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Articles Unsaturation Effect on Gelation Behavior of Aryl Glycolipids George John,*,† Jong Hwa Jung,†,‡ Mitsutoshi Masuda,†,§ and Toshimi Shimizu*,†,§ CREST, Japan Science and Technology Corporation (JST), Nanoarchitectonics Research Center (NARC), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 4, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japan, and Nanoarchitectonics Research Center (NARC), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Received April 24, 2003. In Final Form: December 12, 2003 Structurally simple, renewable-resource-based cardanyl (glucoside)s [1, 1-O-3′-n-(pentadecyl) phenylβ-D-glucopyranoside; 2, 1-O-3′-n-(8′(Z)-pentadecenyl)phenyl-β-D-glucopyranoside; 3, 1-O-3′-n-(8′(Z),11′(Z)pentadecadienyl)phenyl-β-D-glucopyranoside, 4; 1-O-3′-n-(8′(Z),11′(Z),14′-pentadecatrienyl)phenyl-β-Dglucopyranoside; and the mixture 5] form thermally reversible transparent gels in a water/alcohol mixture and a number of organic solvents, strongly influenced by the unsaturation of the aliphatic alkyl chain. DSC studies revealed that the Tgel of 1 in water/ethanol (1:1, vol/vol) gel is 69.0 °C, while of the introduction of a single double bond reduces the value to 30.0 °C in the case of monoene 2, indicating that the stability of the gel is related to the number of double bonds on the lipophilic part of the gelator. Furthermore, XRD measurements showed that the aqueous gel 1 maintains an interdigitated bilayered structure with 3.14 nm long-range ordering, and the corresponding organogel maintains an extended bilayer structure for 4.34 nm, indicating a clear difference in the aggregation behavior in different solvents.
Introduction Organic gelling agents are a growing class of supramolecular assemblies with potential applications in oil chemistry, nutrition, cosmetics, and pharmacology. The development of new thermoreversible physical gels from low molecular mass organic molecules is an emerging area of current interest, though several different categories of gelators have been identified so far.1-6 A major challenge in this field is the elaboration of novel design strategies enabling the synthesis of structurally simple gelator * Corresponding authors. G.J.sCurrent address: Rensselaer Nanotechnology Center & Department of Chemical and Biological Engineering, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, NY 12180. Fax: 1 518 276 4030. E-mail:
[email protected]. T.S.sFax: 81 298 61 4545. E-mail:
[email protected]. † CREST. ‡ Current address: Korea Basic Science Institute (KBSI), 52 Yeoeun-dong, Yusung-gu, Daejeon 305-333, Korea. § Nanoarchitectonics Research Center. (1) For recent comprehensive reviews, see: (a) Otsuni, E.; Kamaras, P.; Weiss, R. G. Angew. Chem., Int. Ed. 1996, 35, 1324. (b) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (c) van Esch, J. H.; Feringa, B. L Angew. Chem., Int. Ed. 2000, 39, 2263. (d) Abdallah, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 1237. (2) (a) Hanabusa, K.; Miki, T.; Taguchi, Y.; Koyama, T.; Shirai, H. J. Chem. Soc. Chem. Commun. 1993, 1382. (b) Hanabusa, K.; Yamada, M.; Shirai, H. Angew. Chem., Int. Ed. Engl. 1996, 35, 1949. (c) Suzuki, M.; Yumoto, M.; Kimura, M.; Shirai, H., Hanabusa, K. Chem. Eur. J. 2003, 9, 348. (3) (a) Loos, M.; Esch, J.; Stokroos, I.; Kellogg, R. M.; Feringa, B. L. J. Am. Chem. Soc. 1997, 119, 12675. (b) van Esch, J.; De Feyter, S.; Kellogg, R. M.; De Schryver, F.; Feringa, B. L. Chem. Eur. J. 1997, 3, 1238. (c) van der Laan, S.; Feringa, B. L.; Kellogg, R. M.; van Esch, J. Langmuir 2002, 18, 7136. (d) Hafkamp, R. J. H.; Kokke, P. A.; Danke, I. M.; Guerts, H. P. M.; Rowan, A. E.; Feiters, M. C.; Nolte, R. J. M. J. Chem. Soc. Chem. Commun. 1997, 545. (e) Hafkamp, R. J. H.; Feiters, M. C.; Nolte, R. J. M. J. Org. Chem. 1999, 64, 412.
