Langmuir 2003, 19, 1017-1025
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Primary Alkyl Amines as Latent Gelators and Their Organogel Adducts with Neutral Triatomic Molecules† Mathew George and Richard G. Weiss* Department of Chemistry, Georgetown University, Washington, D.C. 20057-1227 Received October 1, 2002. In Final Form: December 5, 2002 A series of organogelator salts has been prepared from n-alkylamines by the rapid in situ and isothermal (at room temperature) uptake of a neutral triatomic molecule, CO2, NO2, SO2, or CS2. The organogels have been examined by differential scanning calorimetry, optical microscopy, and X-ray diffraction methods. The efficiency of each gelator has been assessed on the bases of the diversity of liquids it gelled, the minimum amount of it required for gelation, and the temporal and thermal stabilities of its gels. Thus, alkylammonium alkylcarbamates, amine-CO2 adducts, are the most effective gelators and the amineNO2 adducts are the least efficient. Salts from longer n-alkylamines are better gelators than those from shorter homologues. Some of the salts are reconverted to their amine and triatomic constituents by heating, while others are transformed into new compounds. In the case of the CS2 adducts, H2S is expelled and the new species formed, N,N′-dialkylthioureas, are also gelators.
Introduction Interest in low-molecular-mass organic gelators (LMOGs) and their thermally reversible organogels has increased enormously during the past several years.1-10 Organogels are usually comprised of an organic liquid and a small amount of an LMOG. Typically, the LMOG is heated in the liquid until it dissolves and the gel forms as the solution/sol is cooled. Recently, we and others have examined new methodologies for organogel formation in which a “latent gelator” (i.e., a molecule that does not gel a liquid by itself) is treated in situ with chemical or physical triggers to effect gelation.11,12 Regardless of which method is employed, the gelator molecules self-assemble to form usually crystalline fibers, tapes, strands, or other aggregates with high aspect ratios. These elongated objects link at “junction zones”6 to form three-dimensional networks that immobilize the liquid component, primarily by capillary forces and surface tension.2 * To whom correspondence should be addressed. Fax: 202-6876209. Telephone: 202-687-6013. E-mail:
[email protected]. † Dedicated to Professor Kailasam Venkatesan, mentor and friend, on the occasion of his 70th birthday. (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. 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) (a) George, M.; Weiss, R. G. Langmuir 2002, 18, 7124. (b) George, M.; Weiss, R. G. J. Am. Chem. Soc. 2001, 123, 10393. (12) (a) Murata, K.; Aoki, M.; Nishi, T.; Ikeda, A.; Shinkai, S. 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.
Although they can be very simple molecules,13,14 most LMOGs have complex structures,1,15,16 frequently with both lyophilic and hydrophilic or polar regions and several functional groups or two-component systems that act via specific H-bonding interactions.7,10,17 In addition, an exceedingly broad range of organic liquids (including quasi-liquids such as supercritical CO218) and water19 have been gelled. The unusual structural and diffusional properties of organogels have led to several interesting applications,20-25 and they can be used as structuredirecting agents for inorganic nanoparticles.25a,26 Long-chain aliphatic amines are an interesting class of LMOGs for several types of organic liquids.8,14,15,17c Recently, we discovered that some alkylammonium alkylcarbamates (2) (Scheme 1), formed in situ upon rapid uptake of carbon dioxide gas by primary amines (1),27-30 some of which are not gelators, become LMOGs, also.11 The amines can be recovered easily by gently heating the carbamate (to accelerate loss of CO2 gas) while passing nitrogen gas over or through it (to remove the CO2 from the proximity). The strong electrostatic forces that supplement the H-bonding and London dispersion forces already available to the aggregated amines must be an important (13) Abdallah, D. J.; Weiss, R. G. Langmuir 2000, 16, 352. (14) Abdallah, D. J.; Lu, L.; Weiss, R. G. Chem. Mater. 1999, 11, 2907. (15) Lu, L.; Weiss, R. G. Chem. Commun. 1996, 2029. (16) Lin, Y.-C.; Kachar, B.; Weiss, R. G. J. Am. Chem. Soc. 1989, 111, 5542. (17) (a) Hanabusa, K.; Tange, J.; Taguchi, Y.; Koyama, T.; Shirai, H. Chem. Commun. 1993, 390. (b) Hanabusa, K.; Miki, T.; Taguchi, Y.; Koyama, T.; Shirai, H. 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. (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) (a) Frkanec, L.; Jokic´, M.; Makarevic´, J.; Wolsperger, K.; Zˇ inic´, M. J. Am. Chem. Soc. 2002, 124, 9716. (b) Kobayashi, H.; Friggeri, A.; Koumoto, K.; Amaike, M.; Shinkai, S. Org. Lett. 2002, 4, 1423. (c) Jung, J. H.; John, G.; Masuda, M. J.; Yoshida, K.; Shinkai, S. Langmuir 2001, 17, 7229.
