Role of Solubilized Water in Micelles Formed by Triton X-100 in 1

Mar 19, 2010 - The effects of added water on the microstructure of Triton X-100 in ILs micelles are investigated. When small amounts of water are adde...
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Role of Solubilized Water in Micelles Formed by Triton X-100 in 1-Butyl-3-methylimidazolium Ionic Liquids Na Li, Shaohua Zhang, Hongchao Ma, and Liqiang Zheng* Key Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of Education, Jinan 250100, China Received January 15, 2010. Revised Manuscript Received March 13, 2010 We demonstrate here the aggregation behavior of a nonionic surfactant Triton X-100 in two 1-butyl-3-methylimidazolium ionic liquids (the hydrophilic IL [bmim][BF4] and the hydrophobic IL [bmim][PF6]) by surface tension measurements. The effects of added water on the microstructure of Triton X-100 in ILs micelles are investigated. When small amounts of water are added to Triton X-100 in [bmim][PF6] micelles, the water molecules are first bound to the ethylene oxide (EO) units of Triton X-100 and then form the water pool in the core of the microemulsion. When water molecules are added to the Triton X-100 in [bmim][BF4] micelles, there is no microemulsion formed; these water molecules are first solubilized in [bmim][BF4]. When the solubilization is saturated, the water molecules start to bind to the EO group of Triton X-100; these results are confirmed by UV-vis, FTIR, and 1H NMR spectra.

Introduction The self-assembly of surfactant molecules is of fundamental interest and is important in many applications such as nanoparticle preparation and organic or bioorganic synthesis.1-3 The majority of the aggregation studies use water or volatile organic solvents as the solvent. Recently, attempts have been made to prepare and study aggregates formed in ionic liquids (ILs).4-22 In these aggregates, water and organic solvents have been replaced by ILs. Aggregations in IL solvents have a number of distinct advantages due to the unique chemical and physical properties of *Corresponding author: Tel þ86 531 88366062; Fax þ86 531 88564750; e-mail [email protected].

(1) Schwuger, M.; Stickdorn, K.; Schomaecker, R. Chem. Rev. 1995, 95, 849. (2) Qi, L. M.; J. Ma, M. J. Colloid Interface Sci. 1998, 197, 36. (3) Holmberg, K. Adv. Colloid Interface Sci. 1994, 51, 137. (4) Gao, Y. A.; Li, N.; Zheng, L. Q.; Zhao, X. Y.; Zhang, S. H.; Han, B. X.; Hou, W. G.; Li, G. Z. Green Chem. 2006, 8, 43. (5) Li, N.; Gao, Y. A.; Zheng, L. Q.; Zhang, J.; Yu, L.; Li, X. W. Langmuir 2007, 23, 1091. (6) Eastoe, J.; Gold, S.; Rogers, S. E.; Paul, A.; Welton, T.; Heenan, R. K.; Grillo, I. J. Am. Chem. Soc. 2005, 127, 7302. (7) Li, N.; Zhang, S.; Zheng, L. Q.; Gao, Y. A.; Yu, L. Langmuir 2008, 24, 2973. (8) Gao, Y. A.; Li, N.; Zheng, L. Q.; Zhao, X. Y.; Zhang, J.; Cao, Q.; Zhao, M. W.; Li, Z.; Zhang, G. Y. Chem.;Eur. J. 2007, 13, 2661. (9) Li, N.; Cao, Q.; Gao, Y.; Zhang, J.; Zheng, L. Q.; Bai, X. T.; Dong, B.; Li, Z.; Zhao, M. W.; Yu, L. ChemPhysChem 2007, 8, 2211. (10) He, Y. Y.; Lodge, T. P. Chem. Commun. 2007, 2732. (11) He, Y. Y.; Boswell, P. G.; Buhlmann, P.; Lodge., T. P. J. Phys. Chem. B 2007, 111, 4645. (12) Evans, D. F.; Yamauchl, A.; Roman, R.; Casassa., E. Z. J. Colloid Interface Sci. 1982, 88, 89. (13) Evans, D. F.; Yamauchl, A.; Jason Wel, G.; Bloomfield, V. A. J. Phys. Chem. 1983, 87, 3537. (14) Anderson, J. L.; Pino, V.; Hagberg, E. C.; Sheares, V. V.; Armstrong, D. W. Chem. Commun. 2003, 2444. (15) Fletcher, K. A.; Pandey, S. Langmuir 2004, 20, 33. (16) He, Y.; Li, Z.; Simone, P.; Lodge, T. P. J. Am. Chem. Soc. 2006, 128, 2745. (17) He, Y.; Lodge, T. P. J. Am. Chem. Soc. 2006, 128, 12666. (18) Patrascu, C.; Gauffre, F.; Nallet, F.; Bordes, R.; Oberdisse, J.; de LauthViguerie, N.; Mingotaud, C. ChemPhysChem 2006, 7, 99. (19) Zhang, S. H.; Li, N.; Zheng, L. Q.; Li, X. W.; Gao, Y. A.; Yu, L. J. Phys. Chem. B 2008, 112, 10228. (20) Wu, J. P.; Li, N.; Zheng, L. Q.; Li, X. W.; Gao, Y.; Inoue, T. Langmuir 2008, 24, 9314. (21) Li, N.; Zhang, S. H.; Zheng, L. Q.; Wu, J. P.; Li, X. W.; Yu, L. J. Phys. Chem. B 2008, 112, 12453. (22) Li, N.; Zhang, S. H.; Zheng, L. Q.; Dong, B.; Li, X. W.; Yu, L. Phys. Chem. Chem. Phys. 2008, 10, 4375.

