Versatile Supramolecular Gelators That Can Harden Water, Organic

May 31, 2012 - ABSTRACT: We developed novel supramolecular gelators with simple molecular structures that could harden a broad range of solvents: ...
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Versatile Supramolecular Gelators That Can Harden Water, Organic Solvents and Ionic Liquids Nami Minakuchi,† Kazuki Hoe,† Daisuke Yamaki,‡ Seiichiro Ten-no,‡ Kazunori Nakashima,† Masahiro Goto,§ Minoru Mizuhata,† and Tatsuo Maruyama†,* †

Department of Chemical Science and Engineering and ‡Graduate School of System Informatics, Kobe University, 1-1 Rokkodai, Nada-ku, Kobe 657-8501, Japan § Department of Applied Chemistry, Graduate School of Engineering, Center for Future Chemistry, Kyushu University, 744 Motooka, Fukuoka 819-0395, Japan S Supporting Information *

ABSTRACT: We developed novel supramolecular gelators with simple molecular structures that could harden a broad range of solvents: aqueous solutions of a wide pH range, organic solvents, edible oil, biodiesel, and ionic liquids at gelation concentrations of 0.1−2 wt %. The supramolecular gelators were composed of a long hydrophobic tail, amino acids and gluconic acid, which were prepared by liquid-phase synthesis. Among seven types of the gelators synthesized, the gelators containing L-Val, L-Leu, and L-Ile exhibited high gelation ability to various solvents. These gelators were soluble in aqueous and organic solvents, and also in ionic liquids at high temperature. The gelation of these solvents was thermally reversible. The microscopic observations (TEM, SEM, and CLSM) and small-angle X-ray scattering (SAXS) measurements suggested that the gelator molecules self-assembled to form entangled nanofibers in a large variety of solvents, resulting in the gelation of these solvents. Molecular mechanics and density functional theory (DFT) calculations indicated the possible molecular packing of the gelator in the nanofibers. Interestingly, the gelation of an ionic liquid by our gelator did not affect the ionic conductivity of the ionic liquid, which would provide an advantage to electrochemical applications.

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INTRODUCTION Gels are classified depending on the types of solvents they are prepared from and include hydrogels, organogels, and ionogels (gels of ionic liquids). Conventionally, gels are prepared from polymers including biomacromolecules and synthetic polymers. These gels are widely used in industry and throughout daily life. In the past decade, supramolecular gels, especially hydrogels, have attracted much attention owing to the fact that they are thermoreversible, easily designable, and rapidly responsive to external stimuli.1−5 In the supramolecular gels, small-molecule building blocks (low-molecular-weight gelators) self-assemble to form one-dimensional nanofibers and the three-dimensional entanglement of the nanofibers induces the gelation of solvents. The supramolecular hydrogel, being a synthetic compound, can be designed at the molecular level depending on its proposed use, because of its relatively simple molecular structure. In the past decade, several series of supramolecular hydrogelators have been reported. Some of them were designed to have sol−gel transition responsive to various external stimuli. Several groups also investigated the gelation of water/organic solvent mixture using supramolecular gelators.3,6,7 Peptide amphiphiles (PAs) are one of the most studied supramolecular gelators for hydrogenation. Stupp et al. reported a wide variety of PA-based supramolecular gelators with various functional properties and proposed their biological applications.8−10 Although supramolecular organogel preceded supramolecular hydrogel in their © 2012 American Chemical Society

