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Supramolecular Structure of a Sugar-Appended Organogelator Explored with Synchrotron X-ray Small-Angle Scattering Kazuo Sakurai,*,† Yeonhwan Jeong,† Kazuya Koumoto,† Arianna Friggeri,‡ Oliver Gronwald,‡ Shinichi Sakurai,§ Shigeru Okamoto,| Katuaki Inoue,⊥ and Seiji Shinkai∇ Department of Chemical Processes & Environments, The University of Kitakyushu, 1-1, Hibikino, Wakamatsu-ku, Kitakyushu, Fukuoka, 808-0135, Japan, Chemotransfiguration Project, Japan Science and Technology, Kurume Research Center Building, 2432 Aikawa, Kurume, Fukuoka 839-0861, Japan, Department of Polymer Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Kyoto 606-8585, Japan, Material Science and Engineering, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466-8555, Japan, SPring-8, Japan Synchrotron Radiation Research Institute (JASRI), 323-3 Mihara, Mikazuki, Sayo, Hyogo 679-5198, Japan, and Faculty of Engineering Department of Chemistry & Biochemistry, Graduate School of Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka, Fukuoka 812-8581 Japan Received April 21, 2003. In Final Form: July 7, 2003 Small-angle X-ray scattering (SAXS) from an organogel system prepared from methyl 4,6-O-benzylideneR-D-mannopyranoside and p-xylene was carried out with a synchrotron X-ray source at SPring-8, which revealed that hexagonally packed fibrils are formed in the gel state. The spacing between the fibrils can be evaluated to be about 60 Å, and this value was almost independent of both the gelator concentration and the temperature. Furthermore, the spacing is larger than the gelator molecular size. Upon heating, this supramolecular structure completely disappeared. Time-resolved SAXS revealed that phase separation takes place initially and subsequently the hexagonal structure is formed. Wide-angle X-ray diffraction (WAXD) showed that there is no crystalline peak at all and the diffraction pattern is consistent with being amorphous. 1H NMR spectral data show that the gelator molecules still maintain thermal motion in the gel state. The present SAXS, WAXD, and NMR results contrast with those of “dry gels” in which the gel fibers consist of the crystal of the gelators. Our results suggest that the solvent molecules are incorporated into the gel fiber and the present gel can be classified as a “wet gel”.
Introduction Organogelators are a class of molecules that can undergo self-organization in a particular organic solvent to yield a fine fibrous structure.1-5 As shown in a transmission electron microscopic (TEM) image for an organogel presented in Figure 1b, the fibrous structures (gel fiber) can be bundled to each other to form a micrometer-scale cylinder and these microcylinders are connected to each †
The University of Kitakyushu. Japan Science and Technology. Kyoto Institute of Technology. | Nagoya Institute of Technology. ⊥ JASRI. ∇ Kyushu University. ‡ §
(1) (a) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.; Komori, T.; Osheto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994, 116, 6664. (b) Terech, P.; Rodriguez, V.; Barnes, J. D.; McKenna, G. B. Langmuir 1994, 10, 3418. (c) Ostuni, E.; Kamaras, P.; Weiss, R. G. Angew. Chem., Int. Ed. Engl. 1996, 35, 1324. (d) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (e) Esch, J. V.; Schoonbeek, F.; Loos, M. D.; Kooijman, H.; Spek, A. L.; Kellogg, R. M.; Feringa, B. L. Chem. Eur. J. 1999, 5, 937. (f) van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263. (g) Sakurai, K.; Ono, Y.; Jung, J. H.; Okamoto, S.; Sakurai, S.; Shinkai, S. J. Chem. Soc., Perkin Trans. 2 2001, 108. (h) Simmons, B. A.; Taylor, C. E.; Landis, F. A.; John, V. T.; McPherson, G. L.; Schwartz, D. K.; Moore, R. J. Am. Chem. Soc. 2001, 123, 2414. (2) (a) Yang, H.; Coombs, N.; Ozin, G. A. Nature 1997, 386, 692. (b) Tanev, P. T.; Liang, Y.; Pinnavaia, T. P. J. Am. Chem. Soc. 1997, 119, 8616. (c) Kim, S. S.; Zhang, T. P. Science 1998, 282, 1302. (d) Jung, J. H.; Ono, Y.; Sakurai, K.; Sano, M.; Shinkai, S. J. Am. Chem. Soc. 2000, 122, 8648. (e) Jung, J. H.; Ono, Y.; Shinkai, S. Chem. Eur. J. 2000, 6, 4552. (3) Hafkamp, R. J. H.; Kokke, P. A.; Danke, I. M.; Guerts, H. P. M.; Rowan, A. E.; Feiters, M. C.; Nolte, R. J. M. Chem. Commun. 1997, 545.
