Chemically Reversible Organogels: Aliphatic Amines as "Latent" Gelators with Carbon Dioxide

7124 Langmuir 2002, 18, 7124-7135 Chemically Reversible Organogels via “Latent” Gelators. Aliphatic Amines with Carbon Dioxide and Their Ammonium Carbamates† Mathew George and Richard G. Weiss* Department of Chemistry, Georgetown University, Washington, D.C. 20057-1227 Received January 15, 2002. In Final Form: May 2, 2002 Rapid and isothermal (at room temperature) uptake of CO2 by solutions or, in some cases, organogels comprised of a primary or secondary aliphatic amine (1) and an organic liquid leads to in situ chemical transformation to the corresponding alkylammonium alkylcarbamate (2) based gels. Chemical reversibility is demonstrated by removal of CO2 from 2-based gels upon gentle heating in the presence of nitrogen. This is a general strategy for reversible self-assembly or disassembly of molecular aggregates relying on the initiation or termination of ionic interactions. The dependence of the amine structure and the nature of the liquid component on the formation and stability of the 1 and 2 organogels are examined by differential scanning calorimetry, optical microscopy, and X-ray diffraction methods. In most cases, the 2 gelators are more effective (based on the minimum gelator concentration required at room temperature, the gelation temperature, and the duration of time a gel persists without bulk phase separation) and more diverse (based on the classes of liquids gelled) than their corresponding amines. The differences are attributed to the presence of ionic interactions between molecular segments of the alkylammonium alkylcarbamates that are stronger than the hydrogen-bonding interactions available between molecules of amines. The initial stages of aggregation in the gel assemblies (i.e., changes in the degree of aggregation of sols of some 2 gelators) have been examined as a function of concentration and temperature by NMR techniques. Introduction The last several years have witnessed an enormous increase of interest in thermally reversible organogels comprised of (usually) j2 wt % of a low molecular mass organic gelator (an LMOG) and an organic liquid.1-10 These gels are microheterogeneous phases that self-assemble in a wide variety of modes with structures expressed from the molecular to the micrometer distance scales. When sols or solutions of these systems are cooled below their characteristic gelation temperature (Tg), the LMOGs aggregate into fibers, strands, tapes, etc., that join at “junction zones”6 to form networks that immobilize the liquid component, primarily by surface tension.2 A model to describe the stages of aggregation has been presented recently.11 Since the gelator concentration is usually very low, there need be no specific liquid-gelator interactions on the molecular scale. Most LMOGs have complex molecular structures, frequently with both lyophilic and hydrophilic or polar * Corresponding author. E-mail: weissr@georgetown.edu. FAX: 202-687-6209. † This article is part of the special issue of Langmuir devoted to the emerging field of self-assembled fibrillar networks. (1) Abdallah, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 1237. (2) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (3) van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. Engl. 2000, 39, 2263. (4) Shinkai, S.; Murata, K. J. Mater. Chem. 1998, 8, 485. (5) Terech, P.; Weiss, R. G. In Surface Characterization Methods; Milling, A. J., Ed.; Marcel Dekker: New York, 1999; p 286. (6) Terech, P.; Furman, I.; Weiss, R. G. J. Phys. Chem. 1995, 99, 9558. (7) Partridge, K. S.; Smith, D. K.; Dykes, G. M. McGrail, P. T. Chem. Commun. 2001, 319. (8) Lu, L.; Cocker, M.; Bachman, R. E.; Weiss, R. G. Langmuir 2000, 16, 20. (9) Ajayaghosh, A.; George, S. J. J. Am. Chem. Soc. 2001, 123, 5148. (10) Guan, L.; Zhao, Y. J. Mater. Chem. 2001, 11, 1339. (11) Aggeli, A.; Nyrkova, I. A.; Bell, M.; Harding, R.; Carrick, L.; McLeish, T. C. B.; Semenov, A. N.; Boden, N. Proc. Nal. Acad. Sci. U.S.A. 2001, 98, 11857. regions and several functional groups. The unusual structural and diffusional properties of organogels have led to several interesting applications.12-17 An exceedingly broad range of organic liquids (including quasi-liquids such as supercritical CO218) has been gelled, and very diverse types of LMOGs (including two-component systems that act via specific H-bonding interactions7 or single species whose structures can be salts to multifunctional molecules or even simple long-chained n-alkanes19,20) are known.1,21,22 Long-chain aliphatic amines are known to gel a variety of organic liquids.8,20,21,23 Recently, we discovered that some alkylammonium alkylcarbamates, formed in situ and reversibly from the corresponding amines by the rapid uptake or loss of carbon dioxide gas,24-27 are LMOGs, (12) Derossi, D.; Kajiwara, K.; Osada, Y.; Yamauchi, A. Polymer gels: Fundamentals and Biomedical Applications; Plenum Press: New York, 1991. (13) Bhattacharya, S.; Ghosh, Y. K. Chem. Commun. 