10.1021/la026639t CCC: $25.00 © 2003 American Chemical Society Published on Web 01/21/2003
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Scheme 1. Primary Amine Adducts Investigated as Gelatorsa
a
See the text concerning the structures of 3 and 4.
factor in the increased gelling efficiency of the alkylammonium alkylcarbamates. Formation of charged salts from neutral amines by addition of neutral small quadrupolar31 XdYdX molecules with large partial positive charges on the Y atom, like CO2, is a strategy for enhanced reversible self-assembly and gelation. Here, we extend this approach to adducts 3-5, formed from a primary amine 1a-c and NO2, SO2, or CS2. The abilities of 3-5 to act as LMOGs are compared with that of 2, the formally analogous adducts between 1 and CO2. Unlike the 2 adducts, none of the 3-5 adducts reverts to its alkylamine 1 when heated, but the 5 adducts yield new gelators. Experimental Section Instrumentation. Measurements of melting points (corrected) of compounds and polarized optical micrography (POM) of silicone oil gels sandwiched between thin cover slides were performed 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. IR spectra were obtained on a Perkin-Elmer Spectrum One FTIR spectrometer interfaced to a PC. NMR spectra (referenced to internal TMS) were recorded on a Varian 300 MHz spectrometer interfaced to a Sparc UNIX computer using Mercury software. Mass spectra were recorded on a Fisons MD-800 GC-MS instrument at 70 eV for 9a and 30 eV for 9b; the gas chromatograph was equipped with a DB-5 column. Differential scanning (20) Derossi, D.; Kajiwara, K.; Osada, Y.; Yamauchi, A. Polymer gels: Fundamentals and Biomedical Applications; Plenum Press: New York, 1991. (21) Bhattacharya, S.; Ghosh, Y. K. Chem. Commun. 2001, 185. (22) Vidal, M. B.; Gil, M. H. J. Bioact. Compat. Polym. 1999, 14, 243. (23) Pozzo, J.-L.; Clavier, G. M.; Desvergne, J.-P. J. Mater. Chem. 1998, 8, 2575. (24) Loos, M.; Esch, J.; Stokroos, I.; Kellogg, R. M.; Feringa, B. L. J. Am. Chem. Soc. 1997, 119, 12675. (25) (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. (26) (a) Kobayashi, S.; Hamasaki, N.; Suzuki, M.; Kimura, M.; Shirai, H.; Hanabusa, K. J. Am. Chem. Soc. 2002, 124, 6550. (b) Jung, J. H.; Ono, Y.; Shinkai, S. Angew. Chem., Int. Ed. 2000, 39, 1862. (c) Jung, J. H.; Ono, Y.; Hanabusa, K.; Shinkai, S. J. Am. Chem. Soc. 2000, 122, 5008. (d) Ono, Y.; Nakashima, K.; Sano, M.; Hojo, J.; Shinkai, S. J. Mater. Chem. 2001, 11, 2412. (27) Hoerr, C. W.; Harwood, H. J.; Ralston, A. W. J. Org. Chem. 1944, 9, 201. (28) Leibnitz von, E.; Hager, W.; Gipp, S.; Bornemann, P. J. Prakt. Chem. 1959, 9, 217. (29) Lallau, J.-P.; Masson, J.; Guerin, H. Bull. Soc. Chim. Fr. 1972, 3111. (30) Nakamura, N.; Okada, M.; Okada, Y.; Suita, K. Mol. Cryst. Liq. Cryst. 1985, 116, 181. (31) The quadrupole moments of CO2 and CS2 have been calculated and determined experimentally. The absolute values calculated by Das Gupta et al. (ref 31a) are 3.00 × 10-26 esu cm2 (CO2) and 1.84 × 10-26 esu cm2 (CS2). (a) Das Gupta, A.; Singh, Y.; Singh, S. J. Chem. Phys. 1973, 59, 1999 and references cited therein. (b) Vrabec, J.; Stoll, J.; Hasse, H. J. Phys. Chem. B 2001, 105, 12126 and references cited therein.
George and Weiss calorimetry (DSC) was performed on 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. All DSC data are reported at temperatures of maximum heat flow. Thermal gravimetric analyses (TGA) measurements were done 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. X-ray diffraction (XRD) of samples in thin, sealed capillaries (0.5 mm diam; W. Mu¨ller, Scho¨nwalde, Germany) was performed on a Rigaku R-AXIS image plate system with Cu KR X-rays (λ ) 1.54056 Å) generated with a Rigaku generator operating at 46 kV and 46 mA. Data processing and analyses were performed using Materials Data JADE (version 5.0.35) XRD pattern processing software.32 Molecular calculations were performed using the Hyperchem package, release 5.1 Pro for Windows, from Hypercube, Inc. Lowest energy geometries were optimized using the semiempirical Parametric Method 3 (PM3).33 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-Propylamine (99%) and 1-decylamine (95%), from Aldrich, were used as received. 1-Octadecylamine (Aldrich) was distilled twice under vacuum and stored under a nitrogen atmosphere. Salts 2-4 were prepared by bubbling an excess of CO2 (subliming dry ice and passing the gas through a drying column), SO2 (from addition of 75% sulfuric acid to sodium sulfite34), or NO2 (prepared by reaction of concentrated nitric acid and copper powder35) through an amine solution in hexane (1-propylamine and 1-decylamine) or chloroform (1-octadecylamine) (Scheme 1). Salts 5 were prepared by adding 2 molar equivalents of CS2 (99+%, Fisher) to an amine solution. The precipitates were filtered, washed with the corresponding solvent, and dried under house vacuum for 15 min. They were not manipulated further to avoid possible decomposition. Elemental analyses for the adducts from 1c (Desert Analytics, Tucson, AZ) were obtained. For 2c: Calcd for (C18H37NH2)2‚CO2: C, 76.22; H, 13.48; N, 4.80. Found: C, 76.31; H, 13.48; N, 4.88. For 3c: Calcd for (C18H37NH2)2N2O4‚3H2O: C, 63.11; H, 12.35; N, 8.18. Found: C, 63.16; H, 12.31; N, 8.55. For 4c: See below. Found: C, 64.32; H, 12.42; N, 4.30. For 5c: Calcd for (C18H37NH2)2‚CS2: C, 72.24; H, 12.78; N, 4.55. Found: C, 71.85; H, 12.58; N, 4.35. Preparation of Gels and Gelation Temperatures. Weighed amounts of a liquid and a salt were placed into glass tubes (5 mm i.d.) that were flame-sealed in most cases (to avoid evaporation). The tubes were heated twice in a water bath (until all solid material dissolved) and cooled rapidly under tap water to ensure homogeneity. Gelation temperatures (Tg) were determined by the inverse flow method.36 A gel sample in a sealed glass tube was inverted, strapped to a thermometer near the bulb, and immersed in a stirred water bath at room temperature. The temperature of the bath was increased slowly, and the range of Tg was taken from the point at which the first part of the gel was observed to fall to the point at which all had fallen under the influence of gravity.
