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Specialist Gelator for Ionic Liquids Kenji Hanabusa,* Hiroaki Fukui, Masahiro Suzuki, and Hirofusa Shirai Graduate School of Science and Technology, Shinshu University, Ueda 386-8567, Japan Received May 18, 2005. In Final Form: August 22, 2005 Cyclo(L-β-3,7-dimethyloctylasparaginyl-L-phenylalanyl) (1) and cyclo(L-β-2-ethylhexylasparaginyl-Lphenylalanyl) (2), prepared from L-asparaginyl-L-phenylalanine methyl ester, have been found to be specialist gelators for ionic liquids. They can gel a wide variety of ionic liquids, including imizazolium, pyridinium, pyrazolidinium, piperidinium, morpholinium, and ammonium salts. The mean minimum gel concentrations (MGCs) necessary to make gels at 25 °C were determined for ionic liquids. The gel strength increased at a rate nearly proportional to the concentration of added gelator. The strength of the transparent gel of 1-butylpyridinium tetrafluoroborate ([C4py]BF4), prepared at a concentration of 60 g L-1 (gelator 1/[C4py]BF4), was ca. 1500 g cm-2. FT-IR spectroscopy indicated that a driving force for gelation was intermolecular hydrogen bonding between amides and that the phase transition from gel to liquid upon heating was brought about by the collapse of hydrogen bonding. The gels formed from ionic liquids were very thermally stable; no melting occurs up to 140 °C when the gels were prepared at a concentration of 70 g L-1 (gelator/ ionic liquid). The ionic conductivities of the gels were nearly the same as those of pure ionic liquids. The gelator had electrochemical stability and a wide electrochemical window. When the gels were prepared from ionic liquids containing propylene carbonate, the ionic conductivities of the resulting gels increased to levels rather higher than those of pure ionic liquids. The gelators also gelled ionic liquids containing supporting electrolytes.
Introduction Very recently, ionic liquids have received much attention as eccentric materials in chemistry.1-3 Ionic liquids composed of anions and cations have useful properties such as extremely low volatility, high thermal stability, wide liquid phase temperature range, nonflammability, high chemical stability, high ionic conductivity, and a wide electrochemical window. These advantages imply that ionic liquids will play an important role in such technological applications as dye-sensitized solar cells,4 electrochemical devices,5 and wet double-layer capacitors.6 Quasi-solidification of ionic liquids by chemical or physical gelation may also play an important role in the fabrication of devices, since gelation is a simple and promising method * To whom correspondence should be addressed. Phone: +81268-21-5487. Fax: +81-268-21-5608. E-mail: hanaken@ giptc.shinshu-u.ac.jp. (1) Welton, T. Chem. Rev. 1999, 99, 2071. (2) (a) Rogers, R. D.; Seddon, K. R. Ionic Liquids as Green Solvents: Progress and Prospects; ACS Symposium Series 856; American Chemical Society: Washington, DC, 2003. (b) Rogers, R. D.; Seddon, K. R. Science 2003, 302, 792. (3) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis; WileyVCH: 2003. (4) (a) Wang, P.; Zakeeruddin, S. M.; Moser, J.-E.; Humphry-Baker, R.; Gra¨tzel, M. J. Am. Chem. Soc. 2004, 126, 7164. (b) Wang, P.; Zakeeruddin, S. M.; Moser, J.-E.; Gra¨tzel, M. J. Phys. Chem. B 2003, 107, 13280. (c) Wang, P.; Zakeeruddin, S. M.; Humphry-Baker, R.; Gra¨tzel, M. Chem. Mater. 2004, 16, 2694. (d) Stathatos, E.; Lianos, P.; Zakeeruddin, S. M.; Kiska, P.; Gra¨tzel, M. Chem. Mater. 2003, 15, 1825. (e) Yamanaka, N.; Kawano, R.; Kubo, W.; Kitamura, T.; Wada, Y.; Watanabe, M.; Yanagida, S. Chem. Commun. 2005, 740. (f) Matsui, H.; Okada, K.; Kawashima, T.; Watanabe, M. J. Photochem. Photobiol., A 2004, 164, 129. (5) (a) Lu, W.; Fadeev, A. G.; Qi, B.; Smela, E.; Mattes, B. R.; Ding, J.; Spinks, G. M.; Mazurkiewicz, J.; ZhFou, D.; Wallace, G. G.; MacFarlane, D. R.; Forsyth, S. A.; Forsyth, M. Science 2002, 297, 983. (b) Lu, W.; Fadeev, A. G.; Mattes, B. R. J. Electrochem. Soc. 2004, 151, H33. (c) Yang, C.; Sun, Q.; Qiao, J.; Li, Y. J. Phys. Chem. B 2003, 107, 12981. (d) Kim, K.; Lang, C.; Moulton, R.; Kohl, P. A. J. Electrochem. Soc. 2004, 151, A1168. (e) Yoshizawa, M.; Narita, A.; Ohno, H. Aust. J. Chem. 2004, 57, 139. (6) (a) Ue, M.; Takeda, M.; Toriumi, A.; Kominato, A.; Hagiwara, R.; Ito, Y. J. Electrochem. Soc. 2003, 150, A499. (b) Ue, M.; Takeda, M.; Takehara, M. Electrochem. Solid State Lett. 2002, 5, A119. (c) Sato, T.; Masuda, G.; Takagi, K. Electrochim. Acta 2004, 49, 3603.
