chemically reversible organogels via “latent” gelators ... · chemically reversible organogels...

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, organogelscomprised of a primary or secondary aliphatic amine (1) and an organic liquid leads to in situ chemicaltransformation to the corresponding alkylammonium alkylcarbamate (2) based gels. Chemical reversibilityis demonstrated by removal of CO2 from 2-based gels upon gentle heating in the presence of nitrogen. Thisis a general strategy for reversible self-assembly or disassembly of molecular aggregates relying on theinitiation or termination of ionic interactions. The dependence of the amine structure and the nature ofthe liquid component on the formation and stability of the 1 and 2 organogels are examined by differentialscanning calorimetry, optical microscopy, and X-ray diffraction methods. In most cases, the 2 gelators aremore effective (based on the minimum gelator concentration required at room temperature, the gelationtemperature, 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 attributedto the presence of ionic interactions between molecular segments of the alkylammonium alkylcarbamatesthat are stronger than the hydrogen-bonding interactions available between molecules of amines. Theinitial stages of aggregation in the gel assemblies (i.e., changes in the degree of aggregation of sols of some2 gelators) have been examined as a function of concentration and temperature by NMR techniques.

Introduction

The last several years have witnessed an enormousincrease of interest in thermally reversible organogelscomprised of (usually) j2 wt % of a low molecular massorganic gelator (an LMOG) and an organic liquid.1-10 Thesegels are microheterogeneous phases that self-assemble ina wide variety of modes with structures expressed fromthe molecular to the micrometer distance scales. Whensols or solutions of these systems are cooled below theircharacteristic gelation temperature (Tg), the LMOGsaggregate into fibers, strands, tapes, etc., that join at“junction zones”6 to form networks that immobilize theliquid component, primarily by surface tension.2 A modelto describe the stages of aggregation has been presentedrecently.11 Since the gelator concentration is usually verylow, there need be no specific liquid-gelator interactionson the molecular scale.

Most LMOGs have complex molecular structures,frequently with both lyophilic and hydrophilic or polar

regions and several functional groups. The unusualstructural and diffusional properties of organogels haveled to several interesting applications.12-17 An exceedinglybroad range of organic liquids (including quasi-liquidssuch as supercritical CO2

18) has been gelled, and verydiverse types of LMOGs (including two-component sys-tems that act via specific H-bonding interactions7 or singlespecies whose structures can be salts to multifunctionalmolecules or even simple long-chained n-alkanes19,20) areknown.1,21,22

Long-chain aliphatic amines are known to gel a varietyof organic liquids.8,20,21,23 Recently, we discovered that somealkylammonium alkylcarbamates, formed in situ andreversibly from the corresponding amines by the rapiduptake or loss of carbon dioxide gas,24-27 are LMOGs,

* Corresponding author. E-mail: [email protected]. FAX:202-687-6209.

† This article is part of the special issue of Langmuir devoted tothe 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.

(12) Derossi, D.; Kajiwara, K.; Osada, Y.; Yamauchi, A. Polymergels: Fundamentals and Biomedical Applications; Plenum Press: NewYork, 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.

7124 Langmuir 2002, 18, 7124-7135

10.1021/la0255424 CCC: $22.00 © 2002 American Chemical SocietyPublished on Web 06/13/2002

also.28 Their ability to gel organic liquids depends on thenature of the alkyl group(s) and whether the precursoramine is primary or secondary.20,28 Here, we report ingreater detail the gelation properties of a wider varietyof selected primary and secondary amines (1) and theiralkylammonium alkylcarbamates (2) (Scheme 1). Theprocess that transforms the 1-based organogels to (andfrom) the 2-based ones is novel since it involves chemical(as well as) thermal reversibility.29 Several of the aminesinvestigated are “latent” LMOGs because they, alone, donot form gels with a variety of liquids that are gelled ratherefficiently by the corresponding 2. The transformationbetween solution (or sol) and gel in these cases is effectedonly by the nature of the gas bubbled through thecondensed phase.

This is a general strategy for reversible self-assemblyor disassembly of molecules based on the initiation ortermination of ionic interactions. It should be applicableto formation of many other aggregates besides thoseresponsible for gelation. It is a completely differentphenomenon than the gelation of supercritical CO2 (asthe liquidcomponent).18 Inaddition, thegelationprocedureoffers a convenient, rapid, and efficient method to se-quester (reversibly) and sense the presence of atmosphericCO2.30

Experimental SectionMelting points (corrected) were measured on a Leitz 585 SM-

LUX-POL microscope equipped with crossed polars, a Leitz 350heating stage, and an Omega HH503 microprocessor thermom-eter connected to a J-K-T thermocouple. IR spectra were obtainedon a Perkin-Elmer Spectrum One FT-IR spectrometer interfacedto a PC. NMR spectra (referenced to internal TMS) were recordedon a Varian 300 MHz spectrometer connected with a variable-temperature controller and interfaced to a Sparc UNIX computerusing Mercury software. Samples were equilibrated at eachtemperature for 5 min prior to recording spectra. Low and high

temperatures were calibrated with the chemical shift of the OHpeak of methanol and ethylene glycol, respectively.31

Materials. Silicone oil (tetramethyltetraphenylsiloxane, Dowsilicone oil 704) was used as received. Other liquids for thepreparation 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,N-dioctadecylamine (>99%) from Fluka were used as received.1-Hexadecylamine, 1,12-diaminododecane, and N-methyl octa-decylamine from Aldrich were recrystallized from chloroformunder a nitrogen atmosphere. 1-Octadecylamine (Aldrich) wasdistilled twice under vacuum and stored under a nitrogenatmosphere. Alkylammonium alkylcarbamates (2) were preparedby passing CO2 gas through a hexane solution (1-decylamineand N,N-dioctylamine) or chloroform solution (other amines) for15 min. The precipitates were filtered and dried. Melting pointsof amines and their alkylammonium alkylcarbamates arereported in Supporting Information. Decylammonium chloridewas prepared by bubbling dry hydrogen chloride gas through ahexane 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 precipitatedproduct was washed with chloroform (to remove any decylam-monium decylcarbamate that might have been formed) anddried: 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 ) 6.8Hz), 1.42 (2H, m), 1.27 (14H, s), 0.88 (3H, t, J ) 6.8 Hz) ppm.

