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Effect of gelator structures on electrochemical properties of ionic-liquid supramolecular gel electrolytes
Electrochimica Acta 55 (2010) 2275–2279
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Effect of gelator structures on electrochemical properties of ionic-liquid supramolecular gel electrolytes Xuelin Dong a , Hong Wang a,∗ , Fang Fang a , Xue Li a , Yajiang Yang b,∗∗ a
Institute of Analytical Chemistry, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China Institute of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Luoyu Rd. 1037, Wuhan 430074, China b
a r t i c l e
i n f o
Article history: Received 15 July 2009 Received in revised form 12 November 2009 Accepted 14 November 2009 Available online 24 November 2009 Keywords: Chemical structure Gelator Ionic liquid Supramolecular gel Electrolytes
a b s t r a c t Bis(4-acylaminophenyl)methane (G1) and bis(4-acylaminophenyl)ether (G2) with varied acyl chains were found to be efficient gelators for the gelation of imidazole-based ionic liquids. The supramolecular gel electrolytes were formed via the self-assembly of these gelators in ionic liquids. The minimum gelator concentrations (MGCs) for the gelation of ionic liquids depend on the chemical structures of the gelators. The longer the acyl chains, the lower the MGCs. Polarized optical microscopy images of the ionic-liquid gels reveal the formation of spherical crystallites resulting from the fibrillar aggregates of the gelators. In addition, the phase transition temperatures of the ionic-liquid gels increase with an increase of the acyl chain length of the gelators. The impedance spectra of the ionic-liquid gels indicate that the temperature dependence of the conductivity follows the classical Arrhenius equation. The conductivities of ionic-liquid gels also decrease with an increase of the acyl chain length, but the differences in conductivities between the gels and corresponding ionic liquids are in one order of magnitude. The ionic-liquid gels possess a stable electrochemical window. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Supramolecular gels are semi-solid materials formed by the self-assembly of the low-molecular weight gelators in organic or aqueous liquids [1–4]. Ionic liquids are molten salts at room temperature, which have recently received increasing attention because of their unique properties, such as nonvolatility, wide range of liquid phase temperature, nonflammability, high ionic conductivity, and a wide electrochemical window [5–7]. Ionic liquids can also be gelatinized in the presence of gelators to form ionic-liquid supramolecular gels, which reduce their fluidity and overcome leakage in certain applications such as gel electrolytes [8]. In recent years, Kimizuka and Nakashima [9] reported physical gelation of ionic liquids formed by the addition of amide-group enriched glycolipids and l-glutamic acid derivatives as gelators, providing self-assembling gels. However, these carbohydrates seem to be good gelators only for selected ionic liquids containing hydrophilic bromide as anions, not for those containing hexafluorophosphate (PF6 − ). Shinkai and co-workers prepared ionic-liquid
∗ Corresponding author. ∗∗ Corresponding author. Tel.: +86 27 87547141; fax: +86 27 87543632. E-mail addresses: [email protected] (H. Wang), [email protected] (Y. Yang). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.11.042
gels in the presence of gelators synthesized from cholesteryl chloroformate and 4-nitrophenyl-␣-d-glucopyranoside, but the gelation requires acetone as co-solvent because of the limited solubility of the gelator [10]. Mohmeyer et al. [11] found that amphiphilic cyclohexanecarboxylic acid-[4-(3-tetradecylureido)phenyl] amide was an efficient gelator for binary mixtures of ionic liquids. The resultant ionic-liquid gels make it possible to prepare stable quasi-solid-state dye-sensitized solar cells. Yanagida and co-workers reported a dyesensitized solar cell fabricated using ionic-liquid gel electrolytes formed by N-benzyloxycarbonyl-l-isoleucyl aminooctadecane as gelator, which showed a 5% light-to-electricity conversion efficiency and high temperature stability [12,13]. Hanabusa et al. [8] synthesized two types of gelators which allowed gel formation of a wide variety of ionic liquids including imidazolium, pyridinium, pyrazolidinium, piperidinium, morpholinium and ammonium salts. Most recently, Tu et al. [14] used carbene complexes as efficient gelators to prepare different types of ionic-liquid gels at concentrations as low as 0.5 mg mL−1 . To the best of our knowledge, little further attention has been paid to the effects of chemical structures of gelators on the gelation of ionic liquids and properties of the corresponding ionic-liquids gels, particularly on the electrochemical properties of the ionic-liquid gels. In this work, two types of gelators with different acyl groups were used for the gelation of imidazole-based ionic liquids. The influence of the chemical structure of the gelators on the gelation of the ionic liquids and on the properties of the resultant ionic-liquids
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X. Dong et al. / Electrochimica Acta 55 (2010) 2275–2279
Scheme 1. Chemical structures of two series of gelators (G1 and G2) and two types of ionic liquids.
gels has been investigated via minimum gelator concentrations, sol–gel phase transition temperature and electrochemical properties. The investigations not only increase our understanding of the relationship between the structure of the gelator and the electrochemical properties of ionic-liquid gels, but also are significant for the properties of potentially useful electrolyte materials, modulated by variation of the structures of the gelators.
a scanning rate of 5 mV s−1 . The experiments were conducted at 25 ± 1 ◦ C. Similarly, the ionic-liquid gels were prepared in a glass vessel equipped with a Teflon stopper. Two mirror polished stainless steel electrodes (10 mm × 10 mm) were stably fixed into the Teflon stopper. Ionic conductivity was measured by the impedance technique over a frequency range from 50 MHz to 1 MHz at an oscillation level of 100 mV. The software for data treatment was Zview.
