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Lyotropic Liquid-Crystalline Gel Formation in a Room-Temperature Ionic Liquid Millicent A. Firestone,*,† Julie A. Dzielawa,‡ Peter Zapol,† Larry A. Curtiss,†,‡ So¨nke Seifert,‡ and Mark L. Dietz‡ Materials Science and Chemistry Divisions, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439 Received May 14, 2002. In Final Form: July 11, 2002 Addition of water to 1-decyl-3-methylimidazolium bromide is shown to result in its spontaneous selforganization and the concomitant formation of a liquid-crystalline gel, thus providing a simple means of preparing a supramolecular assembly comprising a room-temperature ionic liquid.
A major area of current interest in the field of nanoscience is the use of self-assembly to prepare functional materials and devices in a spontaneous yet controlled fashion, thereby providing an alternative to more expensive “top down” fabrication strategies. Of particular interest has been the development of a means to direct molecular organization and to generate novel supramolecular structures in soft condensed matter phases. These materials not only are inherently tunable and readily processable but also can combine hierarchical structural organization with a dynamic nature, thus facilitating the development of functional molecular systems.1 Gels comprise an important class of soft matter, constituting an essential component of living organisms and providing the basis of a variety of synthetic systems of technological significance in fields ranging from controlled drug delivery2 and chemical sensor development3 to biomolecule separations4 and nanostructured materials synthesis.5 Recently, the development of liquid-crystalline (LC) gels has attracted considerable attention, particularly in the areas of biomaterials6 and electrooptics.7 Numerous such gels have been described to date, most commonly employing covalent bonding interactions in their design and stabilization (i.e., chemical gels).8 Work in this laboratory has, in contrast, focused on the application of noncovalent interactions in the design and fabrication of novel materials based on physical gels.1,9 Several recent reports have described the preparation of physical gels comprising room-temperature ionic liquids (RTILs). Carlin and Fuller,10 for example, detailed a novel * To whom correspondence should be addressed. Phone: 630252-8298. Fax: 630-252-9151. E-mail:
[email protected]. † Materials Science Division. ‡ Chemistry Division. (1) Firestone, M. A.; Thiyagarajan, P.; Tiede, D. M. Langmuir 1998, 14, 4688. (2) Rethwisch, D. G.; Chen, X.; Martin, B. D.; Dordick, J. S. In Biomolecular Materials by Design; Alper, M., Bayley, H., Kaplan, D., Navia, M., Eds.; Materials Research Society: Pittsburgh, PA, 1994. (3) Tess, M. E.; Cox, J. A. J. Pharm. Biomed. Anal. 1999, 19, 55. (4) Wang, K. L.; Burban, J. H.; Cussler, E. L. Adv. Polym. Sci. 1993, 110, 67. (5) Rees, G. D.; Robinson, B. H. Adv. Mater. 1993, 5, 608. (6) Hafkamp, R. J. H.; Kokke, B. P. A.; Danke, I. M.; Geurts, H. P. M.; Rowan, A. E.; Feiters, M. C.; Nolte, R. J. M. Chem. Commun. 1997, 545. (7) Kato, T.; Mizoshita, N.; Kutsuna, T.; Yabuuchi, K.; Hanabusa, K. Polym. Mater. Sci. Eng. 2000, 82, 334. (8) Kato, T. In Molecular Self-Assembly: Organic vs Inorganic Approaches; Fujita, M., Ed.; Springer-Verlag: New York, 2000. (9) Firestone, M. A.; Williams, D. E.; Seifert, S.; Csencsits, R. Nano Lett. 2001, 1, 129. (10) Carlin, R. T.; Fuller, J. Chem. Commun. 1997, 1345.