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ILs. They have attracted much interest from both theoretical and practical viewpoints. ILs are a class of organic electrolytes with melting points below 373 K.23 They are nonvolatile, thermally stable, and nonflammable and have extremely high ionic conductivity.24-27 A unique advantage of ILs as solvents compared with traditional solvents is that they can be treated as environmentally benign solvents, since their nonvolatile nature can prevent atmospheric pollution.28-31 Also, their physicochemical properties can be modulated by suitable selection of cation, anion, and cation substituent. These properties of ILs make them extensively desirable in many reactions of industrial importance. Molecular self-assemblies formed in ILs are of great interest and may widen the applications of ILs. Micellar aggregations of surfactants in ILs have been intensively studied recently. Evans et al. reported the aggregation behavior of alkyltrimethylammonium bromides, alkylpyridinium bromides, and Triton X-100 in ethylammonium nitrate (EAN).12,13 Anderson et al.14 reported the dry micelle formation of some traditional surfactants in 1-butyl-3-methylimidazolium ILs. The aggregation behavior of a few common anionic, cationic, and nonionic surfactants in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (emimTf2N) was also probed on the basis of their response to solvatochromic probes.15 Our group investigated the aggregation behavior of pluronic triblock copolymer in ILs19 and then performed thermodynamic investigations on the formation mechanism of two aggregates of polyoxyethylene (20) sorbitan monolaurate (Tween 20) in ILs.20 It has been discovered that long-chain ILs (23) Rogers, R. D.; Seddon, K. R. Science 2003, 302, 792. (24) Welton, T. Chem. Rev. 1999, 99, 2071. (25) Kazarian, S. G.; Briscoe, B. J.; Welton, T. Chem. Commun. (Cambridge, U.K.) 2000, 2047. (26) Anderson, J. L.; Ding, J.; Welton, T.; Armstrong, D. W. J. Am. Chem. Soc. 2002, 124, 14247. (27) Anderson, J. L.; Pino, V.; Hagberg, E. C.; Sheares, V. V.; Armstrong, D. W. Chem. Commun. (Cambridge, U.K.) 2003, 2444. (28) Avery, T. D.; Jenkis, N. F.; Kimber, M. C.; Lupton, D. W.; Taylor, D. K. Chem. Commun. 2002, 1, 28. (29) Zerth, H. M.; Leonard, N. M.; Mohan, R. S. Org. Lett. 2003, 5, 55. (30) Huddieston, J. G.; Willauer, H. D.; Swauoski, R. P.; Visser, A. E.; Rogers, K. D. Chem. Commun. 1998, 16, 1765. (31) Wang, P.; Zakeeruddion, S. M.; Comte, P.; Exnar, I.; Gratzel, M. J. Am. Chem. Soc. 2003, 125, 1166.

