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Langmuir 2008, 24, 3150-3156
Anisotropic Ionogels of Sodium Laurate in a Room-Temperature Ionic Liquid Wenqing Jiang and Jingcheng Hao* Key Laboratory of Colloid and Interface Chemistry, Shandong UniVersity, Ministry of Education, Jinan 250100, China
Zhonghua Wu Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China ReceiVed NoVember 20, 2007. In Final Form: December 24, 2007 Anisotropic thermally reversible ionogels of sodium laurate (SL) were prepared in the first discovered roomtemperature ionic liquid (RTIL), ethylammonium nitrate (EAN). Polarized optical microscope images indicate that the gels are birefringent, illuminating the presence of anisotropic structures. Small-angle X-ray scattering results reveal that SL and lauric acid (LA) molecules are arranged to form lamellar structures, but no SL crystallites were confirmed by the X-ray diffraction measurements. With an increase of the SL concentration, the interlayer distance decreases. Rheological measurements indicate that the anisotropic ionogels are highly viscoelastic and the storage modulus (G′) increases with an increase of the SL concentration in EAN. Electrochemical measurements indicate that the anisotropic ionogels may have potential applications in electrochemical fields. The intermolecular hydrogen bond as well as the solvatophobic interaction of SL and LA formed by a chemical reaction, CH3(CH2)10COONa + CH3CH2NH3NO3 f CH3CH2NH2v + NaNO3V + CH3(CH2)10COOH, can play a role in the formation of three-dimensional networks having lamellar structures which are responsible for the anisotropic ionogels. The formation of anisotropic ionogels by surfactants in RTILs could be a new phenomenon, but this is not a very classic case of organogels.
Introduction Increasing attention on self-assembled structures formed by amphiphiles such as surfactants and amphiphilic block copolymers found in room-temperature ionic liquids (RTILs), a class of organic salts with an unusually low melting temperature, has caught scientists’ eyes.1 Due to the unique characteristics of RTILs such as thermal stability, negligible vapor pressure, nonflammability, high ionic conductivity, and wide electrochemical window, RTILs can be used as environmentally benign solvents for green chemistry, catalysts for synthetic chemistry, and electrolytes for batteries in photochemistry and electrosynthesis.2,3 Quasi-solidification of RTILs through chemical or physical gelation may play an important role in the fabrication of devices such as medical electrosensitive gel devices. This is because gelation is a simple and promising method for controlling the fluidity of RTILs.4 Kimizuka named this type of gel an “ionogel”, according to the definitions of a hydrogel and an organogel.5 By using various polymeric materials, such as poly(vinylidene fluoride) hexafluoropropylene copolymer and poly(ethylene glycol), gelation of RTILs has obtained much success.6-8 Hybrid silica material, organically modified silica, and silica nanoparticles were also used in quasi-solidification of RTILs.9-11 Recently, Aida’s group achieved the physical gelation * To whom correspondence should be addressed. Fax/phone: +86-53188366074 (o). E-mail:
[email protected]. (1) Hao, J.; Zemb, Th. Curr. Opin. Colloid Interface Sci. 2007, 12, 129-137. (2) Welton, T. Chem. ReV. 1999, 99, 2071-2084. (3) Rogers, R. D.; Seddon, K. R. Science 2003, 302, 792-793. (4) Hanabusa, K.; Fukui, H.; Suzuki, M.; Shirai, H. Langmuir 2005, 21, 1038310390. (5) Kimizuka, N.; Nakashima, T. Langmuir 2001, 17, 6759-6761. (6) Carlin, R. T.; Fuller, J. Chem. Commun. 1997, 1345-1346. (7) Wang, P.; Zakeeruddin, S. M.; Exnar, I.; Gra¨tzel, M. Chem. Commun. 2002, 2972-2973. (8) Klingshirn, M. A.; Spear, S. K.; Subramanian, R.; Holbrey, J. D.; Huddleston, J. G.; Rogers, R. D. Chem. Mater. 2004, 16, 3091-3097.