Preparation of Gels. Liquid components were saturated withN2 gas by bubbling for 10 min prior to use. Weighed amounts ofa liquid and an amine or alkylammonium alkylcarbamate wereplaced into glass tubes (5 mm i.d.) that were flame-sealed inmost cases (to avoid evaporation). The sealed tubes were twiceheated in a water bath (until all solid material had dissolved)and cooled rapidly under tap water to ensure homogeneity.

Gelation Temperatures. Gelation temperatures (Tg) weredetermined by the inverse flow method33 (i.e., the temperatureat which a gel fell under the influence of gravity when invertedin a sealed glass tube that was placed in a thermostated waterbath). Tg values and heats of melting (∆Hg) of gels with siliconeoil as the liquid were also determined by differential scanningcalorimetry (DSC) using a TA 2910 differential scanningcalorimeter interfaced to a TA Thermal Analyst 3100 controllerequipped with a hollowed aluminum cooling block into whichdry ice was placed for subambient measurements. Thermalgravimetric analysis (TGA) measurements were performed ona TGA 2050 thermogravimetric analyzer (TA Instruments)interfaced to a computer. Heating rates were 5 °C/min; coolingrates were variable and depended on the difference between thecellblock 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 wererecorded on a Leitz 585 SM-LUX-POL microscope equipped withcrossed polars, a Leitz 350 heating stage, a Photometrics CCDcamera interfaced to a computer, and an Omega HH503microprocessor thermometer connected to a J-K-T thermocouple.

X-ray Diffractograms. X-ray diffraction (XRD) data ofsamples in thin capillaries (0.5 mm diameter; W. Muller,Schonwalde, FRG) were collected on a Rigaku R-AXIS imageplate system with Cu KR X-rays generated with a Rigakugenerator operated at 46 kV and 46 mA. Gel samples wereprepared by flowing hot (T > Tg) aliquots into the capillary andsealing both its ends. The samples were then cooled under runningwater. Data processing and analyses were performed usingMaterials Data JADE (version 5) XRD pattern processing.34

Molecular Calculations. Molecular calculations were per-formed using the HYPERCHEM package, release 5.1 Pro forWindows from Hypercube, Inc. Lowest energy geometries wereoptimized using the Parametric Method 3 (PM3) semiempiricalmethod.35

(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 ofcupric 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.

Scheme 1

Chemically Reversible Organogels Langmuir, Vol. 18, No. 19, 2002 7125

Results and DiscussionsGel Formation and Stability and Interconversion

of LMOGs by in Situ Chemical Reaction. Scheme 1includes the amines and corresponding alkylammoniumalkylcarbamates investigated as LMOGs. Several generaltrends are clear from the data summarized in Table 1:most of the 2 gel more of the liquids for longer periods(when kept in sealed vessels at room temperature) andprovide higher Tg values than equivalent concentrationsof the corresponding 1. Figure 1 shows the variation of Tgwith respect to gelator concentration for 1e and 2e insilicone oil. On a macroscopic scale, gelation was consid-ered successful if no sample flow was observable uponinverting the tube. Although the influence of cooling rateon the type and stability of these gels was not investigatedsystematically, samples cooled more slowly were usuallyless stable than those prepared by the above method. Inmost cases, the gel transition occurred over a narrow (ca.1 °C) range. The validity of DSC information from gelswith the more volatile liquids is compromised because ofrapid evaporation at elevated temperatures; for thatreason, most quantitative studies have focused on siliconeoil as the liquid.

The 1 f 2 conversion and consequent differences in gelproperties have been demonstrated by bubbling CO2

through an amine sol/solution (i.e., in a liquid that is notgelled by 1) for several minutes. When the gels of 2 arethen heated slightly (to accelerate loss of CO2) while N2is bubbled through (to ensure displacement of dissolvedCO2 from the liquid component and, thereby, avoidreformation of 2 on cooling), sols/solutions containing 1are obtained anew. The gelation/degelation cycle, initiatedby sequential bubbling of CO2 and N2 gases, can berepeated many times without detectable degradation ofthe system. When gels of 2 are heated in a closed vessel(so that the liberated CO2 can recombine with 1) and thencooled, gels characteristic of the presence of 2 arere-formed.

The appearance of bubbles, presumably from expelledCO2, when a neat 2 is heated rapidly to g100 °C for severalminutes is additional qualitative evidence for the 2 f 1conversion. Quantitative evidence was obtained from theweight loss measured by thermal gravimetric analysiswhen 2e was heated (Supporting Figure 1). A transitionnear 79 °C is accompanied by loss of 7.3% sample weight,in excellent agreement with 7.5% calculated for loss ofCO2. This observation, the lack of a second weight losscorresponding to the evaporation of water (calcd 3.1% forone water molecule per molecule of 2e) below 140 °C, andthe similarity between DSC thermograms of silicone oilgels of 1 and those of the corresponding 2 (after beingheated initially to temperatures at which CO2 is lost;Supporting Figure 2) make unlikely the active participa-tion of water molecules, potentially present in the bulksolids and gel assemblies, in the solid networks. Above ca.150 °C, another larger weight loss is observed fromevaporation of the amine 1e that is produced during the79 °C weight loss. Samples of 2e that are heated to ca. 160°C, cooled, and reheated repeatedly give DSC thermo-grams whose temperatures of transition correspondexactly to those of 1e, but whose heats are lower aftereach cycle.