development, it is still hard to establish a rational strategy for designing a supramolecular organo- or hydrogelator.11−15 Since many of the supramolecular gelators reported had an amide bond in their molecular structure and hydrogen bonds were observed in gelation, hydrogen bonds seemed to play an important role in the organo- and hydrogelation. Hanabusa and Suzuki developed and energetically studied supramolecular organo- and hydrogelators composed of various amino acids.16−18 Das et al. reported amino acid-based amphiphile as a hydrogelator.19 Xu et al. reported various supramolecular hydrogelators composed of short peptides and polyaromatic moieties.20−22 To find a new supramolecular gelator is still an issue of great interest in chemistry and material science. There are a limited number of reports on supramolecular gelators for ionic liquids,23−31 while there are a large number of studies on ionic liquids which have reported the potential of ionic liquids as a novel reaction medium or as a novel solvent alternative to conventional organic solvents.32 Kimizuka and Nakashima first reported that some of ionic liquids can hardened by their synthesized glycolipids.23,24 Hanabusa et al. reported cyclic peptide amphiphile as an efficient supramolecular gelator for a Received: January 31, 2012 Revised: May 26, 2012 Published: May 31, 2012 9259

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Chart 1. Molecular Structures of Gelators 1−7

Table 1. Gelation Properties of Gelators 1−7 for Aqueous Solutions, Organic Solvents and Ionic Liquids gelator solvent

1 Gly

2 L-Ala

3 L-Val

4 L-Leu

5 L-Ile

6 L-Phe

7 D-Val

deionized water HClaq (0.1 M) NaOHaq (0.1 M) phosphate buffer (0.1 M, pH7) NaClaq (20 mM) ethanol 1-propanol 1-butanol acetone toluene isooctane methyl oleate olive oil liquid paraffin [EtMeIm][TFSA] [BuMeIm][TFSA] [HeMeIm][TFSA] [BuPy][TFSA] [BuMeIm][CF3SO3] [MeBu3N][TFSA] [PrMe3N][TFSA] [BuMePi][TFSA]

G(0.8) G(0.5) I I I P G(1.7) G(2.0) I G(2.1) G(1.9) G(1.0) I sol I I G(1.7) G(1.0) G(1.3) I I I

sol sol I I I sol P S I S S sol sol sol I I sol I P I I I

G(0.3) G(0.2) S G(0.5) G(0.5) P G(2.0) G(2.0) G(1.8) G(0.9) G(0.5) G(0.8) G(0.4) G(0.5) G(0.5) G(0.8) G(0.4) G(0.5) G(0.5) sol sol sol

G(0.5) G(0.6) I G(0.5) G(0.6) G(0.8) G(0.8) G(0.7) G(0.4) G(2.0) G(0.8) G(0.9) G(0.4) G(0.1) G(0.6) G(0.6) G(0.5) G(0.5) G(0.3) G(0.6) sol G(0.8)

G(0.4) G(0.4) S G(0.4) G(0.5) G(2.0) G(1.3) G(0.9) G(0.8) G(1.3) G(0.6) G(0.8) G(0.5) G(0.3) G(1.0) G(0.5) G(0.9) G(0.5) G(0.1) G(1.1) sol G(1.0)

G(0.3) G(0.5) I I I S P P PG G(1.2) S sol S G(0.5) G(1.6) G(2.0) sol G(1.0) G(1.9) sol P P

sol sol sol I sol P S S P sol sol sol sol sol sol sol sol sol G(1.0) sol P P

a

G, PG, Sol, P, S, and I represent gel, partial gel, sol, precipitation, soluble, and insoluble. bFigures in parentheses are critical gelation concentrations [wt%].

wide variety of ionic liquids and they first found that the supramolecular gelation did not affect the ionic conductivity of ionic liquids.26 Das et al. reported a novel supramolecular gelator for ionic liquids inspired by their hydro- and organogelators (amino acid-based amphiphiles).29 Recently, a novel imidazolium-type gelator was developed by the Zhou group.31 It seems that there are similar interactions (hydrogen bonding, hydrophobic interactions, etc.) among the supramolecular gelator molecules to form both hydro-, organo-, and ionogels. We speculated that a molecular design similar to supramolecular hydro- and organogelators would be a possible approach to supramolecular ionogelator. To the best of our knowledge, there was no report that single molecules can act as a supramolecular gelator for all three categories of solvents (water, an organic solvent, and an ionic liquid) simultaneously. In the present study, a novel class of supramolecular gelators that can harden a broad range of solvents was designed and synthesized, which had an amide bond, a hydrophobic tail, and polyol as a hydrophilic moiety to be soluble in a broad range of solvents. We found that our designed gelators can harden

aqueous solutions, organic solvents, and ionic liquids without any additives.