of ILs by single-wall carbon nanotubes.12 However, there has been little work on gelation of RTILs by using low molecular weight compounds. Low molecular weight compounds used to harden organic liquids, which are called “gelators”, are of growing interest not only in the area of academic research for chemistry but also in applications such as the environment, industry, and medicine.13,14 Gelators have particular capabilities such as good solubility upon heating as well as the ability to gelate organic liquids at low concentrations. When a hot isotropic solution of a gelator is cooled to room temperature, the gelator molecules can organize themselves into finely dispersed aggregates within the organic solvent, resulting in a three-dimensional structure which causes the gelation. Furthermore, these resultant gels always exhibit a thermally reversible sol to gel phase transition. Kimizuka et al. studied physical gelation of ILs by a compound consisting of glycolipid and L-glutamic acid, but the gelation ability of the compound would be limited to the imidazolium type of ILs that contain a bromide ion.5 Shinkai’s group prepared ionogels of both the imidazolium and pyridinium types of ILs with their gelator.15 However, this gelator was nearly insoluble in ILs. For this reason, the acetone solution of the gelator should be added to the ILs to make a gel. Kubo et al. reported a fabrication of dye-sensitized solar cells using gel electrolytes of an IL by gelating 1-alkyl-3-methylimidazolium iodides with N-benzyloxycarbonyl(9) Stathatos, E.; Lianos, P.; Lavrencic-Stangar, U.; Orel, B. AdV. Mater. 2002, 14, 354-358. (10) Ne´ouze, M. A.; Bideau, J. L.; Gaveau, P.; Bellayer, S.; Vioux, A. Chem. Mater. 2006, 18, 3931-3936. (11) Wang, P.; Zakeeruddin, S. M.; Comte, P.; Exnar, I.; Gra¨tzel, M. J. Am. Chem. Soc. 2003, 125, 1166-1167. (12) Fukushima, T.; Kosaka, A.; Ishimura, Y.; Yamamoto, T.; Takigawa, T.; Ishii, N.; Aida, T. Science 2003, 300, 2072-2074. (13) Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263-2266. (14) Terech, P.; Weiss, R. G. Chem. ReV. 1997, 97, 3133-3160. (15) Ikeda, A.; Sonoda, K.; Ayabe, M.; Tamaru, S.; Nakashima, T.; Kimizuka, N.; Shinkai, S. Chem. Lett. 2001, 1154-1155.
10.1021/la703632g CCC: $40.75 © 2008 American Chemical Society Published on Web 02/23/2008
Anisotropic Ionogels of Sodium Laurate L-isoleucylaminooctadecane.16 The disadvantage of this gelator,
although it is an excellent gelator for many other kinds of familiar organic solvents,17 is that its gelation ability toward ILs is confined to bromide and iodide salts of 1-alkyl-3-methylimidazolium. Recently, Hanabusa et al. reported special gelators for ILs, which solidified a wide variety of ILs at low concentrations without helper additives.4 At present, although some low molecular weight compounds have been demonstrated to form gels in ILs, most other gelators are synthesized through complex organic reactions. In this paper, we used a carboxylic salt, sodium laurate (SL), a classical anionic surfactant to form the ionogels in the earliest studied roomtemperature ionic liquid, ethylammonium nitrate (EAN). The X-ray diffraction (XRD) measurements demonstrate that the ionogels of SL in EAN are not in the crystal phase. The smallangle X-ray scattering (SAXS) measurement results reveal that the interlayer distance, i.e., d spacing, decreases with an increase of the SL concentration. Compared with the gelators used to harden ILs in previous studies, SL is a product of commercial availability. Moreover, the formation of gels is through a chemical reaction between SL