The stability of the gels is assessed on the basis of threedifferent criteria: gelation temperature (Tg), the concen-tration of LMOG necessary to form a gel at roomtemperature, and the period over which a gel phase isstable at room temperature in a sealed vessel. At 2 wt %,amines with low melting points, like 1a and 1f, dissolve

Table 1. Stability Parameters and Appearancesa of Gels of 2 wt % 1 or 2 in Various Liquids

a b c d e f g h i

liquid 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2

hexane S P P P P S P P P TGe

(23)TGe

(22)P P

n-octane S P P P P S P P P TGe

(38)PGb

(<0)PGb

(<0)P

silicone oil PGb

(<0)TGb

(21)PGd

(∼0)TGd

(30)PGf

(11)TGd

(42)TGc (28) TGg

(56-60)TGb

(25)TGg

(59-60)PGb

(<0)TGb

(48)TGc

(36)TGg

(44)TGg

(55)TGg

(54)TGb

(35)PGb

(∼0)ethanol S S P P P S S S PGb

(15)TGe

(30)TGe

(31-36)P P

1-butanol S S P P P S S S S TGe

(38)TGe

(41)P P

1-pentanol S S S S S PGb

(<0)S PGb

(<0)S PGe

(<0)S S S S TGe

(36-38)TGe

(36-39)P P

1-octanol S S S S S PGb

(∼0)PGc

(<0)PGc (<0) P P S S S S TGg

(33-34)PGb

(<0)S P

benzyl alcohol S S S S S PGb

(<0)S PGc

(<0)S TGg

(44)S S S S TGg

(48-50)P S P

toluene S P S P P TGg

(47-48)S S S S TGe

(27)TGe

(25)S P

nitrobenzene S S S S S S PGb

(15)S S P

DMSO P P P TGe

(55)TGc

(27)TGd

(66-68)TGc

(32-40)TGc

(20)TGd

(50-52)TGd

(74-76)S P TGc

(42-43)P P P P P

CCl4 S P P P P P S S S P P P

a Gelation temperatures (°C) are in parentheses. See Scheme 1 for specific 1 and 2 structures. S, P, PG, TG, and OG indicate solution,precipitate, partial gel, turbid gel, and opaque gel, respectively. b Stable for 2 weeks. c Stable for 1 week. d Stable for >8 months. e Stablefor 2 months. f Stable for 1 month. g Stable for >10 months.

Figure 1. Tg values of silicone oil gels as a function ofconcentrations of 1e (O) and 2e (b). The lines have no intendedphysical meaning.

7126 Langmuir, Vol. 18, No. 19, 2002 George and Weiss

in most of the liquids in Table 1. They formed partial gelsonly in silicone oil. However, the corresponding alkylam-monium alkylcarbamates, 2a and 2f, form gels in siliconeoil whose Tg values are above room temperature and thatare stable for 2 weeks at room temperature. Alkylam-monium alkylcarbamates 2a-e are better LMOGs (i.e.,higher Tg values and persistence for longer periods) thanthe corresponding primary amines, 1a-e, and theirgelation efficiency increases with increasing alkyl chainlength. Although 2 wt % of 2d gelled about one-half of theliquids in Table 1, Tg values are relatively low and (exceptfor silicone oil) the periods of stability are rather short.At the same gelator concentrations and in liquids wheregels were formed by both 2d and 2e, those with the latterLMOG persisted for much longer periods at room tem-perature. Although 2 wt % of 2e was able to gel fewerliquids than 2d, it succeeded at higher concentrations.Table 2 summarizes the dependence of 2e concentrationon the appearance and Tg values of its gels with a varietyof liquids. At higher LMOG concentrations, these gels arestable for >7 months, except in hexane where the firstvestiges of phase separation were noted after 3 months.The overall greater stability of the assemblies of the 2than those of 1 and the increasing stability of theassemblies as the n-alkyl chains of 2 become longer can

be attributed to two principal factors: ionic interactionsbetween the cationic and anionic parts of the surfactantsalts 2 are stronger than the H-bonding intermolecularinteractions available to molecules of 1; London dispersionforces are larger between longer n-alkyl chains thanbetween shorter ones.

Whereas 2 wt % of the symmetrical secondary amine1f was able to form a partial gel only in silicone oil, Tg ofthe corresponding 2f gel was ca. 50 °C higher and itpersisted at room temperature for almost 3 weeks. Theunsymmetrical and slightly longer secondary amine 1gproduced gels with silicone oil, nitrobenzene, and DMSOthat persisted <1 week, and its alkylammonium alkyl-carbamate 2g was not an appreciably better gelator. Thesymmetrical secondary amine 1h is a better LMOG than1f or 1g since it forms gels in silicone oil that exhibitmoderately high Tg values and that are stable for severalmonths at room temperature (Table 3).20

Neither the diamine 1i nor its ammonium carbamate2i was an efficient LMOG; each gelled silicone oil poorlyand, additionally, 1i formed an unstable, partial gel withoctane at <0 °C. At 0.5-1.0 wt % concentrations, only 2icould be dissolved in boiling water; the solutions yieldedunstable, quasi-gels when cooled that may have contained1i at least in part. However, 2 wt % of 2i forms gels witha range of water/alcohol mixtures that are stable forseveral months and exhibit relatively high Tg values (Table4).

