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Langmuir 2003, 19, 8168-8176
Detection of Pre-Sol Aggregation and Carbon Dioxide Scrambling in Alkylammonium Alkylcarbamate Gelators by Nuclear Magnetic Resonance† Mathew George and Richard G. Weiss* Department of Chemistry, Georgetown University, Washington, D.C. 20057-1227 Received March 4, 2003. In Final Form: July 1, 2003 The initial stages of aggregation of a series of organogelator salts, prepared from n-alkylamines by the rapid in situ and isothermal (at room temperature) uptake of the neutral triatomic molecule, CO2, have been probed by NMR spectroscopy in the nongelled liquid, chloroform-d. Evidence for specific interactions of the ionic headgroups in the aggregates is presented. The influences of concentration and temperature on the processes leading to pre-sol aggregates of decylammonium decylcarbamate (2b) have been investigated in detail. NMR spectra of selectively deuterated (at the R-methylene group) and selectively 13C-enriched (at the carbonyl carbon) 2b demonstrate that CO2 is scrambled rapidly between the ammonium and carbamate parts of the molecule in chloroform solution. No scrambling of CS2 was detected in alkylammonium alkyldithiocarbamates under the same experimental conditions.
Introduction Low-molecular-mass organic gelators (LMOGs) and their thermally reversible organogels have received increasing attention during the past several years.1 Usually, organogels consist of a small amount of an LMOG and an organic liquid. They are microheterogeneous phases in which the LMOG self-assembles in threedimensional fibrillar networks whose organization can be expressed from the molecular to micrometer-length scales. Molecules that are known to act as LMOGs can have very simple2,3 or very complex structures,1,4,5 and their assembly can involve H-bonding interactions,6 chemical or physical triggers,7 or van der Waals forces.8 Typically, a solid LMOG is heated until it dissolves in a * To whom correspondence should be addressed. Fax: 202-6876209. E-mail:
[email protected]. † Dedicated to Prof. Vaclav Horak on the occasion of the 80th anniversary of his birthday. (1) (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.; Dekker: New York, 1999; p 286. (c) Abdallah, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 1237. (d) van Esch, J. H.; Feringa, B. L. Angew. Chem., Int., Ed. Engl. 2000, 39, 2263. (e) Gronwald, O.; Snip, E.; Shinkai, S. Curr. Opin. Colloid Interface Sci. 2002, 7, 148. (2) Abdallah, D. J.; Weiss, R. G. Langmuir 2000, 16, 352. (3) Abdallah, D. J.; Lu, L.; Weiss, R. G. Chem. Mater. 1999, 11, 2907. (4) Lu, L.; Weiss, R. G. Chem. Commun. 1996, 2029. (5) Lin, Y.-C.; Kachar, B.; Weiss, R. G. J. Am. Chem. Soc. 1989, 111, 5542. (6) (a) Hanabusa, K.; Tange, J.; Taguchi, Y.; Koyama, T.; Shirai, H. J. Chem. Soc., Chem. Commun. 1993, 390. (b) Hanabusa, K.; Miki, T.; Taguchi, Y.; Koyama, T.; Shirai, H. J. Chem. Soc., Chem. Commun. 1993, 1382. (c) Tomioka, K.; Sumiyoshi, T.; Narui, S.; Nagaoka, Y.; Iida, A.; Miwa, Y.; Taga, T.; Nakano, M.; Handa, T. J. Am. Chem. Soc. 2001, 123, 11817. (d) Ahmed, S. A.; Sallenave, X.; Fages, F.; Mieden-Gundert, G.; Mu¨ller, W. M.; Mu¨ller, U.; Vo¨gtle, F.; Pozzo, J.-L. Langmuir 2002, 18, 7096. (e) Willemen, H. M.; Vermonden, T.; Marcelis, A. T. M.; Sudho¨lter, E. J. R. Langmuir 2002, 18, 7102. (f) van der Laan, S.; Feringa, B. L.; Kellogg, R. M.; van Esch, J. Langmuir 2002, 18, 7136. (7) (a) Murata, K.; Aoki, M.; Nishi, T.; Ikeda, A.; Shinkai, S. J. Chem. Soc., Chem. Commun. 1991, 1715. (b) 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. (c) Aggeli, A.; Bell, M.; Boden, N.; Keen, J. N.; Knowles, P. F.; McLeish, T. C. B.; Pitkeathly, M.; Radford, S. E. Nature 1997, 386, 259. (d) Jung, J. H.; Ono, Y.; Sgubjau, S. Tetrahedron Lett. 1999, 40, 8395. (e) Engelkamp, H.; Middelbeek, S.; Nolte, R. J. M. Science 1999, 284, 785. (f) Ihara, H.; Sakurai, T.; Yamada, T.; Hashimoto, T.; Takafuji, M.; Sagawa, T.; Hachisako, H. Langmuir 2002, 18, 7120.