2. Experimental 3. Results and discussion 2.1. Materials 3.1. Gelation of ionic liquids The ionic liquids, 1-butyl-3-methylimidazolium tetrafluoroborate ([C4 mim]BF4 ) and 1-hexyl-3-methylimidazolium tetrafluoroborate ([C6 mim]BF4 ) were purchased from Shanghai Chenjie Chemical Co. Ltd. The purity of ionic liquids is 99.9%. Traces of water in the ionic liquids were removed by applying a vacuum for 12 h at 80 ◦ C [15]. Two series of gelators, bis(4-acylaminophenyl)methane (G1) and bis(4-acylaminophenyl)ether (G2) were synthesized according to the methods described previously [16,17]. The chemical structures of the G1 and G2 gelators and the ionic liquids are shown in Scheme 1. 2.2. Preparation and characterization of the ionic-liquid gels Calculated amounts of gelators and ionic liquids were put into glass vials and heated until the solid completely dissolved. The solution was allowed to cool at room temperature and exhibited no gravitational flow upon inversion of the vials. The required minimum gelator concentrations (MGCs) [17] were measured at an increment of 0.1 mg mL−1 of the gelator. Measurements of gel–sol phase transition temperatures (TGS ) were conducted according to the method of ball falling described elsewhere [18]. A small steel ball (250 mg, ˚ 4 mm) was placed on the top of the ionic-liquid gel in a glass vial. Then the sample was slowly heated (5 ◦ C/h) in a thermostatted oil bath. When the ball fell to the bottom of the vial, the temperature was defined as the gel–sol transition temperature (TGS ). Hot solution mixtures of gelator and ionic liquid were dropped on a pre-heated glass plate and then allowed to cool at room temperature. After the gel sample was kept in a dark place for 4 h, the gel sample was imaged by optical polarized microscope (POM, BH-2, Olympus). 2.3. Electrochemical measurements The ionic-liquid gels were prepared in a three-electrode cell, containing a glass carbon as the work electrode, Pt wire and Pt plate as the reference and counter electrode, respectively [15]. Linear sweep of the ionic-liquid gels was measured using a Zhanner IM6e electrochemical working station under a N2 atmosphere at
In general, the gelation of liquid media is a process of selfassembly of gelator molecules via noncovalent interactions such as hydrogen bonding resulting in a three-dimensional network of supramolecular structures [2]. The molecules of the liquid medium are immobilized in this three-dimensional network by capillary forces. In our experiments, the G1 and G2 gelators were found to be efficient gelation agents for the imidazole-based ionic liquids without any co-solvent. The gelation ability of the gelators can be characterized by minimum gelator concentrations (MGCs), which are summarized in Table 1. For a certain ionic liquid, the MGCs of the G2 gelators were lower than those of the G1 gelators, indicating that the gelation ability of G2 gelators is superior to that of G1. Theoretically, this can be most likely ascribed to the ether oxygen group in the G2 gelators. Herein, the presence of two lone pairs of electrons on the oxygen atoms redounds to the formation of intermolecular hydrogen bonding of G2 gelators. In addition, the data in Table 1 also reveal that the longer the length of the acyl chains in the gelator, the lower the MGCs of the gelators. This may be explained in terms of more solvophilic characteristic of the gelators with higher flexible long acyl chains or more likely that the long acyl chains modify the solvent compatibility of the gelator, resulting in less gelator molecules required for the immobilization of ionic-liquid molecules. Usually gelators self-assemble into fibrillar aggregates in organic media [2,11]. This process, in nature, is a kind of crystallization of the gelators in solvents. In the present work, the morphology of fibrillar aggregates could not be imaged by SEM micrographs because of the nonvolatility of ionic liquids, which hardly convert ionic-liquid gels to xerogels. Thus, the morphology of the gelator aggregates has to be indirectly characterized Table 1 Minimum gelator concentrations (mg mL−1 ) of ionic liquids using G1 and G2 gelators.
C4 mimBF4 C6 mimBF4
G1-5
G1-7
G1-11
G2-5
G2-7
G2-11
30.8 38.5
21.5 15.0
17.8 6.0
7.0 6.0
6.0 5.2
5.2 4.4
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Fig. 1. POM images of ionic-liquid gels formed by (A) G2-5, (B) G2-7 and (C) G2-11 in [C4 mim]BF4 . The concentration of gelators in all samples was 30 mg mL−1 . The magnification is 100×.