The dependence of Tg and periods of stability of gelledsilicone oil on the concentrations of 1 and 2 is summarizedin Table 3. Even at 5 wt %, the least efficient amineLMOGs, 1a and 1f, gelled only silicone oil and at ∼0 °C;the least efficient ammonium carbamate gelator, 2i, didgel silicone oil, but they underwent phase separation as

Table 2. Gelation Properties of 2e in Different Liquids. See Table 1 for Explanations of Termsa

wt % 2e hexane n-octane toluene silicone oil ethanol 1-butanol 1-octanol benzyl alcohol

0.5 P P P TG (4) P P P S1.0 P P P TG (48) P P P S2.0 P P TG (47-48) TG (59-60) P P P TG (44)3.0 OGb (51) OG (45-47) TG (52-55) TG (71-74) OG (45-47) PGc (<0) P TG (50)4.0 OGb (54-56) OG (54-55) TG (52-55) TG (79) OG (54-55) OG (40-43) OG (44-45) TG (50)5.0 OGb (54-56) OG (54-56) TG (56) TG (80) OG (54-56) OG (49-50) OG (40-42) TG (53)

a All gels are stable for >10 months unless indicated otherwise. b (a) Phase separation after 3 months. c (b) Phase separation after <1month.

Table 3. Gelation Temperature Values (°C) of Silicone Oil Gelsa

a b c d e ff g h iwt %gelator 1b 2 1 2 1 2 1b 2e 1 2d 1 2 1c 2 1e 2 1 2

0.5 <0 <0b ∼0 <0 ∼0b 4 <0 ∼0 26 ∼0c 42 40g <0b <0b

1.0 <0 ∼0b 12 ∼0 20b 48 <0 46 35 42c 51 46e ∼0b <0b

2.0 <0 21b ∼0d 30d 11b 42d 28 62-64 25b 59-60 <0 48 36 44e 55 54e 35b ∼0b

3.0 <0 49b 25 70-72 29b 71-74 <0 45 36 47e 60 58e 38b PSg

4.0 <0 52b 26 70-71 34c 79 <0 47 36 49e 63 60e 41b PSe

5.0 <0 55c 15c 71d 16c 73d 27 70-71 35d 80 ∼0 47 35 50e 60 58e 44e PSe

a All gels have turbid appearance. b Phase separation in <1 month. c Phase separation in 3 months. d Stable for >8 months. e Stable for>10 months. f Phase separation in 2 weeks. g Phase separation in 4 months. PS phase separated when heated to T < Tg.

Figure 2. Tg values of silicone oil gels with 2 wt % of primaryamines (4, O) or the corresponding ammonium carbamategelators (b, 2) by the falling drop method (4, 2) and from theheat flow maxima of DSC heating thermograms (O, b) versusthe length of the n-alkyl chain in carbon atoms.

Table 4. Gelation Temperatures of Gels Comprised ofWater/Alcohol Mixtures (vol/vol) and 2 or 5 wt % 2ia

Tg (°C)

liquid 2 wt % 5 wt %

EtOH/H2O (1:1) 62 80EtOH/H2O (3:1) 63 82tBuOHb/H2O (1:1) 45 76tBuOHb/H2O (3:1) 42 88

a All gels were opaque and stable for >6 months. b tert-Butylalcohol.


they were heated more rapidly than the Tg could bemeasured. Lower concentrations of the other 1 and 2yielded gels with relatively long periods of stability andhigh Tg values. As noted previously, Tg values are generallyhigher and periods of stability are usually longer for gelswith 2 than with an equal concentration of the corre-sponding amine.

Thermodynamic Properties of LMOGs and TheirGels. Melting and decomposition temperatures by POMand DSC and transition enthalpies by DSC for the neatLMOGs are included in Supporting Information (Sup-porting Table 1). Especially in those cases where the morphof 1 or 2 in the neat solid and assemblies in the gel is thesame (vide infra), comparisons of heats of melting withthose of heats of gelation (from heating scans) areespecially instructive. Unfortunately, large errors areintrinsic to measurements of the heats per gram-gelatorin the gels and they are exacerbated when the LMOG orliquid is a volatile substance. For these reasons, we havelimited data only for 1d, 1e, and 2e (Supporting Figures3 and 4). To avoid evaporation of the liquid component,DSC measurements of gels have been conducted only insilicone oil. We have confirmed that an inconsequentialamount of silicone oil is lost when its gels are heated tothe highest temperatures employed in our studies. The∆H values of the two amines in their gels approach thevalue of the neat solid at the highest concentrationsexamined, 3 and 5 wt %, respectively. At the sameconcentration of 2e, the ∆H values of its gels areapproaching the value of the neat solid but are stillincreasing with concentration. Both sets of data indicatethat a very small fraction of these LMOGs are dissolvedin silicone oil or become solubilized prior to melting of the

gel assemblies. This result is consistent with the narrowrange of Tg measured by the falling drop method.

Two transitions were evident from POM and DSC when2a was heated initially. On the basis of the temperature(∼61 °C) and low heat (∆H ) -26.5 J g-1), the firsttransition is ascribed to melting of the decylammoniumdecylcarbamate; the temperature (∼75 °C; note 79 °Cdecomposition temperature of 2e discussed above) andlarge heat (∆H ) -248 J g-1) of the second transition areconsistent with the 2a f 1a conversion. Due to thevolatility of 1a, no exotherm during cooling was detectablenor was a second heating scan possible. Similar behaviorwas observed when 2i was heated to above its decomposi-tion temperature because 1i is quite volatile, also.However, when 2b-2f were heated to temperaturesleading to formation of the higher melting (and lessvolatile) 1b-1f, exotherms from solidification of theamines were easily detected. DSC heating thermogramsof 1g appear to include a solid-solid-phase transition thatprecedes melting. The same two endotherms were presentduring the second heating thermograms of 2g (i.e., afterit had been transformed during the first heating to 1g).