molecules, easy to synthesize and available in large quantities from cheap starting materials for high volume applications. Due to our continued interest in the design of novel building blocks for molecular self-assembly using bola- and single head amphiphiles for meso- and nanostructures, we seek new sources for the synthesis of nonpolymeric gelators.7 We envisaged that carbohydratecontaining glycolipids might be appropriate candidates, if both the hydrophobic and the polar segments of these surfactants stem from natural renewable raw materials.8 Recently we have shown that cardanyl (glucoside)s exhibit excellent self-assembly properties in water, leading to (4) (a) Wang, R.; Geiger, C.; Chen, L.; 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. (5) (a) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, K.; Komori, T.; Ohseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994, 116, 6664. (b) Yoza, K.; Amanokura, N.; Ono, Y.; Akao, T.; Shinmori, H. Takeuchi, M.; Shinkai, S.; Reinhoudt, D. N. Chem. Eur. J. 1999, 5, 2722. (c) Kiyonaka, S.; Shinkai, S.; Hamachi, I. Chem. Eur. J. 2003, 9, 976. (6) (a) Menger, F. M.; Caran, K. L. J. Am. Chem. Soc. 2000, 122, 11679. (b) Estroff, L. A.; Hamilton, A. D. Angew. Chem., Int. Ed. 2000, 39, 3447. (c) Wang, G.; Hamilton, A. D. Chem. Eur. J. 2002, 8, 1954. (d) Ajayaghosh, A.; George, S. J. J. Am. Chem. Soc. 2001, 123, 5148. (e) George, M.; Weiss, R. G. J. Am. Chem. Soc. 2001, 123, 10393. (f) Oda, R.; Huc, I.; Candau, S. J. Angew. Chem., Int. Ed. 1998, 37, 2689. (7) (a) Shimizu, T.; Masuda, M. J. Am. Chem. Soc. 1997, 119, 2812. (b) Shimizu, T.; Iwaura, R.; Masuda, M.; Hanada, T.; Yase, K. J. Am. Chem. Soc. 2001, 123, 5947. (c) Jung, J. H.; John, G.; Masuda, M.; Yoshida, K.; Shinkai, S.; Shimizu, T. Langmuir 2001, 17, 7229. (d) Shimizu, T. Macromol. Rapid. Commun. (Feature Article) 2002, 23, 311. (8) (a) Bhattacharya, S.; Acharya, S. N. G. Chem. Mater. 1999, 11, 3504. (b) Aveyard, R.; Binko, B. P.; Chen, J.; Esquena, J.; Fletcher, P. D. J.; Buscall, R.; Davies, S. Langmuir 1998, 14, 4699.
10.1021/la030177h CCC: $27.50 © 2004 American Chemical Society Published on Web 02/06/2004
Gelation Behavior of Aryl Glycolipids Scheme 1
Langmuir, Vol. 20, No. 6, 2004 2061 Table 1. Gelation Behavior of Glycolipids 1-5 in Various Solvents at 25 °Ca 2c
nanofiber formation and further to organic nanotubes.9 The present study reports that the remarkable diversity in gelation and aggregation behavior of this novel family of compounds is strongly influenced by the unsaturation of the alkyl chain. Although much has been reported on the structure and properties of organogels, there is no study pertaining to the influence of unsaturation on the formation, structure, and resulting properties of organoand hydrogels. Compounds 1, 2, and 5 were found to form ordered bilayer aggregates in a number of organic solvents and water/alcohol and gelatinize the liquids. The facile syntheses of glycolipids 1 and 5 were carried out as reported earlier,9 and the structures are given in Scheme 1. The fractionation of cardanyl glucoside 5 to its individual components was achieved by reversed-phase mediumpressure column chromatography and afforded pure compounds such as monoene 2, diene 3, and triene 4 for the gelation experiments. Experimental Section Materials. Compounds 1 and 5 were synthesized according to the method reported previously.9 Cardanyl glucoside 5 mixtures were fractionated using Yamazen fraction collector FR-50N coupled with a gradient mixer GR-200, a variable-wavelength UV detector prep UV-10-V, and a flat minirecorder. The sample was applied to medium-pressure column chromatography on a Yamazen ODS column (100 × 2.6 cm, i.d.) packed with ODS (50 µm particle size). The mobile phase used was methanol-10% aqueous acetic acid (initially 88:12, and after the sample injection the gradient mixer changed to 