Temperature-Induced Microstructural Changes in Ionic Liquid-Based

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Temperature-Induced Microstructural Changes in Ionic Liquid-Based Microemulsions Yanan Gao,† Na Li,† Liane Hilfert,‡ Shaohua Zhang,† Liqiang Zheng,*,† and Li Yu† Key Laboratory of Colloid and Interface Chemistry, Shandong UniVersity, Ministry of Education, Jinan 250100, China, and Chemical Institute, Otto-Von-Guericke-UniVersity Magdeburg, UniVersita¨tsplatz 2, 39106 Magdeburg, Germany ReceiVed October 19, 2008. ReVised Manuscript ReceiVed December 1, 2008 In the present contribution, results concerning the effect of temperature on the nonionic surfactant Triton X-100 based 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4)-in-cyclohexane and bmimBF4-in-toluene ionic liquid (IL) reverse microemulsions are reported. Dynamic light scattering (DLS) along with freeze-fracture transmission electron microscopy (FF-TEM) measurements revealed that the sizes of single microemulsion droplets increased with increasing temperature. However, a decreased temperature led to the appearance of droplet clusters, which have also been observed previously when the single microemulsion droplets were swollen by added bmimBF4 to a certain extent (Gao, Y. A.; Vogit, A.; Hilfert, L.; Sundmacher, K. ChemPhysChem, 2008, 9, 1603-1609). Compared to traditional aqueous microemulsions, IL microemulsions revealed relatively high temperature-independence. The droplet-shaped microstructure was always kept in a large range of temperature. The temperature-independence is ascribed to the temperature-insensitive electrostatic attraction between the solubilized bmimBF4 and Triton X-100, which was considered to be the driving force for solubilizing bmimBF4 into the cores of Triton X-100 aggregates. Two-dimensional rotating frame nuclear Overhauser effect (NOE) experiments (ROESY) further confirmed the microstructural change with temperature.

Introduction Ionic liquids (ILs) are receiving much attention as environmentally benign media for reactions, separations, and multidisciplinary chemistry areas, due to their unique physicochemical properties which can include nonvolatility, high stability, high ionic conductivity, wide electrochemical window, and easy recyclability.1-3 Of particular interest in this regard is the selfassembly of surfactants in contact with ILs, thereby forming micelles,4-11 microemulsions,12-18 liquid crystals,19-25 gels,26-29 and vesicles.30 The micellar aggregation behavior of surfactants in ILs has been investigated in recent years. Evans and co-workers first * Corresponding author. Telephone: 86-531 88366062. Fax: 86-531 88564750. E-mail:[email protected]. † Shandong University. ‡ Otto-von-Guericke-University Magdeburg.

(1) Wasserscheid, P. Nature 2006, 439, 797. (2) Welton, T. Chem. ReV. 1999, 99, 2071–2084. (3) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3773–3789. (4) Anderson, J. L.; Pino, V.; Hagberg, E. C.; Sheares, V. V.; Armstrong, D. W. Chem. Commun. 2003, 2444–2445. (5) Fletcher, K. A.; Pandey, S. Langmuir 2004, 20, 33–36. (6) Evans, D. F.; Yamauchi, A.; Roman, R.; Casassa, E. Z. J. Colloid Interface Sci. 1982, 88, 89–96. (7) Evans, D. F.; Yamauchi, A.; Wei, G. J.; Bloomfield, V. A. J. Phys. Chem. 1983, 87, 3537–3541. (8) Araos, M. U.; Warr, G. G. J. Phys. Chem. B 2005, 109, 14275–14277. (9) Atkin, R.; Warr, G. G. J. Am. Chem. Soc. 2005, 127, 11940–11941. (10) Patrascu, C.; Gauffre, F.; Nallet, F.; Bordes, R.; Oberdisse, J.; de LauthViguerie, N.; Mingotaud, C. ChemPhysChem 2006, 7, 99–101. (11) He, Y. Y.; Li, Z. B.; Simone, P.; Lodge, T. P. J. Am. Chem. Soc. 2006, 128, 2745–2750. (12) Eastoe, J.; Gold, S.; Rogers, S. E.; Paul, A.; Welton, T.; Heenan, R. K. J. Am. Chem. Soc. 2005, 127, 7302–7303. (13) Gao, H. X.; Li, J. C.; Han, B. X.; Chen, W. N.; Zhang, J. L.; Zhang, R. Phys. Chem. Chem. Phys. 2004, 2914–2916. (14) Gao, Y. A.; Li, N.; Zheng, L. Q.; Zhao, X. Y.; Zhang, S. H.; Han, B. X. Green Chem. 2006, 8, 43–49. (15) Gao, Y. A.; Han, S. B.; Han, B. X.; Li, G. Z. Langmuir, 2005, 21, 5681– 5684. (16) Li, N.; Gao, Y. A.; Zheng, L. Q.; Zhang, J.; Yu, L.; Li, X. W. Langmuir 2007, 23, 1091–1097. (17) Atkin, R.; Warr, G. G. J. Phys. Chem. B 2007, 111, 9309–9316.