Fig. 2. TGS of ionic-liquid gels versus concentrations of gelators in [C4 mim]BF4 . Plot A: G1-5 (), G1-7 (䊉) and G1-11 (). Plot B: G2-5 (), G2-7 (䊉) and G2-11 ().
by polarized optical microscopy (POM) [2]. Fig. 1 shows the POM images of the ionic-liquid gels formed by G2 gelators in [C4 mim]BF4 . The typical Maltese cross extinction (birefringence) reveals that the fibrils further aggregate into spherical crystallites. We note that POM can only give birefringence information on the morphology of crystallites, for instance, spherical or non-spherical. Their accurate size is hard to be obtained by POM due to the low magnification. The molecules of the ionic liquids are immobilized by capillary forces in three-dimensional networks within the spherical crystallites. Similar POM images of the ionic-liquid gels formed by other gelators were obtained (not shown here), suggesting that the structures of the gelators exert no significant influence on the morphology of the spherical crystallites. 3.2. The phase transition temperatures (TGS ) of ionic-liquid gels Since supramolecular gels are usually thermoreversible, the gel–sol phase transition temperature (TGS ) is an important parameter for the characterization of their thermal stability. In nature, the TGS values characterize the dissociation temperatures of the gelator aggregates. In other words, the interactions involved in the self-assembly of the gelators are destroyed at higher temperatures. Fig. 2 shows the TGS of the ionic-liquid gels formed by G1 and G2 gelators at varying concentrations. Apparently, the TGS values of ionic-liquid gels are proportional to the concentrations of the gelators. Interestingly, the TGS values are also influenced by the lengths of the acyl chains of the gelators. As shown in Fig. 2, the TGS decrease in the order of G1-11 (G2-11) > G1-7 (G2-7) > G1-5 (G2-5). This phenomenon may involve the solvent compatibility of the gelators as discussed above. Considering that supramolecular gels are viscoelastic materials reinforced by gelator aggregates, a more solvophilic aggregates could be beneficial to reinforce the gels, leading to high TGS . As discussed on the basis of the data in Table 1, the gelator aggregates with flexible long acyl chains exhibit more solvophilic in compari-
son with the case of short acyl chains. In addition, more twistable aggregates are formed in case of gelators with longer acyl chains, resulting in more stable gelator aggregates formed. As a consequence, they possess a comparatively good ability to endure the external force (ball weight) involved in breaking the gels. This is, most likely, the reason why TGS increases with an increase of the lengths of the acyl chains. 3.3. The conductivities and electrochemical windows of ionic-liquid gels Fig. 3 shows the ionic conductivities of [C4 mim]BF4 gels formed by the G2-7 gelators versus temperature. As reference, the ionic conductivity of pure [C4 mim]BF4 was also plotted in Fig. 3. To obtain better gel strength for conductivity measurements, 25 mg mL−1 of G2-7 (higher than its MGC) was applied. The double dashed lines in Fig. 3 represent the TGS values of the [C4 mim]BF4 gels (ca. 135 ◦ C). Obviously, the conductivities of gels in the case of the temperature above the TGS are much closer to corresponding solution states.
Fig. 3. Arrhenius plots of the ionic conductivities of [C4 mim]BF4 gels (䊉) formed by G2-7 (25 mg mL−1 ) and pure [C4 mim]BF4 ().
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Table 2 Ionic conductivities of [C4 mim]BF4 gels formed by G1 and G2 gelators at 25 ◦ C. [C4 mim]BF4 gels formed by
1 /mS cm−1
1 /0∗
G1-5 G1-7 G1-11 G2-5 G2-7 G2-11
2.74 2.7 2.5 2.2 2.1 1.4
0.97 0.96 0.89 0.79 0.75 0.50
The 0 of pure [C4 mim]BF4 was 2.8 mS cm−1 at 25 ◦ C. The concentrations of G1 and G2 gelators were 45 mg mL−1 and 25 mg mL−1 , respectively.
However, the conductivities of the gels are slightly lower than those of pure [C4 mim]BF4 in the case of the temperature was lower than TGS . The differences of ionic conductivity between gels and pure [C4 mim]BF4 are in the range of one order of magnitude. In addition, the relationship between the logarithm of ionic conductivity (log ) and the reciprocal of the temperature (1/T) shows good linearity, indicating that the temperature dependence of the conductivity follows the classical Arrhenius equation. These results are consistent with the assumption made in the discussion of the POM images, namely, that the ionic liquid immobilized by capillary forces in the three-dimensional networks is still microscopically behaving like a liquid. The conductivities ( 1 ) of the [C4 mim]BF4 gels formed by the different gelators are listed in Table 2. The conductivities ( 0 ) of corresponding pure [C4 mim]BF4 was found to be 2.8 mS cm−1 . The ratios ( 1 / 0 ) can be used to evaluate the variation of the ionic conductivities before and after the gelation of the ionic liquids. The data in Table 2 reveal that the length of acyl chains exerts an effect on the
conductivities of the ionic-liquid gels. For example, the conductivities of the [C4 mim]BF4 gels formed by G2-5, G2-7 and G2-11 were 2.2 mS cm−1 , 2.1 mS cm−1 and 1.4 mS cm−1 , respectively, indicating that the conductivity decreased with an increase of the lengths of acyl chain. A similar trend can be also observed for the [C4 mim]BF4 gels formed by the G1 gelators. These results can be related to the density of the three-dimensional network formed by gelator aggregates. Long acyl chains easily entangle and twist to form a higher density of three-dimensional networks, which is interruptive for the mobility of ions within the gels, resulting in a lower conductivity. In addition, it is also correlative with the gelator concentration was applied. In order to obtain stable gels for the measurements of conductivity, 45 mg mL−1 of G1 was applied due to their higher MGCs and 25 mg mL−1 of G2 was applied. Thus, the concentrations of G1-5, G1-7 and G1-11 were 1.46, 2.09 and 2.52 times to their MGCs, respectively. In case of G2, the concentrations of G2-5, G2-7 and G2-11 were 3.57, 4.17 and 4.8 times to their MGCs, respectively. As a result, more dense three-dimensional network was formed in case of G2, leading to the decrease of their conductivities. Based on this explanation, the ratios ( 1 / 0 ) in case of the G1 gelators were higher than those of the G2 gelators, suggesting that the variation of the conductivities of the ionic liquids before and after gelation by G1 gelators was small in comparison with that by G2 gelators. These results are consistent with the conclusions in the gelation of the ionic liquids as discussed about MGCs in Table 1. The electrochemical window of ionic-liquid gels is of immediate importance for the application as gel electrolytes. Fig. 4 shows the linear sweep voltammetry of ionic-liquid gels formed by gelators. As shown in Fig. 4A and B, there is no significant effect of gelators with varying acyl groups on the electrochemical window
Fig. 4. Plot A: The electrochemical windows of pure [C4 mim]BF4 and corresponding gels formed by 45 mg mL−1 of G1-5, G1-7 and G1-11. Plot B: The electrochemical windows of pure [C4 mim]BF4 and corresponding gels formed by 25 mg mL−1 of G2-5, G2-7 and G2-11. Plot C: The electrochemical windows of pure [C4 mim]BF4 and [C6 mim]BF4 and corresponding gels formed by 25 mg mL−1 of G2-7.