to control the fluidity of ionic liquids and to eliminate leakage. Current approaches to quasi-solidification of ionic liquids can be divided into two categories: a method using polymeric materials7-12 and a method using low-molecular-weight gelators.13-18 For quasi-solidification of ionic liquids by polymeric materials, Carlin and co-workers reported a fabrication of a catalytic membrane with an ionic liquid gel using poly(vinylidene fluoride)hexafluoropropylene copolymer.7 A copolymer of more than 30 wt % was needed to make the ionic liquid-polymer gel, and 2-methylpentene as helper additive was necessary to make gel. Gra¨tzel’s group also used poly(vinylidene fluoride)hexafluoropropylene copolymer for making ionic liquid polymer gel elctrolyte.8 Rogers and co-workers reported the formation of an anisotropic viscous solution of 1-butyl3-methylimidazolium chloride by a cellulose polymer. They needed a considerable amount of cellulose (>10 wt %), and the formed matter was a viscous solution rather than a gel.9a They also reported gelation of ionic liquids using (7) (a) Carlin, R. T.; Fuller, J. Chem. Commun. 1997, 1345. (b) Fuller, J.; Breda, A. C.; Carlin, R. T. J. Electroanal. Chem. 1998, 459, 29. (8) Wang, P.; Zakeeruddin, S. M.; Exnar, I.; Gra¨tzel, M. Chem. Commun. 2002, 2972. (9) (a) Swatloski, R. P.; Spear, S. K.; Holbrey, J. D.; Rogers, R. D. J. Am. Chem. Soc. 2002, 124, 4974. (b) Klingshirn, M. A.; Spear, S. K.; Subramanian, R.; Holbrey, J. D.; Huddleston, J. G.; Rogers, R. D. Chem. Mater. 2004, 16, 3091. (10) (a) Stathatos, E.; Lianos, P.; Lavrencic-Stangar, U.; Orel, B. Adv. Mater. 2002, 14, 354. (b) Wang, P.; Zakeeruddin, S. M.; Comte, P.; Exnar, I.; Gra¨tzel, M. J. Am. Chem. Soc. 2003, 125, 1166. (11) Snedden, P.; Cooper, A. I.; Scott, K.; Winterton, N. Macromolecules 2003, 36, 4549. (12) Fukushima, T.; Kosaka, A.; Ishimura, Y.; Yamamoto, T.; Takigawa, T.; Ishii, N.; Aida, T. Science 2003, 300, 2072. (13) Kimizuka, N.; Nakashima, T. Langmuir 2001, 17, 6759. (14) Ikeda, A.; Sonoda, K.; Ayabe, M., Tamaru, S.; Nakashima, T.; Kimizuka, N.; Shinkai, S. Chem. Lett. 2001, 1154. (15) Amaike, M.; Kobayashi, H.; Shinkai, S. Bull. Chem. Soc. Jpn. 2000, 73, 2553. (16) Kubo, W.; Kambe, S.; Nakade, S.; Kitamura, T.; Hanabusa, K.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2003, 107, 4374. (17) Hanabusa, K.; Hiratsuka, K.; Kimura, M.; Shirai, H. Chem. Mater. 1999, 11, 649. (18) Mohmeyer, N.; Wang, P.; Schmidt, H.-W.; Zakeeruddin, S. M.; Gra¨tzel, M. J. Mater. Chem. 2004, 14, 1905.
