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Hexatriacontane Organogels. The First Determination of the Conformation and Molecular Packing of a Low-Molecular-Mass Organogelator in Its Gelled State David J. Abdallah,† Scott A. Sirchio,‡ and Richard G. Weiss*,† Department of Chemistry, Georgetown University, Washington, D.C. 20057-1227, and Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742 Received May 27, 2000. In Final Form: August 7, 2000 On the basis of data from a combination of X-ray diffraction techniques, the BO morph of hexatriacontane [H(CH2)36H], a low-molecular-mass gelator of several classes of liquids (including shorter n-alkanes), has been identified unambiguously as the molecular packing arrangement within the microplatelet units of the organogel superstructures. In addition, the long molecular axes of hexatriacontane molecules have been shown to be perpendicular to the planes of the microplatelets by an optical microscopic method.
Introduction Organogels, viscoelastic materials comprised of an organic gelator and primarily an organic liquid, are able to maintain their shapes under limited directional stress. This rather nebulous definition and others that have been advanced are as enigmatic (e.g., “The colloid condition, the gel, is one which is easier to recognize than to define.”1a) or complicated1b in many respects as the natures of the gels themselves. Recent renewed interest in the properties, applications, and structures of organogels, especially those with low-molecular-mass organic gelators (LMOGs),2 has resulted in many new insights into how gels form. However, the structural requirements for a molecule to be a successful LMOG are still largely relegated to empirical inferences because their packing arrangements in gel superstructures are unknown in most cases. A major impediment to the establishment of a set of structural requirements for a successful LMOG is the dearth of knowledge of the molecular packing within solid assemblies of organogels. Although several structure elucidation techniques, including scanning and transmission electron microscopies,3 small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) measurements,4 and atomic force microscopy,5 allow many features of organogels to be deciphered at the 1-1000 nm distance scales (i.e., the shapes and dimensions of the * To whom correspondence should be addressed. Phone: 202-687-6013. FAX: 202-687-6209. E-mail: weissr@ gusun.georgetown.edu. † Georgetown University. ‡ University of Maryland. (1) (a) Jordon Lloyd, J. Colloid Chemistry; Alexander, J., Ed.; The Chemical Catalog Co.: New York, 1926; Vol. 1, p 767. (b) Flory, P. J. Discuss. Faraday Soc. 1974, 57, 7. (2) (a) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (b) Terech, P.; Weiss, R. G. In Surface Characterization Methods; Milling, A. J., Ed.; Marcel Dekker: New York, 1999; p 286. (c) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.; Komori, K.; Ohseto, F.; Ueda, K.; Shinkai, S. DIC Tech. Rev. 1996, 2, 39. (d) Terech, P. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 1630. (e) Terech, P. In Specialist Surfactants; Robb, I. D., Ed.; Chapman and Hall: London, 1997; p 208. (f) Shinkai, S.; Murata, K. J. Mater. Chem. 1998, 8, 485. (g) Abdallah, D. J.; Weiss, R. G. Adv. Mater., in press. (h) Abdallah, D. J.; Weiss, R. G. J. Braz. Chem. Soc. 2000, 11, 209. (3) (a) Lin, Y.-c.; Kachar, B.; Weiss, R. G. J. Am. Chem. Soc. 1989, 111, 5542. (b) Geiger, C.; Stanescu, M.; Chen, L.; Whitten, D. G. Langmuir 1999, 15, 2241. (4) See for instance: (a) Terech, P.; Coutin, A.; Giroud-Godquin, A. M. J. Phys. Chem. B 1997, 101, 6810. (b) Terech, P.; Rodriquez, V.; Barnes, J. D.; McKenna, G. B. Langmuir 1994, 10, 3406.
