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Langmuir 2000, 16, 352-355
n-Alkanes Gel n-Alkanes (and Many Other Organic Liquids) David J. Abdallah and Richard G. Weiss* Department of Chemistry, Georgetown University, Washington, D.C. 20057-1227 Received June 21, 1999. In Final Form: August 24, 1999 Thermoreversible gelation of relatively short n-alkanes and a variety of other organic liquids has been accomplished using low concentrations of long n-alkanes (chain lengths from 24 to 36 carbon atoms) as gelators. The stability of the gels is discussed in terms of gelator concentration, gelation temperature, and the duration of time gels persist without bulk phase separation. Generally, stability increases with the chain length of the gelator. n-Alkanes are structurally the simplest organic gelators possible and their gels with n-alkanes as liquids are the simplest organogels that can be made. Their existence demonstrates that London dispersion forces alone can provide colloidal networks whose strength is sufficient to immobilize liquids against the pull of gravity.
1. Introduction For centuries, colloidal assemblies, as gels, have found practical uses,1 including religious ones.2 Polymeric or hydrogels, especially, are known to be key ingredients in a number of biological systems (such as the vitreous humor of the eye and the synovial fluid lubricating skeletal joints),3 and they are applied increasingly in industry (e.g., in foods, deodorants, and cosmetics, athletic shoes, and chromatography). Recently, there has been a growth of interest in a different type of gel (denoted as LMOG),4 comprised of a small concentration (usually j2 wt %) of a low molecular mass organic molecule (a gelator) and a liquid component.4 The general definition of gels provided by Flory,5 perhaps the most comprehensive thus far,1 attests to their complexity. Typical LMOGs consist of interacting colloids, supramolecular assemblies of cross-linked strands that hold microscopic domains of the liquid in a nonflowing (yet isotropic) state, primarily via surface tension. Individual strands have very high aspect ratios. In several cases, strand cross sections, typically ∼1-102 nm, are known to be monodisperse,6 and their morphologies sometimes differ from those of “normal” crystals of the same molecules.7 (1) (a) Graham, T. Philos. Trans. R. Soc. 1861, 151, 183. (b) Graham, T. J. Chem. Soc. 1864, 17, 318. (2) Garlaschelli, G.; Ramaccini, F.; Della Sala, S. Nature 1991, 353, 507. (3) Tanaka, T. Sci. Am. 1981, 244, 124. (4) See, for instance: Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (5) Flory, P. J. Discuss. Faraday Soc. 1974, 57, 7. (6) See for instance: (a) Terech, P.; Wade, R. H. J. Colloid Interface Sci. 1988, 125, 542. (b) Wade, R. H.; Terech, P.; Hewat, E. A.; Ramasseul, R.; Volino, F. J. Colloid Interface Sci. 1986, 114, 442. (c) Brotin, T.; Desvergne, J. P.; Fages, F.; Utermo¨hlen, R.; Bonneau, R.; Bouas-Laurent, H. Photochem. Photobiol. 1992, 55, 349. (d) Terech, P.; Desvergne, J. P.; Bouas-Laurent, H. J. Colloid Interface Sci. 1995, 174, 258. (e) Lin, Y.-C.; Kachar, B.; Weiss, R. G. J. Am. Chem. Soc. 1989, 111, 5542. (f) Geiger, C.; Stanescu, M.; Chen, L.; Whitten, D. G. Langmuir 1999, 15, 2241. (g) Shinkai, S.; Murata, K. J. Mater. Chem. 1998, 8, 485. (h) Bhattacharya, S.; Ghanashyam Acharya, S. N.; Raju, A. R. Chem. Commun. 1996, 2101. (i) Smith, J. M.; Katsoulis, D. E. J. Mater. Chem. 1995, 5, 1899. (j) Tachibana, T.; Kayama, K.; Takeno, H. Bull. Chem. Soc. Jpn. 1972, 45, 415. (k) van Nostrum, C. F.; Picken, S. J.; Shouten, S.-J.; Nolte, R. J. M. J. Am. Chem. Soc. 1995, 117, 9957. (l) Hanabusa, K.; Matsumoto, Y.; Miki, T.; Koyama, T.; Shirai, F. J. Chem. Soc., Chem. Commun. 1 1994, 1401. (7) Ostuni, E.; Kamaras, P.; Weiss, R. G. Angew. Chem., Int. Ed. Engl. 1996, 35, 1324.