10.1021/la051323h CCC: $30.25 © 2005 American Chemical Society Published on Web 10/06/2005
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Hanabusa et al. Scheme 1. Synthesis of Gelator 1
a cross-linked poly(ethylene glycol).9b Hybrid silica prepared from poly(propylene oxide) end-capped with triethoxysilane10a and silica nanoparticles10b were used to make quasi-solidification of ionic liquids. Snedden et al. reported a series of composites comprising polymers and ionic liquids prepared by in situ polymerization.11 Recently Aida’s group presented physical gelation of ionic liquids by single-walled carbon nanotubes.12 When crude singlewalled carbon nanotubes are used, they bring about gelation much less efficiently. On the other hand, there has been little work on quasi-solidification of ionic liquids by physical gelation using low-molecular-weight gelators. Kimizuka et al. studied physical gelation of ionic liquids by a compound consisting of glycolipid and L-glutamic acid, but the gelation ability of the compound seems to be limited to the imidazolium type of ionic liquids containing bromide ion.13 Shinkai’s group succeeded in physical gelation of both imidazolium and pyridinium types of ionic liquids with their gelator,14 which was synthesized via three reaction steps from cholesteryl chloroformate and 4-nitrophenyl-β-D-glucopyranoside.15 The disadvantage of this gelator is that it is almost insoluble in ionic liquids; therefore, they needed to add the acetone solution of the gelator to the ionic liquids to make gels. Kubo et al. reported a fabrication of dye-sensitized solar cells using gel electrolytes of ionic liquids.16 They gelled 1-alkyl-3methylimidazolium iodides using N-benzyloxycarbonylL-isoleucylaminooctadecane as the gelator. Although this amino acid derivative is an all-powerful gelator for ordinary solvents,17 the gelation ability toward ionic liquids is regrettably confined to bromide and iodide salts of 1-alkyl-3-methylimidazolium. Gra¨tzel’s group succeeded in fabrication of quasi-solid-state dye-sensitized solar cells with 1,3:2,4-di-O-benzilidene-D-sorbitol derivatives as gelators.18 In recent years there has been considerable interest in developing new types of gelators of organic fluids. Particularly intriguing is the fact that considerably large
volumes of fluid can be immobilized by very low amounts of gelator. Although many low-molecular-weight compounds have been demonstrated to form gels in organic solvents, there is no convenient gelator for ionic liquids as yet. We report here specialist gelators for ionic liquids that can harden a wide variety of ionic liquids at low concentrations without helper additives. Although there are procedures for physical gelation of a few ionic liquids by low-molecular-weight compounds,13,14,16 the proposed gelators appear to be unprecedented in their remarkable gelation ability toward many ionic liquids. Results and Discussion Screening Test of Low-Molecular-Weight Gelators. Low-molecular-weight compounds capable of hardening organic liquids, which are called “gelators”, are of special interest for not only for academic studies but also for potential applications.19-21 Gelators have unique characteristics of both good solubility upon heating and inducement of smooth gelation of organic fluids at low concentration. For example, when a hot solution of gelator is cooled to room temperature, a gel is easily formed in the course of the cooling process. The formed gels always exhibit thermally reversible sol-to-gel phase transition. This is because the driving forces of physical gelation by a gelator are the cooperating noncovalent interactions of gelator molecules, such as hydrogen bonding, van der Waals force, π-π interaction, and electrostatic interaction. With low melting point in mind, 24 ionic liquids were tested for gelation: 12 kinds of 1-alkyl-3-methylimidazolium ([Cnmim]+) salts, 3 kinds of 1-alkylpyridinium ([Cnpy]+) salts, 1-butyl-1-methylpyrazolidinium ([C4pyr]+) salt, 1-butyl-1-methylpiperidinium ([C4pip]+) salt, 1-hexyl-1(19) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (20) Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263. (21) Estroff, L. A.; Hamilton, A. D. Chem. Rev. 2004, 104, 1201.
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Figure 1. Chemical structures of typical ionic liquids and gelators.
methylmorpholinium ([C6mor]+) salt, and 6 kinds of ammonium ([ammo]+) salts. The [Cnmim]+ and [Cnpy]+ salts were prepared by conventional methods,22 and the others were supplied by manufactures. Twelve kinds of low-molecular-weight gelators, which included eight gelators developed by us17,23 and three commercially available ones, were selected for testing (the structures of the 12 kinds of gelators are shown in Figure S1 in the Supporting Information). The commercially available gelators are 1,3:2,4-di-O-dibenzylidene-D-sorbitol, 12hydroxystearic acid, and N-lauroyl-L-glutamic-R,β-dibutylamide. The screening test led us to conclude that cyclo(Lβ-3,7-dimethyloctylasparaginyl-L-phenylalanyl) (1) and cyclo(L-β-2-ethylhexylasparaginyl-L-phenylalanyl) (2)24 are excellent gelators for ionic liquids. The gelators except for 1 and 2 were almost insoluble in ionic liquids upon heating. Esterification of L-asparaginyl-L-phenylalanine methyl ester with alkanol lead to the formation of L-β(22) Bonhoˆte, P.; Dias, A.; Papagerorgiou, N.; Kalyanasundaram, K.; Gra¨tzel, M. Inorg. Chem. 1996, 35, 1168. (23) (a)Hanabusa, K.; Yamada, M.; Kimura, M.; Shirai, H. Angew. Chem., Int. Ed. Engl. 1996, 35, 1949. (b) Hanabusa, K.; Tange, J.; Taguchi, Y.; Koyama, T.; Shirai, H. Chem. Commun. 1993, 390. (c) Hanabusa, K.; Nakayama, H.; Kimura, M.; Shirai, H. Chem. Lett. 2000, 1070. (d) Hanabusa, K.; Shimura, K.; Hirose, K.; Kimura, M.; Shirai, H. Chem. Lett. 1996, 885. (24) Hanabusa, K.; Matsumoto, M.; Kimura, M.; Kakehi, A.; Shirai, H. J. Colloid Interface Sci. 2002, 224, 231.