Results and Discussion Preparation of the LMOGs and Their Thermodynamic Properties. Salts 2-5 were prepared as described in the Experimental Section and as outlined in Scheme 1. Reaction of primary amines with CO2 and CS2 leads to alkylammonium alkylcarbamates (2)27-30 and alkylammonium alkyldithiocarbamates (5),37 respectively. Their spectroscopic properties are consistent with the (32) JADE, release 5.0.35 (SPS); Materials Data Inc.: Livermore, CA. (33) Stewart, J. J. P. J. Comput. Chem. 1989, 10, 209. (34) Inorganic Chemistry; Chambers, C., Holliday, A. K., Eds.; Butterworth Scientific: London, 1982; pp 253-254. (35) Basic Inorganic Chemistry; Cotton, F. A., Wilkinson, G., Gaus, P. L., Eds.; John Wiley & Sons: New York, 1987; p 350. (36) Takahashi, A.; Sakai, M.; Kato, T. Polym. J. 1980, 12, 335.
Primary Alkyl Amines as Latent Gelators
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Table 1. Melting and Decomposition Temperatures (°C) of 2-5 Measured by POM and DSCa 2 (CO2) (ref 11) amine C3H7NH2 (a) C10H21NH2 (b) C18H37NH2 (c)
POM 57.8-60.7 [lit 73-74 (ref 39)] 60.8-61.5, 73.6-79.4 [lit 80 (dec 90) (refs 28,29)] 80.4-95.6 [lit 87 (dec 98) (refs 28,30)]
DSC (∆H, J/g)
3 (NO2)
4 (SO2)
5 (CS2)
POM
DSC (∆H, J/g)
POM
DSC (∆H, J/g)
67.9 (710.8)b
d
d
d
d
61.1 (27.0)b 75.5 (248.0)b
d
d
105.9-113.8
55.4-56.2e 57.2 (44.4)b 70.5 (407.0)b 45.2 (-177.0)c 74.6-75.9 69.3 (20.1)b 74.7 (27.8)b 72.1 (-28.1)c 69.1 (-4.43)c 55.1 (-28.1)c
31.9 (67.01)b 113.1 (0.2)b
80.0e 64.6 (11.6)b 98.3-100.0f 77.9 (13.4)b 206.7-235.1g 90.8 (171.1)b 80.4 (-27.5)c
a Enthalpy changes of transition from DSC (first heating) are in parentheses. b Heating. c Cooling. transition. f Appearance of separate solid and liquid phases. g Decomposition.
assigned structures, and elemental analyses for 2c and 5c are as well. Initial interaction of SO2 with alkylamines is reported to form donor-acceptor complexes which then become pyrosulfites, (RNH3)22+S2O52- (6), and small amounts of alkylthionylamines, R-NdSdO (7, vide infra).38 In accord with this chemistry, elemental analysis of 4c (found: C, 64.32; H, 12.42; N, 4.30) appears to be a mixture of (C18H37NH3)22+S2O52- (calcd: C, 63,11; H, 11.77; N, 4.09), C18H37NdSdO (calcd: C, 68.51; H, 11.82; N, 4.44), and C18H37NH2‚SO2 (calcd: 64.81; H, 11.69: N, 4.20); for instance, a 1:1 mixture of (C18H37NH3)22+S2O52- and C18H37NdSdO gives C, 64.81; H, 11.78; N, 4.20 (vide infra). The amine-NO2 adducts (3b,c) are not LMOGs, and their structures have not been investigated in detail; elemental analysis of the adduct to 1c, 3c, is consistent with the trihydrate of 2[RNH2‚NO2], and its IR spectrum shows a very strong, broad hydroxyl band centered at 3103 cm-1. Compound 3c was a white solid, whereas 3b was a waxy semisolid and therefore not amenable to gelation of organic liquids. Since 3c and 4b were not LMOGs for several liquids examined, the propylamine derivatives 3a and 4a were not prepared. The first heating thermograms of several of the salts show endothermic peaks that can be attributed to melting transitions and decomposition (Table 1). Although the reason for the trends in these enthalpy changes is not completely understood, they can be attributed, at least in part, to differences in the molecular structures. The much large enthalpies in J/g for 2a and 5a than for the longer chained homologues, 2b and 5b, are related to the greater heat needed to break the electrostatic interactions than the dispersive forces between chains (leading to chain melting). Assuming the same general packing motifs, the magnitudes of the enthalpies of the decyl derivatives are determined to a (37) (a) Mathes, R. A. Inorg. Synth. 1950, 3, 48. (b) Chanon, M.; Metzger, J. Bull. Soc. Chim. Fr. 1968, 2842. (c) Datta, K.; Roussel, C.; Metzger, J. Bull. Soc. Chim. Fr. 1974, 2135. (38) (a) Michaelis, A.; Storbeck, O. Justus Liebigs Ann. Chem. 1893, 274, 187. (b) Hata, T.; Kinumaki, S. Nature 1964, 203, 1378. (c) Bodrikov, I. V.; Krasnov, V. L.; Matyukov, E. V.; Chernov, A. N.; Verin, I. A. J. Org. Chem. (USSR) 1987, 23, 1921. (39) Hunter, B. A.; Glenn, H. D. U.S. Patent 2,635,124, U.S. Rubber Co. 1951 (CA 1953, 47, P7814f). (40) POM analyses of 4c showed several changes in appearance at temperatures that correlate with transitions and composition changes detected by DSC and TGA: near 80 °C, where the appearance of the solid changed, DSC showed an endothermic transition at 78.4 °C (∆H ) 20.55 J/g), and the TGA indicated a 2.8% weight loss between 75 and 78 °C; by microscopy, the material separated at 98.3-100.0 °C into solid and liquid phases, while DSC showed an endothermic transition around 100 °C that overlapped with a strong endothermic peak at 90.8 °C; an acrid odor was detected at 172 °C, and the TGA showed a small weight loss near 170 °C; the sample became brown when heated above 200 °C.