other by a so-called “junction point”.1b Therefore, the entire morphology becomes spongelike; thus it is considered suitable to absorb a large amount of solvent molecules by the capillary effect. This is the reason this system behaves as a gel and such compounds are called organogelators. To understand the organogels in the molecular level, there are still several issues pending, such as how the organogelators assemble with each other to form the gel fiber or whether another morphological hierarchy is present to connect the molecules with the gel fiber. Organogels constitute an important class of materials that can be applied to template materials2 and biomimetics,3 so accurate understanding of the organogel structures from the fundamental aspects is essentially important for the molecular design. Weiss and Terech1d are pioneers in this field. They synthesized many organogelators and explored their physicochemical properties. Especially, Terech et al.4 extensively studied small-angle X-ray scattering (SAXS) from the organogels and showed that the scattering can be represented by particle scattering from randomly oriented cylinders with infinite length. Among others, 4-tert-butyl-1-phenylcyclohexanol/decane shows a typical (4) (a) Terech, P.; Furman, I.; Weiss, R. G. J. Phys. Chem. 1995, 99, 9558. (b) Terech, P.; Ostuni, E.; Weiss, R. G. J. Phys. Chem. 1996, 100, 3759. (c) Terech, P.; Coutin, A.; Giroud-Godquin, A. M. J. Phys. Chem. B 1997, 101, 6810. (d) Terech, P.; Allegraud, J. J.; Garner, C. M. Langmuir 1998, 14, 3991. (e) Terech, P.; Coutin, A. Langmuir 1999, 15, 5513. (5) George, M.; Weiss, R. G. J. Am. Chem. Soc. 2001, 123, 10393.
10.1021/la0346752 CCC: $25.00 © 2003 American Chemical Society Published on Web 08/21/2003
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Figure 1. (a) Chemical structure and molecular size of the mannose-appended organogelator methyl 4,6-O-benzylideneR-D-mannopyranoside (R-manno). (b) Typical TEM image of a freeze-dried sample prepared from the R-manno/p-xylene gel.2e The TEM image suggests that the 100 nm diameter fiber is the constitutional unit and those fibers are bundled together to make thicker fibers. The fibers cross and stick together to make the junction point. The blank region had been occupied by the solvent before the sample was freeze-dried. This TEM image is very similar to those for other organogels, so we can consider that this image is a common feature for organogels.
scattering from the cylinder and the cylinder model beautifully agrees with the experimental results.4d For this system, the cylinder radius (36 Å) obtained by SAXS is much smaller than that observed by electron microscopy (estimated to be a few hundred nanometers)4d so that the gel fiber is seemed to consist of a bundle of the many thin cylinders. On the other hand, some of the organogelators studied by Weiss et al.5 show that clear lattice scattering exists in the gel state. For instance, their ammonium carbamate compounds can gelatinize silicon oil, and provide a clear lamellar diffraction pattern with the domain spacing evaluated to be 45 Å.5 Their pattern showed that the crystalline peaks are also present in the gel state. When they compared the gel and the crystalline powder, the peak positions were identical, indicating that their gel fiber essentially consists of the crystal of the gelator molecules. According to them, the size of the lamellar domain spacing is almost twice that of the gelator molecule so that the origin of the lamella can be ascribed to head-to-head arrangement of the gelators.5 Furthermore, Weiss et al.6 showed that hexatriacontane can gelatinize glycidyl methacrylate and silicon oil with forming a lamellar packing. Similarly to the first case, the molecular packing in the gel is exactly the same as that in the crystalline state. All those organogels are composed of crystalline fibers; therefore, the solvent (6) Abdallah, D. J.; Sirchio, S. A.; Weiss, R. G. Langmuir 2000, 16, 7558.