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CnmimBr (n = 10-16) can self-assemble in [bmim][BF4].21 Moreover, we studied the different formation mechanisms of a cationic fluorinated surfactant FC-4 self-assemblies in [bmim][BF4] and [bmim][PF6].22 Owing to the nonvolatile nature of ILs, the above aggregates are more stable than the traditional ones at high temperature. Furthermore, as nonaqueous systems, they have a number of distinct advantages over the aqueous ones, and nonaqueous IL micelles can be considered a new class of reaction media. Water molecules solubilized in heterogeneous media have important effects on the chemistry of the system.32,33 The properties of water in confined media have direct relevance to biological systems.34 Moreover, it is important to study the states or properties of solubilized water in surfactant aggregate templates under different conditions to understand how changes in the microenvironment affect the morphology and properties of the prepared materials.35-38 Our previous studies have shown that small amounts of water have a great effect on the microstructure and stability of IL/O microemulsions.8,9 In the present work, the aggregation behavior of Triton X-100 in [bmim][BF4] and [bmim][PF6] was investigated. The effects of a small amount of water on the microstructure of the micelles were investigated by the use of UV/vis, FTIR, and 1H NMR spectroscopic analysis. These results suggested the added water had different locations in different ILs.

Experimental Section Materials. Triton X-100 was obtained from Alfa Aesar and evaporated under vacuum at 80 °C for 2 h to remove excess water before use. Methyl orange (MO) was provided by Beijing Chemical Reagents Company. [Bmim][BF4] and [bmim][PF6] was prepared in our laboratory by the procedure reported in the literature.39 The purity of the product was further checked by 1 H NMR spectroscopy. To avoid water, the containers with the materials were sealed tightly to avoid any further contact with air before use. Water was doubly deionized and distilled. D2O (99.9%) was provided by Aldrich and used as received. Apparatus and Procedures. The surface tension was determined in a single-measurement conducted on the apparatus of a Model JYW-200B surface tensiometer using the ring method. All measurements were repeated at least twice until the values were reproducible. Temperature was controlled using a super constant temperature trough. The UV/vis spectra were measured at 25 °C with a TU-1901UV spectrometer. FTIR spectra were recorded in KBr pellets with a resolution of 2 cm-1 using a BIORADFTS-165 spectrometer. There was no evident sign that the KBr pellets were eroded by the addition of small amounts of water to the investigated system. 1H NMR measurements were carried out with a Varian ARX 400 NMR spectrometer at 25 °C, operated at a frequency of 400.13 MHz.

Results and Discussion Formation of Micelles. Surface tension measurements were performed to detect the aggregation behavior of Triton X-100 in [bmim][BF4] and [bmim][PF6]. Figure 1 shows the surface tension (32) Mele, A.; Tran, C. D.; Lacerda, S. H. D. Angew. Chem., Int. Ed. 2003, 42, 4364. (33) Huang, J. F.; Chen, P. Y.; Sun, I. W.; Wang, S. P. Inorg. Chim. Acta 2000, 320, 7. (34) Nama, D.; Kumar, P. G. A.; Pregosin, P. S.; Geldbach, T. J.; Dyson, P. J. Inorg. Chim. Acta 2006, 359, 1907. (35) Lynden-Bell, R. M.; Atamas, N. A.; Vasilyuk, A.; Hanke, C. G. Mol. Phys. 2002, 100, 3225. (36) Abraham, M. H.; Zissimos, A. M.; Huddleston, J. G.; Willauer, H. D.; Rogers, R. D.; Acree, W. E., Jr. Ind. Eng. Chem. Res. 2003, 42, 413. (37) Noda, A.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B 2001, 105, 4603. (38) Tasaki, K. J. Am. Chem. Soc. 1996, 118, 8459. (39) Dupont, J.; Consorti, C. S.; Suarez, P. A. Z.; Souza, R. F. Org. Synth. 1999, 79, 236.