catalytic membrane for heterogeneous hydrogenation fabricated by the incorporation of palladium into an ionic liquid-polymer gel. More recently, Ikeda et al.11 employed a cholesterol-based organogelator to induce gelation in various N,N′-dialkylimidazolium and N-alkylpyridinium salts. Similarly, Kimizuka and Nakashima,12 in the course of evaluating the properties of a series of “sugar-philic” RTILs designed to dissolve carbohydrates, noted that physical gelation of certain of these ionic liquids could be induced by dissolution of an appropriate amide groupenriched glycolipid. In this case, gelation was accompanied by the formation of a bilayer structure comprising the glycolipid molecules analogous to that formed by the same glycolipids in aqueous solution. The formation of such supramolecular aggregates in room-temperature ionic liquids has itself attracted increasing recent attention, a result of its obvious fundamental interest and potential utility in a variety of areas of technological importance. Yoshio et al.,13,14 for example, have shown that the addition of an appropriate mesogen (e.g., 6-[2,3-difluoro-4-[trans(4-pentylcyclohexyl)phenyl]phenyloxy]hexane-1-ol13 or n-octadecyl-3-methylimidazolium tetrafluoroborate or hexafluorophosphate14) to 1-ethyl-3-methylimidazolium tetrafluoroborate13,14 or hexafluorophosphate14 can lead to the formation of layered liquid-crystalline assemblies capable of anisotropic ion conduction. In conjunction with our ongoing efforts to exploit soft matter in the design and preparation of novel materials,1,9 we have sought a simple means by which to induce both gelation and mesoscopic ordering (i.e., the formation of a supramolecular assembly) in a room-temperature ionic liquid, one which requires neither chemical derivatization of the ionic liquid12 nor introduction of an organogelator11 or exotic mesogen.13 The structural similarity of the cationic constituents of certain RTILs to conventional cationic surfactants and the well-known tendency of amphiphiles to self-organize in the presence of water suggested to us that an appropriate combination of RTIL and water could provide the desired result. In this report, we describe the preparation and characterization of a lyotropic liquid-crystalline gel formed via the interaction of water with a room-temperature ionic liquid, 1-decyl3-methylimidazolium bromide. Despite a now extensive (11) Ikeda, A.; Sonoda, K.; Ayabe, M.; Tamaru, S.; Nakashima, T.; Kimizuka, N.; Shinkai, S. Chem. Lett. 2001, 1154. (12) Kimizuka, N.; Nakashima, T. Langmuir 2001, 17, 6759. (13) Yoshio, M.; Mukai, T.; Kanie, K.; Yoshizawa, M.; Ohno, H.; Kato, T. Adv. Mater. 2002, 14, 351. (14) Yoshio, M.; Mukai, T.; Kanie, K.; Yoshizawa, M.; Ohno, H.; Kato, T. Chem. Lett. 2002, 320.
10.1021/la0259499 CCC: $22.00 © 2002 American Chemical Society Published on Web 08/30/2002
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Figure 1. Two-dimensional small-angle X-ray scattering patterns, azimuthally averaged intensity as a function of scattering vector, and polarized optical micrographs for the fluid (panels A-C, respectively) and (16% w/w H2O) gel (panels D-F, respectively) states of 1-decyl-3-methylimidazolium bromide (T ) 23 °C).