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molecules are excluded from the gel fibers. In other words, they are classified as a “dry gel”. On the other hand, Whitten et al.7 synthesized a gelator containing a cholesterol tethered to a trans-stilbene and observed morphological changes upon the sol-gel transition (induced by temperature drop) for this gelator with atomic force microscopy (AFM). Their time-resolved AFM images show that many droplets without fibers were first formed, and fine fibers with a width as thin as 6.5 ( 0.5 nm started to form in these droplets. Upon further development, fine fibers became dominant and condensation allowed combination of neighboring fine fibers into thicker ones. Finally, the AFM image becomes very similar to that of Figure 1b. Furthermore, they observed a cross section of the fibril bundle and found that the fiber bundles consist of tens of unit fibers, and the cross section suggests a fluidized surface. Their observation indicated that solvent molecules are involved inside the bundles and the solvent volume fraction was evaluated to be about 30%. Their results evidences that the solvent molecules are incorporated into the gel fiber. As far as we know, this is the first evidence for the “wet gel”. In the meantime, we have studied a series of sugarappended organogelators.8 In a previous paper,8f we presented a model that the p-xylene (hydrophobic molecule) solvent should make the sugar moieties (hydrophilic) assemble together and the hydrogen bonds should be formed between the sugar moieties. Furthermore, we found that methyl 4,6-O-benzylidene-R-D-mannopyranoside (R-manno; see Figure 1 for the structure) can gelatinize p-xylene at considerably low concentrations (less than 1 wt %). One of the peculiar phenomena for the R-manno/p-xylene gel is that a clear thermal transition from the gel to sol state is observed upon heating. Our preliminary SAXS results9 show that a hexagonally packed fibril is formed in the gel state and the solvent molecules can be present between the fibrils. This is clear evidence to show that gelator molecules and solvents can cooperate to form a particular supramolecular structure. Such a system should be classified as a “wet gel”. The present paper explores the structural aspects for this “wet gel”, using mainly synchrotron SAXS. Experimental Section We synthesized the R-manno samples according to a method described elsewhere8c and prepared 0.5, 1.0, 2.0, and 3.0 wt % p-xylene gels. The gel-sol transition temperatures (Tgel) for these solutions were determined to be 39, 55, 64, and 72 °C for 0.5, 1.0, 2.0, and 3.0 wt %, respectively. These values agree with the results reported by Gronwald et al.8d The samples were loaded into a quartz cell (Mark-Rohrchen) with 2 mm diameter, and then the cell was sealed with an epoxy adhesive. SAXS experiments were carried out at the BL40B2 and BL45XU stations at SPring-8 in Japan.10 Temperature and (7) (a) Geiger, C.; Stanescu, M.; Chen, L.; Whitten, D. G. Langmuir 1999, 15, 2241. (b) Wang, R.; Geiger, C.; Chen, L.; Swanson, B.; Whitten, D. G. J. Am. Chem. Soc. 2000, 122, 2399. (8) (a) Amanokura, N.; Yoza, K.; Shinmori, H.; Shinkai, S.; Reinhoudt, D. N. J. Chem. Soc., Perkin. Trans. 2 1998, 2585. (b) Gronwald, O.; Luboradzki, R.; Ikeda, A.; Shinkai, S. Chem. Lett. 2000, 1148. (c) Gronwald, O.; Sakurai, K.; Luboradzki, R.; Kimura, T.; Shinkai, S. Carbohydr. Res. 2001, 331, 307. (d) Gronwald, O.; Shinkai, S. Chem. Eur. J. 2001, 7, 4328. (e) Yoza, K.; Amanokura, N.; Ono, Y.; Akao, T.; Shinmori, H.; Takeuchi, M.; Shinkai, S.; Reinhoudt, D. N. Chem. Eur. J. 2001, 5, 2722. (f) Gronwald, O.; Shinkai, S. J. Chem. Soc., Perkin. Trans. 2 2001, 1933. (9) Sakurai, K.; Kimura, T.; Gronwald, O.; Inoue, K.; Shinkai, S. Chem. Lett. 2001, 746. (10) (a) Fujisawa, T. J. Synchrotron Radiat. 1999, 12, 194. (b) Fujisawa, T.; Inoko, Y.; Yagi, N. J. Synchrotron Radiat. 1999, 6, 1106. (c) Fujisawa, T.; Inoue, K.; Oka, T.; Iwamoto, H.; Uruga, T.; Kumasaka, T.; Inoko, Y.; Yagi, N.; Yamamoto, M.; Ueki, T. J. Appl. Crystallogr. 2000, 33, 797.