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Figure 1. Surface tension versus concentration plots obtained for Triton X-100 solutions in [bmim][BF4] (a) and [bmim][PF6] (b) at 25 °C.

Figure 2. FF-TEM images for Triton X-100 in [bmim][BF4]. TX100 content is 40 wt %.

versus concentration plot obtained from the solution of Triton X-100 in [bmim][BF4] and [bmim][PF6]. The pure interfacial tension values for [bmim][BF4] and [bmim][PF6] are 48.4 and 47.5 mN/m, respectively. As can been seen from the figure, micelles are formed in both [bmim][BF4] and [bmim][PF6] solutions. Figure 2 shows the typical FF-TEM images of Triton X-100 in [bmim][BF4] above the cmc (for example). Irregular spherical aggregates 30-50 nm in diameter are formed in [bmim][BF4]. The FF-TEM results further confirmed the formation of the micelle of TX-100 in ILs. Solubilization of Water. The details of water molecules solubilized in heterogeneous media have attracted much attention.40,41 Understanding the properties of water in confined media has a direct relevance to biological systems.42 In addition, it is also (40) Mele, A.; Tran, C. D.; Lacerda, S. H. D. Angew. Chem. 2003, 115, 4500. (41) Huang, J. F.; Chen, P. Y.; Sun, I. W.; Wang, S. P. Inorg. Chim. Acta 2000, 320, 7. (42) Nama, D.; Kumar, P. G. A.; Pregosin, P. S.; Geldbach, T. J.; Dyson, P. J. Inorg. Chim. Acta 2006, 359, 1907.

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Figure 3. (a) Absorption spectra of methyl orange in the Triton X-100/[bmim][PF6] micelles (TX-100 content is 40 wt %) as a function of water content. Probe concentrations from a to i: 1.5  10-5, 3.0  10-6, 4.0  10-6, 5.0  10-6, 6.0  10-6, 7.0  10-6, 8.0  10-6, 9.0  10-6, and 1.0  10-5 M, respectively. (b) Variation of λm of MO in Triton X-100/[bmim][PF6] micelles as a function of water content.

Figure 4. (a) Absorption spectra of riboflavin in Triton X-100/[bmim][PF6] micelles as a function of water content. (b) Maximum absorbance intensities (446 nm) of riboflavin in the Triton X-100/[bmim][PF6] micelles with different water content. (The components of the samples are the same as those in (a).)

important to study the states or properties of solubilized water in micelle media under different conditions in order to control the morphology and properties of materials. When water is added to traditional inverse micelles in hydrocarbon media, microemulsion forms.2,43-45 Zhu and co-worker43 reported when water was added to Triton X-100 in mixed solvent (30% benzene and 70% n-hexane) reverse micelle, it first bound the ethylene oxide (EO) units of Triton X-100 and then formed water pool. Qi et al.2 investigated the inverse micelles of Triton X-100, n-hexanol in cyclohexane using methyl orange (MO) and methylene blue (MB) as absorption probes, and they got the similar results. The effects of water on the sodium bis(2-ethylhexyl)sulfo succinate (AerosolOT, AOT) in isooctane inverse micelle were studied using FTIR spectra.44 Microemulsion was also formed in this system. In the current system, water has high solubility in both [bmim][BF4] and Triton X-100 but low solubility in [bmim][PF6]. In this case, when a small amount of water is added to the Triton X-100/[bmim][BF4] and Triton X-100/[bmim][PF6] inverse micelles, the solubilization sites of the water molecules are still unclear. UV/vis Spectra and FTIR Spectra. The absorption spectra of MO in different solvents have been studied, and the absorption maximum is red-shifted with increasing polarities of the pure solvents.44,45 The electronic transition energy of MO molecules, as reflected in the absorption maximum λmax, is an indication of the “polarity” of the microenvironment where it is located.45 To describe the water molecules’ actual preferential locations, UV/vis spectroscopic analysis with MO as an absorption probe has been applied to determine the solubilization site of added water in the Triton X-100/ILs micelles. (43) Zhu, D. M.; Wu, X.; Schelly, Z. A. J. Phys. Chem. 1992, 96, 7121. (44) Zhou, G. W.; Li, G. Z.; Chen, W. J. Langmuir 2002, 18, 4566. (45) Zhu, D. M.; Schelly, Z. A. Langmuir 1992, 8, 48.