primary LMOG assembly units), they do not provide direct information at the 0.1-1 nm scales (i.e., the shapes and organizations of individual LMOG molecules within primary assembly units). Unfortunately, extrapolation from the supermolecular to the molecular scale using any of these methods is not definitive. Other methods that are capable of providing information at the molecular level6 cannot be applied easily to ascertain how LMOGs are packed within assembly units for one of several reasons. Principal impediments are that many LMOGs are polymorphous and many others have defied attempts to make diffraction quality single crystals.7 Here, we report the first unambiguous determination of the molecular conformation and packing of an LMOG, hexatriacontane (C36; H(CH2)36H), in its organogels8 with hexadecane, 1-octanol, glycidyl methacrylate, and silicone oil (tetramethyltetraphenylsiloxane; Dow 704) as liquids. There are no LMOG structures simpler than n-alkanes, and their gels with n-alkanes as the liquid components are structurally the simplest organogels possible. The C36/ hexadecane combination examined here is an example of that class. Our method of analysis9 is based upon matching the X-ray diffraction (XRD) patterns of a gel and of the neat (5) (a) Kimizuka, N.; Shimizu, M.; Fujikawa, S.; Fujimura, K.; Sano, M.; Kunitake, T. Chem. Lett. 1998, 967. (b) Jorgensen, M.; Bechgaard, K. J. Org. Chem. 1994, 59, 5877. (c) Wang, R.; Geiger, C.; Chen, L.; Swanson, B.; Whitten, D. G. J. Am. Chem. Soc. 2000, 122, 2399. (6) (a) Tata, M.; John, V. T.; Waguespack, Y. Y.; McPherson, G. L. J. Phys. Chem. 1994, 98, 3809. (b) Tata, M.; John, V. T.; Waguespack, Y. Y.; McPherson, G. L. J. Am. Chem. Soc. 1994, 116, 9464. (c) Tata, M.; John, V. T.; Waguespack, Y. Y.; McPherson, G. L. J. Mol. Liq. 1997, 72, 121. (d) Menger, F. M.; Yamasaki, Y.; Catlin, K. K.; Nishimi, T. Angew. Chem., Int. Ed. Engl. 1995, 34, 585. (e) Snijder, C. S.; de Jong, J. C.; Meetsma, A.; van Bolhuis, F.; Feringa, B. L. Chem. Eur. J. 1995, 1, 594. (f) Hanabusa, K.; Miki, T.; Taguchi, Y.; Koyama, T.; Shirai, H. J. Chem. Soc., Chem. Commun. 1993, 1382. (g) Vassilev, V. P.; Simanek, E. E.; Wood, M. R.; Wong, C.-H. J. Chem. Soc., Chem. Commun. 1998, 1865. (7) In the most impressive indirect structural determination of LMOG packing in gels, Terech et al. found that X-ray diffractograms of the organogels, aerogels, and crystalline powders of either chiral or racemic 12-hydroxyoctadecanoic acid (HOA) have almost the same structural features.4b Although a single-crystal X-ray analysis of neither chiral nor racemic HOA could be obtained, peaks in the XRD pattern that characterize the crystalline nature of the network structure could be correlated with X-ray diffractograms of structurally related compounds, such as octadecanoic acid, whose single-crystal structures are known. (8) (a) Srivastava, S. P.; Saxena, A. K.; Tandon, R. S.; Shekher, V. Fuel 1997, 76, 625. (b) Abdallah, D. J.; Weiss, R. G. Langmuir 2000, 16, 352.
10.1021/la000730k CCC: $19.00 © 2000 American Chemical Society Published on Web 09/06/2000
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LMOG in one of its (various) morphs. If single-crystal diffraction or other structural analyses are available for the matched morph, the conformation and packing of the gelator in the gel are known. In addition, we have ascertained the orientations of the LMOG molecules with respect to the basic aggregate units in their gel assemblies using optical microscope techniques.10 Thus, the details of gelator packing are revealed from the macro (i.e., millimeter) to molecular scales in all parts of the gel superstructures except the “junction zones”2a that crosslink gelator units (vide infra). Experimental Section Materials. 