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gelator-concentration dependences of the SAXS intensity (I) were examined at BL40B2, where the camera length and the X-ray wavelength were adjusted to 100 cm and 1.0 Å, respectively. The scattering intensity was accumulated for 60 or 240 s in the range of q ) 0.02-0.67 Å-1 with a Rigaku R-AXIS IV++ system (a 30 cm × 30 cm imaging plate), where q is the magnitude of the scattering vector. Time-resolved SAXS measurement upon quenching from the sol to gel states was carried out for the 1.0 wt % solution at the BL45XU and BL40B2 stations. The BL45XU station was equipped with an X-ray image intensifier with a cooled CCD camera.10b,c This camera enabled us to measure I exposing the sample to the X-ray for 1 s. We measured I in the range of q ) 0.008-0.2 Å-1 with a 180 cm camera length and 1.0 Å wavelength, respectively. Every sample was heated above Tgel and slowly cooled to room temperature about 24 h before the measurement in order to erase thermal hysteresis. The obtained two-dimensional scattering pattern of SAXS consisted of concentric circles, and the I versus q profile was independent of the azimuthal angle. Therefore, we took a circular average of I, and the resultant I versus q plot was used for discussion. We confirmed that degradation of the samples was negligibly small during exposure to the synchrotron X-ray, comparing the SAXS profiles before and after exposure for 200 s. Furthermore, we loaded a drop of the 3.0 wt % gel on a piece of quartz glass. Then the gel was extended unidirectionally, and subsequently SAXS was measured from it at the BL40B2 station. The unidirectional extension should orient the gel fibers to the extended direction.11 Wide-angle X-ray diffraction (WAXD) from the 3.0 wt % gel was measured on a Rigaku XRD-DSC-X II at room temperature in the 3°-35° range scanning at 0.05°/min. Temperature dependence of 1H NMR for the 1.0 wt % gel was carried out on a Bruker ARX 300 using fully deuterated p-xylene as a solvent. For comparison, SAXS from a solid crystalline powder and SAXS from a freeze-dried gel were measured at room temperature. The powder sample was prepared by evaporating the column chromatography solvent (ethyl acetate/n-hexane mixture) and the freeze-dried gel was made from the 1.0 wt % p-xylene gel in the same way to prepare the electron microscopy samples.7a
Results and Discussion Temperature Dependence of SAXS and Hexagonally Packed Molecules. Figure 2 compares the scattering profiles measured at different temperatures (T) upon heating for the 1.0 wt % sample. In Figure 2a, to make comparison easy, each profile is shifted vertically by multiplying a certain number (indicated in parentheses) and the q ) 0.42-0.46 Å-1 range is magnified and presented above the panel. For the q < 0.2 Å-1 range, the data only for the gel state are replotted double logarithmically in Figure 2b, where the vertical shift is not applied. Figure 2a shows that a typical lattice scattering is present below Tgel (55 °C), and the peak intensities decrease with increasing temperature and finally the peaks disappear at T > Tgel. These features indicate that an ordered lattice is present in the gel state and the lattice structure can be related to the gel nature. The peak positions seem independent of temperature; however, when we carefully observe the position in the magnified profiles, the position slightly shifts toward the low q with increasing temperature. Another characteristic feature of the SAXS profiles is that the gel samples have an abrupt upturn of the intensity at low q and this upturn completely disappears at the sol state. According to the scattering theory12 for hexagonally packed cylinders, the lattice peak position for the Miller index of (k,l) is given by
qkl ) 2πa-1(2/x3)xk2 + kl + l2
(1)
Here, qkl is the peak position corresponding to (k,l) and a is the distance between the nearest adjacent lattices
Figure 2. Temperature dependence of SAXS profiles for 1.0 wt % R-manno/p-xylene gel upon heating. In (a), the numbers attached to the peaks show the values of the square root in eq 1. The profiles are shifted vertically to make comparison easy, and the shift factors are indicated in parentheses. The sixth peaks are magnified above the panel. For the q < 0.2 Å-1 range, the data only for the gel state are replotted double logarithmically in (b).