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RESULTS AND DISCUSSION We synthesized novel gelators (Chart 1) having simplified structures by liquid-phase synthesis. The gelator molecules consisted of an amino acid, gluconic acid, and long hydrocarbons resulting in an amphiphilic structure. We conjectured that this amphiphilic structure and an amide bond play an essential role in constructing self-assembled fibers in both aqueous and organic phases. Six L-amino acids having different side chain residues (glycine, alanine, valine, leucine, isoleucine, and phenylalanine) as well as D-valine were used (gelators 1− 7). The gels were prepared by heating solutions containing the gelators and subsequently by cooling them down to room temperature. The synthesized gelators except gelators 2 and 7 were found to be an efficient gelator for deionized water and 0.1 M HCl aqueous solution, where the critical gelation concentrations 9260

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Chart 2. Structures of Ionic Liquids Used for the Gelation Tests

(cgcs) were from 0.2 wt % to 0.8 wt % (Table 1). A NaOH aqueous solution (0.1 M) could not be hardened by any of the prepared gelators, probably due to the possible hydrolysis of the ester bonds of the gelators. A phosphate buffer solution (0.1 M, pH 7) and a 20 mM NaCl solution (Table 1) were gelated by gelators 3−5. In particular, gelator 3 (L-Val-containing) can harden phosphate-citrate buffer solutions at a wide range of pHs. Since it is gluconic acid, the hydrophilic moiety of the gelator is unlikely to be affected by the pH of an aqueous solution. The gelation under physiological conditions has great importance from the viewpoint of biological and biochemical applications.5,8−10,20−22 Gelator 2 did not exhibit gelation ability under the present conditions, although there were several reports on alanine-containing gelators.8,22 One of the major differences between our present study and previous reports is the fact that our gelator was composed of single amino acid and did not have a peptide structure. It should be also noted that D-Val-containing gelator (gelator 7) did not harden aqueous solutions and organic solvents at 2 wt %, while that containing L-Val worked as an efficient gelator. The steric conformation of the side chain of Val seemed critical for the gelation. We also found that the gelators except for 2 and 7 could harden organic solvents such as ethanol, 1-propanol, 1-butanol, acetone, toluene, isooctane, methyl oleate (example of biodiesel), and olive oil (edible oil) (Table 1). The cgcs for organogelation ranged from 0.1 to 2.1 wt % depending on the types of gelators and solvents. Interestingly, the gelators could harden several kinds of ionic liquids at 0.1−2.0 wt % (Table 1). Widely studied ionic liquids like imidazolium- and pyridiniumbased ionic liquids (Chart 2) were gelated by many types of the tested gelators at relatively low concentrations. Of the ammonium salt-based ionic liquids tested, only methyl(tributyl)ammonium [TFSA] ([MeBu3N][TFSA]) was hard-