The Tg values (from both falling drop and DSC methods)of 2 wt % gels of either the primary amines with one alkylchain or the corresponding 2 are plotted as a function ofchain length in Figure 2; see also Supporting Table 2. Thevalues appear to plateau when the chains reach 16 carbonatoms in length, but the experimental error in Tg and thelack of 1 and 2 with chains longer than 18 carbon atomsmake this conclusion somewhat speculative. Both methodsshow the same trend with increasing chain length. Formost of the gels, the Tg values measured by the fallingdrop method and by the maximum heat flow in DSCheating thermograms are comparable, especially at thehigher concentrations. Since the falling drop methodreports the temperature at which a three-dimensional gelnetwork breaks into pieces and the DSC method indicatesthe temperature at which the network melts, the twoprocesses must be occurring virtually concurrently at thehigher concentrations. At lower concentrations, where theLMOG networks are less extensive, the force of gravitycannot be resisted as well by the gels and Tg values by thefalling drop method are generally lower than by DSC. Insome cases,36,37 “junction zones”2 connecting strands areknown to “melt” at significantly lower temperatures thanthe strands, themselves.

The mole fraction (øg) of the LMOGs, 1e (a long-chainedprimary amine), 1h (a long-chained symmetrical second-ary amine), and their alkylammonium alkylcarbamates,2e and 2h, are plotted versus Tg

-1 for silicone oil gels inFigures 3 and 4 according to the Schroder-van Laarequation (eq 1). Although the assumption implicit in theseanalyses, that the gels form ideal solutions on heating,38

is not completely valid because sols are formed when thegels melt, the calculations provide useful insights regard-less. In this equation, ∆Hfus and Tfus are the enthalpy ofmelting of the gel assemblies and the melting temperatureof the neat gelator, respectively. The isomorphous natureof the packing arrangements of the neat solids and thegels of these gelators (vide infra) allows interestingcomparisons to be made between the values of ∆Hfus and∆H from DSC heating thermograms using the neat solids.We hypothesize that the nearer the two values, the smaller

(36) Fuhrhop, J.-H.; Helfrich, W. Chem. Rev. 1993, 93, 1565.(37) Fuhrhop, J.-H.; Svenson, S.; Boettcher, C.; Rossler, E.; Vieth,

H.-M. J. Am. Chem. Soc. 1990, 112, 4307.(38) (a) Atkins, P. W. Physical Chemistry, 5th ed.; Freeman, New

York, 1994; p 227. (b) Moore, W. J. Physical Chemistry, 4th ed.; PrenticeHall: Englewood, Cliffs, NJ, 1972; p 249.

Figure 3. Semilog plots of the mole fraction of 1e (b) and 2e(O) in silicone oil versus the inverse of gelation temperature.

Figure 4. Semilog plots of the mole fraction of 1h (b) and 2h(O) in silicone oil versus the inverse of gelation temperature.


the influence of the liquid component on the dissolution ofthe gelator network at Tg. The values of ∆Hfus of the amines1e and 1h calculated from eq 1 are near the ∆H values(Table 5). However, the ∆Hfus values of the corresponding2 are less than half as large as the ∆H from DSCdeterminations using the neat solids. The differences arelarger than can be rationalized on the basis of the relativelylarge errors in the measurements. Since it is unlikely thatthe alkylammonium alkylcarbamates are more solublethan the amines in silicone oil at room temperature (i.e.,that the disparity in heats can be attributed to a part ofthe 2 being dissolved in silicone oil prior to heating), thelower heats from the 2-based gels must be due tointeractions between the gelator molecules and the siliconeliquid at the time of melting and/or retardation of loss ofCO2. We suspect that the disparity between Tg values andthe true melting temperatures of the gel assemblies (thatmay not include loss of CO2 when the ammoniumcarbamates are in silicone gels) are larger for the 2-basedgels than for those employing 1; the Tg values of the 1 gels

are associated with strand melting to a greater extentthan those of 2.

Polarizing Optical Microscopy. Optical micrographs(OMs) of gels of 1e and 2e in silicone oil presented inFigure 5 are typical of those found from other silicone oilgels with primary amines and their alkylammoniumalkylcarbamates as LMOGs. As can be seen, aggregatesof the 2e gel are more elongated and strandlike than thoseof the 1e gel. Individual stands of 2e are difficult to discernwithin bundles at this magnification, but their lengthsare clearly in the 10-100 µm range. Although OMs of gelswith the more volatile liquids have not been investigatedextensively because rapid evaporation compromises thequality and reliability of the images, this trend is observedwith the other liquids also. (See, for instance, SupportingFigure 5.) The strand appearances (as well as the higherdissolution temperatures) of the 2e assemblies are con-sistent with their more efficient gelation than those of 1e.

The OMs of 1i and 2i in silicone oil (Figure 6) are uniquewithin the gels examined. The aggregates appear to betapes, ribbons, or (perhaps) tubules.39-43 Details of theirmorphology will be discussed elsewhere. However, the

(39) Zubarev, E. R.; Pralle, M. U.; Sone, E. D.; Stupp, S. I. J. Am.Chem. Soc. 2001, 123, 4105.