d
POM
DSC (∆H, J/g)
87.8-92.4 106.5 (368.0)b [lit 95 73.6e (45.9)b (ref 37c)] 71.3e 84.1-86.2 91.1 (68.8)b 114.9 (113.2)b 58.6 (-95.6)c 93.7-102.0 93.3 (25.0)b 101.7 (85.3)b 79.7 (-121.6)c
Not determined. e Solid-solid phase
Figure 1. TGA (dashed line) and DSC (solid line) thermograms of salts derived from 1-octadecylamine (1c): (a) 2c, (b) 3c, (c) 4c, and (d) 5c. The insets are the second heating and cooling cycles; the axes are the same as in the first cycle. Arrows indicate the direction of temperature change.
greater extent by the energy needed to disrupt the weaker dispersive chain-chain interactions, while the enthalpies of the propyl homologues are weighted more by contributions from separation of the charged centers. When the packing arrangement of homologues of one salt type is not constant as a function of chain length, the above arguments do not hold necessarily. An example appears to be 2c, whose enthalpy is higher than that of 2b (and lower than that of 2a). Representative TGA and DSC thermograms of salts derived from 1-octadecylamine (1c) are collected in Figure 1. Alkylammonium alkylcarbamates 2 expel carbon dioxide and regenerate 1 when heated to above their melting temperatures.11 For example, TGA of 2c showed a transition near 80 °C due to loss of CO2 from the carbamate (weight loss ) 7.3%, theoretical for loss of one CO2 per salt molecule ) 7.5%) and a gradual weight loss above 150 °C from evaporation of the amine (Figure 1a). The heats of the transitions measured by DSC decreased with successive runs due to slow evaporation of the amine. The amine-NO2 adduct 3c became an isotropic liquid at 74.6-75.9 °C. DSC and TGA analyses of 3c indicate that it is fairly stable thermally and does not revert to
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Figure 2. IR spectra of 4c (a) before and (b) after heating to 125 °C. Characteristic peaks of the donor-acceptor complex 4c are indicated by arrows in (a). Scheme 2
Scheme 3
amine and NO2 when it finally decomposes at higher temperatures. Its DSC thermogram during the first heating and cooling contained three endotherms and three exotherms (Table 1). The second heating thermogram contained only two endothermic peaks, at 68.0 and 75.3 °C, whereas the cooling thermogram retained the three exothermic peaks (at 55.1, 69.1, and 72.4 °C). Subsequent heating and cooling thermograms were like the second one. The enthalpy changes of transition of these salts during several heating and cooling cycles are listed in Supporting Table 1. No weight loss of 3c was discernible to 150 °C (Figure 1b); the weight loss shown above 150 °C is 69%. Complex thermal behavior was observed for the SO2 adducts (4). TGA measurements indicate that 4b is fairly stable to 200 °C. However, samples heated to >100 °C became yellow, and decomposition, in the form of significant weight loss, commenced above 220 °C. DSC thermograms showed broad endotherms at 31.9 °C (∆H ) 67.0 J/g) and at 113.1 °C (∆H ) 0.2 J/g). TGA of 4c showed weight losses of 2.8% at 75.0-78.0 °C and 31.5% gradually from 231.0 to 290.0 °C (Figure 1c); in addition, the IR spectrum of the 1c-SO2 adduct (Figure 2a) shows characteristic peaks of a donor-acceptor complex like 4c.38b After being heated to 125 °C, the complex was transformed into a new species (Figure 2b). On the bases of these collective data and from the thermal reactions of pyrosulfites,38 we tentatively ascribe the principal species formed near 78 °C to a dimeric form of the amine-SO2 adduct, 8c (Scheme 2).40 The 64.6 °C endotherm in the first DSC heating thermogram (to 125 °C) of 4c is replaced by one at 45.3 °C in the second heating thermogram, and the temperatures and heats of endotherms and exotherms are reproducible in heating and cooling cycles after the first (Supporting Figure 1). From these results and the
George and Weiss
Figure 3. IR spectra of 5c (a) before heating to 125 °C and (b) after being heated (to 125 °C) and cooled three times. The peaks indicated by arrows in (a) are absent in (b), and those marked in (b) are absent in (a).
