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Self-Assembly of a Sugar-Based Gelator in Water: Its Remarkable Diversity in Gelation Ability and Aggregate Structure Jong Hwa Jung,† George John,† Mitsutoshi Masuda,‡ Kaname Yoshida,‡ Seiji Shinkai,§ and Toshimi Shimizu*,†,‡ CREST, Japan Science and Technology Corporation (JST), Nanoarchitectonics Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 4, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japan, Nanoarchitectonics Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan, and Chemotransfiguration Project, Japan Science and Technology Corporation (JST), 2432 Aikawa, Kurume, Fukuoka 839-0861, Japan Received June 22, 2001. In Final Form: August 16, 2001 A new sugar-based gelator 1 was synthesized, and its gelation ability was evaluated in organic solvents and water. Very surprisingly, 1 was found to gelate organic solvents as well as water, indicating that 1 can act as an amphiphilic gelator. We characterized on superstructures of an aqueous gel from 1 using SEM, TEM, NMR, IR, and XRD. The aqueous gel 1 formed a three-dimensional network with 20-500 nm diameter puckered fibrils. In addition, the chiral aggregate was found to be largely twisted helical ribbons with ca. 85 nm width, ca. 315 nm pitch, and up to several micrometer length, whose helicity was exclusively left-handed. XRD diagrams indicate that an aqueous gel 1 maintains a bilayered structure with 2.90 nm long-range spacing. This gives the first example of the formation of well ordered bilayer-based aqueous gel. The XRD, FT-NMR, and FT-IR results suggested that the aqueous gel 1 is stabilized by a combination of the hydrogen bonding, π-π interactions, and hydrophobic forces.
The recent focus in suprarmolecular chemistry is mainly on organizing monomeric species in a desired superstructure. There has been intense interest in the development of efficient and tunable small molecule gelators for industrial purposes (e.g., in foods, deodorants, cosmetics, athletic shoes, and chromatography), as a consequence of versatile gel functions in both a microscopic and macroscopic scale.1-5 SEM and TEM observations have established that fibrous aggregates of low-molecular-weight compounds formed by noncovalent interactions are responsible for such gelation phenomena. Particularly, “aqueous gels” are usually made of polymeric molecules * To whom correspondence should be addressed. Fax: +81-29861-2659. E-mail:
[email protected]. † CREST. ‡ Nanoarchitectonics Research Center. § Chemotransfiguration Project. (1) (a) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.; Komori, T.; Ohseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994, 116, 6664 and references therein. (b) James, T. D.; Murata, K.; Harada, T.; Ueda, K.; Shinkai, S. Chem. Lett. 1994, 273. (c) Jeong, S. W.; Murata, K.; Shinkai, S. Supramol. Sci. 1996, 3, 83. (d) Shinkai, S.; Murata, K. J. Mater. Chem. (Feature Article) 1998, 8, 485. (d) Yoza, K.; Amanokura, N.; Ono, Y.; Akao, T.; Shinmori, H.; Takeuchi, M.; Shinkai, S.; Reinhout, D. L. Chem. Eur. J. 1999, 5, 2722. (2) (a) Wang, R.; Geiger, C.; Chen, L.; Swanson, B.; Whitten, D. G. J. Am. Chem. Soc. 2000, 122, 2399. (b) Duncan, D. C.; Whitten, D. G. Langmuir 2000, 16, 6445. (c) Geiger, C.; Stanescu, M.; Chen, L.; Whitten, D. G. Langmuir 1999, 15, 2241. (3) (a) For recent comprehensive reviews, see:Terech, P.; Weiss, R. G. Chem. Rev. 1997, 3313. (b) Otsuni, E.; Kamaras, P.; Weiss, R. G. Angew. Chem. Int. Ed. 1996, 35, 1324 and references therein. (c) Terech, P.; Furman, I.; Weiss, R. G. J. Phy. Chem. 1995, 99, 9558 and references therein. (d) Abdallach, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 1237. (4) (a) Hanabusa, K.; Yamada, M.; Kimura, M.; Shirai, H. Angew. Chem., Int. Ed. Engl. 1996, 35, 1949. (b) Loos, M.; Esch, J. v.; Stokroos, I.; Kellogg, R. M.; Feringa, B. L. J. Am. Chem. Soc. 1997, 119, 12675. (c) Schoonbeek, F. S.; Esch, J. v.; Hulst, R.; Kellogg, R. M.; Feringa, B. L. Chem. Eur. J. 2000, 6, 2633. (c) Esch, J. v.; Feringa, B. L. Angew. Chem., Int. Ed. Engl. 2000, 39, 2263.