(for hexagonal packing, a equals the unit cell length). Table 1 compares the lattice peak positions at room temperature and those calculated from eq 1. Discrepancy between theory and experiment is within 1%. Therefore, we can conclude that the R-manno molecules form a cylinder-like fibril (hereinafter, we denote this cylinder as an elemental fibril) and these elemental fibrils assemble hexagonally in the gel state. Using eq 1, a is evaluated to be 60.0 Å at room temperature and slightly increases with increasing temperature. At 52 °C (just below Tgel), it reaches 60.2 Å (i.e., 0.5% increment). From these a values, the thermal linear expansion coefficient of the hexagonal lattice can be approximately evaluated to be 1.6 × 10-4 K-1; this order of magnitude is close to that for organic solid compounds.13 Scattering contrast in SAXS arises from difference in the electron density.14 Generally, the electron density of the sugar moiety is lower than that of p-xylene and the aromatic moiety of R-manno; furthermore, the electron densities of the aromatic moiety and p-xylene should be of similar magnitude. Therefore, the sugar moieties only provide the contrast for X-ray. In other words, the (11) Lescanne, M.; Colin, A.; Olivier, M.-M.; Heuze, K.; Fages, F.; Pozzo, J.-L. Langmuir 2002, 18, 7151. (12) Tadokoro, H. Structure of Crystalline Polymers; Robert E. Krieger Publishing Co.: Malabar, FL, 1990. (13) Nippon Kagakukai Hen. Kagaku Binran Kisoheiben II; Maruzen Co. Ltd.: Japan, 2000. (14) Glatter, O.; Kratky, O. Small-Angle X-ray Scattering; Academic Press: London, 1982.
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Table 1. Comparison of the Peak Positions between Experiment and a Hexagonal Packing Modela Miller index
(1 0)
(1 1)
(2 0)
(2 1)
(3 0)
(2 2)
(3 1)
(3 2)
(4 0)
(4 1)
+ kl + qkl/q01 theory peak position/Å-1 qkl/q01 measurement comparison
1 1 0.12 1 1.00
x3 1.73 0.21 1.71 1.01
2 2.00 0.25 2.01 1.00
x7 2.64 0.32 2.64 1.00
3 3.00 0.37 2.98 1.01
x12 3.47
x13 3.60 0.44 3.59 1.00
x19 4.35 0.54 4.37 1.00
4 4.00
x21 4.58 0.56 4.59 1.00
(k2
a
l2)1/2
(4 0) and (2 2) diffractions cannot be observed because these are essentially higher order diffractions for (1 0) and (1 1), respectively.
hexagonally packed elemental fibrils observed by SAXS are directly related to how the sugar moieties assemble each other. As mentioned above, the distance between the elemental fibrils is 60 Å at room temperature. This distance is relatively larger than the size of the R-manno molecule (ca. 12 Å; see Figure 1). Even a head-to-head arrangement5 of the gelator only gives 24 Å, which is much smaller than 60 Å. To rationalize such a large a value, we have to suppose that some amount of the solvent molecules exists between the elemental fibrils. Therefore, the SAXS results suggest that the R-manno/p-xylene gel can be classified as a “wet gel”. As shown in Figure 2, the SAXS profiles at the sol state (57 and 73 °C) are typical of homogeneous solutions. This means that the gelator molecules completely dissolve above Tgel. On the other hand, there is an abrupt upturn of the intensity at low q when T < Tgel. When we replot the data double logarithmically in Figure 2b, the data points can be fitted by a line with a slope of -4, indicating that the Porod law15 holds in this range. This means that the scattering at q ) 0.02-0.05 Å-1 can be explained by the ideal two-phase model with a smooth surface.15 To be exact, the presence of such a scattering indicates that our system consists of two phases. The q range providing the Porod law corresponds to 100-300 Å in the real space. The morphology of organogels in this range is expected to consist of the gel fibers and the solvent phases (see Figure 1). Therefore, the two phases observed by SAXS should be related to the gel fiber and solvent. The TEM image (Figure 1b) shows that the morphology of the gel consists of the gel fibers with different diameters, their bundles, and the junction points. This morphological diversity can be characterized as scattering from a fractal object.15 The fractal scattering is usually observed in the range of 1/R , q , 1/a, where R and a are the overall dimensions of the object and the size of the basic building units of the structure, respectively.15 In our case, a should be the diameter of the gel fiber, which is larger than 1000 Å. Therefore, our q range is too large to observe the fractal scattering, so that ultra-small-angle X-ray or neutron scattering is needed to observe the fractal for our system. Supramolecular Structure of the Gel Fiber. Figure 3 shows the two-dimensional scattering pattern from the unidirectionally extended gel. As shown in the figure, the extension direction is parallel to the meridian direction and all SAXS peaks appear at the equator direction (perpendicular to the extended direction). Since the gel fiber should be oriented to the extended direction,11 this result indicates that the hexagonally packed elemental fibrils are parallel to the gel fiber. On the basis of the SAXS results described above, we can present a more elaborate model for the supramolecular structure, as illustrated in Figure 4. Here, the gel fibers observed by TEM should consist of the hexagonally packed elemental fibrils, and the elemental fibrils are parallel to the gel fiber. The hexagonally packed elemental fibrils consist of sugar moieties, and the sugar moieties form (15) Roe, R.-J. Methods of X-Ray and Neutron Scattering in Polymer Science; Oxford University Press: New York, 2000.