literature on the physicochemical properties of RTILs and their application as alternatives to conventional organic solvents in various synthetic, catalytic, and electrochemical applications, this seemingly obvious approach to the formation of a liquid-crystalline “ionogel” has not been described. Prior investigations have clearly established that with sufficiently long (n g 12) alkyl chains appended, 1-alkyl3-methylimidazolium (abbreviated [Cn-mim+]) salts can display liquid-crystalline behavior, frequently exhibiting mesophases over an extended temperature range.15-17 In contrast, analogues bearing shorter (n ) 2-10) alkyl chains have been reported to be isotropic liquids at room temperature.15,17 Consistent with these reports, polarized optical microscopy (POM) of a dried sample (defined here as one containing e1.6% w/w water) of 1-decyl-3-methylimidazolium bromide18 ([C10-mim+][Br-]) reveals no evidence of liquid crystallinity (Figure 1, panel C). Addition of water to the sample to yield a composition containing ∼5-40% w/w H2O, however, results in its nearly immediate conversion from a viscous liquid to a stable, homogeneous gel that resists flow against gravity for an indefinite period of time. Examination of this material by POM indicates that gelation is accompanied by the appearance of optical birefringence (e.g., for a composition containing 16% w/w H2O: Figure 1, panel F), demonstrating that the gel is, in fact, liquid-crystalline. Upon a modest increase in temperature, the gel converts to a free-flowing, nonbirefringent (i.e., optically isotropic) fluid. Both the gel and the fluid phases exhibit excellent optical transparency and good stability. Cycling through the phase transition, in fact, produces no obvious deleterious effects on the macroscopic properties of either phase. To characterize the thermotropic behavior of the gel, in particular, to better define the birefringent to nonbire(15) Bowlas, C. J.; Bruce, D. W.; Seddon, K. R. Chem. Commun. 1996, 1625. (16) Gordon, C. M.; Holbrey, J. D.; Kennedy, A. R.; Seddon, K. R. J. Mater. Chem. 1998, 8, 2627. (17) Holbrey, J. D.; Seddon, K. R. J. Chem. Soc., Dalton Trans. 1999, 2133. (18) The ionic liquid was prepared via the method of Huddleston.19 Following drying in vacuo at 70 °C for 48 h, the IL was characterized by 1H and 13C NMR and by high-resolution FAB-MS (Washington University Mass Spectrometry Resource, St. Louis, MO). The water content of the ionic liquid was determined by Karl Fischer titration (ANL Analytical Services Department).
fringent phase transition observed under polarized light, a differential scanning calorimetry (DSC) profile20 was obtained over the range 10-80 °C. This profile is dominated by a single, narrow, endothermic transition (Tm) centered at 44.6 °C. The occurrence of a phase transition in this region is consistent with the disappearance of optical birefringence at elevated temperatures observed by POM. To probe the mesoscale (i.e., nanometer to micrometer scale) structure of the ionic liquid and the gel formed from it upon addition of water, small-angle X-ray scattering (SAXS)21 was employed. Figure 1 compares the twodimensional SAXS patterns and the corresponding plots of the azimuthally averaged intensity as a function of scattering vector for the dried (i.e., fluid) and hydrated (i.e., gel; 16% w/w H2O) [C10-mim+][Br-] samples. For the dried sample, only a single Bragg peak centered at Q ) 0.24 Å is observed, the breadth of which is indicative of significant lattice and orientational disorder. The isotropic scattering pattern for the sample (panel A) indicates that it consists of microdomains in which all spatial orientations are present. Analysis of the first-order diffraction peak with the Scherrer equation,22 which permits estimation of the size of the quasi-crystalline regions present (thus providing an indication of the extent of long-range ordering in the RTIL), yields an average domain size of only 25 nm. The poor spatial coherence of the sample is undoubtedly the reason that no liquid-crystalline textures are observed for this sample under polarized light. For the gel sample, a strong, anisotropic 2-D pattern featuring two diffraction rings (at Q ) 0.22 and 0.44 Å) is observed (panel D). The anisotropy of the scattering pattern, with the scattered X-ray intensity directed predominantly along the equato(19) Huddleston, J. G.; Willauer, H. D.; Swatloski, R. P.; Visser, A. E.; Rogers, R. D. Chem. Commun. 1998, 1765. (20) DSC profiles were obtained as previously described.1 A 10 min equilibration time prior to the start of a heating/cooling scan and a heating rate of 10 °C/min were employed throughout. (21) SAXS measurements were performed as described in a previous report.9 All samples were measured in sealed glass capillaries. The sample-to-detector distance was set so as to provide for the detection of the range of momentum transfers 0.02 < Q < 0.85 Å. The area detector images were corrected for background scattering by subtraction of the image for an empty glass capillary. For loading, ionogel samples were warmed until free-flowing and drawn into Pasteur pipettes, which were cut to yield capillaries ca. 9 cm in length, then sealed at both ends. (22) Guinier, A. Crystals, Imperfect Crystals, and Amorphous Bodies; Dover: New York, 1994.