Table 1. Gelation Abilitya of 1 in Organic Solvents and Water solvent
1
methanol ethanol 1-butanol t-butanol tetrahydrofuran chloroform dichloromethane n-hexane ethylacetate dimethylformamide (DMF) dimethylsulfoxide (DMSO) waterb
S S G G G G G G G G G G
a Gelator ) 0.1-3.0 wt %. G: stable gel formed at room temperature. S: soluble. b Stable gel formed in the presence of a trace amount of methanol or ethanol.
(i.e., proteins and polymers) whose complicated intermolecular association modes are difficult to define whereas “organogels” are one-dimensional aggregates of lowmolecular-weight compounds, which have reversible superstructures formed by self-assembly. However, there are only a limited number of “aqueous gels” composed of such aggregates of low molecular weight compounds.6 To the best of our knowledge, moreover, the microscopic structures of aqueous gels are not yet characterized clearly by NMR or X-ray-like measurements in detail. We have (5) (a) Melendez, R.; Geib, S. J.; Hamilton, A. D. Molecular SelfAssembly Organic Versus Inorganic Approaches; Fujita, M., Ed.; Springer: New York, 2000. (b) Carr, A. J.; Melendez, R.; Geib, S. J.; Hamilton, A. D. Tetrahedron Lett. 1998, 39, 7447. (c) Shi, C.; Kilic, S.; Xu, J.; Enick, R. M.; Beckman, E. J.; Carr, A. J.; Melendez, R. E.; Hamilton, A. D. Science 1999, 286, 1540. (d) 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.
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Figure 2. 1H NMR spectra of an aqueous gel 1 in D2O and methanol-d4 (1:1 v/v).
Figure 1. EF-TEM pictures of an aqueous gel 1 without negatively staining: (a) low modification, (b) high modification, and (c) SEM picture of the xerogel 1 prepared from water. The arrows indicate a helical fiber.
focused our research effort toward exploitation of novel sugar-based aggregates formed in water:7 a merit of this system is that one can systematically design various aggregates utilizing abundant basic skeletons in a carbohydrate family. In the research process, we found that (6) (a) Estroff, L. A.; Hamilton, A. D. Angew. Chem., Int. Ed. Engl. 2000, 39, 3447. (b) Fuhrhop, J.-H.; Schnieder, P.; Rosenbery, J.; Boekema, E. J. Am. Chem. Soc. 1987, 109, 3387. (c) Fuhrhop, J.-H.; Boettcher, C. J. Am. Chem. Soc. 1990, 112, 1768. (d) Yoza, K.; Ono, Y.; Yoshihara, K.; Akao, T.; Shinmori, H.; Takeuchi, M.; Shinkai, S.; Reinhout, D. L. Chem. Commun. 1998, 907. (e) Menger, F.; Caran, K. J. Am. Chem. Soc. 2000, 122, 11679. (f) Hanbusa, K.; Hirata, T.; Inoue, D.; Kimura, M.; Touhara, H.; Shirai, H. Colloids Surf., A 2000, 169, 387. (g) Oda, R.; Huc, I.; Candau, S. J. Angew. Chem., Int. Ed. Engl. 1998, 37, 2689. (7) (a) John, G.; Masuda, M.; Okada, Y.; Yase, K.; Shimizu, T. Adv. Mater. 2001, 13, 715. (b) Masuda, M.; Hanada, T.; Okada, Y.; Yase, K.; Shimizu, T. Macromolecules 2000, 33, 9233. (c) Nakazawa, I.; Masuda, M.; Okada, Y.; Hanada, T.; Yase, K.; Asai, M.; Shimizu, T. Langmuir 1999, 15, 4757. (d) Shimizu, T.; Masuda, M. J. Am. Chem. Soc. 1997, 119, 2812.