reported reactions of other aliphatic amines with SO2,38 we believe that 4 consists principally of pyrosulfites (6) and alkylthionylamines (7). When heated to 125 °C, compounds 5a and 5b underwent transformations such as those of 5c.41 The 5.5% weight loss suffered by 5c between ca. 100 and 110 °C (Figure 1d) is equal to the calculated loss of one molecule of H2S; the vapors had the characteristic odor of that gas. These observations suggest that the alkylammonium alkyldithiocarbamates become thioureas 9 above their melting temperatures (Scheme 3).42 Consistent with the TGA data, the second and subsequent heating thermograms of 5c consist of two endotherms at 45.2 °C (∆H ) 32.2 J/g, solid-solid transition43) and 88.5 °C (∆H ) 128.5 J/g, melting transition44) (Supporting Figure 6). The assignment of 9c as the thermolysis product from 5c is supported by the IR spectra in Figure 3. The N-H stretch at 3202 cm-1 and C-N stretching bands at 1486 and 1472 cm-1 (coupled with CH2 scissoring45)46 of 5c (41) The mass spectrum of 9a (by heating 5a, Supporting Figure 2a) contains the expected 160 molecular ion peak for 9a and a base peak at 58 attributed to the fragment from C-N bond cleavage. The broad endotherm at 114.9 °C (∆H ) -113.2 J/g) in the first heating thermogram of 5b (Supporting Figure 3) is due to its decomposition. The second and subsequent heating thermograms consisted of only one endotherm at 66.8 °C from melting. The weight loss of 5b measured by TGA at 96 °C varied among several samples but was usually near 13% (12.7% calcd for loss of one molecule of H2S and H2O; i.e., from a hydrate); expulsion of CS2 requires a 19.5% loss. The IR spectrum of 5b heated three times to 125 °C shows peaks characteristic of a thiourea (Supporting Figure 4). The highest mass fragment detected in the mass spectrum from 9b (by heating 5b, Supporting Figure 2b), at 321, corresponds to the molecular ion minus H2S. Compound 9c decomposed on the GC column (since >170 °C is needed to elute it) prior to its MS analysis. The NMR spectra of the thermal products from 5a-c are also in agreement with thiourea structures (Supporting Figure 5). (42) (a) Schroeder, D. C. Chem. Rev. 1955, 55, 181. (b) Williams, A.; Ibrahim, I. T. J. Am. Chem. Soc. 1981, 103, 7090. (c) Foye, W. E.; Lasala, E. F.; Georgiadis, M.; Meyer, W. L. J. Pharm. Sci. 1965, 54, 557. (d) Erickson, J. G. J. Org. Chem. 1956, 21, 483. (e) Harber, Wm. I. Iowa State Coll. J. Sci. 1940, 15, 13. (43) The DSC peak at ca. 45 °C appears to be due to a solid-solid transition of a small amount of another substance formed during the initial heating of 5c. Compound 9c prepared by refluxing 5c in toluene lacked this endotherm. Moreover, the POM of 5c heated thrice to 125 °C showed no changes near 45 °C and melted at 94.1-95.2 °C. (44) Melting points of 95-96 °C (ref 42d) and 88-89 °C (ref 44a) have been reported for N,N′-dioctadecylthiourea. (a) McKay, A. F.; Garmaise, D. L.; Gaudry, R.; Baker, H. A.; Paris, G. Y.; Kay, R. W.; Just, G. E.; Schwartz, R. J. Am. Chem. Soc. 1959, 81, 4328. (45) Spectrometric Identification of Organic Compounds; Silverstein, R. M., Bassler, G. C., Morrill, T. C., Eds.; John Wiley & Sons: New York, 1991; Chapter 3. (46) Peak assignments are based on IR spectra of dithiocarbamate salts (ref 46a,b). (a) Sceney, C. G.; Magee, R. J. Inorg. Nucl. Chem. Lett. 1974, 10, 323. (b) Nakamoto, K.; Fujita, J.; Condrate, R. A.; Morimoto, Y. J. Chem. Phys. 1963, 39, 423.
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Table 2. Gelation Abilities of 2-5 in Different Liquidsa liquid
2b (ref 11) 2 wt %
hexane
P
n-octane
P
silicone oil
TGb
ethanol
S
1-butanol
S
1-octanol
S
benzyl alcohol
S
DMSO
P
toluene
P
CCl4
P
2c (ref 11) 2 wt %
5 wt %
OGe (54-56) P OGc (56-60) TGc TGc (59-60) (80) P OGc (54-56) P OGc (49-50) P OGc (40-42) TGd TGc (44) (53) TGc TGc (74-76) (90-92) TGc TGc (47-48) (56) P OGc (40-42) P
3c
4b
4c
2 wt % 5 wt % 2 wt % 5 wt %
5b
5c
2 wt %
5 wt %
2 wt %
5 wt %
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
PGf (∼0) P
P
P
P
P
P
P
PGf (∼0) P
OGh (63) TGh (62) P
TGf (62) P
TGi (68) P
OGf (72-78) TGb TGb (57-61) (88) P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
S
S
P
P
P
P
P
PGg TGh (19-22) (28-30) P P
S
S
P
P
P
P
P
P
P
P
P
P
P
P
P
TGf (46)
P
P
P
TGh (46) TGh (61)
2 wt % 5 wt %
P
OGi (62) OGi (63) TGi (61) TGj (71-76) PGf (17 months. d Stable for >17 months; pale yellow color developed after 5 months. e Phase separation in 3 months. f Stable for 2 weeks. g Stable for 2 months. g Stable for 12 months. j Stable for >12 months; pale yellow color developed after 6 months.