90:10, v/v, methanol:10% aqueous acetic acid) and was delivered with a Yamazen Peristatic pump at 8 mL/min. The fractions were collected and dried to constant weight under vacuum at room temperature. The compounds corresponding to the saturated 1, monoene 2, diene 3, and triene 4 components were isolated and analyzed by standard methods (Supporting Information). Gelation Experiments. In a typical gelation experiment, a weighed amount (ca. 1-2 mg) of the glycolipid and 1 mL of the solvent were put in a sample bottle, after which the sample bottle was tightly sealed with a screw cap. The bottle was then heated with shaking until all the solid material had dissolved. The solution was set aside and allowed to cool to 25 °C or at 6-10 °C. Gelation was considered to have occurred if the solid aggregate mass was stable to inversion when the sample bottle was turned upside down. Both aqueous and organogels of 1 and 2 are transparent at lower (0.15 wt %) concentrations, while they are opaque at higher concentrations. (9) John, G.; Masuda, M.; Okada, Y.; Yase, K.; Shimizu, T. Adv. Mater. 2001, 13, 715.
solvents
1
methanol methanol/water (1:1)b ethanol ethanol/water (1:1) 2-propanol 2-propanol/water (1:1) n-butanol n-butanol/water (1:1) water acetone acetone/water (1:1) diethylene glycol glycerol dimethylformamide (DMF) DMF/water (1:1) dimethyl sulfoxide (DMSO) DMSO/water (1:1) tetrahydrofuran (THF) THF/water (1:1) n-hexane petroleum ether cyclohexane toluene p-xylene m-xylene
S G S G (69.0) S G S S P S G G G S G S G S G P P G G (50.0) G G
25 °C ∼10 °C S S S S S S S S P S S S S S S S S S S P P Gp Gp Gp Gp
S G S G (30.0) S G S S P S G G G S Gp S Gp S G P P G G (37.0) G G
3 4 S S S S S S S S P S S S S S S S S S S S P S S S S
S S S S S S S S P S S S S S S S S S S S P S S S S
5 S S S S S S S S P S S S S S S S S S S P P G G Gp Gp
a Gelator ) 0.1-3.0 wt %. G ) gel; Gp ) partial gel; P ) precipitation; S ) solution. The value given in parentheses is the Tgel measured in °C. b 1:1 (vol/vol) mixture of solvents. c Gelation occurs at a lower temperature of 6-10 °C.
Gelation Temperatures. Gelation temperatures (Tgel) were determined by the inverse flow method10 (i.e., the temperature at which gel fell under the influence of gravity when inverted in a sealed glass tube that was placed in a thermostated water bath). Tgel values were also measured by differential scanning colorimetry (DSC) using a Seiko 6100 high-sensitivity differential scanning colorimeter equipped with a nitrogen gas cooling system. The gel was hermetically sealed in a silver pan and measured against alumina as the reference. The thermograms were recorded at a heating rate of 1 °C/min. X-ray Diffractograms. X-ray diffraction (XRD) data of samples in thin capillaries (0.5 mm diameter; W. Muller, Schonwalde, FRG) were collected on a Rigaku R-AXIS (IV) image plate system with Cu KR X-rays generated with a Rigaku generator (4037) operated at 40 kV and 30 mA. Gel samples were prepared by flowing hot (T > Tgel) aliquots into the capillary and sealing its ends. The samples were cooled to 10 °C for complete gelation. The capillary was sealed and the XRD was measured. The typical exposure time was 5 min for self-assembled structures with 150-mm camera length. Data processing and analyses were performed using Materials Data JADE. SEM and TEM Observations. A JEOL JSM-5600 scanning electron microscope was used for taking the SEM pictures. The gel samples were prepared by a freezing-and-pumping method from their gel phases below the gel-sol transition temperature. For energy-filtering transmission microscopy (EF-TEM) a piece of the gel was placed on a carbon-coated copper grid (400 mesh) and removed after 1 min, leaving some small patches of the gel on the grid. Then, it was dried for 1 h at low pressure. The specimen was examined with Carl Zeiss EM 902, using an accelerating voltage of 80 kV and a 16-mm working distance.