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of corresponding ionic-liquid gels. From Fig. 4C, it was found that the cations of the ionic liquids have also no significant effect on the electrochemical window in case the same gelator was applied. In addition, the linear sweep voltammetry of ionic-liquid gels indicates that the electrochemical window is stable for all samples, suggesting that the gel formation has no influence on the redox behavior of the ionic liquids.
Acknowledgements
4. Conclusions
[1] P. Terech, R. Weiss, Chem. Rev. 97 (1997) 3133. [2] G. Mathew, R. Weiss, Acc. Chem. Res. 39 (2006) 489. [3] R.G. Weiss, P. Terech (Eds.), Molecular Gels. Materials with Self-assembled Fibrillar Networks, Springer, Dordrecht, The Netherlands, 2005. [4] M. Kimura, S. Kobayashi, T. Kuroda, K. Hanabusa, H. Shirai, Adv. Mater. 16 (2004) 335. [5] N.L. Lancaster, P.A. Salter, T. Welton, G.B. Young, J. Org. Chem. 67 (2002) 8855. [6] C.J. Bradaric, A. Downard, C. Kennedy, A.J. Robertson, Y. Zhou, Green Chem. 5 (2003) 143. [7] Y. Ito, T. Nohira, Electrochim. Acta 45 (2000) 2611. [8] K. Hanabusa, H. Fukui, M. Suzuki, H. Shirai, Langmuir 21 (2005) 10383. [9] N. Kimizuka, T. Nakashima, Langmuir 17 (2001) 6759. [10] A. Ikeda, K. Sonoda, M. Ayabe, S. Tamaru, T. Nakashima, N. Kimizuka, S. Shinkai, Chem. Lett. 30 (2001) 1154. [11] N. Mohmeyer, D. Kuang, P. Wang, H.-W. Schmidt, S.M. Zakeeruddin, M. Grätzel, J. Mater. Chem. 16 (2006) 2978. [12] W. Kubo, T. Kitamura, K. Hanabusa, Y. Wada, S. Yanagida, Chem. Commun. 4 (2002) 374. [13] W. Kubo, S. Kambe, S. Nakade, T. Kitamura, K. Hanabusa, Y. Wada, S. Yanagida, J. Phys. Chem. B 107 (2003) 4374. [14] T. Tu, X. Bao, W. Assenmacher, H. Peterlik, J. Daniels, K.H. Dötz, Chem. Eur. J. 15 (2009) 1853. [15] A.M. O’Mahony, D.S. Silvester, L. Aldous, C. Hardacre, R.G. Compton, J. Chem. Eng. Data 53 (2008) 2884. [16] Y. Meng, Y. Yang, Electrochem. Commun. 9 (2007) 1428. [17] X. Chang, L. Wang, Y. Yang, X. Yang, H. Xu, Mater. Chem. Phys. 99 (2006) 61. [18] A. Friggeri, B.L. Feringa, J. van Esch, J. Control. Release 97 (2004) 241.
Our investigation of the formation of ionic-liquid gels indicates that the lengths of the acyl chains in the gelators have a significant influence on their gelation ability. The measurements of minimum gelator concentrations (MGCs) suggest that the gelation ability of gelators becomes stronger with an increase of the length of the acyl chains. POM images of ionic-liquid gels reveal that the gelators selfassemble into fibrillar aggregates in ionic liquids and further form spherical crystallites. The molecules of the ionic liquids are immobilized by capillary forces within the three-dimensional network formed by the gelator aggregates. TGS values of ionic-liquid gels also increase with an increase of the length of acyl chains. The conductivities of ionic-liquid gels and the corresponding pure ionic liquid possess values within the range of one order of magnitude. The temperature dependence of the conductivities of ionic-liquid gels follows the classical Arrhenius equation. The linear sweep of ionicliquid gels shows a stable electrochemical window. The present results provide strategies to modulate the properties of ionic-liquid gels as novel electrolytes by variation of the structures of the gelators.