ened by gelators 4 and 5, while propyl(trimethyl)ammonium [TFSA] ([PrMe3N][TFSA]) could not be hardened by any of the tested gelators. Pyrrolidinium-based ionic liquid ([BuMePi][TFSA]) was also hardened by gelators 4 and 5. To harden solvents, the supramolecular gelator needs to be soluble in the solvent under the preparation conditions. Indeed, all of the successful gelators were soluble in many of the aqueous solutions, organic solvents, and ionic liquids at high temperature. These unique phenomena would be attributed to the molecular structure of the gelators. Gluconic acid in the hydrophilic moiety contributed to the solubilization of the gelators in aqueous solutions and ionic liquids.33 The hydrophobic moiety (C16) helped the gelators be soluble in organic solvents and also possibly in hydrophobic ionic liquids (e.g., [HeMeIm][TFSA]). Similar to the supramolecular gels reported in the literature, the hydrogels, organogels, and ionogels prepared with our gelators were thermally reversible.1−5,34,35 Gel−sol transition temperatures (Tgel) for each gel were measured by differential scanning calorimetry (Figure S1 of the Supporting Information, SI). They were about 85 °C for hydrogel (deionized water, 1.0 wt % gelator 3), 70 °C for organogel (toluene, 1.0 wt % gelator 3), and 121 °C for the ionogel ([BuMeIm][TFSA], 1.0 wt % gelator 3). Tgel would be influenced with the solubility of the gelator among the solvents at high temperature. Solvents would also affect the interactions between the gelator molecules, which led to the differences in Tgel among the solvents. The effect of gelator concentrations on Tgel of hydrogels (deionized water) was examined (Figure S2 of the SI). Gelator concentrations of gelators 3 and 5 were varied from cgcs to approximately 6 wt %. The Tgel value did not change much. Cgcs gave the slightly low Tgel. To evaluate the viscoelastic properties of the gel, we carried out rheological measurements with the dynamic time sweep 9261

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literatures.4,22 The similar results on the rheological properties were obtained in the ionogel ([BuMeIm][CF3SO3], gelator 3). In some cases, especially in supramolecular gelators recently reported, gelator molecules are thought to form gels through the entanglement of nanofibers that are built up by noncovalent bonds between gelator molecules.2−10 To confirm this, we observed gel matrices using a transmission electron microscope (TEM, Figure 2a), a field-emission scanning electron microscope (FE-SEM, Figure 2b−d) and a confocal laser scanning microscope (CLSM). The TEM observation of the hydrogel (gelator 3) revealed nanofibers of 100−200 nm in width in the xerogel prepared from the hydrogel of gelator 3. The FE-SEM image of the hydrogel (Figure 2b) showed the threedimensional entanglement of the nanofibers of 90−500 nm in width in the xerogel, as previously reported.36 The TEM and SEM observations were available only for xerogels, which were the lyophilized forms of gels. We then carried out the CLSM observation of hydrogel stained by rhodamine B, where rhodamine B was thought to be localized in the hydrophobic space of the nanofibers. We successfully obtained in situ evidence of fibers in the hydrogel (Figure 2e), which agreed with the previous reports studying the CLSM observations of nanofibers formed by supramolecular gelators.36,37 Intriguingly, the FE-SEM observations also demonstrated that the organogel of 1-propanol and the ionogel of [BuMeIm][CF3SO3] had entangled nanofibers (Figure 2c,d). The nanofibers observed in 1-propanol were 280−800 nm in width, which were relatively thicker than those in the hydrogel. In fact, the organogel of 1-propanol was opaque (Inset of Figure 2c). The nanofibers in [BuMeIm][CF3SO3] were 80− 340 nm in width. The diameters of the nanofibers were not homogeneous, especially in a hydrogel, and also varied with the kinds of solvents. It is noteworthy that our gelators can selfassemble to form three-dimensional networks of nanofibers in aqueous solutions, organic solvents, and ionic liquids. We carried out SAXS measurements to obtain structural information of the nanofibers (Figure 3). The SAXS profiles of the hydrogel (gelator 3) had two peaks at 0.21 and 0.41 Å−1 with the same intervals, indicating that the nanofibers in the

(Figure 1a). The value of G′ (storage elastic modulus) and G″ (loss elastic modulus) rapidly increased by about 150 s, and

Figure 1. Viscoelastic properties of the hydrogel and the ionogel prepared using 1.0 wt % gelator 3 (solvent: deionized water and [BuMeIm][TFSA]). (a) Dynamic time sweep at the frequency of 1 rad/s (30 °C) and (b) dynamic frequency sweep (30 °C) of the hydrogel; (c) Dynamic time sweep at the frequency of 1 rad/s (30 °C) and (d) dynamic frequency sweep (30 °C) of the ionogel.