2001, 185. (14) Vidal, M. B.; Gil, M. H.; J. Bioact. Compat. Polym. 1999, 14, 243. (15) Pozzo, J.-L.; Clavier, G. M.; Desvergne, J.-P. J. Mater. Chem. 1998, 8, 2575. (16) Loos, M.; Esch, J.; Stokroos, I.; Kellogg, R. M.; Feringa, B. L. J. Am. Chem. Soc. 1997, 119, 12675. (17) (a) Jung, J. H.; Ono, Y.; Shinkai, S. Chem. Eur. J. 2000, 6, 4552. (b) Terech, P. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 1630. (18) (a) Shi, C.; Huang, Z.; Kilic, S.; Xu, J.; Enick, R. M.; Beckman, E. J.; Carr, A. J.; Melendez, R. E.; Hamilton, A. D. Science 1999, 286, 1540. (b) Placin, F.; Desvergne, J.-P.; Cansell, F. J. Mater. Chem. 2000, 10, 2147. (19) Abdallah, D. J.; Weiss, R. G. Langmuir 2000, 16, 352. (20) Abdallah, D. J.; Lu, L.; Weiss, R. G. Chem. Mater. 1999, 11, 2907. (21) Lu, L.; Weiss, R. G. Chem. Commun. 1996, 2029. (22) Lin, Y.-C.; Kachar, B.; Weiss, R. G. J. Am. Chem. Soc. 1989, 111, 5542. (23) 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. (24) Hoerr, C. W.; Harwood: H. J. Ralston, A. W. J. Org. Chem. 1944, 9, 201. (25) Leibnitz von E.; Hager, W.; Gipp, S.; Bornemann, P. J. Prakt. Chem. 1959, 9, 217. 10.1021/la0255424 CCC: $22.00 © 2002 American Chemical Society Published on Web 06/13/2002 Chemically Reversible Organogels Langmuir, Vol. 18, No. 19, 2002 7125 Scheme 1 temperatures were calibrated with the chemical shift of the OH peak of methanol and ethylene glycol, respectively.31 Materials. Silicone oil (tetramethyltetraphenylsiloxane, Dow silicone oil 704) was used as received. Other liquids for the preparation of gels were reagent grade or better (Aldrich). 1-Decylamine (95%), 1-dodecylamine (99+%), N,N-dioctylamine (98%), and 1-tetradecylamine (95%) from Aldrich and N,Ndioctadecylamine (>99%) from Fluka were used as received. 1-Hexadecylamine, 1,12-diaminododecane, and N-methyl octadecylamine from Aldrich were recrystallized from chloroform under a nitrogen atmosphere. 1-Octadecylamine (Aldrich) was distilled twice under vacuum and stored under a nitrogen atmosphere. Alkylammonium alkylcarbamates (2) were prepared by passing CO2 gas through a hexane solution (1-decylamine and N,N-dioctylamine) or chloroform solution (other amines) for 15 min. The precipitates were filtered and dried. Melting points of amines and their alkylammonium alkylcarbamates are reported in Supporting Information. Decylammonium chloride was prepared by bubbling dry hydrogen chloride gas through a hexane solution of decylamine and collecting the precipitate. Sodium decylcarbamate was prepared by a reported procedure32 using sodium hydride (1.1 equiv) as the base. The precipitated product was washed with chloroform (to remove any decylammonium decylcarbamate that might have been formed) and dried: mp 93 °C (dec, by DSC); IR (neat) 3325 (NsH), 2918, 2849 (C-H), 1566 (CdO) cm-1; 1H NMR (CDCl3) 2.68 (2H, t, J ) 6.8 Hz), 1.42 (2H, m), 1.27 (14H, s), 0.88 (3H, t, J ) 6.8 Hz) ppm. Preparation of Gels. Liquid components were saturated with N2 gas by bubbling for 10 min prior to use. Weighed amounts of a liquid and an amine or alkylammonium alkylcarbamate were placed into glass tubes (5 mm i.d.) that were flame-sealed in most cases (to avoid evaporation). The sealed tubes were twice heated in a water bath (until all solid material had dissolved) and cooled rapidly under tap water to ensure homogeneity. Gelation Temperatures. Gelation temperatures (Tg) were determined by the inverse flow method33 (i.e., the temperature at which a gel fell under the influence of gravity when inverted in a sealed glass tube that was placed in a thermostated water bath). Tg values and heats of melting (∆Hg) of gels with silicone oil as the liquid were also determined by differential scanning calorimetry (DSC) using a TA 2910 differential scanning calorimeter interfaced to a TA Thermal Analyst 3100 controller equipped with a hollowed aluminum cooling block into which dry ice was placed for subambient measurements. Thermal gravimetric analysis (TGA) measurements were performed on a TGA 2050 thermogravimetric analyzer (TA Instruments) interfaced to a computer. Heating rates were 5 °C/min; cooling rates were variable and depended on the difference between the cellblock and ambient temperatures. Unless stated otherwise, the reported Tg values are from the inverse flow method. Optical Micrographs. Polarizing optical micrographs (OMs) of silicone oil gels sandwiched between thin cover slides were recorded on a Leitz 585 SM-LUX-POL microscope equipped with crossed polars, a Leitz 350 heating stage, a Photometrics CCD camera interfaced to a computer, and an Omega HH503 microprocessor thermometer connected to a J-K-T thermocouple. 