Figure 4. IR spectra of neat 5c (a), of a 5 wt % 5c in silicone oil gel (b), of a 5 wt % 5c in silicone oil gel after heating to 110 °C (c), and of neat 5c after heating to 125 °C (d). The peak at 938 cm-1, indicated by arrows in (a) and (b), is absent in (c) and (d), and a new peak at 1361 cm-1 from 5c that had been heated to 110 °C is indicated by arrows in (c) and (d).
appear at 3237 cm-1 45 and at 1575 and 1471 cm-1,47,48 respectively, after heating the sample three times to 125 °C and cooling. The IR spectrum of the heated sample (mp 94.1-95.2 °C) and that of 9c prepared in refluxing toluene (mp 95.1-95.8 °C) are very similar. Gelation Properties. Of the amines, only 1-octadecylamine 1c formed stable room-temperature gels at 2 wt % and only in silicone oil and DMSO.11 The gelating ability of 2 and 5 wt % of the salts derived from 1a-c and the triatomic molecules in a variety of organic liquids is summarized in Table 2. The minimum LMOG concentration required for gelation has not been determined for the (47) (a) Jensen, K. A.; Nielsen, P. H. Acta Chim. Scand. 1966, 20, 597. (b) Infrared Determination of Organic Structures; Randall, H. M., Fowler, R. G., Fuson, N., Dangel, R., Eds.; Van Nostrand: New York, 1949. (c) Gosavi, R. K.; Agarwala, U.; Rao, C. N. R. J. Am. Chem. Soc. 1967, 89, 235. (48) (a) Yamaguchi, A.; Penland, R. B.; Mizushima, S.; Lane, T. J.; Curran, C.; Quagliano, J. V. J. Am. Chem. Soc. 1958, 80, 527. (b) Rao, C. N. R.; Venkataraghavan, R.; Kasturi, T. R. Can. J. Chem. 1964, 42, 36.
Figure 5. DSC thermograms of a gel consisting of 5 wt % 5c in silicone oil: first (a), second (b), and third (c) heating (to 110 °C) and cooling cycles. The arrows indicate the direction of temperature change.
salts. However, Tg of a 1 wt % 2c/silicone oil gel is 48 °C, and it has been stable for >17 months; at 0.5 wt % of 2c, a gel forms only below room temperature. Evidence for aggregation of the carbamates 2 prior to nucleation and gelation has been obtained from NMR studies.11a,49 Such aggregation processes must also occur with the salts from the other three triatomic molecules. Generally, the salts from propylamine (1a) were poor LMOGs. A partial gel was formed by 2 wt % 3c in silicone oil, and 3a and 3b did not gel any of the liquids examined. The shorter amine-SO2 adduct, 4b, was also not an LMOG, but the longer 4c salt was able to gel some of the liquids, albeit with low Tg values and temporal stabilities at room temperature. Most of the salts derived from 1-octadecylamine 1c gel liquids more efficiently than their shorter homologues.
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Figure 6. Polarizing optical micrographs (room temperature) of 5 wt % (a) 2b, (b) 2c, (c) 5b, and (d) 5c gels in silicone oil. The black space bars are 100 µm. The images were taken with a full-wave plate.
At concentrations of >2 wt %, both 2c and 5c gel several liquids such as n-octane, alcohols, DMSO, and CCl4 with remarkably higher stability and Tg values as indicated in Table 2. Whereas gels from 4c separated macroscopically into a solid and a liquid phase after 2 weeks, several of the 5c gels at equal weight concentrations and in the same liquids have been stable for >12 months. Octadecylammonium octadecylcarbamate (2c) at 5 wt % is able to gel all the liquids in Table 2 and with very high Tg values. With the exception of the hexane gel, they are stable for more than 17 months. As mentioned above, neat samples and toluene solutions of the alkylammonium alkyldithiocarbamates (5a-c) appear to form the corresponding thioureas (9a-c) when heated above characteristic temperatures. Since (1) their gels with silicone oil are retained when heated to slightly above the same temperatures and (2) infrared spectroscopy confirms that 9 is being formed (Figure 4), the thioureas are also organic gelators. Although IR spectra of the 9 gel samples are complicated by the signals from silicone oil, the formation of the C band48 of thiourea at 1360 cm-1 and the disappearance of the peak at 938 cm-1 (characteristics of 5c) in the heated gels are clearly discernible. The Tg value of the gel is lowered after the first heating but is constant thereafter (Figure 5). The endothermic transition at 88.8 °C (∆H ) 7.0 J/g) in the first heating is assigned to the melting/decomposition of the network; the melting endotherm occurred at 82.0 °C (∆H ) 6.2 J/g) in the second heating. A broad peak at 19.5 °C (∆H ) 1.7 J/g) in the second heating thermogram is from a solid-solid phase transition of 9c. Heating silicone oil gels of 5b also led to formation of a thiourea, 9b. Supporting Figures 7 and 8 show the IR spectra of 5b before and after being heated and the corresponding heating and cooling thermograms. The Tg values of silicone oil gels of 5b and 5c decrease between
the first and second heating thermograms, but not thereafter. For example, gels of 5 wt % 5b and 5c in silicone oil exhibited an endotherm at 69.4 °C (∆H ) 11.52 J/g) and 88.8 °C (∆H ) 7.0 J/g), respectively, during the first heating and at 57.4 °C (∆H ) 6.3 J/g) and 82.0 °C (∆H ) 6.2 J/g), respectively, during the second. Polarizing Optical Microscopy. Optical micrographs (OMs) of 5 wt % gels containing 2b, 2c, 5b, or 5c in silicone oil are shown in Figure 6. Elongated and strandlike aggregates are seen in the images of 2c, 5b, and 5c. Consistent with its lower Tg value, 55 °C, the 2b gel has shorter aggregates. Although individual stands are difficult to discern within bundles at the magnifications in Figures 6 and 7, strand lengths are clearly in the 10-100 µm range. The appearances of strands in the 5c gel are consistent with their higher dissolution temperatures and the higher efficiency of 5c as a gelator as compared to any of the other LMOGs in Scheme 1. Qualitatively, the aspect ratios of the aggregates in optical micrographs of the silicone oil gels with 5 wt % 9b and 9c (Figure 7) and the corresponding gels of 5b and 5c correlate with the relative Tg values. X-ray Diffraction. The diffraction peaks of the LMOG assemblies of 2, 4, and 5 were obtained by subtracting the “amorphous scatter” of the liquid components from the total gel diffractograms.50 They were compared with diffraction patterns of the neat salts (Figures 8-10 and Supporting Figures 9-13 and 15-19). Diffraction patterns of 3c, 4b (Supporting Figure 14), and salts derived from 1a were not examined in detail. Despite the weakness of some peaks from gels of carbamates 2b and 2c, it is clear that most coincide with peaks of the neat powders; the molecular packing arrangements of the gel assemblies and the bulk crystals
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Figure 7. Polarizing optical micrographs (room temperature) of 5 wt % (a) 9b and (b) 9c gels in silicone oil. The black space bars are 100 µm. The images were taken with a full-wave plate.