Figure 5. Polarizing optical micrographs (room temperature) of 2 wt % (a) 1e and (b) 2e gels in silicone oil. Black space bars are100 µm.

Figure 6. Polarizing optical micrographs (room temperature) of 4 wt % gels of (a) 1i and (b) 2i in silicone oil. Black space barsare 100 µm.

Table 5. Melting Enthalpies (J g-1) of Some NeatGelators from DSC Heating Thermograms (∆H) and from

Thermograms of Gels and the Schro1der-van LaarEquation (∆Hfus)

gelator ∆H (neat) ∆Hfus (from slope) ∆Hfus (from intercept)

1e 399 339 ( 24 320 ( 262e 398 133 ( 12 109 ( 131h 313 323 ( 10 253 ( 102h 345 141 ( 13 132 ( 14

ln øg )∆Hfus

RTg+

∆Hfus

RTfus(1)


previously noted higher Tg value of 1i than 2i gels insilicone oil (Table 1) may be related to the breadths of theaggregates; those of 2i are significantly narrower thanthose of 1i and both are much broader and longer thanthose of the other 1 and 2 in their gels.

NMR Investigations. 1H NMR spectra in chloroform-dsolutions of the amines can be explained on the basis offirst-order analyses, and spectra of 1a are independent ofconcentrationwithin the0.079-0.79Mrange investigated.1H and 13C NMR spectra of several alkylammoniumalkylcarbamates in chloroform-d, a nongelled liquid, areconcentration dependent and not easily interpreted. Aqualitative explanation of their spectra is presented hereusing data from 2a, the most soluble 2 from primaryamines (Figure 7; Supporting Figure 6); a quantitativetreatment of the data will be presented in the future.44

The 1H NMR spectrum of sodium dodecylcarbamate(recorded at a low concentration only) and the concentra-tion-independent 1H and 13C NMR spectra of decylam-monium chloride are the bases for the peak assignmentsthat follow. The R- and â-methylene protons of thecarbamate portion of 2a give rise to a triplet at 2.68 ppmand a multiplet at 1.45 ppm, respectively, whose positionsare independent of concentration. At concentrations lowerthan ∼0.06 M, the R- and â-methylene protons of theammonium portion are not detected; the â-methylenesignals may fall under the large methylene peak at ca. 1.3

ppm, but that does not explain the absence of theR-methylene signal. At the same concentration, 13C NMRspectra (see Supporting Figure 6) included no discerniblecarbonyl and only one R-methylene carbon peak at thechemical shift of the carbamate portion, ca. 41 ppm.

As the concentration of 2a is increased, 1H MMR spectradevelop a new triplet (from the R-methylene protons ofthe ammonium portion) that becomes larger while re-maining at 3.02 ppm, and a multiplet from the â-protonsthat both becomes larger and moves slightly downfield(from an initial value of ca. 1.47 ppm at above 0.06 M toa final one at ca. 1.56 ppm at 0.6 M). Concurrently, severalother spectral features change: the triplet from thecarbamate portion at 2.75 ppm decreases in intensity; thesum of the triplets at 2.75 and 3.02 ppm integrate for ca.four protons throughout but their ratio is altered; a broadsinglet remains at ca. 4.3 ppm while another moves rapidlydownfield. The singlets are assigned to the carbamateNsH proton and the ammonium NsH protons, respec-tively, based on their integration ratios, their loss whena drop of D2O is added to the solution, and nuclearOverhauser effect (NOE) experiments that lead to thedisappearance of each peak when the other is irradiated.At the higher concentrations of 2a, new peaks whosepositions are concentration independent appear in the13C NMR spectra at 163.2 (carbonyl carbon) and 41.9(carbamate R-methylene carbon). The position of the peakfrom the R-methylene carbon of the ammonium part shiftsto higher field with increasing concentration: at 0.062 M,δ ) 42.3 ppm; at 0.62 M, δ ) 40.29 ppm. Similar to thebehavior of the 3.02 and 2.75 ppm 1H triplets, the 13Cresonance of the R-methylene carbon of the ammoniumpart decreases in intensity and the one at 41.9 ppmincreases as concentration is increased.

(40) Terech, P.; de Geyer, A.; Struth, B.; Talmon, Y. Conference onSelf-AssembledFibrillarNetworks2001,Autrans,France,2001,Abstract34.

(41) Selinger, J. V.; Spector, M. S.; Schnur, J. M. J. Phys. Chem. B2001, 105, 7157.

(42) Zastavker, Y. V.; Asherie, N.; Lomakin, A.; Pande, J.; Donovan,J. M.; Schnur, J. M.; Benedek, G. B. Proc. Nal. Acad. Sci. U.S.A. 1999,96, 7883.

(43) Schnur, J. M.; Shashidhar, R. Adv. Mater. 1994, 6, 971.(44) George, M.; Weiss, R. G. To be published.

Figure 7. Concentration dependence of 1H NMR spectra of 2a in CDCl3 at room temperature: (a) 0.062 M, (b) 0.097 M, (c) 0.25M, and (d) 0.62 M. Inset is the expanded spectrum of b showing the broad peak at 4.3 ppm.