liquid and the gel forms as the solution/sol is cooled below the characteristic gelation temperature, Tg. The details of the initial steps in the formation of such networks are not well understood, and several different pathways may be operative, depending upon the system.9 Regardless, the smaller aggregates (sols) extend into fibers, strands, and tapes, etc.,7c,10 that join at “junction zones”11 to form the three-dimensional networks that immobilize the liquid component.1a Although analytical techniques, such as small-angle (SAXS) and wide-angle (WAXD) X-ray diffraction, small-angle neutron scattering (SANS), NMR spectroscopy, fluorescence, and circular dichroism have been used to probe solvent effects on polymorph selection,5,11-13 few studies have attempted to monitor the in situ development of ordered precursors that precede the crystallization process from supersaturated solutions.14 Here, we probe the initial (pre-sol) aggregation properties of and chemical exchange within amine-derived (1), (8) (a) Lin, Y. C.; Weiss, R. G. Macromolecules 1987, 20, 414. (b) Pozzo, J.-L.; Clavier, G. M.; Colomes, M.; Bouas-Laurent, H. Tetrahedron 1997, 53, 6377. (c) Placin, F.; Colome`s, M.; Desvergne, J.-P. Tetrahedron Lett. 1997, 38, 2665. (d) Clavier, G. M.; Mistry, M.; Fages, F.; Pozzo, J.-L. Tetrahedron Lett. 1999, 40, 9021. (9) Terech, P.; Talmon, Y. Langmuir 2002, 18, 7240. (10) (a) Aggeli, A.; Fytas, G.; Vlassopoulos, D.; McLeish, T. C. B.; Mawer, P. J. Boden, N. Biomacromolecules 2001, 2, 378. (b) Aggeli, A.; Nyrkova, I. A.; Bell, M.; Harding, R.; Carrick, L.; McLeish, T. C. B.; Semenov, A. N.; Boden, N. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 11857. (c) Hughes, R. E.; Hart, S. P.; Smith, D. A.; Movaghar, B.; Bushby, R. J. Boden, N. J. Phys. Chem. B 2002, 106, 6638. (11) Terech, P.; Furman, I.; Weiss, R. G. J. Phys. Chem. 1995, 99, 9558. (12) (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) Waguespack, Y. Y.; Banerjee, S.; Ramannair, P.; Irvin, G. C.; John, V. T.; McPherson, G. L. Langmuir 2000, 16, 3036. (e) Simmons, B. A.; Irvin, G. C.; Agarwal, V.; Bose, A.; John, V. T.; McPherson, G. L.; Balsara, N. P. Langmuir 2002, 18, 624. (13) (a) Menger, F. M.; Yamasaki, Y.; Catlin, K. K.; Nishimi, T. Angew. Chem., Int. Ed. Engl. 1995, 34, 585. (b) Snijder, C. S.; de Jong, J. C.; Meetsma, A.; van Bolhuis, F.; Feringa, B. L. Chem. Eur. J. 1995, 1, 594. (c) 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. (d) Liu, L. M.; Li, S. C.; Simmons, B.; Singh, M.; John, V. T.; McPherson, G. L.; Agarwal, V.; Johnson, P.; Bose, A.; Balsara, N. J. Dispersion Sci. Technol. 2002, 23, 441.
10.1021/la0343829 CCC: $25.00 © 2003 American Chemical Society Published on Web 08/19/2003
Aggregation of a Series of Organogelator Salts
Langmuir, Vol. 19, No. 20, 2003 8169 Scheme 1
alkylammonium alkylcarbamate (2), and alkylammonium alkyldithiocarbamate (3) gelators15 in chloroform-d, a nongelled liquid, by NMR spectroscopy. Due to the poor solubility of salts derived from 1c in chloroform, the present studies are limited to 2a,b and 3a,b (Scheme 1), and an emphasis is placed on studies involving 2b and its isotopically enriched analogues. The 1H and 13C NMR experiments on 2a,b at different concentrations and temperatures reported here demonstrate that CO2 exchanges rapidly between the ammonium and carbamate parts of the salts. However, CS2 exchange was not observed by NMR in solutions of the corresponding alkylammonium alkyldithiocarbamates (3). As expected, aggregation increases with decreasing temperature or increasing concentration.
Chart 1
Experimental Section Instrumentation. IR spectra were obtained on a PerkinElmer Spectrum One FT-IR spectrometer interfaced to a PC. Elemental analyses were performed on a Perkin-Elmer 2400 CHN elemental analyzer. NMR spectra (referenced to internal tetramethylsilane (TMS) for 1H and to CDCl3 for 13C) were recorded on a Varian 300 MHz spectrometer or a 500 MHz INOVA spectrometer with a variable-temperature controller and interfaced to a Sparc UNIX computer using Mercury software. Solutions for room-temperature investigations were placed in closed NMR tubes under air. Solutions for temperature-dependent studies were flame-sealed in tubes after one freeze-pumpthaw cycle at