Figure 3. Two-dimensional scattering pattern from the unidirectionally extended gel.
hydrogen bonding with each other to stabilize the molecular packing. Between the elemental fibrils, the solvent molecules exist and the aromatic moieties of R-manno come face to face outside the elemental fibril; probably the π-π interaction can stabilize the interface between the aromatic moiety and the p-xylene solvent. As mentioned above, the sugar moiety has a lower electron density than both p-xylene and the aromatic moiety. Therefore, the electron density profile for the cross section of the hexagonal domain should be like that schematically illustrated in the figure. This profile can explain the SAXS results. Furthermore, the relatively small expansion coefficient of the hexagonal lattice suggests that the solvent molecules are tightly bound between the elemental fibrils. Comparison between the Gel and Solid States. Figure 5a presents the patterns from the 3.0 wt % gel, a crystalline powder, and p-xylene. The pattern from the gel sample shows only amorphous peaks and no trace of crystalline peak at all; on the other hand, the pattern from the powder shows a typical feature from a crystalline powder solid. This result indicates that the gelator molecules in the gel are in a disordered state, so as not to give a diffraction peak. The present results are quite different from those obtained for the “dry gels” by Weiss et al.6 The SAXS profile from the crystalline powder is presented Figure 5b, which exhibits a strong peak around 0.44 Å-1. This strong peak is also observed in WAXD (indicated by the arrow) and may be assigned to the long spacing in the crystal. The scattering feature of SAXS
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Figure 4. Schematic model showing how R-manno molecules form a fibril and the fibrils and p-xylene form the hexagonal supramolecular structure. On the basis of this gel structure, we also proposed the electron density profile to explain the SAXS data.
from the powder is completely different from that of the gel (Figure 2), indicating that the hexagonally packed cylinder structure is characteristic of the gel state. When we prepared a freeze-dried sample (xerogel) from the p-xylene gel and measured SAXS from it, the obtained SAXS profile consisted of the powder and gel peaks as shown in Figure 5b. The intensity ratio of the powder to gel origin peaks was found to strongly depend on the sample preparation procedure, especially the freeze-drying condition. The scattering profile from the xerogel sample shows that the supramolecular structure in the xerogel is not the same as the original one that is formed in the gel state. The freeze-dry technique is frequently used when the TEM specimen is prepared. The present result instructs (or warns) us that the TEM image does not always reflect the original morphology in the gel. The WAXD and SAXS results for the R-manno/p-xylene system are different from those observed for the “dry gels” by Weiss et al.6 All our results consistently indicate that the molecular assembling in the gel state is completely different from that in the crystalline state and it bears all characteristics of “wet gels”. Temperature Dependence of NMR and Molecular Mobility. Terech et al.16 studied various organogels made from p-octylbenzohydroxamic acid and found that the gel fiber consists of the crystal of the gelator (i.e., “dry gel”). They carried out the temperature dependence of 1H NMR measurements for this “dry gel” and found that the gelatororiginated peaks completely disappeared in the gel state, while they were present in the sol state. This feature is anticipated for “dry gels”, and it can be rationalized by the fact that the gelator molecules almost terminate thermal motion in the crystal so that there is no peak in 1 H NMR.17 Therefore, disappearance of the 1H NMR peaks in the gel state seems to be one criterion for “dry gels”. (16) Terech, P.; Coutin, A.; Giroud-Godquin, A. M. J. Phys. Chem. B 1997, 101, 6810. (17) Bovey, F. A.; Jelinski, L.; Mirau, P. A. Nuclear Magnetic Resonance Spectroscopy; Academic Press: San Diego, 1988.