This work was financially supported by Hi-Tech Research and Development Plan of China (No. 2007AA03Z246). We thank the Analytical Test Center of HUST for all measurements. References
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Effect of gelator structures on electrochemical properties of ionic-liquid supramolecular gel electrolytes Xuelin Dong a , Hong Wang a,∗ , Fang Fang a , Xue Li a , Yajiang Yang b,∗∗ a
Institute of Analytical Chemistry, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China Institute of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Luoyu Rd. 1037, Wuhan 430074, China b
a r t i c l e
i n f o
Article history: Received 15 July 2009 Received in revised form 12 November 2009 Accepted 14 November 2009 Available online 24 November 2009 Keywords: Chemical structure Gelator Ionic liquid Supramolecular gel Electrolytes
a b s t r a c t Bis(4-acylaminophenyl)methane (G1) and bis(4-acylaminophenyl)ether (G2) with varied acyl chains were found to be efficient gelators for the gelation of imidazole-based ionic liquids. The supramolecular gel electrolytes were formed via the self-assembly of these gelators in ionic liquids. The minimum gelator concentrations (MGCs) for the gelation of ionic liquids depend on the chemical structures of the gelators. The longer the acyl chains, the lower the MGCs. Polarized optical microscopy images of the ionic-liquid gels reveal the formation of spherical crystallites resulting from the fibrillar aggregates of the gelators. In addition, the phase transition temperatures of the ionic-liquid gels increase with an increase of the acyl chain length of the gelators. The impedance spectra of the ionic-liquid gels indicate that the temperature dependence of the conductivity follows the classical Arrhenius equation. The conductivities of ionic-liquid gels also decrease with an increase of the acyl chain length, but the differences in conductivities between the gels and corresponding ionic liquids are in one order of magnitude. The ionic-liquid gels possess a stable electrochemical window. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Supramolecular gels are semi-solid materials formed by the self-assembly of the low-molecular weight gelators in organic or aqueous liquids [1–4]. Ionic liquids are molten salts at room temperature, which have recently received increasing attention because of their unique properties, such as nonvolatility, wide range of liquid phase temperature, nonflammability, high ionic conductivity, and a wide electrochemical window [5–7]. Ionic liquids can also be gelatinized in the presence of gelators to form ionic-liquid supramolecular gels, which reduce their fluidity and overcome leakage in certain applications such as gel electrolytes [8]. In recent years, Kimizuka and Nakashima [9] reported physical gelation of ionic liquids formed by the addition of amide-group enriched glycolipids and l-glutamic acid derivatives as gelators, providing self-assembling gels. However, these carbohydrates seem to be good gelators only for selected ionic liquids containing hydrophilic bromide as anions, not for those containing hexafluorophosphate (PF6 − ). Shinkai and co-workers prepared ionic-liquid
∗ Corresponding author. ∗∗ Corresponding author. Tel.: +86 27 87547141; fax: +86 27 87543632. E-mail addresses: [email protected] (H. Wang), [email protected] (Y. Yang). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.11.042
gels in the presence of gelators synthesized from cholesteryl chloroformate and 4-nitrophenyl-␣-d-glucopyranoside, but the gelation requires acetone as co-solvent because of the limited solubility of the gelator [10]. Mohmeyer et al. [11] found that amphiphilic cyclohexanecarboxylic acid-[4-(3-tetradecylureido)phenyl] amide was an efficient gelator for binary mixtures of ionic liquids. The resultant ionic-liquid gels make it possible to prepare stable quasi-solid-state dye-sensitized solar cells. Yanagida and co-workers reported a dyesensitized solar cell fabricated using ionic-liquid gel electrolytes formed by N-benzyloxycarbonyl-l-isoleucyl aminooctadecane as gelator, which showed a 5% light-to-electricity conversion efficiency and high temperature stability [12,13]. Hanabusa et al. [8] synthesized two types of gelators which allowed gel formation of a wide variety of ionic liquids including imidazolium, pyridinium, pyrazolidinium, piperidinium, morpholinium and ammonium salts. Most recently, Tu et al. [14] used carbene complexes as efficient gelators to prepare different types of ionic-liquid gels at concentrations as low as 0.5 mg mL−1 . To the best of our knowledge, little further attention has been paid to the effects of chemical structures of gelators on the gelation of ionic liquids and properties of the corresponding ionic-liquids gels, particularly on the electrochemical properties of the ionic-liquid gels. In this work, two types of gelators with different acyl groups were used for the gelation of imidazole-based ionic liquids. The influence of the chemical structure of the gelators on the gelation of the ionic liquids and on the properties of the resultant ionic-liquids
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X. Dong et al. / Electrochimica Acta 55 (2010) 2275–2279
Scheme 1. Chemical structures of two series of gelators (G1 and G2) and two types of ionic liquids.
gels has been investigated via minimum gelator concentrations, sol–gel phase transition temperature and electrochemical properties. The investigations not only increase our understanding of the relationship between the structure of the gelator and the electrochemical properties of ionic-liquid gels, but also are significant for the properties of potentially useful electrolyte materials, modulated by variation of the structures of the gelators.
a scanning rate of 5 mV s−1 . The experiments were conducted at 25 ± 1 ◦ C. Similarly, the ionic-liquid gels were prepared in a glass vessel equipped with a Teflon stopper. Two mirror polished stainless steel electrodes (10 mm × 10 mm) were stably fixed into the Teflon stopper. Ionic conductivity was measured by the impedance technique over a frequency range from 50 MHz to 1 MHz at an oscillation level of 100 mV. The software for data treatment was Zview.