It is reasonable to associate the spectral changes uponincreasing concentrations to aggregation of 2a molecules;45

sols are able to form in chloroform although gels do not.Despite this conclusion, the absence of signals from theR-protons of the ammonium portion at low concentrationsand the loss of intensity of the signals from the R-protonsof the carbamate portion as concentration of2a is increasedare especially difficult to understand. Clearly, the motionsof groupsnear the ionic centers must bestrongly influencedby concentration changes in ways that are very dependenton the two halves of the molecule. In addition to inves-tigating aggregation at room temperature by increasingconcentration, we have recorded 1H spectra of 0.22 and0.062 M 2a in CDCl3 at several temperatures. The moreconcentrated 0.22 M sample is partially aggregated atroom temperature (Figure 8c) and J ) 7.0 Hz for the 3.02ppm triplet in the 1H NMR spectrum. As temperature islowered, the triplet gradually becomes a doublet ofdoublets (at -5 °C, J ) 7.0 Hz, J ) 12.5 Hz) (Figure 8d,Supporting Figure 7); the â-methylene protons are nolonger motionally averaged. As temperature is increasedto promote deaggregation, the 3.02 ppm signal is lost, the

2.75 ppm triplet loses intensity, and the NsH proton signalat 4.2 ppm is lost (Figure 8a-d). At 52 °C, the spectrumprovides no indications of aggregation (Figure 8a).

The more dilute 0.062 M 2a sample appears to beunaggregated at room temperature according to its 1HNMR spectrum (Figure 8e). However, even at 10 °C, 2amolecules are partially aggregated as indicated by theNsH singlets at 1.8 and 4.2 ppm and the characteristic3.05 ppm triplet (Figure 8f). As temperature was decreasedto -17 °C (the lowest temperature possible withoutprecipitation), the relatively sharp 1.8 ppm singlet shiftedto∼2.6 ppm and broadened, the triplet at 3.05 ppm becamea doublet of doublets, and the broad singlet at 4.2 ppmsplit into a triplet (Figure 8g). As temperature wasincreased, from -17 °C (spectra not shown), the lowerfield broad singlet moved upfield, and it and the peak at4.2 ppm disappeared finally at 55 °C. At the same time,the doublet of doublets at 3.02 ppm became a triplet andthen disappeared.

Consistent with the concentration-dependent dataobtained at room temperature, molecules of 2a aggregateas temperature is lowered and deaggregate as temperatureis raised. The aggregates in chloroform may be thenucleation sites from which the three-dimensional gelassemblies eventually grow in other liquids.11,45 At thispoint, the NMR evidence demonstrates that the ionic headsof the ammonium carbamate salts interact much morestrongly and specifically than their alkyl tails and suggestsan inverted micellar organization.

X-ray Diffraction Investigations. XRD patterns of1 and 2 as neat powders and in their gels have been

(45) (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) 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. (g) Waguespack, Y. Y.; Banerjee, S.; Ramannair, P.; Irvin,G. C.; John, V. T.; McPherson, G. L. Langmuir 2000, 16, 3036.

Figure 8. Temperature dependence of 1H NMR spectra of 0.22 M (a-d) or 0.062 M (e-g) 2a in CDCl3: (a) 52 °C, (b) 38 °C, (c)24 °C, (d) -5 °C, (e) 24 °C, (f) 10 °C, and (g) -17 °C. The region between 2.3 and 4.3 ppm of spectrum g is expanded in the inset.It covers only the peak from residual CHCl3 in other spectra.


compared. The diffraction peaks of the LMOGs wereobtained by subtracting the “amorphous” scatter of theliquid component from the total gel diffractogram.46 Sincethe diffraction patterns of most of the 1 and 2 in gels ofsilicone oil (as well as other liquids) are coincident withthose from the neat powders (e.g., Figures 9 and 10), themolecular packing arrangements responsible for the gelassemblies are the same as those in the bulk crystals.19,46

The positions of the liquid-subtracted diffraction peaks ofthe silicone oil gel with 2g corresponded to those of theneat powder, but the relative intensities within the twodiffractograms differed, indicating that the assembliesare oriented within the capillaries (Figure 11). In fact, itis more common for the gel and bulk solid morphs ofLMOGs to differ than for them to be the same.46

A low-angle peak in the patterns of many of the gels isconsistent with lamellar organizations of 1 and 2 withintheir strands. In many cases, the low angle (and other)peaks of amine gels were very weak even at 5 wt % gelatorconcentrations after 12 h of data collection. (See, forexample, Figure 12d.) However, the positions of thesepeaks could be ascertained reasonably well in amplifiedsolvent-subtracted traces (Figure 9). Diffraction patternsof the gels with 2 contained much stronger lowest anglepeaks that were easily discernible without amplification.Diffraction patterns of several other gelators and theirgels are included as Supporting Information.

The Bragg distances (d) of the low-angle peaks representthe thicknesses of layers in the lamellar assemblies. Theyand the calculated lengths of the fully extended aminesand alkylammonium alkylcarbamates are collected inTable 6.47 The calculated lengths of 1 in their extendedconformations are consistently somewhat more than one-half the layer spacings from the X-ray analyses. Thesedata and the structures of the molecules suggest that pairs

(46) Ostuni, E.; Kamaras, P.; Weiss, R. G. Angew. Chem., Int. Ed.Engl. 1996, 35, 1324.

Figure 9. X-ray diffraction patterns (room temperature) of1b: (a) powder; (b) 5 wt % gel in silicone oil; (c) neat siliconeoil; (d) diffractogram b subtracted from diffractogram c andamplified 5 times.

Figure 10. X-ray diffraction patterns (room temperature) of2b: (a) powder; (b) 5 wt % gel in silicone oil; (c) neat siliconeoil; (d) diffractogram b subtracted from diffractogram c.

Figure 11. X-ray diffraction patterns (room temperature) of2g: (a) powder; (b) 5 wt % gel in silicone oil; (c) neat siliconeoil; (d) diffractogram b subtracted from diffractogram c.

Figure 12. X-ray diffraction patterns (room temperature) of1h: (a) powder; (b) 5 wt % gel in silicone oil; (c) neat siliconeoil; (d) diffractogram b subtracted from diffractogram c.