alkylasparaginyl-L-phenylalanine methyl ester which was then transformed into cyclo(L-β-alkylasparaginyl-L-phenylalanyl) by heating (Scheme 1). L-Asparaginyl-L-phenylalanine methyl ester is a commodity known as the artificial sweetener “Aspartame”. Considering that structurally simple compounds are desirable from both fundamental and practical standpoints, the above gelators 1 and 2 are especially promising because they can be prepared easily in large amounts from available cheap materials. It is important to note that gelator 1 also shows versatile gelation ability for a variety of organic solvents such as alcohols, ketones, aromatic molecules, glycerides, edible oils, and silicone oils24 (see Table S1 in the Supporting Information). Nine other kinds of gelators, besides 1 and 2, could not gel ionic liquids owing to their low solubility, although N-benzyloxycarbonyl-L-isoleucylaminooctadecane and N-benzyloxycarbonyl-L-valyl-L-valylaminooctadecane were able to gel [C6mim]Br and [C6mim]I. The chemical structures of typical ionic liquids and gelators are shown in Figure 1. The results of a gelation test of ionic liquids by gelators 1 and 2 are summarized in Table 1, where the values mean the minimum gel concentrations (MGC) necessary to harden ionic liquids at 25 °C. The MGCs were determined by the upside-down test tube method. When a test tube filled with a gelled sample could be turned
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Table 1. Results of Gelation of Ionic Liquids by Gelators 1 and 2 at 25 °Ca ionic liquidb
1
2
[C2mim]BF4 [C2mim]TFSI [C3mim]BF4 [C3mim]TFSI [C4mim]BF4 [C4mim]PF6 [C4mim]TFSI [C4mim]CF3SO3 [C4mim]ClO4 [C6mim]BF4 [C6mim]PF6 [C6mim]I [C4py]BF4 [C6py]TFSI [C1C6py]TFSI [C4pyr]TFSI [C4pip]TFSI [C6mor]TFSI [ammo-1]TFSI [ammo-2]TFSI [ammo-3]BF4 [ammo-3]TFSI [ammo-4]BF4 [ammo-4]TFSI
Tl-Gel (3) Tl-Gel (4) Tl-Gel (4) Tl-Gel (5) Tl-Gel (5) Tl-Gel (5) Tl-Gel (8) Tl-Gel (20) Tl-Gel (12) Tp-Gel (8) Tl-Gel (8) Tl-Gel (>50) Tp-Gel (3) Tl-Gel (12) Tl-Gel (10) Tl-Gel (5) Tl-Gel (3) Tl-Gel (3) Tl-Gel (7) Tl-Gel (3) Tl-Gel (5) Tl-Gel (9) Tl-Gel (5) O-Gel (25)
Tl-Gel (5) Tl-Gel (7) Tl-Gel (6) Tl-Gel (9) Tl-Gel (7) Tl-Gel (8) Tl-Gel (12) Tl-Gel (30) Tl-Gel (15) Tp-Gel (12) Tl-Gel (12) Tl-Gel (>100) Tp-Gel (5) Tl-Gel (12) Tl-Gel (12) Tl-Gel (7) Tl-Gel (5) Tl-Gel (5) Tl-Gel (9) Tp-Gel (3) Tl-Gel (10) Tl-Gel (13) Tl-Gel (7) O-Gel (30)
Figure 2. Strength of ionic liquid gels formed by gelator 1: (9) [C4mim]BF4; (2) [C4py]BF4.
a Tp-Gel: transparent gel. Tl-Gel: translucent gel. O-Gel: opaque. The values mean the minimum gel concentrations at 25 °C; the unit is g L-1 (gelator/ionic liquid). b The structures of the ionic liquids are shown in Figure S2 in the Supporting Information.
upside-down without significant flow, we judged it a “gel”. The gelators gelled all of the ionic liquids to form translucent or transparent gels, generally even with less than 1 wt % of gelator. For example, 3 g of gelator 1 can convert 1 L of [C2mim]BF4 into the corresponding translucent gel. The results in Table 1 illustrate that the nature of the ionic liquids has no influence on the formation of the three-dimensional networks by the gelator molecules responsible for gelation. Photos of the actual gels of [C4mim]BF4 formed by gelator 1 and [ammo-2]TFSI by gelator 2 are shown in the Table of Contents graphic. The formed gels are so stable that crystallization does not occur even after 2 years. Gel Strength and Thermal Stability. Gel strength, which is an important factor in the application of gels, has been evaluated by measuring the elastic storage modulus G′ and loss modulus G′′ values at different concentrations of gelator.25 However, in our study we evaluated gel strength as the power necessary to sink a cylindrical bar (10 mm in diameter) 4 mm deep in the gels. The gel strengths of [C4mim]BF4 and [C4py]BF4 gels are plotted against the concentration of 1 (Figure 2). It is clear that gel strength increases at a rate nearly proportional to the concentration of added gelator. A similar relation between gel strength and the amount of added gelator was observed in the case of another gelator.17 The strength of the transparent gel of [C4py]BF4, prepared at a concentration of 60 g L-1 (1/[C4py]BF4), was ca. 1500 g cm-2. The formed gels show thermally reversible sol-to-gel transition because the three-dimensional networks responsible for gelation are built up by the noncovalent interactions. The sol-to-gel phase diagrams for [C4mim]BF4 and [C4py]BF4 are shown in Figure 3, where the MGCs are plotted against the temperature of the ionic liquids. The area above the plots is the gel phase and that below them is the sol phase. The transition temperature from (25) Mohmeyer, N.; Schmidt, H.-W. Chem. Eur. J. 2005, 11, 863.