one glucose-based gelator can gelate water.7a The finding implies that if we carefully search for sugars as well as for appropriate hydrophobic groups, several new aqueous gelators may be further discovered, which may be useful to specify what is the basic structural requirements to design aqueous gelators. With these objectives in mind, we designed a new gelator bearing a sugar moiety, an aminophenyl, and a long alkyl chain group. The long alkyl chain at one side of gelator enhances its solubility in organic solvents but also promotes association among the fibers, through van der Waals forces, and eventual gel formation. Here, we report the characterization of aggregation properties of such a novel gelator 1 in water by energy-filtering transmission electron microscopy (EFTEM),7c NMR, FT-IR, and XRD.
A typical procedure for studying gel formation ability is as follows: a weighed sample was mixed with water or an organic liquid in a sealed test tube and the mixture was heated until the solid dissolved. The resulting solution was allowed to cool to 25 °C for 1 h, and then the gelation was studied. The gelator and the solvent were put in a septum-capped test tube and heated in an oil bath until the solid was dissolved. The solution was cooled at 25 °C. If the stable gel was observed at this stage, it was classified as G in Table 1. The gelation ability of 1 was examined in 10 organic solvents and water. The results are summarized in Table 1. The gelator 1 could gelate 8 out of 10 organic solvents, such as chloroform, tetrahydrofuran, 1-butanol, and ethyl acetate. Of particular interest is the
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Figure 3. Powder XRD spectra of (a) the xerogel 1 from prepared water, (b) an aqueous gel 1, and (c) possible self-assembled microstructure of an aqueous gel 1.
gelation of water in the presence of a trace amount of alcohol (ca. 1 wt %) by gelator 1 at concentrations less than 0.1 wt % (> 50000 molecules of water per one gelator molecule). These results indicate that 1 can act as an amphiphilic gelator in water and organic solvents. To obtain visual insights into chiral aggregation mode which may arise from the sugar moiety, we took EF-TEM and SEM images of an aqueous gel 1. Figures 1a and 1c show typical pictures obtained from an aqueous gel 1. They clearly reveal that the gelator forms a threedimensional network with 20-500 nm diameter puckered fibrils. Judging from the SEM picture of an aqueous gel 1, some ribbon structures are twisted, and have a lefthanded helicity. In addition, TEM analysis of the chiral aggregate clearly showed that the fibers are twisted helical ribbons with ca. 85 nm width, ca. 315 nm pitch, and up to several micrometers in length, whose helicity is exclusively left-handed (Figure 1b). These helical aggregates can account for the formation of a metastable gel instead of thermally stable crystals. Alternative evidence supporting the self-aggregation of an aqueous gel 1 was provided by 1H NMR measurements. As shown in Figure 2a, very interestingly, aromatic peaks of aggregate 1 in the gel phase appeared at 7.43 (d, J ) 8.61 Hz, Hb) and 7.38 ppm (d, J ) 8.61 Hz, Ha) at 27 °C. Upon heating, the new peaks appeared gradually at 7.60 (d, J ) 8.61 Hz, Hb) and 7.28 ppm (d, J ) 8.61 Hz, Ha) with disappearance of original peaks for 7.43 and 7.38 ppm. The difference in chemical shift between the aromatic protons Ha and Hb may arise from π-π stacking and the hydrogen-bond interactions. It may be explained by considering the fact that the induction effect of the hydrogen bonding interaction is only too large to cancel the upfield shifts of Ha due to π-π stacking interaction.8 The similar phenomenon was observed with the anomeric proton of the C-1 position of sugar moiety (Figure 2b). The appearance of the separated signals of the aggregated (8) (a) Jung, J. H.; Takehisa, C.; Sakata, Y.; Kaneda, T. Chem. Lett. 1996, 147. (b) Lee, S. S.; Jung, J. H.; Yu, S. H.; Cho, M. H. Thermodyn. Acta 1995, 259, 133.