reported the aggregation behavior of alkyltrimethylammonium bromides, alkylpyridinium bromides, and Triton X-100 in a low melting fused salt, ethylammonium nitrate (EAN).6,7 Recently, the formation of micelles in ILs 1-butyl-3-methylimidazolium chloride (bmimCl) and hexafluorophosphate (bmimPF6) was explored using inverse gas chromatography by Armstrong et al.4 They found that the surface tension of ILs was depressed with dissolution of surfactants in a manner analogous to aqueous solutions. Merrigan et al. have demonstrated that imidazolium cations with attached long fluorous tails act as surfactants and appear to self-aggregate within imidazolium-based ILs.31Studies on the aggregation behavior of a series of alkyl poly(oxyethyleneglycol) nonionic surfactants in ILs have revealed a high critical micellar concentration (CMC) and small hydrodynamic radii.10 The possible aggregate formation by nonionic surfactants Brij-35, Brij700, Tween-20, and Triton X-100 in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (emimTf2N) was observed on (18) Gao, Y. A.; Li, N.; Zheng, L. Q.; Bai, X. T.; Yu, L.; Zhao, X. Y. J. Phys. Chem. B 2007, 111, 2506–2513. (19) Evans, D. F.; Kaler, E. W.; Benton, W. J. J. Phys. Chem. 1983, 87, 533–535. (20) Tamura-Lis, W.; Lis, L. J.; Quinn, P. J. J. Phys. Chem. 1987, 91, 4625– 4627. (21) Tamura-Lis, W.; Lis, L. J.; Quinn, P. J. Biophys. J. 1988, 53, 489–492. (22) Wang, L. Y.; Chen, X.; Chai, Y. C.; Hao, J. C.; Sui, Z. M.; Zhuang, W. C. Chem. Commun. 2004, 2840–2841. (23) Greaves, T. L.; Weerawardena, A.; Fong, C.; Drummond, C. J. J. Phys. Chem. B 2007, 111, 4082–4088. (24) Greaves, T. L.; Weerawardena, A.; Fong, C.; Drummond, C. J. Langmuir 2007, 23, 402–404. (25) Wang, Z. N.; Liu, F.; Gao, Y.; Zhuang, W. C.; Xu, L. M.; Han, B. X. Langmuir 2005, 21, 4931–4937. (26) Kimizuka, N.; Nakashima, T. Langmuir 2001, 17, 6759–6761. (27) He, Y. Y.; Lodge, T. P. Chem. Commun. 2007, 2732–2734. (28) He, Y. Y.; Boswell, P. G.; Buhlmann, P.; Lodge, T. P. J. Phys. Chem. B 2007, 111, 4645–4652. (29) Lee, J.; Panzer, M. J.; He, Y.; Lodge, T. P.; Frisbie, C. D. J. Am. Chem. Soc. 2007, 129, 4532–4523. (30) Hao, J. C.; Song, A. X.; Wang, J. Z.; Chen, X.; Zhuang, W. C.; Shi, F. Chem.sEur. J. 2005, 11, 3936–3940. (31) Merrigan, T. L.; Bates, E. D.; Dorman, S. C.; Davis, J. H. Chem. Commun. 2000, 2051–2052.