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rial axis, indicates that the mesogens are well ordered, with a preferred domain orientation perpendicular to the capillary axis. Application of the Scherrer equation yields an average domain size of 164 nm, considerably greater than that observed for the dried sample, thus demonstrating that a substantial increase in the structural order in the ionic liquid is induced by an increase in water content. The presence of a second-order diffraction ring at integral order spacing is consistent with a lamellar structure, here with a repeat distance/lattice spacing of 28.6 Å. Electronic structure calculations using ab initio molecular orbital theory23 at the Hartree-Fock level using the 6-31G* basis set indicate that an isolated, geometryoptimized [C10 -mim+][Br-] molecule incorporating an alltrans conformationally ordered alkyl chain will exhibit a maximum C-C length of ∼12.5 Å, while the same molecule incorporating a pair of gauche defects into the alkyl chain will be only ∼10.6 Å in length. Thus, regardless of the state of the alkyl chains, the observed lattice spacing exceeds twice the molecular length, indicating the presence of a water channel between lamellae 3.6 Å or more in thickness. Given that previous solid-state structural studies of long-chain (n > 12) [Cn-mim+] salts suggest that some degree of alkyl chain interdigitation might reasonably be expected,15 this thickness may well be significantly greater. To gain insight into the molecular basis for the onset of gelation in [C10-mim+][Br-] and to further define the gel structure, samples of the RTIL in the fluid and gel states were examined by transmission FTIR spectroscopy. Gelation was found to have no measurable effect on either the shape or position of the symmetric and asymmetric methylene stretching bands for [C10-mim+], modes known to be sensitive probes of alkyl chain packing.1 Thus, gelation is unlikely to arise from two-dimensional alkyl chain packing/ordering effects. Rather, the infrared results suggest that gelation arises from diminution of hydrogen bonding between the imidazolium ring and the bromide counterion24 and the concomitant formation of an Hbonded network comprising water, bromide ion, and the
imidazolium cation. Such network formation apparently serves to enhance the segregation of the hydrophilic and hydrophobic segments of the [C10-mim+], thereby leading to the regions of confined water and, ultimately, to the onset of gelation. The simultaneous appearance of a smectic liquid-crystalline phase indicates that the formation of H-bonds between imidazolium rings, mediated by a channel of water, provides for enhanced ordering of the parallel molecular layers. In conclusion, the results of the present study demonstrate that a room-temperature ionic liquid can be induced to form a liquid-crystalline ionogel simply by addition of an appropriate concentration of water. We expect that the chemical simplicity of this material, together with the unique combination of properties exhibited by it, in particular, its existence as a liquid crystalline gel at room temperature, the wide range of water concentrations (540% w/w) over which the gel is formed, and the nature of the gel structure (with its segregated organic and aqueous domains), will open the door to wide-ranging studies of supramolecular chemistry in ionic liquids. Although we have confined our comments here to 1-decyl-3-methylimidazolium bromide, there is no reason to expect that the observed behavior will prove to be unique to this ionic liquid. In fact, our preliminary SAXS studies of related compounds incorporating a variety of other anions indicate that the presence of even small quantities of water is sufficient to induce a measurable degree of order in certain of these liquids. Work to define the structure of the ordered moieties produced and to explore the application of supramolecular assemblies of ILs in chemical separations and materials synthesis is now underway in this laboratory.
(23) Hehre, W. J.; Radom, L.; Schleyer, P. V. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; John Wiley: New York, 1986.
(24) Elaiwi, A.; Hitchock, P. B.; Seddon, K. R.; Srinivasan, N.; Tan, Y.; Weldon, T.; Zora, J. A. J. Chem. Soc., Dalton Trans. 1995, 3467.
Acknowledgment. This work was performed under the auspices of the Office of Basic Energy Sciences, Divisions of Materials Sciences (MAF, PZ, LAC) and Chemical Sciences (JAD, SS, MLD), United States DepartmentofEnergy,underContractNumberW-31-109-ENG-38. LA0259499