Figure 3. Sol-to-gel phase diagrams. Plots of MGC of gelator 1 against temperature of ionic liquids: ([) [C2mim]TFSI; (9) [C4mim]BF4; (2) [C4py]BF4.
gel to sol increases with increasing concentration of gelator. For instance, the phase transition temperature of gel to sol, for gel prepared at a concentration of 70 g L-1 (1/ionic liquid), was more than 140 °C. In other words, cooling the isotropic solution lowers the MGCs necessary to build up the three-dimensional networks. From the nature of the gel phases it follows that a necessary requirement is that the pure gelator molecules must be solid at the temperature of their gels. In fact, the melting points of gelators 1 and 2 were 180-181 °C and 178-180 °C, respectively. In conclusion, we succeeded in preparing thermally stable gels by physical gelation of ionic liquids using gelator 1. As long as gels are prepared using gelator 1, the highest temperature at which the gel state can be held should be less than the melting point of 180 °C. Electron Microscope and FT-IR Spectroscope. The gelation of fluids by low-molecular-weight gelators is an attractive phenomenon in which the flow of bulk fluids is turned off macroscopically by the self-assembly of gelator molecules. When low-molecular-weight gelators cause physical gelation, three-dimensional networks in the gels are often observed by electron microscopy. This is because the driving force responsible for gelation is thought to be the formation of three-dimensional networks consisting of highly intertwined fibers. The first stage of physical
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Figure 5. Variable temperature FT-IR spectra for the mixture of gelator 1 and [ammo-3]TFSI.
Figure 4. SEM (a) and FE-SEM (b) images of [C4mim]BF4 gel formed by gelator 1.
gelation is the self-aggregation of gelator molecules. Figure 4 shows SEM and FE-SEM images of [C4mim]BF4 gel formed by gelator 1. The sample was prepared by washing with water to remove [C4mim]BF4 in the gel and then shaded by Au. It is unclear at this time whether the observed structures are directly related to those originally present in the ionic liquid gels. Nevertheless, a threedimensional image of a huge, steric, and deep aggregate is visible in the SEM, where the width of the fibrous aggregates range from 500 nm to several micrometers. The FE-SEM observation indicates that the broad fibrous aggregates are composed of many slender aggregates whose diameter is ca. 120 nm. However, the diameter of the aggregates in FE-SEM is still much larger than 16.7 nm as the molecular length of gelator 1. Although the FE-SEM image does not reveal the inner structure of the aggregates, their diameter indicates that they are probably superstructures consisting of molecular fibers. When sufficiently long, juxtaposed, and intertwined, these structures may encircle and immobilize the ionic liquid molecules in three-dimensional networks, leading to the physical gelation. The three-dimensional networks responsible for gelation are built up by the noncovalent interactions. In particular, intermolecular hydrogen bonding plays a crucial role in the case of gelators 1 and 2. IR spectra give useful information on the formation of hydrogen bonding. Figure
5 shows variable temperature FT-IR spectra in the range of 25-160 °C for the mixture of 1 and [ammo-3]TFSI. It should be mentioned that the macroscopic phase transition temperature from gel to sol for the gel consisting of 1 and [ammo-3]TFSI (14 wt %) was ca. 150 °C visually. The spectra in the range of 25-80 °C are characterized by 1670 cm-1, which is attributed to a CdO stretching vibration for hydrogen bonding. With increasing temperature above 100 °C, the peak gradually shifted to the upper field of frequency and finally settled to 1685 cm-1 at 160 °C, which indicates a CdO stretching vibration for nonhydrogen bonding. The gradual change of spectra near 150 °C coincides with the sol-to-gel transition temperature; namely, the phase transition from gel to sol is brought about by the collapse of hydrogen bonding. The change at around 150 °C is eventually linked to a macroscopic change from a gel state to a solution state. It should be mentioned that the attachment of a branched alkyl group onto the cyclo(dipeptide) core is also essential for gelators. The branched alkyl group not only enhances solubility in ionic liquids but also prompts association among the fibers, through van der Waals interaction, and more importantly prevents crystallization. Actually, the gel fails to form if the branched alkyl chain is replaced with a long normal alkyl group. Furthermore, deleting the branched alkyl chain in the gelators prevents gelation and readily permits crystallization in the solution. Conductivity of Ionic Liquid Gels. Solid electrolytes, which are electrically conductive solids with ionic carriers, are becoming increasingly important due to their potential use in the fields of solid-state batteries, fuel cells, energy storage, and chemical sensors.26-29 The gelation of ionic liquids by a gelator is a conventional method for making gel electrolytes, and it is expected to play a role in the field of solid electrolytes. Figure 6 shows the ionic conductivities of eight ionic liquids’ gels formed by gelator 1. In general, TFSI- (bis(trifluoromethylsulfonyl)imide) and BF4- salts of [C4mim]+ show higher ionic conductivity. For comparison, the ionic conductivities of pure ionic liquids without gelator are also plotted on the left side in Figure 6. The ionic conductivities of the gels are very similar to those of the pure ionic liquids themselves. Namely, the conductivity decreased very slightly, while (26) Bruce, P. G. Solid State Elecrochemistry; Cambridge University Press: 1995. (27) Munshi, M. Z. A. Handbook of Solid State Batteries & Capacitors; World Scientific: 1995. (28) Balbuena, P. B.; Wang, Y. Lithium-Ion Batteries: SolidElectrolyte Interphase; Imperial College Press: 2004. (29) Gray, F. M. Solid Polymer Electrolytes: Fundamentals and Technological Applications; VCH: 1991.