10.1021/la803452m CCC: $40.75  2009 American Chemical Society Published on Web 01/08/2009

Microstructural Changes in IL-Based Microemulsions

the basis of the response of solvatochromic probes.5 Moreover, Tran and Yu have reported that nonionic surfactants Ndodecylsultaine (SB-12) and caprylyl sulfobetaine (SB3-10) formed micelles in bmimPF6 and emimTf2N.32 Recently, self-assemblies of surfactants within microemulsions using ILs as a substitute for water or organic solvents have been intensively investigated. Han’s group first discovered that 1-butyl3-methylimidazolium tetrafluoroborate (bmimBF4) assembled in polar nanosized droplets when dispersed in cyclohexane as solvent and the IL microemulsion showed a regular swelling behavior similar to water-in-oil (W/O) microemulsions; that is, the volume of the dispersed nanodroplets is directly proportional to the amount of added IL.13 Small-angle neutron scattering (SANS) measurements indicated a regular increase in droplet volume as micelles were progressively swollen by the added bmimBF4, which is in accordance with the traditional W/O microemulsions, indicating that these unusual systems behave akin to common W/O microemulsions.12 Recent work in our laboratory has concerned the formation mechanism of the surfactant aggregations, and more attention was especially paid to the IL-containing micelles and microemulsions stabilized by nonionic surfactants.18,33,34 The driving force of IL microemulsion formation was considered to be the electrostatic interaction between the hydrophilic group of Triton X-100 and the imidazolium cation of bmimBF4.33 It was also demonstrated that a hydrogen-bonding network within the palisade layer is formed when small amounts of water are added to the bmimBF4-inbenzene microemulsion,18,34 whereas water molecules were mainly located in the periphery of the polar core of the bmimBF4in-cyclohexane microemulsion droplets and led to the decrease of the microemulsion droplet size.35 The effect of temperature on various self-assemblied structures formed in ILs has been investigated. For instance, Kunz and coworkers reported that a mixture of two surfactant-like ILs 1-hexadecyl-3-methyl-imidazolium chloride (C16mimCl) and 1-hexadecyl3-methyl-imidazolium tetrafluoroborate (C16mimBF4) aggregated in EAN. The investigated aggregates are stable up to greater than 200 °C.36 Temperature-independent micellar morphologies of amphiphilic poly((1,2-butadiene)-block-ethylene oxide) (PBPEO) diblock copolymers in bmimPF6 between 25 and 100 °C were also observed by cryogenic transmission electron microscopy (cryo-TEM).11,37 Moreover, it has been reported that a decrease in temperature results in a decrease in lattice spacing for the nonionic Brij 97/bmimBF4 hexagonal liquid crystalline but an increase in lattice spacing for the Brij 97/bmimPF6 system.25 Also, the ion gels of poly(N-isopropyl acrylamide-b-ethylene oxide-b-N-isopropyl acrylamide) (PNIPAm-PEO-PNIPAm) in bmimPF6 were found to be thermally stable up to at least 100 °C and possess significant mechanical strength.28 A common feature for these self-assemblies is that they have revealed high thermostability. However, to our knowledge, the influence of temperature on the microstructure of IL microemulsions has not been reported so far. Therefore, it would be of interest and of importance to investigate their microstructural change with temperature. Although the IL microemulsions have been intensively investigated, their properties and features are still not very clear. For (32) Tran, C. D.; Yu, S. F. J. Colloid Interface Sci. 2005, 283, 613–618. (33) Gao, Y. A.; Zhang, J.; Xu, H. Y.; Zhao, X. Y.; Zheng, L. Q.; Li, X. W. ChemPhysChem 2006, 7, 1554–1561. (34) Gao, Y.; Li, N.; Zheng, L. Q.; Zhao, X. Y.; Zhang, J.; Cao, Q.; Zhao, M. W.; Li, Z.; Zhang, G. Y. Chem.sEur. J. 2007, 13, 2661–2670. (35) Gao, Y.; Hilfert, L.; Voigt, A.; Sundmacher, K. J. Phys. Chem. B 2008, 112, 3711–3719. (36) Thomaier, S.; Kunz, W. J. Mol. Liq. 2007, 130, 104–107. (37) He, Y. Y.; Lodge, T. P. J. Am. Chem. Soc. 2006, 128, 12666–12667.