Results and Discussion To test gelation, the glycolipid and liquid are heated in a sealed glass tube until the solid dissolves and then cooled to room temperature (Table 1). The macroscopic manifestation of successful gelation is the absence of observable flow when a sample is inverted. Interestingly, compound 1 formed gel at low concentrations in common organic (10) Takahashi, A.; Sakai, M.; Kato, T. Polym. J. 1980, 12, 335.
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Figure 1. Proposed model for different aggregation modes of 1 and 2 in polar and nonpolar solvents.
solvents and aqueous solutions. Of particular interest is the gelation of a 1:1 mixture of water/alcohol and water/ acetone and other protic solvents at a concentration of 15 000 solvent molecules per gelator molecule). By contrast, compound 5 formed no stable gels under the same conditions but formed a fibrous precipitate (water/ alcohol, 1:1 mixture, vol/vol) at low temperature, indicating that these molecules can also induce further gelation.
John et al.
The gelation phenomena in a 1:1 mixture of water/ alcohol or water/acetone could be explained in terms of the difference in crystallization of the alkyl part of the glycolipids 1 and 5. In the case of unsaturated analogue 5 comprising of a mixture of multiple double-bonded components, the cis double bond possesses a nonmovable bend with an angle of 30° in the alkyl chain as discussed (cf. Figure 5). Due to this bend, the crystallinity of the alkyl chain is disturbed and the fluidity of the side chain is increased, which retards the gelation. To study the influence of unsaturation on the gelation phenomena, we fractionated the mixture of compounds to individual pure components by reversed-phase column chromatography. The gelation behavior of monoene 2 fractionated from the mixture was tested under similar conditions to the saturated analogue, which does not form any stable gel at 25 °C, using the water/alcohol mixture. However, 2 formed a gel with water/alcohol mixture at slightly lower temperature (∼10.0 °C) and was stable under ambient conditions. Interestingly, the diene 3 and triene 4 hardly form gel with any of the solvent system selected, even on cooling to -6 °C (Table 1). Therefore, the present study discusses the gelation behavior of compounds 1, 2, and 5 toward mixed solvent systems of water/alcohol, water/
Figure 2. (a) SEM micrograph of the 1-water/ethanol system, (b) EF-TEM image of 1-water/ethanol (inset, TEM micrograph showing twisted morphology), (c) SEM image of 1-cyclohexane gel, (d) 2-water/ethanol gel, (e) 2-cyclohexane gel (inset, wrinkled sheets at high magnification), and (f) 5-cyclohexane gel, showing sheets and macroporous solid structures.
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Figure 3. Schematic representation of the xerogel formation of 2 and 5 from cyclohexane.
acetone, or pure apolar solvents such as cyclohexane or toluene. A possible mechanism of gelation of compounds 1 and 2 in a mixed solvent system (for example, water/alcohol or water/acetone) could be explained by the presence of alcohols or acetone, which may take part in hydrogen bonding, contributing to the stabilization of gel in mixtures with water, since the compounds with a long saturated/ mono-unsaturated alkyl moiety have no strong hydrogenbond acceptor.11 When the water content in methanol solution was varied gradually from 0 to 100%, we observe a remarkable change in the self-assembled state of 1 from monodispersed homogeneous solution to crystalline fiber precipitate via entangled network or partial fusion of individual fibers for 45-55% of water. Thus, intermediate solubility of the glycolipids in a mixed solvent gelatinizes the solvent system; however, a more hydrophobic alcohol, i.e. n-butanol, induces solubilization of 1, irrespective of the presence of 50% water (Table 1). Polar solvents such as DMSO, DMF, or glycerol could play an intermediate role in gelating behavior toward 1. They can form either gel upon long standing or spontaneous gel formation in the presence of water. Since the gels formed from the mixed solvent systems with 1 and 2 showed the ability to trap water molecules upon gelation, these gels could be of use in certain hydrogel applications, too. When gels from 1 and 2 were heated to ∼70 and 30 °C, respectively, the samples turn into clear isotropic fluids, and upon gradual cooling the transparent hydrogels could be reproduced, showing their excellent thermoreversible nature. Gels of water/alcohol, water/acetone, ethylene glycol, and glycerol are stable for months with 1 and 2. While in the case of 2, the gelation ability and Tgel reduced significantly as compared to the saturated analogue, which might be due to the inadequate packing of the long alkyl chain possessing a cis-double bond and hence poor aliphatic crystallization via hydrophobic interactions. Besides the polar solvents described above, 1, 2, and 5 also form gels with other aprotic solvents such as toluene, cyclohexane, and xylene, but the gels thicken at slightly higher concentrations (3-5 g L-1). Compounds 1, 2, and 5 are not soluble in chlorinated solvents, Et2O, or EtOAc, and no gels are formed in alcohols alone. These results illustrate the marked influence that the gelator structure and the nature of the liquid have on the network of gelator strands and the stability of the gels. The attachment of C15 alkyl chain with a phenyl moiety onto the hydrogenbonded core enhances its solubility in organic solvent but also promotes association among the fibers, through van der Waals forces, π-π-stacking, and eventual gel formation.12 (Figure 1). (11) Boettcher, C.; Schade, B.; Fuhrhop, J.-H. Langmuir 2001, 17, 873.