and nonaggregated species demonstrates that the chemical exchange is slow compared with the NMR time scale. These results are the first observation as 1H NMR spectral evidence for aggregates stabilized by the hydrogen bonding and π-π stacking interactions in gel phase, and strongly support the view that the aromatic units induce rigidity in the structure and also help with a linear fashion to form an aqueous gel. Since it is very difficult or almost impossible to obtain useful information about intermolecular hydrogen-bonding interactions in an aqueous gel from a FT-IR study, we thus observed evidence of the intermolecular hydrogen-bonding interaction in a D2O system. The FT-IR spectrum of the deuterated aqueous gel 1 is characterized by the absorption bands around 1645 (-CdO, amide I) and 1514 (-NH, amide II) cm-1. In addition, the similar results were obtained from the cyclohexane gel 1, which gave the IR bands at 3398 (-OH), 3298 (-NH), and 1658 (-CdO) cm-1. These results indicate that the amide groups of an aqueous gel 1 not only form the intermolecular hydrogen bond in the gel phase, but also enjoy the much stronger hydrogen-bonding interaction in comparison to the cyclohexane gel 1. Recently, an X-ray crystallographic methodology for ascertaining the molecular packing of gelators in the gel phase has been reported, and this method is being used to clarify the gelation mechanism of low molecular-weight gelators.3a,9 However, the correlation between the molecular packing of gelator molecules and the physical gelation properties is still unknown. The xerogel 1 obtained from water by a freezing method resulted in the spongelike aggregate, but not the typical crystal. We obtained information about the molecular packing mode of gelator molecules in a neat gel from an X-ray diffraction pattern of the xerogel. The diffraction pattern of the xerogel 1 shows periodical diffraction peaks (Figure 3a), indicating that 1 indeed assembles into a lamellar organization. The (9) (a) Hanabusa, K.; Matsumoto, M.; Kimura, M.; Kakehi, A.; Shirai, H. J. Colloid Interface Sci. 2000, 224, 231. (b) Abdallah, D. J.; Sirchio, S.; Weiss, R. G. Langmuir 2000, 16, 7558. (c) Sakurai, K.; Ono, Y.; Jung, J. H.; Okamoto, S.; Sakurai, S.; Shinkai, S. J. Chem. Soc., Perkin Trans. 2 2001, 108.
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long spacing d of the aggregate obtained by the XRD method is about 2.90, 1.46, and 0.97 nm, which are almost exactly 1:1/2:1/3 and smaller than twice of the extended molecular length of 1 (2.45 nm, by the CPK molecular modeling) but larger than the length of one molecule 1. Very surprisingly, the 2.90 nm peak observed from xerogel 1 has successfully been observed in the gel state (Figure 3b). These results strongly indicate that an aqueous gel 1 maintains a interdigitated bilayered structure with a 2.90 nm thickness of molecular sheet corresponding to the (100) plane (Figure 3c). In addition, wide-angle region of the X-ray diagram for an aqueous gel 1 reveals a series of sharp reflection peaks, supporting the view that presumably long alkyl chain groups form highly ordered layer packing by the interdigitated hydrophopic interaction. This is the first example for the formation of well
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ordered bilayer-based aqueous gel. On the basis of XRD, FT-NMR, and FT-IR results, an aqueous gel 1 was found to be stabilized by a combination of the hydrogen bonding, π-π interactions, and hydrophobic forces. In conclusion, the present study has demonstrated that the sugar-based gelator 1 can form well ordered bilayer aggregates by self-assembly, through intermolecular hydrogen-bonding interaction, π-π stacking and interdigitated hydrophobic interaction in water, which is juxtaposed by hydrophobic interaction. Moreover, the gelator can act as an amphiphilic gel in water and organic solvents. The findings reveal that the synergetic operation of several difference forces is essential to successfully design the aqueous gel system. LA0109516