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 image ¨ plate system with Cu KR X-rays generated with a Rigaku generator operated at 46 kV and 46 mA. Gel samples were prepared by flowing hot (T > Tg) aliquots into the capillary and sealing both its ends. The samples were then cooled under running water. Data processing and analyses were performed using Materials Data JADE (version 5) XRD pattern processing.34 Molecular Calculations. Molecular calculations were performed using the HYPERCHEM package, release 5.1 Pro for Windows from Hypercube, Inc. Lowest energy geometries were optimized using the Parametric Method 3 (PM3) semiempirical method.35 also.28 Their ability to gel organic liquids depends on the nature of the alkyl group(s) and whether the precursor amine is primary or secondary.20,28 Here, we report in greater detail the gelation properties of a wider variety of selected primary and secondary amines (1) and their alkylammonium alkylcarbamates (2) (Scheme 1). The process that transforms the 1-based organogels to (and from) the 2-based ones is novel since it involves chemical (as well as) thermal reversibility.29 Several of the amines investigated are “latent” LMOGs because they, alone, do not form gels with a variety of liquids that are gelled rather efficiently by the corresponding 2. The transformation between solution (or sol) and gel in these cases is effected only by the nature of the gas bubbled through the condensed phase. This is a general strategy for reversible self-assembly or disassembly of molecules based on the initiation or termination of ionic interactions. It should be applicable to formation of many other aggregates besides those responsible for gelation. It is a completely different phenomenon than the gelation of supercritical CO2 (as the liquid component).18 In addition, the gelation procedure offers a convenient, rapid, and efficient method to sequester (reversibly) and sense the presence of atmospheric CO2.30 Experimental Section Melting points (corrected) were measured on a Leitz 585 SMLUX-POL microscope equipped with crossed polars, a Leitz 350 heating stage, and an Omega HH503 microprocessor thermometer connected to a J-K-T thermocouple. IR spectra were obtained on a Perkin-Elmer Spectrum One FT-IR spectrometer interfaced to a PC. NMR spectra (referenced to internal TMS) were recorded on a Varian 300 MHz spectrometer connected with a variabletemperature controller and interfaced to a Sparc UNIX computer using Mercury software. Samples were equilibrated at each temperature for 5 min prior to recording spectra. Low and high (26) Lallau, J.-P.; Masson, J.; Guerin, H. Bull. Soc. Chim. Fr. 1972, 3111. (27) Nakamura, N.; Okada, M.; Okada, Y.; Suita, K. Mol. Cryst. Liq. Cryst. 1985, 116, 181. (28) George, M.; Weiss, R. G. J. Am. Chem. Soc. 2001, 123, 10393. (29) The closest analogies we have been able to find in the literature involve Cu(I)a,b and Cu(II)c,d alkoxides and their reaction with CO2. There is an unpublished report that bubbling CO2 through solutions of cupric methoxidec in methanol transforms them into gels.e (a) Tsuda, T.; Chujo, Y.; Saegusa, T. J. Chem. Soc., Chem. Commun. 1976, 415. (b) Yamamoto, T.; Kubota, M.; Yamamoto, A. Bull. Chem. Soc. Jpn. 1980, 53, 680. (c) Tsuda, T.; Saegusa, T. Inorg. Chem. 1972, 11, 2561. (d) Vlekova, J.; Bartoo, J. J. Chem. Soc., Chem. Commun. 1973, 306. ` ´ ` (e) Berrie, B. Private communication. (30) (a) Messaoudi, B.; Sada, E. J. Chem. Eng. Jpn. 1996, 29, 193, 534. (b) Sada, E.; Kumazawa, H.; Han, Z. Q. Chem. Eng. J. 1985, 31, 109. (c) Sada, E.; Kumazawa, H.; Ikehara, Y.; Han, Z. Q. Chem. Eng. J. 1989, 40, 7. (31) Amman, C.; Meier, P.; Merbach, A. E. J. Magn. Reson. 1982, 46, 319. (32) Waldman, T. E.; McGhee, W. D. Chem. Commun. 1994, 957. (33) Takahashi, A.; Sakai, M.; Kato, T. Polym. J. 1980, 12, 335. (34) Materials Data Inc., Release 5.0.35 (SPS), Livermore, California. (35) Stewart, J. J. P. J. Comput. Chem. 1989, 10, 209. 7126 Langmuir, Vol. 18, No. 19, 2002 George and Weiss Table 1. Stability Parameters and Appearancesa of Gels of 2 wt % 1 or 2 in Various Liquids a liquid hexane n-octane 1 S b 2 1 c 2 1 d 2 P S P P TGb 1 P silicone oil PGb PGf TGd (42) ethanol (