Figure 8. X-ray diffraction patterns (room temperature) of 2c: (a) powder; (b) 5 wt % gel in silicone oil; (c) neat silicone oil; and (d) diffractogram b subtracted from diffractogram c.
are the same.13,50 Unfortunately, we have been unable to obtain single crystals of these LMOGs and perform singlecrystal X-ray analyses that would determine the molecular packing in the gel strands. Aggregates of 4c, 5b, and 5c in gels with several liquids are a different morph from that of their neat powders (Figures 9 and 10 and Supporting Figures 16 and 17). In fact, it is more common for LMOGs in gel assemblies and their neat solids to pack differently.2,50-54 Diffractograms of all of the LMOGs as neat powders and in gels possess a low-angle peak (and, in some cases, higher order diffractions of it) that is indicative of lamellar organizations within the aggregates. The layer thicknesses (d) have been calculated from the low-angle peaks using Bragg’s law (Table 3). The d values of the neat solid and gel assembly of each LMOG are the same even when the high-angle peaks demonstrate that they are different morphs; the organizations within layers differ. The diffractogram of the 5 wt % 5c in 1-butanol gel appears to include some of the morph of the neat solid that may or may not be a part of the gelator assembly (Supporting Figure 17). This is the only case for which the coexistence (49) George, M.; Weiss, R. G. To be published. (50) Ostuni, E.; Kamaras, P.; Weiss, R. G. Angew. Chem., Int. Ed. Engl. 1996, 35, 1324. (51) (a) Lin, Y.-C.; Ph.D. Thesis, Georgetown University, Washington, DC, 1987. (b) Lin, Y.-C.; Kachar, B.; Weiss, R. G. J. Am. Chem. Soc. 1989, 111, 5542. (c) Lin, Y.-C.; Weiss, R. G. Macromolecules 1987, 20, 414. (52) (a) Mukkamala, R.; Weiss, R. G. Chem. Commun. 1995, 375. (b) Mukkamala, R.; Weiss, R. G. Langmuir 1996, 12, 1474. (53) Lu, L.; Weiss, R. G. Langmuir 1995, 11, 3630. (54) Furman, I.; Weiss, R. G. Langmuir 1993, 9, 2084.
of more than one morph in a gel can be documented. The POM of each of the gels for which the diffraction pattern matches that of the neat solid provides no evidence for nonfibrillar LMOG crystallites; if they are present, they must be less than ca. 10 microns in size. From this and the presence of fibrillar assemblies (e.g., Figure 6), we are confident that the diffraction patterns are from gelator networks. In all cases, the d values are shorter than the calculated lengths of the LMOGs in their extended linear conformations.55 Partial interdigitation of extended molecules in orthogonally packed layers, as shown in Figure 11a, is possible but unlikely because the cross-sectional areas of the amine-based headgroups and n-alkyl chains are very similar. Bent conformations along the alkyl chains are also unlikely within the neat solid phases and those of the gel fibrils. Alternatively, the long axes of the molecules may not be tilted as in Figure 11b and/or the positively and negatively charged parts of each molecule may not be collinear (as in Figure 11c). Assuming an arrangement like that in Figure 11b, the tilt angles have been calculated using the lamellar spacings in Table 3. Our data do not allow a clear distinction among these models or, for that matter, others that may include some of the features of each. Diffractograms of 9b and 9c indicate that the same lamellar morph is responsible for their neat phases and gelator networks (Supporting Figures 18 and 19). Furthermore, their d values are in good agreement with the tip-to-tip lengths calculated for these bent molecules55 (Table 3 and Figure 11d). The layers do not appear to be interdigitated. Unlike the salts 5a-c, packing of thioureas 9a-c must depend in large part on intermolecular H-bonding interactions. The greater directional specificity of their H-bonds may be responsible for the lack of interdigitation by the lamellae of 9. A Model for Stepwise Gel Formation. In other studies with salts of 2, we have used NMR spectroscopy to probe the initial steps of aggregation leading eventually to gel networks.11a,49 A reasonable model for gel formation from a (dispersed) solution starts with aggregation of small numbers of salt molecules leading to inverted micellelike structures, followed by the appearance of larger aggregates (sols). Eventually, as the liquid molecules are expelled, nucleation occurs and the aggregates reorganize into microcrystallites with large aspect ratios. Since the (55) Calculated by the Hyperchem (version 5.1) molecular modeling system at the PM3 level, adding the van der Waals radii of the terminal atoms.
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Table 3. Lamellar Spacings (d in Å) and Potential Tilt Angles (Degrees, See Text) from the Lowest Angle Peaks in X-ray Diffraction Patterns of Powders and Gels of 2-5 and 9 and Extended Molecular Lengths from Calculationsa 2
3
d LMOG
calcdb
X-ray
tilt angle
a b c
17.4 34.9 55.0
13.6 31.8 52.4
39 24 18
a
4
d
b
calcdb
55.0
5
d X-ray
9
d
d
tilt angle
calcdb
X-ray
tilt angle
calcdb
X-ray
tilt angle
calcdc
X-ray
55
35.1 55.1
26.1 38.4
42 46
18.1 36.0 55.8
12.2 28.3 46.6
48 38 33
12.7 27.8 45.1
11.2 27.6 47.3
31.8
c
Reference 55. See Figure 11a-c. See Figure 11d.