It is generally believed4 that a molecule must be able to self-associate through interactions that are more specific and stronger than London dispersion forces to be a successful gelator. Thus, the presence of heteroatoms, because of the stronger van der Waals and other forces they induce, is considered necessary for a molecule to be an LMOG gelator; in the extreme, polymeric gelators rely upon covalent cross-linking interactions to stabilize their gels.8 An ongoing question has been, “How simple can the structures of LMOG gelators and their gelled liquids be?” The simplest, generally known LMOG gelators are partially fluorinated n-alkanes (in relatively high concentrations)9 and n-alkanes into which one N or S heteroatom has been inserted.10 However, there are reports in the petroleum-related literature that long n-alkanes, alone, gel shorter n-alkanes.11 These are the structurally simplest class of LMOG gelator and liquid molecules conceivable. Here, we investigate the properties of those gels and of others in which the liquid component is one of a wide variety of non-hydrocarbon liquids. 2. Experimental Section Materials. Silicone oil 704 (tetramethyltetraphenylsiloxane) was from Dow-Corning. All other liquids were reagent grade or better and were used without purification. The n-alkane gelators (Humphrey Chemicals Co.) were recrystallized several times from toluene (n-tetracosane (C24) and n-octacosane (C28)) or petroleum ether (n-dotriacontane (C32) and n-hexatriacontane (C36)) until >99% pure by GLPC analyses: C24, mp 50.1-51.8 °C (lit. mp 50.6 °C15); C28, mp 61.6-62.6 °C (lit. mp 61.2 °C15); C32, mp 69.8-71.1 °C (lit. mp 68.8 °C12); C36, mp 77.0-77.6 °C (lit. mp 75.9 °C15). (8) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953; pp 381-3. (9) (a) Twieg, R. J.; Russell, T. P.; Siemens, R.; Rabolt, J. F. Macromolecules 1985, 18, 1361. (b) Rabolt, J. F.; Russell, T. P.; Siemens, R. L.; Twieg, R. J.; Farmer, B. L. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1986, 27, 223. (c) Pugh, C.; Ho¨pken, J.; Mo¨ller, M. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1988, 29, 460. (d) Ho¨pken, J. Ph.D. Thesis, University of Twente, Enschede, The Netherlands, 1991, Chapter 3. (e) Ku, C. Y.; LoNostro, P.; Chen, S. H. J. Phys. Chem. B 1997, 101, 908. (10) Abdallah, D.; Lu, L.; Weiss, R. G. Mater. Chem., in press. (11) Srivastava, S. P.; Saxena, A. K.; Tandon, R. S.; Shekher, V. Fuel 1997, 76, 625 and references cited therein. (12) Sirota, E. B.; King, H. E., Jr.; Singer, D. M.; Shao, H. H. J. Chem. Phys. 1993, 98, 5809.
10.1021/la990795r CCC: $19.00 © 2000 American Chemical Society Published on Web 10/07/1999
n-Alkanes Gel n-Alkanes
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Table 1. Transition Temperatures, Tg (°C), and Periods of Stability (at Room Temperature)a of Gels with n-Alkane Gelatorsb gelator concentration (M) C24 liquid cyclohexane heptane decane dodecane hexadecane benzene toluene CH2Cl2 ethyl acetate ethanol 1-butanol 1-pentanol 1-octanol silicone oil
0.03c
0.04
s s 35 (