Figure 4. Powder XRD pattern of (A) (a) 1-water/ethanol gel and (b) 2-water/ethanol gel and (B) (a) 1-water/ethanol gel and (b) 1-cyclohexane xerogel.
Figure 5. (a) Proposed packing mode of the saturated analogue 1-water/alcohol gel system and (b) the influence of unsaturation on the packing of the 2-water/alcohol system.
The textures of the gels were examined with scanning electron microscopy (SEM) of a loose gel (the gel obtained was further diluted with excess solvent/solvent mixture, which was dropped on the SEM grid and frozen in liquid nitrogen) of 1 (water/ethanol) on the gold-coated specimen. The images revealed a three-dimensional collection of intertwisted and interlocked fibers (Figure 2a). The diameter of the smallest fiber is several tens of nanometers; it seems that even the smallest fiber is formed by gathering together a number of molecular sheets, which may interact with each other and grow into aggregates responsible for gelation. The pitch is 150-300 nm for a fiber with a width (12) (a) Philip, D.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1996, 35, 1154. (b) Smits, E.; Engberts, J. B. F. N.; Kellogg, R. M.; Van Doren, H. A. Liq. Cryst. 1997, 23, 481. (c) Shimizu, T.; Masuda, M. J. Am. Chem. Soc. 1997, 119, 2812.
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Figure 6. Variable-temperature 1H NMR studies of the gel of 1 in toluene-d8.
of 20-30 nm and a length exceeding 1 µm, and all individual fibers showed twisted morphology by EF-TEM13 (Figure 2b and inset TEM image). The SEM image of a xerogel of 1 in cyclohexane revealed the formation of entangled nanostructures in organic solvents (Figure 2c). The SEM analysis of the gel structure of compound 2 in water/ethanol showed the formation of the network structure observed as discussed for compound 1 (Figure 2d). Surprisingly, the SEM micrographs of 2 and 5 xerogels prepared under exactly the same conditions as that of 1 from cyclohexane showed a marked change in the morphology of unsaturated systems in lipophilic solvents. It appears that the cardanyl glucoside and monoene derivative gelled in cyclohexane resulted in extended bilayer membranes, which also might be forming a fibrous network for gelation. However, the SEM images showed a porous material with stacks of sheets (Figure 2e) for monoene 2 and (Figure 2f) for compound 5 in cyclohexane. The SEM image shown in Figure 2e shows how the xerogel is comprised of curved, wrinkled, and interconnected sheets. Increasing the instrumental magnification revealed the fibrous nature of the fine structure as in Figure 2e (inset). To clarify and confirm the original gel structure in the solvent system, we examined the wet gel by light microscopic methods. Interestingly, we could observe the fibrous structure formation in the gel systems. Therefore, we propose that the macroscopic fibers, as observed by light microscopy, did not survive the drying process. The microfibers pack themselves into sheetlike structures and finally to a macroporous structure on freezing-pumping. A similar observation was reported by Menger and coworkers for the glycouril-based gelating systems and its xerogel.14 This has been explained on the assumption that the molecular sheets self-assembled to fibers, which (13) Nakazawa, I.; Masuda, M.; Okada, Y.; Hanada, T.; Yase, K.; Asai, M.; Shimizu, T. Langmuir 1999, 15, 4757. (14) (a) Kolbel, M.; Menger, F. M. Chem. Commun. 2001, 275. (b) Kolbel, M.; Menger, F. M. Adv. Mater. 2001, 13, 1115.
