Langmuir 2005, 21, 5681-5684
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TX-100/Water/1-Butyl-3-methylimidazolium Hexafluorophosphate Microemulsions Yanan Gao,†,‡ Shuaibing Han,‡ Buxing Han,*,† Ganzuo Li,*,‡ Dong Shen,† Zhonghao Li,† Jimin Du,† Wanguo Hou,‡ and Gaoyong Zhang§ Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China, Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, People’s Republic of China, and China Research Institute of Daily Chemical Industry, Taiyuan 030001, People’s Republic of China Received January 11, 2005. In Final Form: May 5, 2005 Both ionic liquids and water are typical green solvents. In this work, the phase behavior of the ternary system consisting of ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6), TX-100, and water was determined at 25.0 °C. The water-in-bmimPF6, bicontinuous, and bmimPF6-in-water microregions of the microemulsions were identified by cyclic voltammetry method using potassium ferrocyanide K4Fe(CN)6 as the electroactive probe. Dynamic light scattering (DLS) and the UV-vis method were used to characterize the microemulsions. It was demonstrated that the hydrodynamic diameter (Dh) of the bmimPF6-in-water microemulsions is nearly independent of the water content but increases with increasing bmimPF6 content due to the swelling of the micelles by the ionic liquid. The UV-vis further confirmed the existence of water domains in the water-in-bmimPF6 microemulsions, and the salt potassium ferricyanide K3Fe(CN)6 could be dissolved in the water domains.
Introduction Ionic liquids (ILs) are organic salts which are liquids at ambient conditions. They are nonvolatile, nonflammable and thermally stable and have adjustable solvent properties. These advantages make them very promising replacements for traditional volatile organic solvents. In recent years, the fundamental properties of ILs and their applications in different fields, such as in organic synthesis,1-5 separations,6-9 and materials preparation,10-12 have been studied extensively. As potential neoteric solvents, they afford significant environmental benefits and can contribute much to green chemistry and technology. Microemulsions are thermodynamically stable dispersions of two or more immiscible liquids which are stabilized by an adsorbed surfactant film at the liquid-liquid interface. Microemulsions have been an interesting subject for the past decades. They have been successfully applied to separation,13 chemical reactions,14 and materials syn†
Chinese Academy of Sciences. Shandong University. § China Research Institute of Daily Chemical Industry. ‡
(1) Welton, T, Chem. Rev. 1999, 99, 2071. (2) Avery, T. D.; Jenkins, N. F.; Kimber, M. C.; Lupton, D. W.; Taylor, D. K. Chem. Commun. 2002, 28. (3) Huddleston, J. G.; Willauer, H. D.; Swatloski, R. P.; Visser, A. E.; Rogers, R. D. Chem. Commun. 1998, 1765. (4) Smietana, M.; Mioskowski, C. Org. Lett. 2001, 3, 1037. (5) Anderson, J. L.; Pino, V.; Hagberg, E. C.; Sheares, V. V.; Armstrong, D. W. Chem. Commun. 2003, 2444. (6) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772. (7) Anderson, J. L.; Ding, J.; Welton, T.; Armstrong, D. W. J. Am. Chem. Soc. 2002, 124, 14247. (8) Chun, S.; Dzyuba, S. V.; Bartsch, R. A. Anal. Chem. 2001, 73, 3737. (9) Blanchard, L. A.; Brennecke, J. F. Ind. Eng. Chem. Res. 2001, 40, 287. (10) Kim, K.; Demberelnyamba, D.; Lee, H. Langmuir 2004, 20, 556. (11) Zhou, Y.; Antonietti, M. J. Am. Chem. Soc. 2003, 125, 14960. (12) Taubert, A. Angew. Chem., Int. Ed. 2004, 43, 5380. (13) Zhang, R.; Liu, J.; Han, B. X.; He, J.; Liu, Z. M.; Zhang, J. L. Langmuir 2003, 19, 8611. (14) Spiro, M.; de Jesus, D. M. Langmuir 2000, 16, 2464.