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Figure 6. Dependence of the ionic conductivities of the ionic liquid gels on the concentration of gelator 1 at room temperature: (O) [ammo-3]BF4; (4) [C4mim]TFSI; (b) [ammo-3]TFSI; (9) [C4mim]BF4; ([) [C4mim]PF6; (2) [C4py]BF4; (0) [C6min]BF4; (]) [C6mim]PF6. Table 2. Comparison of Ionic Conductivities of Pure Ionic Liquids and the Corresponding Gels Formed by Gelator 1 at Room Temperaturea ionic liquid [C4mim]BF4 [C4mim]PF6 [C4mim]TFSI [C6mim]BF4 [C6mim]PF6 [C4py]BF4 [C6py]TFSI [C6C1py]TFSI [ammo-1]TFSI [ammo-2]TFSI [ammo-3]TFSI [ammo-3]BF4 [ammo-4]TFSI [ammo-4]BF4 [C4pip]TFSI [C4pyr]TFSI [C6mor]TFSI
σ0 (S cm-1) 10-3
3.5 × 1.7 × 10-3 3.9 × 10-3 1.2 × 10-3 4.2 × 10-4 1.5 × 10-3 1.6 × 10-3 1.7 × 10-3 9.3 × 10-4 5.0 × 10-5 3.9 × 10-3 4.8 × 10-3 2.2 × 10-3 1.1 × 10-3 8.4 × 10-4 2.4 × 10-3 1.9 × 10-4
σ30 (S cm-1) 10-3
3.0 × 1.6 × 10-3 3.4 × 10-3 1.1 × 10-3 3.8 × 10-4 1.3 × 10-3 1.4 × 10-3 1.5 × 10-3 8.4 × 10-4 4.6 × 10-5 3.4 × 10-3 4.4 × 10-3 2.1 × 10-3 9.6 × 10-4 8.2 × 10-4 2.2 × 10-3 1.8 × 10-4
σ30/σ0 0.87 0.96 0.87 0.92 0.90 0.85 0.88 0.88 0.90 0.92 0.87 0.92 0.95 0.87 0.98 0.92 0.95
a σ ) conductivity of pure ionic liquids, σ 0 30 ) conductivity of ionic liquid gels formed by 30 g L-1 of 1.
increasing the amount of gelator. Gels of ionic liquids were characterized by high ionic conductivity; for instance, the ionic conductivities of [ammo-3]BF4 and [C4mim]TFSI gels formed by 1 (10 g L-1) were 4.75 and 3.61 mS cm-1, respectively. The results in Figure 6 also indicate that the gelator molecules hardly interfere with the mobility of anions and cations, which are components of ionic liquids. The ionic conductivities of various ionic liquids gels are compared with those of pure ionic liquids in Table 2, where the concentration of gelator 1 is kept constant at 30 g L-1 higher than each MGC. To assess the decrease in ionic conductivity by adding gelator, the ratios of σ30/σ0 were calculated. The σ0 and σ30 are ionic conductivities of pure ionic liquids and ionic liquid gels formed at a concentration of 30 g L-1, respectively. Despite the formation of stiff gels, the σ30/σ0 values suggest the ionic conductivities barely decreased. The temperature dependence of the ionic conductivity was studied at a constant concentration of 1 in [C4mim]BF4 (50 g L-1). Figure 7 shows the plots of ionic conductivities against 1/T for [C4mim]BF4 gel. The conductivities of pure [C4mim]BF4 itself are also plotted in Figure 7. The straight line was obtained between 2.3 × 10-3 and 2.8 × 10-3 T-1, suggesting that the temperature
Figure 7. Arrhenius plots for the ionic conductivity of [C4mim]BF4 gel and pure [C4mim]BF4: (9) [C4mim]BF4 gel formed by gelator 1; ([) pure [C4mim]BF4. Right and left broken lines correspond to 138 and 142 °C.