Figure 9. X-ray diffraction patterns (room temperature) of 4c: (a) powder; (b) 5 wt % gel in benzyl alcohol; (c) neat benzyl alcohol; and (d) diffractogram b subtracted from diffractogram c.
Figure 10. X-ray diffraction patterns (room temperature) of 5c: (a) powder; (b) 5 wt % gel in silicone oil; (c) neat silicone oil; and (d) diffractogram b subtracted from diffractogram c.
salts precipitate from nongelled liquids as powders whose aspect ratios are much smaller, we suggest that the sols consist of elongated aggregates that must be layered as well and analogous in some aspects to giant wormlike micelles.56 The point in this process at which the liquid molecules are expelled is unknown at this time. However, this expulsion is crucial to the development of crystallites and their assembly into three-dimensional networks capable of immobilizing macroscopically the liquid components. Although the X-ray results indicate that molecules of the liquid are largely excluded from the crystalline gel networks, they must play a very important role in the assembly of strands and their disassembly when gels are heated. Conclusions Primary amines can be converted very efficiently to salts by reaction with the quadrupolar triatomic molecules, CO2, SO2, NO2, and CS2. In this way, the uncharged amines (56) (a) Lequeux, F.; Candau, S. J. In Theoretical Challenges in the Dynamics of Complex Fluids; McLeish, T., Ed.; NATO ASI Series, Series E, Vol. 339; Kluwer Academic: Dordrecht, The Netherlands, 1997; p 181. (b) van der Schoot, P.; Wittmer, J. P. Macromol. Theory Simul. 1999, 8, 428. (c) Aliotta, F. Trends Phys. Chem. 1997, 6, 31.
are converted to ionic species without addition of Brønsted acids. The structure of the products depends on the nature of the triatomic molecule added; CO2 and CS2 give alkylammonium carbamates and dithiocarbamates, respectively. The complexes with SO2 are pyrosulfites and small amounts of alkylthionylamines.38 The structures of the amine-NO2 adducts have not been investigated here, and we have not found them in the literature. In general, the salts are better gelators than their precursor amines.11a A likely principal reason for this improvement is believed to involve the strengthening of intermolecular interactions near the headgroups: London (dispersive) interactions and H-bonding are the strongest forces holding together the headgroups of the primary amines; electrostatic interactions allow the headgroups of the salts to associate more strongly. The efficiency of the salts as LMOGs depends primarily on the nature of the triatomic molecule added to an amine and the length of the alkyl chain: alkylammonium alkylcarbamates are better LMOGs than the salts from the other triatomic molecules, and the efficiency within one family of salts increases with increasing alkyl chain length. The latter may be due to tighter cation-anion interactions when the ionic centers of a salt are more shielded from stabilizing (i.e., polar or protic) molecules of the liquid component by the longer alkyl chains and to their larger London dispersion forces. Since the alkylammonium alkylcarbamate salts expel their triatomic molecular adduct, CO2, when heated to melting points above ca. 80 °C, melting and reversion to alkylamines are linked processes. That is not the case for the other three classes of salts investigated here. Heating the dithiocarbamates and SO2 and NO2 adducts to somewhat above their melting temperatures results in formation of new species that are not the alkylamines. However, heating these salts only to their melting temperatures does not initiate an irreversible chemical change. Hence, the measured Tg values derive only from dissociation of aggregates of salt molecules. The phenomena described here can also be thought of as a way for amines to sequester quadrupolar triatomic molecules, and the transformation from solution to gel constitutes a visual sensor for the uptake (and, therefore, the presence) of these triatomic molecules. In fact, the transformation of solutions of primary amines and subsequent formation of gels may be useful to detect and absorb undesirable gases from petroleum, coal, or natural gas.58 We have demonstrated that polyamines are able to perform this task as well.58 In addition, a pH-sensitive system based on latent gelators of carboxylic acids in which the active gas is ammonia, an amine, or another type of base may be possible. Acknowledgment. We thank Dr. Veeradej Chynwat for assistance in obtaining the NMR spectra and Professors (57) (a) Davini, P. Carbon 2001, 39, 2173. (b) Hercilio, R.; Xiomara, G. Acta Cient. Venez. 1999, 50, 54. (58) Carretti, E.; Dei, L.; George, M.; Baglioni, P.; Weiss, R. G. To be published.
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Figure 11. Three possible packing arrangements for LMOGs 2 and 5 in gel aggregates (a-c), the energy minimized structure (ref 55) of 9b (d), and distances d corresponding to the low-angle X-ray diffraction peaks. The general features of the packing arrangements of 3 and 4 are assumed to be like those of 2 and 5.
Travis Holman and C. L. Khetrapal for helpful discussions. We are grateful to the National Science Foundation and the Petroleum Research Fund (administered by the American Chemical Society) for their support of this research. Supporting Information Available: Temperatures (°C) and heats of transitions (J/g) of neat salts 2-5 measured by
DSC; DSC thermograms of 4c, 5c, and 9c powder samples; DSC thermograms of neat 5c and its 5 wt % gel in silicone oil; IR spectra of neat 5b and its 5 wt % gel in silicone oil before and after DSC; XRD patterns of several gels and neat powder samples; and MS and NMR spectra of thioureas 9. This material is available free of charge via the Internet at http://pubs.acs.org.
LA026639T