and EAN, which not only represents a new approach to the preparation of gels, but also expands the potential use of RTILs. Experimental Section Materials. Sodium laurate (purity >98%) was purchased from Tokyo Chemical Industry Co., Ltd. and used as received, without further purification. An ethylamine solution of water (65-70 wt %) was obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Nitric acid (65-68 wt %) was obtained from Zibo Chemical Reagent Factory (Shandong Province, China). Water used was triply distilled. Synthesis of the RTIL EAN. EAN was synthesized as described by Evans et al.,18 by slow addition of ∼3 mol‚L-1 nitric acid to the ethylamine solution with stirring and cooling in an ice bath. Most of the water was removed with a rotary evaporator; the final amounts of water were removed with a lyophilizer (Martinchrister ALPHA12). The product was stored in a vacuum desiccator. Its melting point was about 286 K, and the density at room temperature was approxiamately 1.2 g‚mL-1. These measurements agree with previous reports.18 The residual water content, as determined by Karl Fischer titration, was around 0.6 wt %. 1H NMR and FT-IR spectra indicate that the resulting product is EAN. 1H NMR: R-H (2H, 3.10 ppm), ω-H (3H, 1.32 ppm). 13C NMR: R-C (34.56 ppm), ω-C (11.36 ppm). IR (liquid membrane, cm-1): 3066.75 (νN-H), 1358.13 (νN-O). Gelation. A typical gelation is performed according to the following: A mass of SL is mixed with 2 mL of EAN in a test tube with a screw cap. The mixture is then heated until the solid completely dissolves. The resulting solution is cooled at 25 °C, and then the gelation is checked visually. The macroscopic manifestation of a successful gelation is the absence of observable gravitational flow upon inversion of the test tube. Characterization. Polarized optical microscopy was carried out with a Zeiss Axioskop 40 with cross polarizers to examine the anisotropies of ionogels. Samples were placed between thin microslides (all slides were washed in acetone and dried at room temperature before use). To avoid the influence of shearing forces due to contact with the glass, the thin microslides with samples sandwiched between them were all stored at 25.0 ( 0.1 °C for at least 1 week. (16) Kubo, W.; Kambe, S.; Nakade, S.; Kitamura, T.; Hanabusa, K.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2003, 107, 4374-4381. (17) Hanabusa, K.; Hiratsuka, K.; Kimura, M.; Shirai, H. Chem. Mater. 1999, 11, 649-655. (18) Evans, D. F.; Yamauchi, A.; Roman, R.; Casassa, E. Z. J. Colloid Interface Sci. 1982, 88, 89-96. (19) Mohmeyer, N.; Wang, P.; Schmidt, H.; Zakeeruddin, S. M.; Gra¨tzel, M. J. Mater. Chem. 2004, 14, 1905-1909.
Langmuir, Vol. 24, No. 7, 2008 3151 SL crystals and ionogels were characterized by XRD, which was carried out on a Bruker ADVANCE D8 (Germany) diffractometer using Cu KR radiation (λ ) 1.5406 Å) from a rotating anode X-ray generator operating at 40 kV and 40 mA. SAXS measurements were carried out at 298 K on the beamline 4B9A synchrotron radiation X-ray small-angle system at the Beijing Synchrotron Radiation Facility. The imaging plate is Mar 345 with a resolution factor of 3450 × 3450. The two-dimensional scattering patterns of SAXS obtained consisted of concentric circles, and the intensity (I) versus scattering vector (q) profile was independent of the azimuthal angle. Therefore, we took a circular average of I, leaving the resultant I versus q plot for discussion. The range of scattering vectors was chosen from q ) 0.5 nm-1 to q ) 14 nm-1 (q ) (4π/λ) sin(θ/2), θ and λ, respectively, the