2. Experimental 3. Results and discussion 2.1. Materials 3.1. Gelation of ionic liquids The ionic liquids, 1-butyl-3-methylimidazolium tetrafluoroborate ([C4 mim]BF4 ) and 1-hexyl-3-methylimidazolium tetrafluoroborate ([C6 mim]BF4 ) were purchased from Shanghai Chenjie Chemical Co. Ltd. The purity of ionic liquids is 99.9%. Traces of water in the ionic liquids were removed by applying a vacuum for 12 h at 80 ◦ C [15]. Two series of gelators, bis(4-acylaminophenyl)methane (G1) and bis(4-acylaminophenyl)ether (G2) were synthesized according to the methods described previously [16,17]. The chemical structures of the G1 and G2 gelators and the ionic liquids are shown in Scheme 1. 2.2. Preparation and characterization of the ionic-liquid gels Calculated amounts of gelators and ionic liquids were put into glass vials and heated until the solid completely dissolved. The solution was allowed to cool at room temperature and exhibited no gravitational flow upon inversion of the vials. The required minimum gelator concentrations (MGCs) [17] were measured at an increment of 0.1 mg mL−1 of the gelator. Measurements of gel–sol phase transition temperatures (TGS ) were conducted according to the method of ball falling described elsewhere [18]. A small steel ball (250 mg, ˚ 4 mm) was placed on the top of the ionic-liquid gel in a glass vial. Then the sample was slowly heated (5 ◦ C/h) in a thermostatted oil bath. When the ball fell to the bottom of the vial, the temperature was defined as the gel–sol transition temperature (TGS ). Hot solution mixtures of gelator and ionic liquid were dropped on a pre-heated glass plate and then allowed to cool at room temperature. After the gel sample was kept in a dark place for 4 h, the gel sample was imaged by optical polarized microscope (POM, BH-2, Olympus). 2.3. Electrochemical measurements The ionic-liquid gels were prepared in a three-electrode cell, containing a glass carbon as the work electrode, Pt wire and Pt plate as the reference and counter electrode, respectively [15]. Linear sweep of the ionic-liquid gels was measured using a Zhanner IM6e electrochemical working station under a N2 atmosphere at
In general, the gelation of liquid media is a process of selfassembly of gelator molecules via noncovalent interactions such as hydrogen bonding resulting in a three-dimensional network of supramolecular structures [2]. The molecules of the liquid medium are immobilized in this three-dimensional network by capillary forces. In our experiments, the G1 and G2 gelators were found to be efficient gelation agents for the imidazole-based ionic liquids without any co-solvent. The gelation ability of the gelators can be characterized by minimum gelator concentrations (MGCs), which are summarized in Table 1. For a certain ionic liquid, the MGCs of the G2 gelators were lower than those of the G1 gelators, indicating that the gelation ability of G2 gelators is superior to that of G1. Theoretically, this can be most likely ascribed to the ether oxygen group in the G2 gelators. Herein, the presence of two lone pairs of electrons on the oxygen atoms redounds to the formation of intermolecular hydrogen bonding of G2 gelators. In addition, the data in Table 1 also reveal that the longer the length of the acyl chains in the gelator, the lower the MGCs of the gelators. This may be explained in terms of more solvophilic characteristic of the gelators with higher flexible long acyl chains or more likely that the long acyl chains modify the solvent compatibility of the gelator, resulting in less gelator molecules required for the immobilization of ionic-liquid molecules. Usually gelators self-assemble into fibrillar aggregates in organic media [2,11]. This process, in nature, is a kind of crystallization of the gelators in solvents. In the present work, the morphology of fibrillar aggregates could not be imaged by SEM micrographs because of the nonvolatility of ionic liquids, which hardly convert ionic-liquid gels to xerogels. Thus, the morphology of the gelator aggregates has to be indirectly characterized Table 1 Minimum gelator concentrations (mg mL−1 ) of ionic liquids using G1 and G2 gelators.
C4 mimBF4 C6 mimBF4
G1-5
G1-7
G1-11
G2-5
G2-7
G2-11
30.8 38.5
21.5 15.0
17.8 6.0
7.0 6.0
6.0 5.2
5.2 4.4
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Fig. 1. POM images of ionic-liquid gels formed by (A) G2-5, (B) G2-7 and (C) G2-11 in [C4 mim]BF4 . The concentration of gelators in all samples was 30 mg mL−1 . The magnification is 100×.