Figure 5. (A) Comparison of WAXD profiles between solvent (a), gel (b), and crystalline powder (c). (B) SAXS profile for the powder and freeze-dried samples. All measurements were carried out at room temperature. The arrows indicate the identical peak.
For the 1.0 wt % R-manno/deuterated p-xylene solution, the temperature dependence of the 1H NMR spectrum is presented in Figure 6 for the protons attached to the aromatic moiety (7.0-7.6 ppm from TMS) in the lower panel and those attached to the sugar moiety (3.4-4.3 ppm from TMS) in the upper panel. Here, the intensities were normalized by the peak area of the solvent (at 6.95 ppm). Figure 6 shows that the gelator can exhibit the peaks in both the sol and gel states, although the peak width becomes sharper with increasing temperature. The peak width was evaluated for the 3.83 ppm peak (one of the sugar protons, the proton attached to the second carbon of the mannose) and plotted against temperature in Figure 7, where the peak width is defined as the full width of the peak at half the maximum height. The figure clarifies that the width is constant at T < 40 °C, decreases below Tgel, and stays the same value at T > Tgel, indicating that the change in the width is correlated to the sol-gel transition. Although the data are not presented, the other peaks show behavior similar to that of Figure 7. The results in Figures 6 and 7 contrast remarkably with the temperature dependence of the “dry gels” reported by Terech et al., that is, the disappearance of the peaks in the gel state. Our gel has the 1H NMR peaks even in the gel state, suggesting that the gelator molecules still keep the thermal motion enough to provide the 1H NMR peak. This feature is consistent with the fact that the present system is classified as a “wet gel”.
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Figure 8. Concentration dependence of SAXS profiles at room temperature. To be easy to compare, the profiles are shifted vertically; however, the baseline intensities are almost identical. Similarly to Figure 2, the sixth peaks are magnified above the panel.
Figure 6. Change in NMR peaks for 1.0 wt % R-manno/pxylene gel upon heating.
Figure 7. Temperature dependence of peak width at 3.83 ppm peak (the proton attached to the second carbon of the mannose), where the peak width is defined as the full width of the peak at half-maximum.
Gelator Concentration Dependence of SAXS. As discussed in Figure 2, the SAXS profiles for the 1.0 wt % gel indicate the presence of the two phases (gel fiber and solvent) and the gel fiber phase corresponds to the hexagonally packed elemental fibrils. It is interesting to explore the gelator concentration dependence of the SAXS, which should provide information on the thermodynamics between the gel fiber and solvent phases. We examined
the R-manno concentration dependence of the SAXS profile, and the results are presented in Figure 8. The peak positions are independent of the concentration; on the other hand, the intensity of each peak increases with increasing concentration. The incremental intensity implies the increment of the volume fraction of the scatterer in this system; that is, the volume fraction occupied by the elemental fibrils (scatterer) is increased with increasing concentration. On the other hand, the unchanging peak position means that a (distance between the nearest adjacent lattices) is independent of the gelator concentration. Supposing that the “wet gel” model illustrated in Figure 4 is valid, there are some solvent molecules present between the elemental fibrils, so that a should be related to the gelator-to-solvent ratio within the gel fiber. Therefore, the unchanging peak position implies that the gelatorto-solvent ratio within the gel fiber is independent of the total gelator concentration. These two results (incremental volume fraction of the gel fiber and independence of the gelator-to-solvent ratio) can be rationalized by assuming the phase diagram illustrated in Figure 9. The R-manno/p-xylene system is a binary mixture; therefore, we suppose that this binary mixture can take a phase diagram with a lower miscibility gap. Suppose that two homogeneous solutions with different gelator concentrations are cooled from T1 to T2 (from A1 and B1 to A2 and B2, respectively), as indicated by each arrow in the figure. At T2, the solutions undergo phase separation; to be precise, the solutions come to consist of two phases and the composition of each phase is determined by the cross-sectional points S and X. Therefore, the composition of each phase is independent of the initial concentrations of A and B; that is, the composition of the X phase (hexagonally packed elemental fibril domain, i.e., gel fiber) is independent of the initial gelator concentration. On the other hand, the volume fraction of the X phase is determined by the lever rule, and it increases with increasing initial concentration. That is, the volume fraction of the X phase in B2 should be larger than that in A2. This is consistent with the incremental volume fraction of the gel fiber with the gelator concentration. If we can assume that the binodal line at the higher concentration (boundary P in Figure 9) hardly depends
SAXS Study of Sugar-Appended Organogelator
Langmuir, Vol. 19, No. 20, 2003 8217
Figure 9. Schematic phase diagram to explain SAXS results. The boundary Q separates the homogeneous region from the phase separation region, and it should correspond to Tgel. The boundary P indicates the composition of the gelator in the hexagonal region.