scattering angle and the incident X-ray wavelength of 0.138059 nm). The distance from the sample to the detector was 26.5 cm, and the data accumulation time was 600 s for each sample. Diffuse reflectance Fourier transform infrared (FT-IR) spectroscopy was conducted with a Nexus 670 infrared spectrometer (ThermoNicolet Electron Corp.). The samples were coated in a circular groove in a stainless steel plate. Spectra were collected from 400 to 4000 cm-1 with a resolution of 2 cm-1. Rheology measurements were carried out on a Haak Rheostree RS75 stress-control rheometer using a cone-plate fixture (Ti, radius 20 mm, cone angle 1°). The distance of the cone plate was adjusted to 52 µm for all measurements. The temperature of the samples was kept at 25 ( 0.1 °C. The viscoelastic properties of all samples were determined from oscillatory measurements that ranged from 0.1 to 10 Hz in frequency. Frequency sweep measurements were performed in the linear viscoelastic region, which was determined by strain sweep measurements. The conductivity measurements were performed on a conductivity meter, DDSJ-308A (China), with a glass electrode, DJS-10C. The electrode was immersed in the sample solution when it was in the isotropic phase. The change of conductivity before and after the formation of the gel was recorded. The cyclic voltammograms of the ionogels were measured with a 600B CHI electrochemistry analytical instrument under a N2 atmosphere. The working electrode used in these experiments is a GC electrode with a 3 mm glassy carbon disk insulated in a 7 mm diameter Teflon rod. All the electrochemistry measurements were performed with a conventional three-electrode cell with a saturated calomel electrode (SCE) and a Pt plate as the reference and the counter electrodes, respectively. The experiments were conducted at an ambient temperature of 26 ( 1 °C. Differential scanning calorimetry (DSC) and thermal gravimetry analysis (TGA) for high-temperature measurements were carried out with the SDT Q600 thermal analysis system (TA). The sample was analyzed in a hermetically sealed aluminum pan. A scan rate of 10 °C min-1 was employed on the samples over the temperature range from room temperature to 900 °C under a N2 atmosphere. The measurements of the gel-to-sol transition temperature were performed by the DSC822e thermal analysis system (Mettler-Toledo, Switzerland). Samples of 10-15 mg were placed and analyzed in a hermetically sealed aluminum pan; an empty pan served as a reference. A scan rate of 5 °C min-1 was employed on all gel samples over the temperature range from 10 to 100 °C under a N2 atmosphere. 1H and 13C NMR spectra were recorded on a Bruker AVANCE 400 Hz NMR spectrometer operating in the Fourier transform mode with quadrature detection. D2O served as the solvent and in calibration.
Results and Discussion The phase behavior of the system with different concentrations of sodium laurate (cSL) in EAN is shown in Figure 1. As we can see from Figure 1, when cSL is 0.2 mol‚L-1, the system cannot form an ionogel, but does form a colorless fluid without birefringence, indicating that the samples are an isotropic solution when cSL is below 0.2 mol‚L-1 in EAN. When the concentration of SL reaches 0.25 mol‚L-1, a slight blue color can be seen in
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Figure 1. Phase behavior of SL in EAN without (top) and with (bottom) polarizers. From left to right, cSL ) 0.2 (isotropic solution), 0.25, 0.35, 0.375, 0.4, and 0.5 mol‚L-1, respectively. All samples were equilibrated at 25.0 ( 0.1 °C for more than 3 months.