then both G′ and G″ increased gently. The plateau value of G′ is about seven times larger than that of G″. The value of G′ exceeded that of G″ within several minutes, which indicates that the gelation occurred within several minutes. For further evaluation, we studied rheological measurements with dynamic frequency of gel (Figure 1b). One hour after cooling the sample, G′ was more than five times larger than G″ in the whole range of angular frequency (0.1−100 rad/s) at 30 °C. These results are consistent with supramolecular gels reported in the

Figure 2. Microphotographs of gels. (a) TEM image of hydrogel (Milli-Q water) stained by 1.0 wt % phosphotungstic acid. FE-SEM images and photographs of hydrogel (b), organogel (c), and ionogel (d) (solvents are Milli-Q, 1-propanol, [[BuMeIm][CF3SO3], respectively). (e) CLSM image of hydrogel stained by 3 μm rhodamine B (solvent: Milli-Q water). Insets are the images of gels in sample tubes. Gelator 3 was used at concentrations of 1.0, 0.5, 2.0, 1.0, and 1.0 wt %. 9262

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liquid paraffin, toluene, and an ionic liquid, we conjectured the similar molecular packing inside the nanofibers in these solvents. However, the gelator molecules at the outermost of the nanofiber, which is the interface between a nanofiber and a solvent, would be affected by the solvents. We imagined that the molecules at the outermost were oriented with solvatophilic moieties toward the solvent (like gluconic acid toward water and hydrophobic moiety toward toluene). We evaluated the validity of these molecular packings based on density functional theory (DFT) calculations in a vacuum. For simplicity, the hydrophobic moiety of gelator 3 (a hexadecyl group), which does not participate in the hydrogen bond, was replaced by a methyl group (Figure 4c). A geometrical optimization of the monomer was performed using molecular mechanics with the Universal force field, and the relative position of the two molecules was optimized at B3LYP/6-31G(d,p), keeping the monomers parallel to each other.41 We used side-by-side and tilted parallel structures for initial geometries of optimizations, and the resulting structures (structures 1 and 2) are depicted in Figure 4d,e, respectively. The binding energies of the structures, −8.4 and −10.3 kcal/mol, indicate that structure 2 is more stable. The calculated electrostatic potential of an optimized monomer (Figure 4f) was positive around the top of the hydrophilic group while that is negative at the base of the group. This fact also supports the preference to the tilted parallel conformation over the side-by-side one (structure 2). Ionic liquids are currently studied as novel electrolyte mediums for use in batteries because of their high ionic conductivity, excellent thermal stability, large electrochemical window, and negligible vapor pressure. The gelation of ionic liquids is a rational strategy to control their fluidity since liquid leak from a battery is still a major concern from a practical viewpoint. We then measured the ionic conductivity of ionic liquids and the ionogels prepared by gelator 3. The ionic conductivity of the ionogels was almost the same as that of the ionic liquids (Figure 5). Although the polymer-based gelation

Figure 3. SAXS profiles of the hydrogel, the organogel (liquid paraffin), and the ionogel [[BuMeIm][CF3SO3] prepared by gelator 3 (5 wt %).

hydrogels had lamellar structures.38 The broad peak at 0.35 Å−1 was derived from a polyimide film of the SAXS apparatus. The long spacings, d, in the lamellar structures of the nanofibers prepared using gelators 3 and 4 were 3.07 and 3.22 nm, respectively. Similar results, which indicated lamellar structures, were obtained in organogel (liquid paraffin) and ionogel ([BuMeIm][CF3SO3]) prepared by gelator 3. The calculated d values for the organogel and ionogel were 3.11 and 3.01 nm, respectively. Interestingly, the d values were very similar among the hydrogels, the organogels (also in toluene, 3.05 nm) and the ionogel, which also suggests the formation of similar nanofibers in all the solvents tested although there are large differences in the polarity among these solvents. In Figure 4a,b, we show the schematic representations of the configurations of the gelator 3 assumed from the SAXS results along with the estimated molecular size of the gelator (3.2 nm).38−40 Since very similar d values were obtained in water,