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example, it is necessary to include a large amount of surfactants to support the IL microemulsions, and a large droplet size was often observed,13,16,17 which suggests that the surfactant aggregation and structure formation are likely different from those observed in “classic” W/O microemulsions. In order to further clarify the microstructure and formation mechanism of these novel self-assemblied structures, the present contribution is focused on the details of the structural change of Triton X-100 based bmimBF4-in-cyclohexane and bmimBF4-in-toluene microemulsions with temperature by dynamic light scattering (DLS), freeze-fracture transmission electron microscopy (FF-TEM), and two-dimensional rotating frame nuclear Overhauser effect (NOE) experiments (2D ROESY spectroscopy analyses). The current study can improve basic understanding of the IL-based microemulsions and therefore establish a better way of using them as new media.

Experimental Section 1. Materials. The nonionic surfactant Triton X-100 and toluene (99.8%) were obtained from Sigma-Aldrich. Triton X-100 was evaporated under vacuum at 80 °C for 4 h to remove any excess water before use. Cyclohexane (99.5%) was purchased from Merck. The IL bmimBF4 was synthesized by the quaternization of 1-methylimidazole with 1-chlorobutane.38 The imidazolium chloride salt was crystallized in ethyl acetate at -30 °C. The postmetathesis product was obtained by ion exchange of bmimCl and potassium tetrafluoroborate in distilled water and then was washed with dichloromethane and dried under high vacuum. The product was checked using 1H NMR spectroscopy. To avoid water, the containers with the materials were sealed tightly to avoid any further contact with air before use. Cyclohexane-D12 (99.5%) was provided by Merck and was used as received. 2. Apparatus and Procedures. The droplet size distributions of the investigated microemulsions were determined by dynamic light scattering (DLS) using Nanotrac Particle Size Analyzer (Nanotrac NPA 250) and the microtrac FLEX application software program. All measurements were made with a laser diode (780 nm wavelength, 3 mWnominal, Class IIIB at the scattering angle of 180°). The temperature of the solution was controlled by using a thermostat (F31C, Julabo) with an accuracy of (0.1°. Freeze-fracture transmission electron microscopy (FF-TEM) observations on the replication of samples were performed by using a JEOL TEM 200CX electron microscope. For the preparation of the replicas, a small amount of sample was placed in a gold cup. The temperature of the gold cup was first kept at 25 °C by high thermal capacity instruments before preparing the sample replicas. The gold cup was then swiftly plunged into liquid Freon which was cooled with liquid nitrogen. However, the temperature of the bmimBF4-in-cyclohexane samples in the gold cup was increased to 60 °C for reflecting the actual morphology of the samples in a high-temperature state at 55 °C. The frozen samples were fractured and replicated in a freeze-fracture apparatus BAF 400 (Bal-Tec, Balzer, Liechtenstein) at -140 °C. Pt/C was deposited at an angle of 45°. 1H NMR measurements were carried out with a Bruker AVANCE 600 NMR spectrometer at 298 K. The instrument was operated at a frequency of 600.13 MHz using tetramethylsilane as an internal reference. The spectrometer was fitted with a 5 mm CPTXI-1H-13C/15N/2H probehead with z-gradients. The standard 2D ROESY pulse sequence was used with a low power spin-lock pulse. The relaxation delay was 2 s. Complex data (2k) were collected in 256 increments with 8 transients each. The spin-lock field strength was 3200 Hz with a mixing time of 200 ms. Phase sensitive twodimensional time domains were recorded and processed using the TPPI protocol. A pure squared cosine window function was used in both dimensions prior to filling and Fourier transformation. (38) Dupont, J.; Consorti, C. S.; Suarez, P. A. Z.; Souza, R. F. Org. Synth. 1999, 79, 236–241.

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Figure 1. Size and size distribution of bmimBF4-in-cyclohexane microemulsion at different temperatures.