Fig. 2. TGS of ionic-liquid gels versus concentrations of gelators in [C4 mim]BF4 . Plot A: G1-5 (), G1-7 (䊉) and G1-11 (). Plot B: G2-5 (), G2-7 (䊉) and G2-11 ().
by polarized optical microscopy (POM) [2]. Fig. 1 shows the POM images of the ionic-liquid gels formed by G2 gelators in [C4 mim]BF4 . The typical Maltese cross extinction (birefringence) reveals that the fibrils further aggregate into spherical crystallites. We note that POM can only give birefringence information on the morphology of crystallites, for instance, spherical or non-spherical. Their accurate size is hard to be obtained by POM due to the low magnification. The molecules of the ionic liquids are immobilized by capillary forces in three-dimensional networks within the spherical crystallites. Similar POM images of the ionic-liquid gels formed by other gelators were obtained (not shown here), suggesting that the structures of the gelators exert no significant influence on the morphology of the spherical crystallites. 3.2. The phase transition temperatures (TGS ) of ionic-liquid gels Since supramolecular gels are usually thermoreversible, the gel–sol phase transition temperature (TGS ) is an important parameter for the characterization of their thermal stability. In nature, the TGS values characterize the dissociation temperatures of the gelator aggregates. In other words, the interactions involved in the self-assembly of the gelators are destroyed at higher temperatures. Fig. 2 shows the TGS of the ionic-liquid gels formed by G1 and G2 gelators at varying concentrations. Apparently, the TGS values of ionic-liquid gels are proportional to the concentrations of the gelators. Interestingly, the TGS values are also influenced by the lengths of the acyl chains of the gelators. As shown in Fig. 2, the TGS decrease in the order of G1-11 (G2-11) > G1-7 (G2-7) > G1-5 (G2-5). This phenomenon may involve the solvent compatibility of the gelators as discussed above. Considering that supramolecular gels are viscoelastic materials reinforced by gelator aggregates, a more solvophilic aggregates could be beneficial to reinforce the gels, leading to high TGS . As discussed on the basis of the data in Table 1, the gelator aggregates with flexible long acyl chains exhibit more solvophilic in compari-
son with the case of short acyl chains. In addition, more twistable aggregates are formed in case of gelators with longer acyl chains, resulting in more stable gelator aggregates formed. As a consequence, they possess a comparatively good ability to endure the external force (ball weight) involved in breaking the gels. This is, most likely, the reason why TGS increases with an increase of the lengths of the acyl chains. 3.3. The conductivities and electrochemical windows of ionic-liquid gels Fig. 3 shows the ionic conductivities of [C4 mim]BF4 gels formed by the G2-7 gelators versus temperature. As reference, the ionic conductivity of pure [C4 mim]BF4 was also plotted in Fig. 3. To obtain better gel strength for conductivity measurements, 25 mg mL−1 of G2-7 (higher than its MGC) was applied. The double dashed lines in Fig. 3 represent the TGS values of the [C4 mim]BF4 gels (ca. 135 ◦ C). Obviously, the conductivities of gels in the case of the temperature above the TGS are much closer to corresponding solution states.
Fig. 3. Arrhenius plots of the ionic conductivities of [C4 mim]BF4 gels (䊉) formed by G2-7 (25 mg mL−1 ) and pure [C4 mim]BF4 ().
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Table 2 Ionic conductivities of [C4 mim]BF4 gels formed by G1 and G2 gelators at 25 ◦ C. [C4 mim]BF4 gels formed by
1 /mS cm−1
1 /0∗
G1-5 G1-7 G1-11 G2-5 G2-7 G2-11
2.74 2.7 2.5 2.2 2.1 1.4
0.97 0.96 0.89 0.79 0.75 0.50
The 0 of pure [C4 mim]BF4 was 2.8 mS cm−1 at 25 ◦ C. The concentrations of G1 and G2 gelators were 45 mg mL−1 and 25 mg mL−1 , respectively.
However, the conductivities of the gels are slightly lower than those of pure [C4 mim]BF4 in the case of the temperature was lower than TGS . The differences of ionic conductivity between gels and pure [C4 mim]BF4 are in the range of one order of magnitude. In addition, the relationship between the logarithm of ionic conductivity (log ) and the reciprocal of the temperature (1/T) shows good linearity, indicating that the temperature dependence of the conductivity follows the classical Arrhenius equation. These results are consistent with the assumption made in the discussion of the POM images, namely, that the ionic liquid immobilized by capillary forces in the three-dimensional networks is still microscopically behaving like a liquid. The conductivities ( 1 ) of the [C4 mim]BF4 gels formed by the different gelators are listed in Table 2. The conductivities ( 0 ) of corresponding pure [C4 mim]BF4 was found to be 2.8 mS cm−1 . The ratios ( 1 / 0 ) can be used to evaluate the variation of the ionic conductivities before and after the gelation of the ionic liquids. The data in Table 2 reveal that the length of acyl chains exerts an effect on the
conductivities of the ionic-liquid gels. For example, the conductivities of the [C4 mim]BF4 gels formed by G2-5, G2-7 and G2-11 were 2.2 mS cm−1 , 2.1 mS cm−1 and 1.4 mS cm−1 , respectively, indicating that the conductivity decreased with an increase of the lengths of acyl chain. A similar trend can be also observed for the [C4 mim]BF4 gels formed by the G1 gelators. These results can be related to the density of the three-dimensional network formed by gelator aggregates. Long acyl chains easily entangle and twist to form a higher density of three-dimensional networks, which is interruptive for the mobility of ions within the gels, resulting in a lower conductivity. In addition, it is also correlative with the gelator concentration was applied. In order to obtain stable gels for the measurements of conductivity, 45 mg mL−1 of G1 was applied due to their higher MGCs and 25 mg mL−1 of G2 was applied. Thus, the concentrations of G1-5, G1-7 and G1-11 were 1.46, 2.09 and 2.52 times to their MGCs, respectively. In case of G2, the concentrations of G2-5, G2-7 and G2-11 were 3.57, 4.17 and 4.8 times to their MGCs, respectively. As a result, more dense three-dimensional network was formed in case of G2, leading to the decrease of their conductivities. Based on this explanation, the ratios ( 1 / 0 ) in case of the G1 gelators were higher than those of the G2 gelators, suggesting that the variation of the conductivities of the ionic liquids before and after gelation by G1 gelators was small in comparison with that by G2 gelators. These results are consistent with the conclusions in the gelation of the ionic liquids as discussed about MGCs in Table 1. The electrochemical window of ionic-liquid gels is of immediate importance for the application as gel electrolytes. Fig. 4 shows the linear sweep voltammetry of ionic-liquid gels formed by gelators. As shown in Fig. 4A and B, there is no significant effect of gelators with varying acyl groups on the electrochemical window
Fig. 4. Plot A: The electrochemical windows of pure [C4 mim]BF4 and corresponding gels formed by 45 mg mL−1 of G1-5, G1-7 and G1-11. Plot B: The electrochemical windows of pure [C4 mim]BF4 and corresponding gels formed by 25 mg mL−1 of G2-5, G2-7 and G2-11. Plot C: The electrochemical windows of pure [C4 mim]BF4 and [C6 mim]BF4 and corresponding gels formed by 25 mg mL−1 of G2-7.