dependence obeyed a classical Arrhenius plot. The conductivities of the [C4mim]BF4 gel are slightly low compared to those of pure [C4mim]BF4; however, there is no difference between their activation energies. The gel-tosol phase transition temperature for the [C4mim]BF4 gel (50 g L-1), determined visually at heating, is 138-142 °C. The transition region is marked with broken lines in Figure 7, where the low-temperature zone across the transition region is the gel phase and the high-temperature zone is the sol phase. The straight relationship between the logarithm of the conductivities and 1/T indicates that the activation energy is unchanged regardless of whether the system is a gel phase or a sol phase. These results led us to conclude that [C4mim]BF4, which is located in the threedimensional networks built up by the gelator molecules, behaves like bulk [C4mim]BF4. When the three-dimensional networks are built up, the ionic liquid component is immobilized on a macroscopic scale but remains isotropic on a microscopic scale at the molecular level. Electrochemical stability is a fundamental requirement in applications of electrochemical devices. We measured the limited reduction and oxidation potentials of ionic liquid gels and evaluated the electrochemical stability of gelator 1. We studied the cyclic voltammograms of gels (14 wt %) of [ammo-3]TFSI and [C4mim]BF4 formed by gelator 1 (see Figure S3 in the Supporting Information). To our surprise, the gels are electrochemically stable over a wide potential range, and the stability of the gels is essentially the same as those of pure [ammo-3]TFSI and [C4mim]BF4 themselves.6c It is known that compounds having an amide segment tend to decompose oxidatively in the high-potential range. The unexpected fact that gelator 1 has electrochemical stability and a wide electrochemical window can be attributed to the 2,5-diketopiperazine structure, which is a very stable compact six-membered ring. The combined use of polar solvents, such as acetonitrile and propylene carbonate (PC), has been proposed, where ionic liquids are used as electrolytes in practical devices.6a,c The addition of polar solvent reduces viscosity and promotes the dissociation of ionic liquids, with a resulting improvement in conductivity. For instance, the fabrication of an electrical double-layer capacitor using [ammo-3]BF4 containing PC was reported.6c This prompted us to study the gelation of a series of mixtures of [C4mim]BF4 and PC by gelator 1, in which the volume ratio of [C4mim]BF4 and
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proportional to the concentration of added gelator, and the strength of the gel prepared at a concentration of 60 g L-1 (gelator/ionic liquid) was ca. 1500 g cm-2. The formed gels of the ionic liquids were very thermally stable; no melting occurs up to 140 °C when the gels were prepared at a concentration of 70 g L-1 (gelator/ionic liquid). The gelator had electrochemical stability and a wide electrochemical window. The ionic conductivities of the gels of the ionic liquids were very similar to those of the pure ionic liquids. When prepared from a binary system of ionic liquids and propylene carbonate, the ionic conductivities of the gels considerably increased in comparison to those of the pure ionic liquids. The gelators also gel the mixture of ionic liquids and supporting electrolytes. The proposed gelators have the potential to provide solidlike materials with an ionic conductivity approaching that of a pure ionic liquid. Figure 8. Ionic conductivities of mixture gels of [C4mim]BF4 and PC: (b) [C4mim]BF4/PC ) 1:1; (9) [C4mim]BF4/PC ) 2:1; (2) [C4mim]BF4/PC ) 5:1; (1) [C4mim]BF4/PC ) 10:1; ([) pure [C4mim]BF4.
PC was varied systematically from 100% [C4mim]BF4 to 100% PC. In this case, ionic liquids in PC are thought to be supporting electrolytes. Gelator 1 could gel mixtures regardless of the PC ratio, and the MGCs were nearly proportional to the ratio of PC in the mixture. The MGCs of gelator 1 necessary to gel 100% of the [C4mim]BF4 and 100% of the PC were 5 g L-1 and 23 g L-1 at 25 °C, respectively. The ionic conductivity of gels composed of [C4mim]BF4, PC, and gelator 1 are shown in Figure 8. The [C4mim]BF4/PC ratios were changed from 1/1 to 10/1 (v/v). The plots on the left side indicate the ionic conductivities of the mixture of [C4mim]BF4 and PC without gelator. Considering that the ionic conductivity of pure [C4mim]BF4 itself is 3.5 mS cm-1, it is apparent that the ionic conductivities of the mixtures increase with the amount of added PC. It is noteworthy that the ionic conductivities decrease very slightly with increasing concentration of gelator. The slight decrease in the ionic conductivities of the gels suggests that the gelators are very useful for making ionic liquid gels. The mixtures of ionic liquids and supporting electrolytes were studied to achieve high ionic conductivity.30,31 In the solution consisting of ionic liquids and supporting electrolytes, ionic carriers are generated from not only ionic liquids but also supporting electrolytes. We studied the gelation ability of gelator 1 toward the solution of LiBF4 and [C4mim]BF4. The addition of 18 g of gelator 1 was needed to gel 1 L of [C4mim]BF4 solution containing LiBF4 of 1.0 M concentration, although the MGC for pure [C4mim]BF4 was 5 g L-1. The lowering of the gelation ability can be explained by taking into account the hindering of the formation of hydrogen bonding by a Li+ ion with large electronegativity. Conclusion The cyclo(dipeptide)s, simply prepared from the Lasparaginyl-L-phenylalanine methyl ester known as “Aspartame”, are specialist gelators for ionic liquids and can gel many kinds of ionic liquids. In particular, cyclo(L-β3,7-dimethyloctylasparaginyl-L-phenylalanyl) could generally gel ionic liquids even with less than 1 wt % of the gelator. The gel strength increased at a rate nearly (30) Sutto, T. E.; DeLong, H. C.; Trulove, P. C. Z. Naturforsch. 2002, 57a, 839. (31) Sasabe, H.; Matsumoto, H. Electrochem. Commun. 2003, 5, 594.