Results and Discussion 1. Size Change and Morphology of Microemulsions. In this work, the weight ratio of cyclohexane/Triton X-100/bmimBF4 is 0.9:1.1:0.2 ([bmimBF4]/[Triton X-100] molar ratio, R ) 0.53) for the cyclohexane-based microemulsion. A bmimBF4-incyclohexane reverse microemulsion can thus be formed according to previous phase behavior studies.35 The self-assemblied microemulsion structure reaches 58 nm at 24 °C, which is larger than common traditional aqueous microemulsions, which are typically considered to be less than 50 nm. It is necessary to mention that for ionic liquid-in-oil (IL/O) microemulsions we have observed maximum droplet sizes of the order of 150 nm.18 The larger microemulsion size may be related to the unique formation mechanism of ionic IL/O microemulsions. It has been proposed that the IL/O microemulsions are mainly driven by the electrostatic attraction between the positively charged imidazolium cation of bmimBF4 and the electronegative oxygen atoms of oxyethylene (OE) chains of Triton X-100.33 The effective area of interaction for OE chains is thus significantly higher in ILs than in water, which would result in a much extended conformation and hence an increased size of IL microemulsions. Besides, a higher surfactant concentration was required to offset the reduced solvophobicity of surfactants in ILs in comparison to aqueous systems,17 which also might contribute to the large aggregate size. Figure 1 shows the temperature-dependence of the microemulsion size and structure variation for the bmimBF4-incyclohexane microemulsions through DLS spectra. It can be seen that the average microemulsion droplets are monodispersed and the diameter linearly increases from 54.4 to 153.2 nm as temperature is raised from 24 to 44 °C (see Figure 1 inset). This phenomenon is similar to the regular swelling behavior of IL/O microemulsions induced by addition of an IL.13 Importantly, the microemulsion was not destroyed with a further increase in temperature. The single phase transparent microemulsion was always observed, even if temperature approaches the boiling point of cyclohexane (74 °C). To our knowledge, such a stable nonionic surfactant supported microemulsion was scarcely reported previously. It is unclear if there is any microstructural transition at temperatures higher than 50 °C because the scattering light strength is too weak to provide any structure information by DLS measurements. However, freeze-fracture transmission electron microscopy (FF-TEM) can provide a direct image of the droplets, aggregates, and their morphology in a liquid sample. Figure 2 shows two typical FF-TEM images of bmimBF4-incyclohexane microemulsions at 25 and 55 °C. The FF-TEM images clearly indicate that the droplet-shaped microstructure was always kept at least below 60 °C and that the microemulsion diameter increased with increasing temperature, which is in accordance with the DLS measurements.

Figure 2. FF-TEM images of bmimBF4-in-cyclohexane microemulsion at 25 °C (top) and 55 °C (bottom).

Nonionic surfactant based aqueous microemulsions are known to be commonly temperature-sensitive due to the considerable temperature-dependence of hydrogen bonds between the hydrophilic OE chains of nonionic surfactants and water molecules. Changing temperature often causes microstructure transition from W/O to oil-in-water (O/W) microemulsions via bicontinuous structure and vice versa39 and even sometime results in phase separation. Compared to the aqueous microemulsions, the presently investigated IL/O microemulsion has shown to be relatively temperature-insensitive. The microemulsion system could find more wide application in organic reactions, chemical separations, or extractions, especially in nanomaterial preparations because one might envision the necessity to keep the microstructure type constant with the change of temperature. The temperature-independence was also observed for the selfassemblied micelles, liquid crystals, and gels in ILs.28,36,37 The reason, we believe, is attributed to the unique electrostatic attraction between the positively charged imidazolium cation of bmimBF4 and the electronegative oxygen atoms of OE chains of Triton X-100. The electrostatic interaction that drove the formation of IL/O microemulsions played a similar role of hydrogen bonds or hydration behavior in the W/O microemulsions. However, the remarkable difference is that the former is temperature-insensitive,40 while the latter is relatively temperature-sensitive. Thus, we can see that the different formation mechanisms led to different properties. (39) Shinado, K. SolVent Properties of Surfactant Solutions; Marcel Dekker: New York, 1967. (40) Mele, A.; Tran, C. D.; De Paoli Lacerda, S. H. Angew. Chem., Int. Ed. 2003, 42, 4364–4366.

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Figure 3. Size and size distribution of bmimBF4-in-toluene microemulsion at different temperatures.