the transparent ionogels. At this concentration, the ionogels present slight birefringence, indicating the presence of anisotropic structure. With an increase of the SL concentration in EAN, the ionogels become semitransparent ivory white and the birefringence gradually becomes strong with polarizers, which could be attributed to the denseness of the three-dimensional networks of the gels. We note that the formed anisotropic ionogels are stable and crystallization of the surfactants is not observed after they are kept for 9 months at T ) 25.0 ( 0.1 °C. The polarized optical micrographs of representative samples are shown in Figure 2. All the anisotropic ionogels are strongly birefringent, having similar striation textures. This further indicates that the three-dimensional networks which are responsible for gelation contribute to the anisotropy. According to a very classic case of organogels, it is well-known that the network structure formed by gelator molecules is crystalline in nature.19 However, the following XRD and SAXS measurements demonstrate that the three-dimensional networks are constructed by the lamellar phase. With an increase of cSL in EAN, we found that, during the course of dissolution of SL upon heating, a white precipitate gradually appeared and an irritative gas, which could turn wetted pH test paper blue, was released. These phenomena indicate that a chemical reaction should exist within this system. We speculated that a proton of amido in EAN was seized by the carboxylate group in SL; that is, the following reaction occurred:
CH3(CH2)10COONa + CH3CH2NH3NO3 f CH3CH2NH2v + NaNO3V + CH3(CH2)10COOH (1) In fact, this is an acid-base neutralization reaction because SL is a weak base and EAN is a weak acid. From the reaction, we can easily draw conclusions that the appearance of a white precipitate is attributed to the separation of superfluous NaNO3 and that the alkaline gas is CH3CH2NH2. This conjecture was supported by the contrastive DSC and TGA results of a white precipitate and pure NaNO3 (Figure 3). From Figure 3, we can see whether the phase transition temperature or percentage of weight loss, the result of precipitation and standard NaNO3, is almost completely consistent, indicating that the precipitates are NaNO3 and that the chemical reaction we speculated is accurate. According to a rough measurement of the mass of the precipitate, we can judge that only part of the SL is exhausted before it is completely dissolved and that the productive rate of lauric acid (LA) is lower than 50%. We
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speculate that this reaction is a dynamic equilibrium process and that the rate of reaction is very slow because we cannot see obvious bubbles even at 70 °C. Moreover, three compounds, SL, LA, and EAN, exist in the system. Generally, the ionogels formed by a low molecular weight gelator are physical gels. The three-dimensional networks responsible for gelation are built by the noncovalent interactions among gelator molecules, such as hydrogen bonds, hydrophobic interactions, van der Waals forces, π-π interactions, electrostatic interactions, and metal ion coordination.13,14,20 For our system, the intermolecular hydrogen bond and solvatophobic interaction play an important role in the building of three-dimensional networks of ionogels. From the previous discussion, we can see that the gelation of EAN is probably caused by the cooperation of SL and LA. To our best acknowledge, fatty acid molecules and the corresponding salt molecules can form an acid-soap complex through intermolecular hydrogen bond interaction.21-25 Lynch’s group26 reported that the structure of the palmitic acidsoap complex was crystalline. We speculate the linkage style of SL and LA is similar to that reported in previous literature. The carboxylate and acid headgroups are arranged in a pairlike fashion to form a bilayer structure, which is the most basic unit of fibers in ionogels. The linkage of the carboxylate and acid headgroup is via two interactions: a hydrogen bond and coordination with a Na atom. Simultaneously, alkyl chains of the acid-soap complex formed by SL and LA molecules arrange and pack through a solvatophobic interaction. Finally, the three-dimensional networks are built. A sketch of the packing style of the acid-soap complex formed by SL and LA molecules is shown in Figure 4. In fact, when the concentration of SL is lower than 0.25 mol‚L-1, SL and LA form an acid-soap complex but the concentration is so low that the alkyl chains cannot pack and arrange to form network structures. One could refuse to accept the interesting new phenomenon of ionogel formation by SL in EAN and instead believe that the gels belong to a very classic case of organogels formed by the networks of microcrystals of the surfactants. We measured the XRD by comparing experiments of SL crystals and ionogels, as shown in Figure 5. One can easily see that no SL crystallites form in SL and EAN mixtures. The pattern of SL powder shows a typical feature from a crystalline powder solid. However, the pattern of the ionogels shows only a very weak peak and no trace of a crystalline peak at all, indicating that the gelator in the ionogels is