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The effect of successive addition of water on the microenvironment in the Triton X-100/[bmim][PF6] reverse micelles is demonstrated by the MO spectra in Figure 3. When the water content is in the range of 0-1.6%, the absorption maximum λmax of methyl orange is red-shifted from 428 to 434 nm. Further increasing of water content, the absorption maximum λmax of MO is constant at 434 nm, suggesting that the further added water has no effect on λmax. In the current system, MO is not soluble in [bmim][PF6],4 so it binds to the EO group of Triton X-100 whatever the water content is. It has been reported that when water is added to Triton X-100 supported reverse micellar solutions, the water molecules are first bound to the EO units, while after the bound water molecules reach the saturation point, the subsequently added water molecules are present in their free form in water pools.2 In this case, when water is added to the Triton X-100/[bmim][PF6] reverse micellar solution, water molecules are bound to the EO groups of Triton X-100 in the polar cores. Here, MO molecules are solubilized in the EO domain. As the water content increases, the polarity of the EO groups of Triton X-100 is increased, so λmax is red-shifted from 428 to 434 nm. After the bound water reaches equilibrium (water content = 1.6%), subsequently added water molecules accumulate to form water pools in the core of the microemulsion. At this stage, polarity of the EO groups of Triton X-100 is not changed anymore, so the λmax of MO presents a constant value. Saturated riboflavin aqueous solution is used to adjust the water content of the Triton X-100/[bmim][PF6] reverse micelle. Thus, there are a series of water/[bmim][PF6] microemulsions with different water contents. Figure 4a shows the UV-vis spectra for riboflavin in microemulsions with different water contents. Over the wavelength range from 320 to 550 nm, there are two absorbance peaks at 363 ( 2 and 445 ( 1 nm. The maximum absorbance of riboflavin (446 nm) increases with the addition of saturated riboflavin solution, but the plot of maximum DOI: 10.1021/la100215z

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Figure 5. (a) Absorption spectra of methyl orange in the Triton X-100/[bmim][BF4] micelles (TX-100 content is 40 wt %) as a function of water content. Probe concentrations from a to g: 0.5  10-5, 1.0  10-5, 1.5  10-5, 2.0  10-5, 2.5  10-5, 3.0  10-5, and 3.5  10-5 M, respectively. (b) Variation of λm of MO in Triton X-100/[bmim][BF4] micelles as a function of water content.