Figure 4. (a,b) Proposed molecular packings of gelator 3 in nanofibers. (c) The structure of the simplified molecule used for DFT calculations. (d) Side-by-side structure optimized by DFT calculations (structure 1). (e) Tilted parallel structure optimized (structure 2). (f) Molecular electrostatic potential (MEP) shown on the iso-surface of electron density. 9263

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residue was dissolved in 200 mL acetone containing conc. HClaq (10 mL) and 1-hexadecyl amino acid ester hydrochloride was precipitated at 4 °C overnight. The precipitate was again added in 100 mL acetone and was reprecipitated at 4 °C overnight. The obtained precipitate was dissolved in chloroform and was mixed with an aqueous solution containing 10 wt % sodium carbonate for neutralization. The chloroform phase was dried over anhydrous Mg2SO4 and was evaporated. The obtained residue was dissolved in ethanol (200 mL). Glucono δ-lactone (0.05 mol) was added to the solution, and the mixture was refluxed at for 5 h. The solvent was evaporated. To remove an excess amount of gluconolactone, the residue was dissolved in 1,4-dioxane at 80 °C, and the solution was filtered at high temperature. The filtrate was again heated at 80 °C and followed by the filtration at high temperature. The solvent was evaporated, and the obtained white powder was recrystallized in acetonitrile unless otherwise stated. If needed, the recrystallization was repeated. The precipitate was dried under reduced pressure. The yields of gelators 1−7 were 63, 62, 60, 52, 50, 66, and 69%. Resultant products were identified by 1H NMR, elemental analysis and MALDI-TOF/MS. 1H NMR spectra were obtained on a 500 MHz Bruker AV-500. FTIR measurements were carried out using Bruker Alpha-E equipped with an ATR unit. 1H NMR assignments and the results of elemental analysis. FTIR and MALDI-TOF/MS are provided in the SI. Preparation of Gels. Gelators were dissolved in solvents at given concentrations in a glass tube (diameter: 8 mm) by heating and then slowly cooling down to room temperature to form gels. Gelation was confirmed by inverting the glass tube containing the solution. TEM, FE-SEM, and CLSM. TEM measurements were carried out using a JEOL JEM-2010. A piece of the gel (1.0 wt %, stained with phosphotungstic acid) was placed on a carbon-coated copper grid and dried for 12 h under vacuum. The grid was observed using an acceleration voltage of 200 kV. Confocal laser scanning microscopy (CLSM) observation was carried out using OLYMPUS FV1000-D. The gel stained with rhodamine B (3 μm) was used for CLSM. FESEM measurements were carried out using a field-emission scanning electron microscope (JSM-7500F, JEOL, Japan). For specimens for FE-SEM, hydrogel and organogel were converted to xerogel by freezedrying under vacuum. The xerogel of the ionogel was prepared by the procedure reported by Hanabusa et al.26 The ionogel was immersed in water for 3 days to exchange the ionic liquid with water, during which time water was replaced by fresh water several times. The gel, after removing ionic liquid, was converted to xerogel by freeze-drying under vacuum. The xerogels were mounted on an aluminum stub and coated with Pd by vapor deposition. Viscoelastic Measurements of Gels. Rheological measurements were carried out using a rheometer (Anton Paar Physica MCR301, Germany) with a parallel plate (diameter = 5.0 cm) at a strain of 0.1% and a gap of 1.0 mm. We loaded the aqueous solution dissolving gelator 3 on the sample plate whose temperature was set at 83 °C. After the sample plate was cooled to 30 °C, the measurement was started. Conductivity Measurements. Conductivity of ionogels was measured at 25 °C using a Hioki Chemical Impedance Meter (3532-80). The isotropic hot solution of ionic liquid containing the gelator was injected into the U-tube, followed by gelation. We measured conductivity of KCl solution (7.44 g/L), and determined the cell constant. SAXS Measurements. Small-angle X-ray scattering (SAXS) of the gels was measured at room temperature using a Nano-Viewer RAMicro 7 (RigaKu Co., Japan). The measurements were made with a Cu Kα radiation generator (λ = 0.1542 nm) operating at 40 kV and 20 mA for 15 min at room temperature. The camera length was 400 mm. The scattering vector q was defined by the following:

Figure 5. Ionic conductivity of ionic liquids and ionogels at 25 °C. The concentration of gelator 3 was 1.0 wt %.

of ionic liquids often remarkably decreases the conductivity,42 the gelation by our gelator did not affect the conductivity. This would be due to the relatively large spaces for liquid between the nanofiber networks (Figure 2) that did not interfere with the movement of ions even in the gels. These results agreed with the previous report on supramolecular ionogel.26

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CONCLUSIONS We have developed novel supramolecular gelators having simple molecular structures. The gelators can harden aqueous solutions within a wide pH window, organic solvents, edible oil, biodiesel, and ionic liquids. To the best of our knowledge, this is the first report that a supramolecular gelator self-assembled to form nanofibers in aqueous solutions, organic solvents, and ionic liquids, which led to the thermally reversible gelation of these different solvents. Intriguingly, the gelation of ionic liquids maintained the ionic conductivity of the ionic liquid itself, indicating that the characteristics of the liquid were kept after gelation. The versatile gelation by our supramolecular gelators has the potential for practical applications in biochemical, fuel, environmental, and electrochemical fields.

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EXPERIMENTAL SECTION

Materials. 1-Hexadecanol and p-toluenesulfonic acid monohydrate were purchased from Kishida Chemical (Osaka, Japan). Glucono δlactone was purchased from Calbiochem. Glycine, L-alanine, L-valine, L-leucine, L-isoleucine, L-phenylalanine, D-valine, sodium carbonate and all solvents used in the syntheses were purchased from Wako Pure Chemical Industries (Osaka, Japan). [EtMeIm][TFSA], [BuMeIm][TFSA], [HeMeIm][TFSA], and [BuPy][TFSA] were synthesized from [EtMeIm][Cl], [BuMeIm][Cl], [HeMeIm][Cl], [BuPy][Br], and [Li][TFSA] as previously reported.43 [EtMeIm][Cl], [BuMeIm][Cl], [HeMeIm][Cl], [BuPy][Br], and [MeBu3N][TFSA] were purchased from Tokyo Chemical Industry (Tokyo, Japan). [Li][TFSA], [PrMe3N][TFSA], and [BuMePi][TFSA] were purchased from Kanto Chemical (Tokyo, Japan). [BuMeIm][CF3SO3] was purchased from Merck. Synthesis. Typically, a mixture of 1-hexadecanol (0.05 mol), ptoluenesulfonic acid monohydrate (0.06 mol), and amino acid (0.05 mol) in toluene (200 mL) was refluxed for 5 h using a Dean−Stark trap. After the reaction, the solvent was evaporated. The residue was dissolved in chloroform (200 mL). Sodium carbonate aqueous solution was added to this solution. The organic phase was collected and dried over anhydrous Mg2SO4. The solvent was evaporated. The

q=

4π sin θ λ

(1)

where λ and 2θ are the wavelength of the X-ray and the scattering angle, respectively. The gelator concentration was 5.0 wt %. Thickness of a sample was ∼1 mm and the cell windows were made of mica. 9264

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ASSOCIATED CONTENT

S Supporting Information *

DSC measurements, the effect of gelator concentrations on Tgel, 1H-NMR assignments, elemental analysis, FTIR data, and MALDI-TOF/MS data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel/fax: +81-78-803-6070. E-mail: [email protected]. ac.jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Nissan Chemical Industries, Ltd., Prof. A. Mori, Prof. T. Takeuchi, and Prof. H. Matsuyama for their technical help and advice. This study was partly supported by the Iwatani Naoji Foundation and the General Sekiyu Foundation.

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