amorphous. The structure information of anisotropic ionogels can be obtained by SAXS measurements. Figure 6 shows the SAXS patterns of anisotropic ionogel samples with different SL concentrations in EAN. All the samples were equilibrated for 3 months at T ) 25.0 ( 0.1 °C before SAXS measurements were taken. As we can see from Figure 6, similar scattering curves were obtained; typically three scattering peaks appear with relative positions of 1:2:3, which correspond to the 001, 002, and 003 planes of the lamellar structure. This is in good agreement with the bilayer structure formed by the SL and LA molecules, which further demonstrates that our judgment about the arrangement of SL and LA molecules is reasonable. In addition, it can be seen from Figure 4 that by increasing the concentration of SL there (20) Estroff, L. A.; Hamilton, A. D. Chem. ReV. 2004, 104, 1201-1218. (21) Mcbain, J. W.; Field, M. C. J. Phys. Chem. 1933, 37, 675-684. (22) Lucassen, J. J. Phys. Chem. 1966, 70, 1824-1830. (23) Ekwall, P. Colloid Polym. Sci. 1988, 266, 279-282. (24) Lynch, M. L. Curr. Opin. Colloid Interface Sci. 1997, 2, 495-500. (25) Kralchevsky, P. A.; Danov, K. D.; Pishmanova, C. I.; Kralchevska, S. D.; Christov, N. C.; Ananthapadmanabhan, K. P.; Lips, A. Langmuir 2007, 23, 35383553. (26) Lynch, M. L.; Wireko, F.; Klein, M. J. Phys. Chem. 2001, 105, 552-561.
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Figure 2. Polarized optical micrographs of anisotropic ionogels with different concentrations of SL in EAN. From (a) to (f), cSL ) 0.3, 0.35, 0.375, 0.4, 0.45, and 0.5 mol‚L-1.
Figure 3. DSC (a) and TGA (b) curves of precipitates obtained from the chemical reaction between SL and EAN and NaNO3. The precipitates were obtained by centrifugation, washing with acetone, and drying in a vacuum drying oven.
is a q shift for the scattering peaks toward larger q values, which corresponds to a decrease of the d spacing (the lattice spacing, calculated from the 001 peak position 2π/q001). The FT-IR spectra of samples with different SL concentrations in EAN are shown in Figure 7. For all samples, two peaks near 1623 and 1519 cm-1 are observed, which can be attributed to the asymmetric and symmetric scissoring vibrations of N-H, respectively. When the concentration of SL reaches 0.25 mol‚L-1 (curve d in Figure 7), a peak near 1558 cm-1 appears which is attributed to the asymmetry stretch of the COO- group in SL.26,27 With an increase of the concentration of SL, the intensity of this (27) Wen, X.; Franses, E. I. J. Colloid Interface Sci. 2000, 231, 42-51.
peak (1558 cm-1) is gradually enhanced. For the samples with 0.2 and 0.15 mol‚L-1 concentrations of SL, this peak becomes very weak and shifts to 1563 cm-1. However, for the pure EAN, there is no peak near this position. From the previous discussion, we know that the minimum concentration of SL for gelating the EAN is 0.25 mol‚L-1. The gradual change of the spectra is in good agreement with the formation of gels. In the following experiments, we added SL and LA to the EAN directly according to the stoichiometric composition of eq 1 and observed whether gels could be formed. However, the result indicates that LA is insoluble in EAN even when the LA melts. This is possibly because the LA cannot come into contact with the SL dissolved in the bulk from a molecular level in this instance. However, through the slow reaction between SL and EAN, the LA molecules gradually created in a bulk solution can sufficiently come into contact with plenty of SL molecules. In the case of chemical reactions, LA and SL form an acid-soap complex but LA does not separate out. It is necessary to note that the alkyl chain of carboxylate has great influence on the formation of ionogels. We found that sodium caprylate cannot form ionogels in EAN even when the concentration exceeds 0.6 mol‚L-1, although a similar chemical reaction between sodium caprylate and EAN exists. This observation could assuredly demonstrate that the solvatophobic interaction plays an important role in the formation of ionogels. Specifically how the alkyl chain of carboxylates influences the formation the ionogels is currently being worked on in our laboratory. Because the three-dimensional networks responsible for the gelation are built by the noncovalent interaction, the formed anisotropic ionogels show thermal reversible sol-to-gel transition. The influence of the concentration of SL on the sol-to-gel transition temperature is shown in Figure 8, where the concentrations of SL are plotted against the temperature. The transition temperature from the gel to the sol increases with increasing concentrations of SL. For instance, the anisotropic ionogels at an SL concentration of 0.35 mol‚L-1 can be transformed into a sol at approxiamately 41.91 °C. In other words, cooling the isotropic sol solution lowers the minimum gel concentration necessary to build the three-dimensional networks. In fact, Figure 8 shows the sol-to-gel phase diagram for SL in EAN. The area above the plot is the anisotropic gel phase, and the area below it is the isotropic sol phase. Hanabusa et al.4 reported that, from the nature of gel phases, the pure gelator molecules must be solid at the temperature of their gels.