absorbance peak versus water contents (also the riboflavin concentrations) is not linear (Figure 4b). The maximum absorbances are distinctly low (relative to the dashed line) for the microemulsion with low water contents, such as 0.7, 1.1, and 1.5. The reason is that with the lower water content, the water molecules are bound to the EO group of Triton X-100, and the behavior of bound water in the microemulsion is markedly different from that of bulk water. It is likely that in these samples a portion of the riboflavin was still in the solid particle form, dispersed in these microemulsion droplets.4 Up to water content = 1.8%, the water pools begin to form. The properties of water in pool are close to the bulk water. The result is the same as that when using MO as a probe. Thus, the formation of water pools is further confirmed by the riboflavin; that is, the IL pools begin to appear when the water content is 1.8%, which is close to the value of 1.6%, as revealed by the MO probe. The water location in Triton X-100/[bmim][PF6] inverse micelles is consistent with that in the traditional inverse micelle in hydrocarbon media mentioned above. When water is added to the Triton X-100/ [bmim][BF4] micelles, it is soluble in both Triton X-100 and [bmim][BF4], so no microemulsion forms. This situation is different from that in the traditional micelle. UV/vis spectroscopic analysis used MO as an absorption probe was also applied to investigate the microenvironment of the Triton X-100/[bmim][BF4] micelles. Figure 5a shows the absorption spectra of MO in Triton X-100/ [bmim][BF4] micelles with different water contents, and the variation of λm of MO as a function of water contents is shown in Figure 5b. When the water content is low, the absorption maximum λmax of MO is red-shifted with the increasing of water content, but when the water content reaches 6%, further increase the water content, the λmax of MO keeps constant. This is similar to the [bmim][PF6] system, but the water location of the system is totally different from the [bmim][PF6] system. It is known that MO associates with the EO groups of TX-100.45 However, in this system, MO is also highly soluble in [bmim][BF4]. Our earlier study indicated that when MO was added to a solution of TX-100/ [bmim][BF4] micelles, it is located in the bulk [bmim][BF4] instead of being solubilized in the polar outer layer of the micelles.5 When the water content is in the range of 0-5%, the absorption maximum λmax of MO is red-shifted from 430 to 433 nm. Therefore, the micropolarity of bulk [bmim][BF4] increases with the addition of water. The polarity of water is much higher than that of [bmim][BF4], and the absorption maximum λmax of MO in water is 464 nm. So in this case, water is solubilized in bulk [bmim][BF4]. Unexpectedly, with further addition of water, when the water content is up to 6%, the absorption maximum λmax remains constant at λmax = 434 nm. The result suggests that the micropolarity of [bmim][BF4] does not change with the further addition of water. So these water molecules are not located in the IL, but 9318 DOI: 10.1021/la100215z

rather they must bind to the EO groups of TX-100. For traditional aggregate solutions, bound water molecules interact strongly with the polar groups of surfactants through counterions that are in the vicinity of the interface.46 The polarity of the micelles is eventually dominated by the competitive contribution from two types of water molecules, bound water and free water, that possess similar hydrogen-bonding characteristics to the bulk phase. When the water content is up to 6%, the water molecules begin to bind to the EO groups of TX-100. This part of bound water has a restricted mobility and lacks the normal hydrogen-bonded structure, and thus the interaction between the bound water and bulk IL can be neglected. So with further addition of water, the polarity of bulk IL keeps constant, and the resulting λmax also remains constant. Then FTIR spectroscopy is applied to confirm the result. FTIR spectroscopy is an important tool for determining the extent of the hydrogen bonding present and thus reflects some structural information about the investigated system. It has been widely used to investigate the states of solubilized water and the structures of microemulsion systems.47-49 Generally speaking, solubilized water molecules have three distinct states: trapped water, bound water, and free water. Trapped water is defined as water species dispersed among long hydrocarbon chains of surfactant molecules and have an O-H stretching vibration at about 3600 cm-1.48 Trapped water exists as monomers (or dimers) and has no hydrogen-bonding interaction with its surroundings. Furthermore, a small amount of water dissolved in a nonpolar solvent is also considered as trapped water.50 As the trapped water molecules are matrix-isolated dimers or monomers, they absorb in the high-frequency region.28 Bound water molecules are hydrogen bonded with the polar headgroups of surfactants, which results in absorption in the low-frequency region of the IR spectrum (about 3400 ( 20 cm-1). Apart from these two types of water species, free water occupies the cores of surfactant aggregates and has strong intermolecular hydrogen bonds; that is, it has properties similar to bulk water, which shifts the O-H stretching band to a lower frequency of 3220 ( 20 cm-1.47 The FTIR spectra of the Triton X-100/[bmim][BF4] micelles were investigated as a function of added water content. Only the O-H stretching bands (3200-3600 cm-1) of the micelles show obvious changes with the added water. Figure 6 shows the dependence of the O-H stretching frequencies of the micelles on the added water contents. As can be seen, the O-H stretching band originally (46) (47) (48) (49) (50) 4505.