hierarchically assembled to bundles and finally to sheets as depicted in Figure 3. Because of the curvature and multiple junctions with one another, cavities are formed to give macroporous solid. However, this finding for a sugar gelating system is unprecedented. Also it is worthy to note that simple molecules with subtle structural variations can induce significant morphological changes in selfassembled systems. 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.15 We obtained information about the molecular packing mode of the gelator molecules in a gel from the wet gel (water/alcohol system) as well as from xerogel (organic solvents) prepared by a freezing-pumping method. X-ray diffraction patterns showed a d spacing of 3.14 and 3.90 nm for compounds 1 and 2, respectively, in water/alcohol system (Figure 4A), which is characteristic of π-π stacked interdigitated lamellar packing of the molecule in water/ethanol gel and also the self-assembled isolated fibers in water, while that of the cyclohexanederived gel of 1 gave a periodicity of 4.34 nm (Figure 4B), indicating a clear difference in the aggregation behavior in different solvents. The difference in the d spacing of the saturated 1 and unsaturated 2 systems may be explained by the influence of a cis-double bond in the lipophilic chain (Figure 5). Variable-temperature 1H NMR studies of 1 in toluened8 between 27 and 80 °C showed the appearance of wellresolved signals above 70 °C that correspond to the various protons, indicating the disordering of the π-π stacked self-assembly to the isotropic form. The chemical shift for the aromatic protons in the gel state (a) and solution (c) are depicted in Figure 6. In the gel state, small broad (15) (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.
Gelation Behavior of Aryl Glycolipids
peaks corresponding to the aromatic protons of 1 were seen in the 1H NMR spectrum and are assigned as follows, based on the molecular simulation values: h, 6.87 (d); i, 7.10 (t); j, 6.80 (d); k, 6.84 (s). The peaks corresponding to i and k are shown in the figure for clarity, as they merged to the peak corresponding to toluene-d8. On heating there is a dramatic shift for the proton signals of h [6.96 (d)], i [7.15 (t)], and k [7.05 (s)] on disruption of the π-π-stacking, and hence aggregates formed at elevated temperature (80.0 °C), as shown in the spectrum (Figure 6c). It is clear evidence for the π-π-stackingassisted self-assembly of the gelator molecules in combination with the intermolecular hydrogen-bonding interactions of the sugar moieties in organogels. Similar changes were noticed for monoene 2 gel in toluene-d8 on variable-temperature 1H NMR studies. We investigated the Tgel of the water/ethanol gel of 1 using differential scanning calorimetry (DSC). During heating, the melting of the gel occurred over a broad temperature range from 67.5 to 88.7 °C, indicating that the transitions took place gradually. This can be attributed to the relatively low degree of ordering that is present in the gel compared to that of crystalline organic compounds, which normally give sharp peaks. Upon cooling, the exothermic peak at 30.5 °C indicates that gelation reoccurred in the system. The Tgel of the monoene analogue 2 in water/ethanol was measured as 30 °C, lowered by
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∼40 °C by the introduction of a single double bond on the lipophilic chain. This suggests that one could regulate the structure and stability of a gel system by tuning the unsaturation on the lipophilic part of the gelator molecule for desired applications. A collective analysis of the available data supports the involvement of hydrogenbonding and π-π-stacking-assisted self-assembly of molecules of 1, 2, and 5 in the the formation of gel structures. Interestingly, the gelating ability and self-assembly behavior is dramatically affected by the unsaturation in the side chain of the alkyl part of the molecule. This also supports the influence that the structural diversity of natural systems has on the fine-tuning of material properties. In conclusion, we have designed and synthesized a new class of low molecular weight gelators of simple glycolipids fully derived from renewable resources, having the ability to gel in a water/alcohol mixture as well as in pure organic solvents, strongly depending on the unsaturation of the pentadecyl phenolic chain. Supporting Information Available: Detailed procedure for the synthesis and fractionation of cardanyl glucoside 5 and the complete characterization of the individual components (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. LA030177H