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Figure 4. Packing scheme of acid-soap units formed by SL and LA.
Figure 5. XRD patterns of SL and ionogels with cSL ) 0.4 mol‚L-1 in EAN at 25.0 ( 0.1 °C. The inset is the magnification of the pattern of the ionogels. Figure 7. FT-IR spectra of samples with different concentrations of SL in EAN. From a to g, cSL ) 0.0, 0.15, 0.2, 0.25, 0.3, 0.4, and 0.5 mol‚L-1. T ) 25.0 ( 0.1 °C.
Figure 6. SAXS patterns of ionogel samples with different SL concentrations in EAN. cSL ) 0.35 (a), 0.4 (b), and 0.45 (c) mol‚L-1. T ) 25.0 ( 0.1 °C.
The macroproperties of anisotropic ionogels with different concentrations of SL in EAN were characterized by rheological measurements of the oscillatory shear. We found that all the gel samples show similar rheological behavior. Two typical rheograms of the oscillatory shear for two ionogel samples having different concentrations of SL at 25.0 ( 0.1 °C are shown in Figure 9. By comparing these two rheograms, one can see that the rheological behavior of these two samples is similar. Over the range from 0.1 to 10 Hz, both the storage modulus (G′) and loss modulus (G′′) are nearly frequency-independent, and G′ is higher than G′′. The complex viscosity (|η*|) has a linear decrease with a slope of -1. These results indicate that the gels are highly viscoelastic, which is in agreement with previous reports on the gels formed in organic solvents.28 The storage modulus is an important parameter which characterizes the strength of gels, which can estimate the degree
Figure 8. Minimum cSL necessary for gelation to occur against the temperature. The two inset photos are of representative sol and gel samples.
of resistance against mechanical stress.28 Figure 10 shows the variation of G′ of ionogel samples with the concentration of SL. All moduli are nearly independent of the frequency. From Figure 10, one can see that with an increase of the SL concentration from 0.30 to 0.50 mol‚L-1, the G′ increases gradually, implying that three-dimensional networks become much denser with an increase of the SL concentration. Both the stability and strength of the anisotropic ionogel networks increased. In addition, it was (28) Mohmeyer, N.; Schmidt, H. Chem. Eur. J. 2005, 11, 863-872.
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Figure 11. Steady-shear rheological data for ionogels with different concentrations of SL. All data were collected at 25.0 ( 0.1 °C.
Figure 9. Rheograms for two ionogel samples. The concentration of SL is 0.35 (a) and 0.40 (b) mol‚L-1, respectively. G′, G′′, and |η*| as a function of the frequency. T ) 25.0 ( 0.1 °C.