thesis.15 Using hydrophobic ILs instead of the conventional organic solvents to prepare an IL-based microemulsion may be an attractive topic. Several papers related with both ILs and aggregation behavior of surfactants have been published. 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.16 A communication presenting the evidence of dry micelle formation of several traditional surfactants in 1-butyl-3-methylimidazolium chloride and 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6) has appeared in the literature.5 Binks et al. have successfully prepared some novel emulsions of ILs stabilized solely by silica nanoparticles.17 Furthermore, on the basis of the response of solvatochromic probes, the aggregation behavior of several common surfactants in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide has been investigated.18 In the previous study, we have demonstrated that ILs can exist as nanosized polar domains dispersed in cyclohexane with the aid of TX-100.19 A major limitation to the broader use of ILs is their inability to dissolve a number of chemicals including some hydrophilic substances, although the properties of ILs can be tuned.17,18 To overcome this problem, the use of surfactants to create microemulsions containing water is an alternative method, as it is obvious that hydrophilic substances can be solubilized in the dispersed water phase. Furthermore, the microemulsion may possess some unexpected advantages in applications due to unique properties of ILs. However, to our knowledge, the microemulsions composed of ILs and water have not been reported in the open literature. In this work, TX-100/H2O/ (15) Summers, M.; Eastoe, J.; Davis, S. Langmuir 2002, 18, 5023. (16) Merrigan, T. L.; Bates, E. D.; Dorman, S. C.; Davis, J. H., Jr. Chem. Commun. 2000, 2051. (17) Binks, B. P.; Dyab, A. K. F.; Fletcher, P. D. I. Chem. Commun. 2003, 2540. (18) Fletcher, K. A.; Pandey, S. Langmuir 2004, 20, 33. (19) Gao, H. X.; Li, J. C.; Han, B. X.; Chen, W. N.; Zhang, J. L.; Zhang, R.; Yan, D. D. Phys. Chem. Chem. Phys. 2004, 2914.
10.1021/la0500880 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/25/2005
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IP ) 4.463 × 10-4nFAc(nF/RT)1/2D1/2υ1/2
Figure 1. Phase diagram of the TX-100/H2O/bmimPF6 ternary system at 25 °C; the dished lines are chosen for apparent diffusion coefficient study. A-C are water-in-bmimPF6, bicontinuous, and bmimPF6-in-water regions, respectively.
bmimPF6 microemulsions are prepared and characterized using different techniques. Experimental Section TX-100 was obtained from Farco Chemical, Hong Kong. Potassium ferrocyanide K4Fe(CN)6 was purchased from Shanghai Experimental Reagent Co., Ltd. The potassium ferricyanide K3Fe(CN)6 used was provided by Fisher. Methyl orange (MO) was produced by Beijing Chemical Reagent Factory. Double distilled water was used. The bmimPF6 used was prepared by the procedures reported in the literature.20 The phase behavior of the system was determined by the direct observation method. Cyclic voltammetry (CHI832A CH Instrument, Austin, TX), dynamic light scattering (DLS; Brookhaven Instrument Co., BI-200SM goniometer and BI-9000AT correlator), and UV-vis (Beijing Instrument Co., TU-1201) were used to characterize the structure of the microemulsions, and a tensiometer (model TX-550A, Bowing Industry Corp.) was used to determine the interfacial tension between bmimPF6 and H2O.
Results and Discussion The interfacial tension between bmimPF6 and H2O is about 11.06 mN/m at 25 °C, which is smaller than those of traditional oil/H2O systems (generally 20-40 mN/m). The phase diagram of the TX-100/H2O/bmimPF6 ternary system at 25 °C is illustrated in Figure 1. It is obvious that a continuous single-phase microemulsion region can be observed over a large water content range. This singlephase region is suitable for the study of the microstructure and structural transition of the microemulsions.21,22 Cyclic voltammetry measurement is a widely used technique to study the microstructure and structural transition of micelles and microemulsions.21-26 The apparent diffusion coefficient of the probe in the system can be determined by this method, and the structural transition of the microdroplets can be identified on the basis of the change of the diffusion coefficient. In cyclic voltammetry, the peak current IP for a reversible system is described by the Randles-Sevcik equation.27,28 (20) Dupont, J.; Consorti, C. S.; Suarez, P. A. Z.; Souza, R. F. Org. Synth. 1999, 79, 236. (21) Mo, C. S.; Zhong, M. H.; Zhong, Q. J. Electroanal. Chem. 2000, 493, 100. (22) Mo, C. S. Langmuir 2002, 18, 4047. (23) Raymond, A. M. Anal. Chem. 1990, 62, 1084. (24) Geetha, B.; Mandal, A. B. Langmuir 1995, 11, 1464. (25) Mandal, A. B.; Nair, B. U. J. Phys. Chem. 1991, 95, 9008. (26) Mandal, A. B. Langmuir 1993, 9, 1932. (27) Kissinger, P. T.; Heineman, W. R. J. Chem. Educ. 1988, 60, 702.