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of corresponding ionic-liquid gels. From Fig. 4C, it was found that the cations of the ionic liquids have also no significant effect on the electrochemical window in case the same gelator was applied. In addition, the linear sweep voltammetry of ionic-liquid gels indicates that the electrochemical window is stable for all samples, suggesting that the gel formation has no influence on the redox behavior of the ionic liquids.
Acknowledgements
4. Conclusions
[1] P. Terech, R. Weiss, Chem. Rev. 97 (1997) 3133. [2] G. Mathew, R. Weiss, Acc. Chem. Res. 39 (2006) 489. [3] R.G. Weiss, P. Terech (Eds.), Molecular Gels. Materials with Self-assembled Fibrillar Networks, Springer, Dordrecht, The Netherlands, 2005. [4] M. Kimura, S. Kobayashi, T. Kuroda, K. Hanabusa, H. Shirai, Adv. Mater. 16 (2004) 335. [5] N.L. Lancaster, P.A. Salter, T. Welton, G.B. Young, J. Org. Chem. 67 (2002) 8855. [6] C.J. Bradaric, A. Downard, C. Kennedy, A.J. Robertson, Y. Zhou, Green Chem. 5 (2003) 143. [7] Y. Ito, T. Nohira, Electrochim. Acta 45 (2000) 2611. [8] K. Hanabusa, H. Fukui, M. Suzuki, H. Shirai, Langmuir 21 (2005) 10383. [9] N. Kimizuka, T. Nakashima, Langmuir 17 (2001) 6759. [10] A. Ikeda, K. Sonoda, M. Ayabe, S. Tamaru, T. Nakashima, N. Kimizuka, S. Shinkai, Chem. Lett. 30 (2001) 1154. [11] N. Mohmeyer, D. Kuang, P. Wang, H.-W. Schmidt, S.M. Zakeeruddin, M. Grätzel, J. Mater. Chem. 16 (2006) 2978. [12] W. Kubo, T. Kitamura, K. Hanabusa, Y. Wada, S. Yanagida, Chem. Commun. 4 (2002) 374. [13] W. Kubo, S. Kambe, S. Nakade, T. Kitamura, K. Hanabusa, Y. Wada, S. Yanagida, J. Phys. Chem. B 107 (2003) 4374. [14] T. Tu, X. Bao, W. Assenmacher, H. Peterlik, J. Daniels, K.H. Dötz, Chem. Eur. J. 15 (2009) 1853. [15] A.M. O’Mahony, D.S. Silvester, L. Aldous, C. Hardacre, R.G. Compton, J. Chem. Eng. Data 53 (2008) 2884. [16] Y. Meng, Y. Yang, Electrochem. Commun. 9 (2007) 1428. [17] X. Chang, L. Wang, Y. Yang, X. Yang, H. Xu, Mater. Chem. Phys. 99 (2006) 61. [18] A. Friggeri, B.L. Feringa, J. van Esch, J. Control. Release 97 (2004) 241.
Our investigation of the formation of ionic-liquid gels indicates that the lengths of the acyl chains in the gelators have a significant influence on their gelation ability. The measurements of minimum gelator concentrations (MGCs) suggest that the gelation ability of gelators becomes stronger with an increase of the length of the acyl chains. POM images of ionic-liquid gels reveal that the gelators selfassemble into fibrillar aggregates in ionic liquids and further form spherical crystallites. The molecules of the ionic liquids are immobilized by capillary forces within the three-dimensional network formed by the gelator aggregates. TGS values of ionic-liquid gels also increase with an increase of the length of acyl chains. The conductivities of ionic-liquid gels and the corresponding pure ionic liquid possess values within the range of one order of magnitude. The temperature dependence of the conductivities of ionic-liquid gels follows the classical Arrhenius equation. The linear sweep of ionicliquid gels shows a stable electrochemical window. The present results provide strategies to modulate the properties of ionic-liquid gels as novel electrolytes by variation of the structures of the gelators.
This work was financially supported by Hi-Tech Research and Development Plan of China (No. 2007AA03Z246). We thank the Analytical Test Center of HUST for all measurements. References