Figure 10. Comparison of the G′ values versus frequency at different concentrations of SL in EAN. T ) 25.0 ( 0.1 °C.
found that the storage modulus values of all samples were higher than 40 kPa, indicating a good resistance against mechanical stress. To further explore the rheological behavior of anisotropic ionogels, steady-shear measurements of ionogels with different concentrations of SL were carried out. Typical steady-shear rheograms, i.e., apparent viscosity as a function of shear rate, for ionogel samples with different amounts of SL in EAN are shown in Figure 11. It was found that all ionogel samples show similar rheological behavior. Two distinct regions are evident in the plot. At the lower shear rate range, the apparent viscosity, η, decreases noticeably with increasing shear rate, indicating shear thinning behavior. This is because the network of the gels is destroyed gradually with increasing shear rate. While the rheological behavior of the anisotropic ionogels prefers Newtonian fluid at a higher shear rate range, the apparent viscosity is nearly constant with variation of the shear rate, indicating that the
Figure 12. Cyclic voltammograms of GC electrodes in a gel sample with a 0.4 mol‚L-1 concentration of SL.
networks collapse completely due to the high shear rate and that the samples have turned into sols. Solid electrolytes, which are electrically conductive solids with ionic carriers, are becoming increasingly important due to their potential application in solid-state batteries, fuel cells, energy storage, and chemical sensors. The gelation of ionic liquids by a gelator is a conventional method for making gel electrolytes due to the high conductivity and wide electrochemical window of ionic liquids, and it is expected to play a role in the field of solid electrolytes. Conductivity measurements were carried out on the selected ionogel sample of 0.4 mol‚L-1 SL. The ionogel sample was first heated, causing it to convert to a sol, and afterward cooled to 25 °C. The measurement starts when the sample is in the isotropic sol phase. The conductivity of the sol sample was 19.9 ms‚cm-1, but after the sample formed into a gel, the conductivity became 17.5 ms‚cm-1. The nearly 12% decrease of conductivity is probably due to the three-dimensional network of the gel, which causes the mobility of anions and cations to become weaker. However, the conductivity of the ionogels still remains a high value, indicating that although EAN is located in the three-dimensional networks built by the gelator molecules, its behavior is similar to that of the bulk EAN. This result is consistent with Hanabusa’s conclusion that the ionic liquid component is immobilized on a macroscopic scale when the three-dimensional networks are built up, but remains isotropic on a microscopic scale at the molecular level.4 Electrochemical stability is an elementary requirement in applications of electrochemical devices. We measured the limited reduction and oxidation potentials of an ionogel and evaluated its electrochemical stability. The cyclic voltammogram of a selected ionogel sample with 0.4 mol‚L-1 SL in EAN is shown in Figure 12. It was found that there are nearly no redox peaks for the ionogels over the range from -3.0 to +3.0 V, indicating
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the ionogels are electrochemically stable in wide potential ranges.4,29 It is expected that these gels can have important applications in electrochemical fields such as batteries. The wider electrochemical windows of the ionogels could be mainly attributed to the good electrochemical stability of RTILs.
Conclusions An ionogel was prepared in room-temperature ionic liquids due to a chemical reaction between SL and ionic liquids. The gelation of the ionic liquid is due to the formation of an acidsoap complex between SL and LA through hydrogen bond interaction and the solvatophobic interaction between the alkyl chains of SL and LA. SAXS results further indicate the SL and (29) Li, Z.; Liu, H.; Liu, Y.; He, P.; Li, J. J. Phys. Chem. 2004, 108, 1751217518.
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LA molecules formed a lamellar structure. Polarized optical microscopy results indicate an anisotropic structure of the ionogels. With an increase of the SL concentration in EAN, the sol-to-gel transition temperature gradually increases. Rheological data indicate that the ionogels are highly viscoelastic and that the gel strength increases with an increase of the SL concentration. Moreover, the ionogels have good resistance against mechanical stress even at a lower concentration of gelator. The electrochemical properties indicate that the gels have higher conductivity, better electrochemical stability, and a wide electrochemical window, therefore illuminating the potential application in the field of electrochemical devices. Acknowledgment. This paper was financially supported by the NSFC (Grant No. 20625307, J.H.). LA703632G