(1)
where n is the number of electrons involved in oxidation or reduction, F is Faraday constant, A is the area of the electrode, c is the concentration of the electroactive probe, D is the diffusion coefficient of the electroactive probe, and υ is the scan rate. The equation indicates that IP should increase linearly with υ1/2, and D can be obtained from the slope of the line. In our experiments, water-soluble potassium ferrocyanide K4Fe(CN)6 was used as the electroactive probe, and a glass carbon working electrode, a saturated calomel reference electrode, and a platinum counter electrode were used in the experiments. The analysis is made in the single-phase microemulsion region, and the bmimPF6-to-TX-100 weight ratios (I) are 0.18, 0.11, and 0.05, respectively, as shown by the dashed lines in Figure 1. As an example, Figure 2 shows the D of K4Fe(CN)6 in the microemulsions with I ) 0.18 as a function of water content. It is obvious that the D increases with increasing water content over the entire single-phase microemulsion region. As the water content is lower about 33% or higher about 65%, the curve increases gradually; an abrupt increase of D is observed in the water content range from about 33% to 65%. The changes can be explained as follows: K4Fe(CN)6 probes the water environment due to its insoluble nature in bmimPF6 and TX-100. The system forms a water-in-bmimPF6 microemulsion as the water content is below 33%, and bmimPF6 is the continuous phase. The D corresponds to the diffusion coefficient of K4Fe(CN)6 in water-in-bmimPF6 microemulsion droplets, and the value is small. D changes slowly with the water content in this region, showing that the change of the microenvironment of the microemulsion is not considerable. Similarly, bmimPF6-in-water microemulsions are formed when the water content is above 65%. D changes slowly with water content because the structure of the microemulsions is not sensitive to the water content. In this region, the probe exists in the water continuous phase, and the value of D is larger. A dramatic change in D is observed in the water content range from 33% to 65%. This indicates a considerable change of the microenvironment, and the microemulsion is different from water-inbmimPF6 and bmimPF6-in-water types. On the basis of the principle to distinguish subregions in microemulsions using the apparent diffusion coefficient,29 it can be known that a bicontinuous microemulsion is formed in this region. Therefore, we can recognize three types of microstructures: water-in-bmimPF6, bmimPF6-in-water, and bicontinuous in the microemulsions. The subregions identified are also marked in Figure 1. The DLS method is used to study the size of the microemulsions at some typical compositions, and corresponding hydrodynamic diameters are listed in Table 1. Only the bmimPF6-in-water microemulsions are investigated in this work because it is very difficult to carry out the experiments for water-in-bmimPF6 microemulsions due to the high viscosity. It is clear from Table 1 that the hydrodynamic diameter (Dh) of the microemulsions is nearly independent of water content at a fixed bmimPF6-to-surfactant molar ratio (R). This hints that the structure change of the micelles or microemulsions with water content is not considerable in this region as R is fixed, which is consistent with the conclusion derived from the apparent diffusion coefficient discussed above. (28) Chokshi, K.; Qutubuddin, S.; Hussam, A. J. Colloid Interface Sci. 1989, 129, 315. (29) Guering, P.; Lindman, B. Langmuir 1985, 1, 464.
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Figure 2. Apparent diffusion coefficient of K4Fe(CN)6 as a function of water content at 25 °C (initial amounts: 1.2 g of bmimPF6, 6.8 g of TX-100, and 0.065 g of K4Fe(CN)6). Dashed lines are used to distinguish three types of microstructures.
Figure 4. Absorption spectra of the K3Fe(CN)6 in water-inbmimPF6 reverse microemulsions with I ) 4.0 at 25.0 °C and different K3Fe(CN)6 concentrations (C). a, W0 ) 0, no K3Fe(CN)6 powder dissolved; b, W0 ) 2, saturated K3Fe(CN)6 solution; c, W0 ) 6, C ) 0.10 g/L; d, W0 ) 6, C ) 0.20 g/L; e, W0 ) 6, C ) 0.30 g/L; f, W0 ) 6, C ) 0.40 g/L. Table 2. λmax of MO in the Microemulsions at Different bmimPF6-to-TX-100 Weight Ratios (I) and Water-to-TX-100 Molar Ratios (W0; [MO] ) 2.4 × 10-4 M)
Figure 3. Dependence of hydrodynamic diameter Dh on bmimPF6-to-surfactant molar ratio R. 9, Concentration of TX-100 ≈ 15 wt %; b, concentration of TX-100 ≈ 21 wt %; 2, concentration of TX-100 ≈ 26 wt %. Table 1. Hydrodynamic Diameter (Dh) of the Microemulsions Obtained by the DLS Method number
water, wt %
TX-100, wt %
bmimPF6, wt %
R
Dh, nm
1 2 3 4 5 6 7 8 9