Jain, T. K.; Varshney, M.; Maitra, A. J. Phys. Chem. 1989, 93, 7409. Zhou, G. W.; Li, G. Z.; Chen, W. J. Langmuir 2002, 18, 4566. Zeng, H. X.; Li, Z. P.; Wang, H. Q. Acta Phys.-Chim. Sin. 1999, 15, 522. Li, Q.; Weng, S. F.; Wu, J. G.; Zhou, N. F. J. Phys. Chem. B 1998, 102, 3168. Zhou, N. F.; Li, Q.; Wu, J.; Chen, J.; Weng, S.; Xu, G. Langmuir 2001, 17,

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Figure 6. Dependence of the O-H stretch of the Triton X100/[bmim][BF4] micelles (TX-100 content is 40 wt %) on added water content.

appears at 3388 cm-1, which is due to the terminal O-H group of Triton X-100, and shifts greatly to 3430 cm-1 when the water content reaches 6 wt % of the micelle system. After that, the wavenumber of the O-H stretching band remains almost unchanged with the further addition of water. A possible reason for this behavior is that when water is just added to the micelle, as mentioned above, these water molecules are trapped in the bulk [bmim][BF4]. As the O-H stretching vibration of trapped water is at 3600 cm-1, which is much larger than the terminal O-H stretching vibration of Triton X-100, with the addition of water the O-H stretching band rapidly moves to a high-frequency region. When the water content reached 6%, the added water molecules are bound to the EO units of Triton X-100 by hydrogen-bonding interactions instead of being trapped in the bulk [bmim][BF4]. The O-H stretching vibration of bound water is about 3400 ( 20 cm-1, which is close to the 3430 cm-1 observed for the micelle system with 6% water. So the wavenumber of the O-H stretching band remains almost unchanged (about 3430 cm-1) with the further addition of water. Furthermore, there is no free water in the micelle, for if free water were to appear in the system, the overall O-H stretching band would move to a fairly lowfrequency region due to the low-frequency vibration of free water. The O-H stretching band never declined to the low-frequency region even when the amount of water added reached 8 wt %. 1 H NMR. 1H NMR spectra give more detailed information about the intermolecular interactions and thus provide insight into the solubilization of added water molecules in the micelles. In our recent reports, 1H NMR spectra have been applied to investigate the microstructural characteristics of IL microemulsions and the effect of water on the microstructure of IL microemulsions.10,11 We also have used 1H NMR spectra to investigate the formation mechanism of surfactant aggregates in ILs.23,24 In the current study, 1H NMR measurements were carried out for Triton X-100/ILs micelles as a function of water concentration. Figure 7 shows 1H NMR spectra of the Triton X-100/[bmim][BF4] micelles without water (a) and the same micelles with 8% water (b). The 1H NMR spectra of Triton X-100/[bmim][PF6] micelles are similar (not shown here). The chemical structure and atom numbering for [bmim][BF4], ([bmim][PF6])and Triton X-100 is shown in Chart 1. In Triton X-100/[bmim][BF4] micelles, the chemical shifts for H1 and H2 in the bmimþ cation exhibit a shift toward high magnetic field with the addition of water. These changes suggest that there are some interactions between the added water molecules and [bmim][BF4]. The higher field shifts experienced by the H1 and H2 are because the electronegative oxygen atoms of the added water molecules tend to attract the electropositive imidazolium rings, and thus the electron density of the imidazolium ring rises, the shielding effect increases, and δ shifts toward higher field. The chemical shifts of all the other protons in the bmimþ cation remain constant with the addition of Langmuir 2010, 26(12), 9315–9320

Figure 7. 1H NMR spectra obtained for the Triton X-100/[bmim][BF4] micelles (TX-100 content is 40 wt %) without (a) and with 8% water (b) at 298 K. Chart 1. Chemical Structure and Atom Numbering for [bmim][BF4] ([bmim][PF6]) and Triton X-100

