. B 2008, 112, 4735– 4740. (25) Fazio, B.; Triolo, A.; Di Marco, G. J. Raman Spectrosc. 2008, 39, 233–237. (26) Iimori, T.; Iwahashi, T.; Kanai, K. J. Phys. Chem. B 2007, 111, 4860–4866. (27) Zhang, L. Q.; Xu, Z.; Wang, Y. J. Phys. Chem. B 2008, 112, 6411– 6419. (28) Sun, B. J.; Jin, Q.; Tan, L. S.; Wu, P. Y.; Yan, F. J. Phys. Chem. B 2008, 112, 14251–14259. (29) Dominguez-Vidal, A.; Kaun, N.; Ayora-Canada, M. J.; Lendl, B. J. Phys. Chem. B 2007, 111, 4446–4452. (30) Lopez-Pastor, M.; Ayora-canada, M. J.; Valcarcel, M.; Bendl, B. J. Phys. Chem. B 2006, 110, 10896–10902. (31) Wu, B.; Liu, W. W.; Zhang, Y. M.; Wang, H. P. Chem.sEur. J. 2008, 15, 1804–1810. (32) Wu, B.; Zhang, Y. M.; Wang, H. P. Chem.sEur. J. 2009, 15, 6889– 6893. (33) Hanke, C. G.; Lynden-Bell, R. M. J. Phys. Chem. B 2003, 107, 10873–10878. (34) Hanke, C. G.; Atamas, N. A.; Lynden-Bell, R. M. Green. Chem. 2002, 4, 107–111. (35) Jiang, W.; Wang, Y.; Voth, G. A. J. Phys. Chem. B 2007, 111, 4812–4818. (36) Wang, Y.; Li, H. R.; Han, S. J. J. Phys. Chem. B 2006, 110, 24646– 24651. (37) Kelkar, M. S.; Maginn, E. J. J. Phys. Chem. B 2007, 111, 4867– 4876. (38) Moreno, M.; Castiglione, F.; Mele, A. J. Phys. Chem. B 2008, 112, 7826–7836. (39) Porter, A. R.; Liem, S. Y.; Popelier, P. L. A. Phys. Chem. Chem. Phys. 2008, 10, 4240–4248. (40) Remsing, R. C.; Liu, Z. W.; Sergeyev, I.; Moyna, G. J. Phys. Chem. B 2008, 112, 7363–7369. (41) Noda, I. Appl. Spectrosc. 1993, 47, 1329–1336. (42) Wu, B.; Zhang, Y. M.; Wang, H. P. J. Chem. Eng. Data 2008, 53, 983–985. (43) Wu, B.; Zhang, Y. M.; Wang, H. P. J. Phys.Chem. B 2008, 112, 6426–6429. (44) Wu, B.; Zhang, Y. M.; Wang, H. P. J. Phys.Chem. B 2008, 112, 13163–13165.
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