Experimental Section Instrumentation. Elemental analysis was performed with a Perkin-Elmer 240B analyzer. Infrared spectra were recorded on a Jasco FT-IR-7300 spectrometer using a KBr plate. The strength of the gels was measured with a Sun Science RHEO TEX SD-305. SEM and FE-SEM were done with a Hitachi S-2380 scanning electron microscope and a Hitachi S-5000 field emission scanning electron microscope. Impedance spectra were recorded on a Solartron 1255B frequency response analyzer and SI 1287 electrochemical interface between 1.0 Hz and 1.0 MHz at an oscillation level of 100 mV. Cyclic voltammograms of the gels were measured with a Solartron 1255B frequency response analyzer and SI 1287 electrochemical interface at the scanning rate of 5 mV sec-1. Synthesis of Gelator. Cyclo(L-β-3,7-dimethyloctylasparaginyl-L-phenylalanyl) (1) was prepared by esterification of Lasparaginyl-L-phenylalanine methyl ester with 3,7-dimethyl-1octanol, which is commercially available as the artificial sweetener “Aspartame”, followed by cyclization by heating. A mixture of p-toluenesulfonic acid (PTS) monohydrate (67.91 g, 0.357 mol) and benzene (200 mL) in a 1 L flask was refluxed for 1.5 h using a Dean-Stark trap until water was taken off. To the resulting solution 3,7-dimethyl-1-octanol (59.20 g, 0.374 mol), L-asparaginyl-L-phenylalanine methyl ester (“Aspartame”) (100.0 g, 0.357 mol), and benzene (1000 mL) were added. This mixture was refluxed for 11 h using a Dean-Stark trap, after which time no additional water was separated. The cooled solution was extracted with NaOH(aq) (20.00 g, 0.50 mol), washed 2 times with water (300 mL), and dried by MgSO4. After evaporating the filtrate without MgSO4, the resulting oil was heated to 140 °C for 3 h under reduced pressure using an aspirator. The oil began to solidify after 30 min under heating. The formed solid matter was dissolved in hot 1-propanol (1.6 L) and cooled at room temperature to make the gel. The gel was sufficiently broken up using a mechanical stirrer for 9 h, filtered off, and dried. The complete destruction of the gel by the mechanical stirrer was essential for further suction filtration. Cyclo(L-β-3,7-dimethyloctylasparaginyl-L-phenylalanyl) (1) was obtained in a yield of 89.13 g (65%). The purification procedure with 1-propanol was repeated for the elemental analysis sample. Elemental analysis Calcd for C23H34N2O4: C, 68.63; H, 8.51; N, 6.96. Found: C, 68.41; H, 8.27; N, 7.14. Cyclo(L-β-2-ethylhexylasparaginyl-L-phenylalanyl) (2) was obtained with 2-ethyl-1-hexanol by a similar procedure. Gelation Test and Gel Strength. A typical gelation test is as follows: a weight of gelator and ionic liquid (2.0 mL) were heated in a glass tube with a screw cap (inside diameter, 14 mm) until the solid dissolved. The resulting solution was cooled at 25 °C for 2 h, and then the gelation was checked visually. The macroscopic manifestation of successful gelation is the absence of observable flow upon inversion of the test tube. We evaluated quantitatively the gelation ability by determining the minimum gel concentration (MGC), which is the minimum concentration of gelator necessary for gelation at 25 °C. The unit of MGC is g L-1 (gelator/ionic liquid).
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The gel strength of the gels was measured and evaluated as the power necessary to sink a cylinder bar (10 mm in diameter) 4 mm deep in the gels. Electrode for Conductivity Measurement. A Teflon spacer (15 mm × 20 mm, 2 mm thickness) having a hole of 6 mm diameter was sandwiched between two mirror-finished stainless steel (SUS) electrodes (15 mm × 30 mm) and subjected to the impedance measurements. The isotropic hot solution of ionic liquid containing the gelator was injected into the hole sandwiched with the SUS plates and then cooled to form gel. Specimens for SEM and FE-SEM. For specimens for SEM, [C4mim]BF4 gel was prepared using gelator 1 at 3 g L-1. The gel was immersed in water for 4 days, during which time water was replaced by fresh water several times. The gel, after removing [C4mim]BF4, was converted to xerogel by freeze-drying under vacuum. The xerogel was mounted on an aluminum stub and
Hanabusa et al. coated to 100-150 Å thick with Ag by sputtering. To prepare the specimens for FE-SEM, the xerogel was shadowed to ca. 100 Å thick with Pt-Pd by sputtering.
Acknowledgment. This work was supported by the Grant-in-Aid for 21st Century COE Program and a Grant (No. 15350132) by the Ministry of Education, Culture, Sports, Science, and Technology of Japan. Supporting Information Available: Gelation ability of 1 for organic solvents; structures of the 12 kinds of gelators; structures of ionic liquids; electrochemistry of gels. This material is available free of charge via the Internet at http://pubs.acs.org. LA051323H