1-Octanol, glycidyl methacrylate, and hexadecane (Aldrich) and tetramethyltetraphenyltrisiloxane (Dow silicone oil no. 704; Dow Corning) were used as received. n-Hexatriacontane (Humphrey Chemicals Co.) was recrystallized several times from petroleum ether until >99% pure by gas/liquid phase chromatography (GLPC) analyses: mp 77.0-77.6 °C (lit. mp 75.9 °C11). Methods. Weighed quantities of C36 and a liquid were heated until a solution was obtained. An isotropic aliquot was poured into a 2 mm deep (2 cm wide) glass dish, heated on a hot plate until the mixture became isotropic, and placed on a flat horizontal surface to cool. By visual inspection, gels formed within a minute. After setting for ca. 10 min, the samples were analyzed at room temperature by X-ray diffraction using a Bruker D8 advance diffractometer and Cu KR radiation (λ ) 1.54178 nm) from 0.5° to 30° in 0.2° steps at a scan rate of 0.48°/min. For comparison purposes, a hot isotropic C36/glycidyl methacrylate sample was cooled more slowly by leaving it on the disconnected hot plate until room temperature was achieved. Once the sample and hot plate returned to room temperature the, sample was analyzed. Some regions in the XRD patterns exhibit small regions where no data were collected due to a sporadic instrumental problem. Repeat scans showed no peaks in those regions. Due to differences in alignment of the samples, linear calibration corrections (e0.30°/ 2Θ) were made for each diffractogram. Optical micrographs were obtained with an RS Photometrics CoolSNAP CCD camera attached to a Leitz 585 SM-LUX-POL microscope equipped with crossed polars. Samples were prepared by allowing a hot (T > Tgel) aliquot to flow into a flattened glass capillary (0.8 mm i.d.) which was then sealed at the ends. The sealed sample was cooled, reheated, and recooled before recording micrographs. Theoretical XRD patterns were generated for C36 from monoclinic (BM) single-crystal data12a (taken from the Cambridge Structural Database) and orthorhombic forms,12b using the Lazy Pulverix12c program included with the X-SEED software.12d A line graph for the orthorhombic form (BO) was generated from a table of reflections supplied by the International Centre for Diffraction Data.12e
Results and Discussion Because data in Figures 1 and 2 were collected during ca. 1 h in open pans, only relatively low vapor pressure liquids were used to ensure that a thin layer of xerogel was not being analyzed. For instance, no weight loss occurred when the silicone oil gel was opened to the atmosphere for several days. To increase signal-to-noise (9) Ostuni, E.; Kamaras, P.; Weiss, R. G. Angew. Chem., Int. Ed. Engl. 1996, 35, 1324. (10) (a) Lovinger, A. J.; Nuckolls, C.; Katz, T. J. J. Am. Chem. Soc. 1998, 120, 264. (b) Livolant, F.; Levelut, A. M.; Doucet, J.; Benoit, J. P. Nature 1989, 339, 724. (11) Schaerer, A. A.; Busso, C. J.; Smith, A. E.; Skinner, L. B. J. Am. Chem. Soc. 1955, 77, 2017. (12) (a) Shearer, H. M. M.; Vand, V. Acta Crystallogr. 1956, 9, 379. (b) Teare, P. W. Acta Crystallogr. 1959, 12, 294. (c) Yvon, K.; Jeitschko, W.; Parthe, E. J. Appl. Crystallogr. 1977, 10, 73. (d) Siemens Analytical X-ray Instruments, Madison, Wisconsin, USA.; Barbour, L. University of MissourisColumbia, 1999. (e) PDF-Card no. 38-1975; Schulz, D.; McCarthy, G. North Dakota State University, Fargo, ND, ICDD Grantin-Aid 1987.
Figure 1. Line representation of the XRD pattern for C36 in its BO phase12b and XRD patterns for gels composed of C36 at (a) 4 wt % in 1-octanol, (b) 2 wt % in 1-octanol, and (c) 4 wt % in hexadecane. The small regions lacking data (due to an intermittent instrumental problem) contain no peaks.
Figure 2. XRD patterns for C36 in its (a) BM12a and (b) BO12b phases and for gels composed of 4 wt % C36 in (c) glycidyl methacrylate and (d) silicone oil. Neat silicone oil is shown as curve e.
in the XRD patterns, gelator concentrations are larger than the minimum necessary for gelation.8b Broad humps in Figures 1 and 2 are from the “isotropic” liquids; compare spectra d and e of Figure 2. The XRD pattern of the gel composed of 4 wt % C36 in silicone oil was also collected between 0.5° < 2Θ e 70° at a slow scan rate (0.12°/min; 0.1° steps) in an attempt to detect wide angle peaks (corresponding to distances in the range of 1-2 Å). However, none was observed. In theory, any of the four known layered morphs of C3613,14 (or a new one) may be responsible for the gels. (13) For a review of the solid phases of n-alkanes, see: Turner, W. R. Ind. Eng. Chem. Prod. Res. Dev. 1971, 10, 238. For a summary of powder XRD data for neat phases of n-alkanes, see: Heyding, R. D.; Russel, K. E.; Varty, T. L. Powder Diffr. 1990, 5, 93.
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Figure 3. Optical micrograph of a 4 wt % C36/1-octanol gel viewed through crossed polars (a), a cartoon representation of the microplatelets in (a) showing the orientations of the long molecular axes of C36 molecules (b), and the molecular packing of C36 in its BO12b phase viewed from the b-axis (c). The distance bar in (a) is 100 µm.