83.7 78.1 71.8 83.3 77.1 70.5 81.5 74.8 68.9
15.0 20.3 26.2 15.1 20.7 26.7 15.7 21.4 26.4
1.1 1.6 2.0 1.6 2.2 2.8 2.8 3.8 4.7
0.17 0.17 0.17 0.24 0.24 0.24 0.41 0.41 0.41
8.3 8.5 9.0 12.6 12.8 11.9 18.9 18.3 17.9
As expected, Dh of the microemulsions at the larger R is larger, indicating that the micelles are swollen by bmimPF6. Figure 3 indicates that the radius of the droplet shows a nearly linear behavior as a function of R, indicating the spherical structure droplets.19 Furthermore, Figure 3 shows that the Dh value is nearly independent of the surfactant concentration, indicating that the droplet interaction is essentially negligible. MO is often used as a solvatochromic probe.30 The UVvis absorbance maximum (λmax) of MO is sensitive to the local environment, and λmax increases with the polarity of the environment. Moreover, MO is not soluble in bmimPF6 and, thus, can be used to confirm the existence of water domains in the system. In this work, the λmax of MO in the water-in-bmimPF6 microemulsions at different water-toTX-100 molar ratios (W0) was determined. The results are listed in Table 2. The λmax increases with increasing W0, suggesting that the polarity of the water domains is stronger as the W0 is larger. We also investigated the dissolution of metal salt, potassium ferricyanide K3Fe(CN)6, in water-in-bmimPF6 microemulsions by direct observation and the UV-vis (30) Zhu, D. M.; Schelly, Z. A. Langmuir 1992, 8, 48.
I
W0
λmax
I
W0
λmax
1.5 1.5 1.5 1.5
2 5 8 10
424 429 433 436
4.0 4.0 4.0 4.0
2 5 8 10
433 434 435 two phase
techniques. In the experiments, K3Fe(CN)6 is used as a probe to take advantage of the distinctive color, and the highly ionic compound requires complete hydration for its dissolution in a water environment.31 The experiments were conducted at I ) 1.5 and 4.0, respectively. The UVvis spectra of K3Fe(CN)6 in the microemulsions at I ) 4.0 are shown in Figure 4. K3Fe(CN)6 is a solid and not soluble in bmimPF6, TX-100, and their mixture even after prolonged ultrasonic of the mixture and, therefore, shows no absorption (Figure 4a). However, in the presence of a little amount of water (W0 ) 2), the microemulsion showed a distinct Kelly color and the characteristic absorption peak appeared at about 422 nm (Figure 4, spectrum b), revealing that K3Fe(CN)6 was dissolved in the aqueous domains. Spectra c-f in Figure 4 demonstrate that, as expected, at fixed water content (W0 ) 6), absorbance increases with increasing concentration of K 3Fe(CN)6 (0.1, 0.2, 0.3, and 0.4 g/L), and the absorbance at the maximum wavelength increases linearly with the K3Fe(CN)6 concentration as shown by the inset of the figure. In other words, the absorbance obeys the LambrtBeer law32 in the concentration range studied. At I ) 1.5, the results are similar. The UV-vis results provide further evidence for the formation of water domains. It is well-known that waterin-oil microemulsions can be used to prepare nanomaterials15 or used as nanoreactors for some reactions.14 In many cases, metal salts are required to be dissolved in the water-in-oil microemulsions. In this work, we use the IL to replace conventional organic solvents in the microemulsions. Therefore, the microemulsions have potential applications for preparing some nanomaterials or conducting interfacial reactions, which can avoid the use of volatile organic solvents, and may have some other features due to the unusual properties of ILs. Conclusion At suitable conditions bmimPF6/TX-100/water system can form microemulsions, which can be divided into waterin-bmimPF6, bicontinuous, and bmimPF6-in-water sub(31) Hutton, B. H.; Perera, J. M.; Grieser, F.; Stevens, G. W. Colloids Surf., A 2001, 189, 177. (32) Meziani, M. J.; Sun, Y. P. Langmuir 2002, 18, 3787.
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regions. DLS study shows that the hydrodynamic diameter (Dh) of the bmimPF6-in-water microemulsions is nearly independent of water content but increases with increasing bmimPF6 content due to the swelling of the micelles by the IL. Water domains exist in the water-in-bmimPF6 microemulsions, which can dissolve salts such as MO and K3Fe(CN)6, and the polarity of the water domains increases with increasing water content. This kind of clean microemulsion has potential applications in different fields, such
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as preparation of nanomaterials and chemical reactions, which is a benefit to environmental protection and may have other advantages. Acknowledgment. The authors are grateful to the National Natural Science Foundation of China (20133030) and Ministry of Science and Technology for financial support (G2000078103). LA0500880