To further confirm our deduction, another common IL microemulsion, bmimBF4-in-toluene microemulsion, was also investigated. Figure 3 shows the size and size distribution of the microemulsion at different temperatures. It can be seen that the average size of a single microemulsion droplet increased from 41.6 to 200.7 nm with increasing temperature from 8 to 24 °C. The increase in temperature leading to the increase of droplet size is same with the bmimBF4-in-cyclohexane microemulsions. Besides, we can notice that some larger aggregates appeared, which was reflected by the scattering peaks at large size region. The larger aggregates are actually microemulsion droplet clusters, which were formed owing to the polar nature of toluene, special structure, and properties of both bmimBF4 and Triton X-100.41 These droplet clusters are inclined to appear at a high solubilized IL content. In that case, the excess amount of additional bmimBF4 penetrated the palisade layer of Triton X-100 and obscured the initially distinct surfactant interfacial film between toluene and bmimBF4 and thus facilitated cluster formation.41 In a similar way, the decreased temperature can lead to the decrease of microemulsion size and some bmimBF4 molecules are thus squeezed out of the palisade layer and result in the appearance of clusters. Therefore, at this point, a decrease in temperature corresponds to the increase of bmimBF4 concentration. In addition, it can also be seen from Figure 3 that the diameter of droplet clusters (scattering peaks at large size region) increased with decreasing temperature and scattering strength was also enhanced at a relatively low temperature, indicating that both cluster size and number were increased. The same phenomenon was also obtained when increasing the concentration of bmimBF4. So, the result further confirms that the scattering peaks at large size region are attributed to the appearance of large-sized droplet clusters. Similarly, FF-TEM measurements were also carried out, and two typical FF-TEM images of a bmimBF4-in-toluene microemulsion obtained at 8 and 24 °C are shown in Figure 4. It is clear that single microemulsion droplets were accompanied by the appearance of some large droplet clusters at a relatively low temperature of 8 °C and these large cluster aggregates (about 300 nm and marked by circles in Figure 4a) are visually composed of many small droplets that have an average diameter of 70 nm. With increasing temperature to a relatively high temperature of 24 °C, monodispersed microemulsion droplets dominate in the image and large clusters basically disappear (Figure 4b). The average diameter of the monodispersed microemulsion droplets is 230 nm, which is larger than that obtained at 8 °C. The result is also in accordance with the DLS measurements. 2. Mechanism Analysis. From the above analyses, we can see that the diameters of both bmimBF4-in-cyclohexane and bmimBF4-in-toluene microemulsions increased with increasing (41) Gao, Y. A.; Vogit, A.; Hilfert, L.; Sundmacher, K. ChemPhysChem 2008, 9, 1603–1609.

Figure 4. FF-TEM images of bmimBF4-in-toluene microemulsion at 8 °C (top) and 24 °C (bottom).

temperature. The increase of microemulsion size suggests that the interfacial curvature of the surfactant film was decreased. This may be due to the enhanced solubility of the surfactant tail groups in continuous organic solvents when raising the temperature because the electrostatic interaction between the positively charged imidazolium cation and the electronegative oxygen atoms of Triton X-100 is temperature-independent and will not cause a remarkable change of interaction between IL and IL-philic OE groups. However, for the aqueous microemulsions, an increased temperature not only enhances the solubility of surfactant tail groups in the continuous oil phase but also decreases the hydrogen bonds or hydration interactions between the head groups of surfactants and dispersed water molecules, both of which can decrease the interfacial curvature and thus lead to an obvious increase in aggregate size. This is why the IL/O microemulsions are relatively thermo-insensitive, whereas traditional W/O microemulsions usually demonstrate obvious temperature-dependence. We need to mention that although hydrogen bonds may also happen to the terminal hydroxyl of Triton X-100 and the electronegative fluorine atoms of the BF4- anion, to the acidic C-2, C-4, and C-5 protons of the imidazolium cation and the electronegative oxygen atoms of OE chains, hydrogen bond connection sites relative to the terminal hydroxyl of Triton X-100 are very limited because there is only one hydroxyl group per Triton X-100 molecule. Besides, hydrogen bonds due to C-2, C-4, and C-5 protons are very weak because of their extremely

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Gao et al. Scheme 1. Schematic Illustration of the bmimBF4-in-Oil Microemulsion Structure, Accompanied by the Curvature Change of Triton X-100 Interfacial Film at bmimBF4/Oil Two Phases with Changing Temperature

Chart 1. Chemical Structure and Atom Numbering for bmimBF4 and Triton X-100

Figure 5. 2D ROESY spectra of bmimBF4-in-cyclohexane microemulsion with R ) 0.53 at different temperatures.

weak acidity.42 Therefore, we consider that the electrostatic attraction actually contributed much to the formation of IL/O microemulsions compared to the hydrogen bond interactions. (42) Headley, A. D.; Jackson, N. M. J. Phys. Org. Chem. 2002, 15, 52–55.