on the concentration and the other binodal line (boundary Q) has an upward curvature as presented in the figure, the gelator concentration dependence of Tgel and the temperature dependence of a can be explained. According to the phase diagram, Tgel can be defined by the temperature where the arrow crosses the boundary Q; therefore, Tgel increases with increasing initial gelator concentration. On the other hand, the vertical boundary of P can give a characteristic composition which is scarcely dependent upon temperature; namely, the a value is independent of temperature. The above discussion based on the phase diagram can explain both gelator concentration and temperature dependences of SAXS results, although it is still qualitative. To enrich and deepen this discussion, it may be necessary to measure Tgel and SAXS in a wider range of gelator concentration and to construct an accurate phase diagram. This will be the next plan in this series of work. Time-Resolved SAXS. To observe the development of the hexagonal structure as well as the gel structure, we carried out time-resolved SAXS measurements when the cell temperature was changed from 100 to 20 °C for the 1.0 wt % gel. The temperature of the SAXS cell holder was kept at 20 °C, and the cell was heated to 100 °C outside the cell holder. After we confirmed that the sol state emerged, we immediately transferred the cell to the cell holder (temperature 20 °C). The cell temperature reached 20 °C within 20 s. Figure 10 illustrates the time development of the scattering profiles obtained in that experiment, with the initial change in the lower panel and the later stage in the upper panel. As shown in the lower panel, within 400 s, the scattering at the low q regime appears to be saturated. This scattering can be ascribed to heterogeneity, which is probably related to the phase separation for the gelatorrich phase and the solvent-rich phase. However, when we tilted the cell after 40 s, the solution was still in the sol state (i.e., it could flow). The upper panel in the figure presents the time development between 35 and 4530 s. It is interesting that the hexagonal peaks do not appear until 1540 s and they grow slowly after 1540 s. Further examination reveals that the sol-gel transition occurs
Figure 10. Time development of the scattering profile after temperature was changed from 100 (sol) to 20 (gel) °C for 1.0 wt % gel. The initial change is the lower panel, and the later stage is the upper panel.
around 1000 s. Figure 10 clarifies that the hexagonal structure develops after the phase separation, and the gelation takes place just before the development of the hexagonal structure. The phase separation and subsequent fibril formation are consistent with the AFM observation for a “wet gel” system by Wang et al.7b Concluding Remarks SAXS from an organogel system prepared from methyl 4,6-O-benzylidene-R-D-mannopyranoside and p-xylene revealed that a hexagonally packed fibril is formed in the gel state. The spacing between the fibrils can be evaluated to be about 60 Å, and this value was almost independent of both the gelator concentration and temperature. The spacing is larger than the gelator molecular size, indicating that the present gel system can be classified as a “wet gel”. Time-resolved SAXS revealed that phase separation takes place initially and subsequently the hexagonal structure is formed. WAXD shows that there is no crystalline peak at all and the diffraction pattern is consistent with being amorphous. 1H NMR measurement shows that the gelator molecules still maintain thermal motion in the gel state. All SAXS, WAXD, and NMR results evidence that the solvent molecules are incorporated into the gel fiber. Acknowledgment. This work was performed under the approval of the SPring 8 Advisory Committee (2000B0227-NL -np). K.S. and S.S. thank the SORST program in JST for financial support. LA0346752