water. Plots of the chemical shifts for H1 and H2 against the water concentration are shown in Figure 8. The δH1 and δH2 exhibit a shift toward higher magnetic field. At first, the slope decreases steeply, but after the water content reached 6%, the slope is much smaller than that below 6%. When water was added to the micelle system, it was solubilized in the [bmim][BF4] solvent. The water molecules only interact with the electropositive imidazolium rings of [bmim][BF4], so the δ of H1 and H2 in the imidazolium ring exhibit a shift toward higher magnetic field,8 but the δ of other protons in the bmimþ cation are unchanged. When the water content is 6%, the solubilization of water in [bmim][BF4] is saturated, so upon further addition of water, the water molecules reach the interior of the micelle where they are bound to the EO units of Triton X-100 and become isolated from the [bmim][BF4]. In this case, the interaction between these molecules and the electropositive imidazolium rings becomes much weaker, the change of δ of H1 and H2 diminishes, and the slope is much smaller than before. This result is consistent with the results obtained from the UV/vis and FTIR spectra. For Triton X-100/[bmim][[PF6] micelles, the signals of all bmimþ cation in [bmim][[PF6] do not shift with the addition of water. This result is different from the [bmim][BF4] system. When water is added to the system, the proton signals of EO units in Triton X-100 (Hh-y) shifted downfield. The downfield shift experienced by the EO proton resonances is because of the fact that the added water molecules form hydrogen bonds with the EO units, and thus the electron cloud density of oxygen atoms of the EO units is decreased. The electropositivity of the carbon atoms adjacent to DOI: 10.1021/la100215z

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Figure 8. 1H NMR chemical shifts of H1 and H2 in [bmim][BF4] as a function of water content at 298 K.

Figure 9. 1H NMR chemical shifts of Hh-y in Triton X-100 as a function of water content at 298 K.

the oxygen atoms is enhanced owing to the induction effect. As a consequence, the hydrogen atoms attached to the carbon atoms are deshielded and resonate in a downfield position.11 Figure 9 shows the plots of the chemical shifts for Hh-y against the water concentration. As can be seen, the slope increases steeply at first, but after the water content reached 1.5%, the slope is much smaller than before. When water is added to the Triton X-100/ [bmim][PF6] micelles, it is bound to the EO units of Triton X-100. There is some interaction between water molecules and EO group, so the δ of Hh-y increases steeply. While after the bound water molecules reach saturation point (1.5%), the subsequently added water molecules are present in their free form in water pool; in this case, the interaction between the further added water and EO group is much weaker than before, so the δ of Hh-y increases slowly. This result is also consistent with the UV/vis results.

Conclusion In conclusion, the aggregation behavior of a nonionic surfactant, Triton X-100, in [bmim][BF4] has been investigated and

9320 DOI: 10.1021/la100215z

compared with the behavior in [bmim][PF6]. FF-TEM showed that Triton X-100 aggregates are formed in ILs. The addition of small amounts of water to the IL micelles was intensively investigated. When water is added to the Triton X-100/[bmim][PF6] and Triton X-100/[bmim][BF4] micelles, different cases appear. A microemulsion is formed in the former but not the latter. UV/vis spectra showed that upon addition of water the water molecules are initially trapped in the [bmim][BF4] solvent, up to a water content of 6 wt %. Further added water molecules are solubilized in the EO units of Triton X-100, which was confirmed by the FTIR spectra. 1H NMR spectroscopic analysis indicated that the added water interact with the imidazolium rings of [bmim][BF4] intensely at first, but when the water in [bmim][BF4] becomes saturated, the interaction between the further added water molecules and imidazolium rings becomes much weaker. For the [bmim][PF6] system, UV/vis spectra with different probes (MO and riboflavin) both suggested when water was added to the Triton X-100/[bmim][PF6] micelle, water molecules were first bound to the EO group of Triton X-100; after the bound water molecules were saturated (about 1.6%), the further added water molecules are present in water pools. 1H NMR spectroscopic analysis further confirmed the results. Such findings will contribute to our understanding of ionic liquids as assembly solvents. Acknowledgment. The authors are grateful to the National Natural Science Foundation of China (No. 20773081, No. 50972080), National Basic Research Program (2007CB808004, 2009CB930101), and the Natural Scientific Foundation of Shandong Province of China (Z2007B06). This work was partially supported by the Key Laboratory of Photochemical Conversion and Optoelectronic Materials, TIPC, CAS. We also thank Dr. Pamela Holt for editing the manuscript.

Langmuir 2010, 26(12), 9315–9320