3. 2D ROESY Spectra. 2D ROESY spectroscopic analysis was also used to assess the structural change of microemulsions with changing temperature. The analysis of intermolecular ROEs was carried out by volume integration of the cross peaks attributed to intermolecular interactions.40 As an example, Figure 5 shows the contour plot of the 2D ROESY spectra for the bmimBF4in-cyclohexane microemulsion at different temperatures with a mixing time of 200 ms. First, it can be seen from Figure 5a that much stronger ROE intensities were observed for those interactions involving imidazolium ring protons H-2, H-4, and H-5 and protons of OE chains of Triton X-100 such as 2/h, 4/h, and 5/h cross peaks when compared to other ones. The same phenomenon was also found in our previous report, further suggesting that there is widely electrostatic attraction between the positively charged imidazolium ring and the hydrophilic OE chains of Triton X-100 and the electrostatic interaction played an important role in inducing the formation of bmimBF4-in-oil microemulsions.41 Second, all proton signals of bmimBF4 including H-2, H-4, H-5, H-6, H-7, H-8, H-9, and H-10 did not interact with the hydrophobic group of Triton X-100, which can be reflected by the fact that there are no cross peaks between the imidazolium ring protons of bmimBF4 and the hydrophobic alpha-[4-(1,1,3,3-tetramethylbutyl)phenyl] tail of the surfactant. The result indicated that bmimBF4 was separated from the continuous organic solvent by the surfactant interfacial film and solubilized into the hydrophilic cores of Triton X-100 aggregates with a hydrophobic shell pointing toward the oil phase and with OE chains penetrating into the IL interior, where a palisade layer consisting of OE chains and some bonded IL molecules surrounds the IL core or

Microstructural Changes in IL-Based Microemulsions

pool. The schematic molecular arrangement of various components in the microemulsion is shown in Scheme 1. Third, the ROE intensities among the hydrophobic group of Triton X-100, involving H-a/H-d, H-a/H-e, H-b/H-d, H-b/H-e (Figure 5b), H-a/ H-c, H-b/H-c, and H-a/H-b (Figure 5c) increased as temperature increased from 294 to 299 K, whereas the ROE intensities among the hydrophilic OE units of Triton X-100 (H-f/H-g, H-g/H-h, and H-f/H-h) decreased (Figure 5d). In other words, increasing temperature results in two different trends: (1) interaction between the hydrophilic OE units of the surfactant decreased, and (2) the opposite result was obtained for interactions among the hydrophobic groups of Triton X-100 molecules. This finding reveals that the interfacial curvature of the Triton X-100 film between the bmimBF4 and cyclohexane decreased with increasing temperature. The interfacial curvature change in microemulsions with changing temperature is also shown in Scheme 1. The decreased interfacial curvature means that the microemulsion size increased, which is in accordance with the DLS and FFTEM measurements.

Conclusion In summary, the effect of temperature on the 1-butyl-3methylimidazolium tetrafluoroborate (bmimBF4)-in-cyclohexane

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and bmimBF4-in-toluene ionic liquid (IL) reverse microemulsions was investigated. The results showed that the size of single microemulsion droplets was increased with increasing temperature. A low temperature resulted in the appearance of droplet clusters for the bmimBF4-in-toluene microemulsions. This is because the decrease in temperature led to the decreased size of microemulsion droplets and the solubilized bmimBF4 molecules were squeezed out of the palisade layer of the microemulsion droplets and therefore resulted in the appearance of droplet clusters. The IL microemulsions revealed a relatively high temperature-independence compared to the traditional aqueous systems, because in the IL microemulsions electrostatic attraction, that played a similar role of hydrogen bonds or hydration that happened in aqueous systems and was considered to drive the formation of IL microemulsions, was highly temperatureindependent. The droplet-shaped microstructure was thus kept in a large temperature range, which can make the IL microemulsion systems available in more wide fields. Acknowledgment. This work was supported by the Natural Scientific Foundation of China (Grant No. 20773081) and the National Basic Research Program (2007CB808004). LA803452M