The correspondence between the sharp diffraction peaks in Figure 1 and the superimposed bar graph identifies the morph of C36 in the hexadecane and 1-octanol gels as the lamellar, orthorhombic (BO) phase. The XRD patterns with glycidyl methacrylate and silicone oil (Figure 2) also contain a very small monoclinic (BM) component.12a The relative amounts of BO and BM diffraction components were virtually the same when the glycidyl methacrylate gel was prepared by cooling its sol state much more slowly than normal or when the C36 concentration in the 1-octanol gel was doubled. Both BO and BM morphs of C36 are known to crystallize from dodecane under different experimental conditions.15 Although the BM solid may be a part of the gel networks or an extraneous suspended material, its small contribution to the total diffraction suggests that it is not important to gelation. Additionally, there is no evidence from optical microscopy and differential scanning calorimetry (DSC) measurements for a change in gelator morph in gels with n⊥ for the lamellar crystals of all long n-alkanes. The index of refraction in the direction of the chain axis (nc) of a crystal of tritriacontane (C33) at 25 °C is 1.588, and the average value perpendicular to the c-axis (n⊥) is 1.521.17a Polarizability measurements on liquid n-alkanes17b and crystalline polyethylene17c support this assumption. (a) Piesczek, W.; Strobl, G. R.; Malzahn K. Acta Crystallogr. 1974, B30, 1278. (b) Lamotte, M.; Lesclaux, R.; Merle, A. M.; Joussot-Dubien, J. Faraday Discuss. 1975, 58, 253. (c) Boyd, R. H.; Kesner, L. Macromolecules 1987, 20, 1802. (18) See for instance: (a) Taggart, A. M.; Voogt, F.; Clydesdale, G.; Roberts, K. J. Langmuir 1996, 12, 5722. (b) MacDowell, L. G.; Guillaume, F.; Ryckaert, J.-P.; Girard, P.; Rodriguez, V.; Dianoux, A. J. Physica B 1997, 234, 106. (c) Ryckaert, J.-P. Physica A 1995, 213, 50. (19) Broadhurst, M. G. J. Res. Natl. Bur. Stand. 1962, 66A, 241.
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In support of this hypothesis, we find that gels from alkane gelators form more readily if their sols are cooled rapidly.8b Also, they separate macroscopically into a solid and liquid over time, possibly due to an orthorhombic f monoclinic phase transition within the solid network. Despite the successful determination reported here, several important questions concerning LMOGs remains unanswered. Among these are, “Why do these (and other) LMOG gels form?” and “Is there a generally applicable method for determining the molecular packing in LMOG gels?” To answer the former, the processes leading from sols to gels, including the nucleation of gelator aggregates and the assembly of the aggregates into the colloidal superstructures (i.e., the nature of “junction zones”2 between microplatelets or the primary aggregate units of other LMOGs), must be investigated more thoroughly. Although many investigations have been made and several models advanced to explain nucleation of organic molecules, including n-alkanes,20 there is a dearth of such work on organogel systems. The latter question will require an approach very different from that used here because of the dearth of information concerning molecular packing of LMOGs in single crystals. However, the use of polarized optical microscopy may be widely applicable to determining the preferred orientations of LMOGs in their gel assemblies even when packing details are unknown. In many respects, organogels in general, and LMOG gels in particular, remain as enigmatic as when Jordon Lloyd made her prophetic statement more than 70 years ago.1a Acknowledgment. The National Science Foundation is gratefully acknowledged for its support of this research. D.J.A. thanks the Achievement Rewards for College Scientists Foundation for a graduate fellowship. The authors are grateful to Professor Timothy Swager and Dr. Stephan Holger Eichhorn (Massachusetts Institute of Technology) and Dr. James Fettinger (University of Maryland) for use of and assistance with their powder diffractometers. Mr. T. Mathew Cocker (Georgetown) and Dr. Eric B. Sirota (ExxonMobil) are thanked for several useful discussions. LA000730K (20) See for instance: (a) Kraack, H.; Sirota, E. B.; Deutsch, M. J. Chem. Phys. 2000, 112, 6873. (b) Sirota, E. B. J. Chem. Phys. 2000, 112, 492. (c) Sirota, E. B.; Herhold, A. B. Science 1999, 283, 29.