Table 6. Lamellar Spacings (Å) from the Lowest AnglePeaks in X-ray Diffraction Patterns of Powders and Gels

of 1 and 2 and Extended Molecular Lengths fromCalculations

1 2

molecule structure of 1 calcd X-ray calcd X-ray

a C10H21NH2 16.6 a 34.9 31.8b C12H25NH2 18.5 32.5 40.0 37.1c C14H29NH2 21.0 36.6 44.0 42.0d C16H33NH2 24.1 47.5 50.0 47.3e C18H37NH2 26.0 45.4 55.0 52.4f C8H17NHC8H17 24.2 a 24.2 22.6g C18H37NHCH3 28.0 41.3 28.0b 40.8d

28.4c

h C18H37NHC18H37 49.3 49.0 49.4b 49.0e

49.2c

i H2NC12H24NH2 19.6 19.7d 20.3f 19.423.6g

22.0h

a Not detected. b Ammonium. c Carbamate parts. d Weak dif-fraction. e From neat powder; exceedingly weak diffraction fromgel samples; see Supporting Figure 16. f 1,12-Diammoniumdode-cane. g Dodecane-1,12-dicarbamate. h 1-Ammoniumdodecane-1-carbamate.


ofaminemoleculesdefine the lamellar thicknesses inhead-to-head (hydrogen-bonded) arrangements. In addition, thelong axes of the molecular pairs must be nonorthogonalwith respect to the lamellar planes and/or chains must bebent somewhat. Interdigitation is unlikely since the cross-sectional areas of amine headgroups and n-alkyl chainsare very similar. Figure 13 is an illustration of theproposedarrangement of the primary amine gelator as-semblies.

Layer spacings of the secondary ammonium carbamates2f and 2h are about the same as that of the corresponding1f and 1h. The carbamate and ammonium chains appearto adopt a bent conformation with the polar moietiesprojecting at one end so that they can stabilize the bilayersthrough electrostatic interactions. The layer spacing ofthe unsymmetrical secondary ammonium carbamate 2gis ∼1.5 the extended chain length of the amine, suggesting

that the molecules deviate from orthogonality or areinterdigitated in the layered structure.

Except for 2i, the calculated extended lengths for theprimary 2 and the lamellar thicknesses are approximatelythe same. Single, completely extended molecules of 2whose long axes are orthogonal to the lamellar planesmust define the layer thicknesses. Furthermore, the strongelectrostatic interactions between the positively chargedammonium and negatively charged carboxylate head-groupsmustallowthealkyl tails of therespectivesegmentsto be projected along a common axis.

The distance corresponding to the lowest angle peak of2i is approximately the same as the calculated extendeddistance of one molecule of the amine 1i. This and thepresence of functional groups at both molecular endssuggest that the gelator assemblies and bulk solid of 2iconsist of linear polymers in which chains are packed ina parallel fashion and the polymethylene segments withineach molecule are oriented perpendicular to the interfacialplanes defined by opposing headgroups. It is possible that

(47) Calculated by Hyperchem (version 5.1) molecular modelingsystem at the PM3 level, adding the van der Waals radii of the terminalatoms.

Figure 13. Proposed packing arrangement of gelator molecules in gel aggregates and distances d associated with low-angle X-raydiffractions: (a) 1a-e, (b) 2a-e, (c) 1i, and (d) 2i (as an intramolecular ammonium carbamate; see text). The proposed packingarrangements of the secondary amines, 1f-h, and of 2f-h are somewhat analogous to (a) and (b) (See text).


“monomers” of 2i along each chain are alternatingdicarbamates and diammonium ions, ammonium car-bamates arranged in a head-to-tail fashion, or a randomcombination of the two. Figure 13d presents the proposedpacking arrangement, assuming the ammonium carbam-ate form. The other two possible structure types lead tovery similar packing diagrams. Since preparation of singlecrystals of any of the 2 suitable for X-ray analysis hasbeen unsuccessful thus far, resolution of this aspect ofthe packing arrangement of 2i must await furtherstudies.

X-ray diffraction patterns of gels with octadecylam-monium octadecylcarbamate 2e in liquids other thansilicone oil have been analyzed, also. Somewhat surpris-ingly,8,48 the same morph present in the silicone oil gelsis found in assemblies of 2e gels with n-octane (Figure 14)and toluene (Figure 15). Gels with ethanol and 1-octanolalso showed the same morph as that of the silicone oil gel(see Supporting Information). Unlike most other classesof LMOGs,2,46,48-51 the alkylammonium alkylcarbamatesseem reluctant to form more than one morph, regardlessof the conditions under which their precipitation isconducted.

Conclusions

A complex picture emerges for the formation of gelsfrom solutions of 1 that are exposed to CO2. From our

NMR studies, initial aggregation involves the cationic andanionic parts of molecules of 2; the ionic headgroups aremore tightly associated than the long alkyl chains.Molecules of the liquid component are then expelled,allowing the aggregates to crystallize in lamellae. It iscurrently unknown the stage at which this expulsionoccurs during the development of crystallites into strands(and possibly tubules in the cases of 1i and 2i) and theirassembly into three-dimensional networks. The data pointto the ionic interactions, initiated as part of the 1 f 2transformations, as the major factor in creating strongergel assemblies than those from molecules of 1 that relyon H-bonding interactions among NsH groups. In addi-tion, the greater stability of gels from 1 or 2 with longeralkyl chains indicates that London dispersion forcescontribute to stronger assemblies. Although molecules ofthe liquid appear to be excluded from the crystalline gelnetworks, they do play a role in disassembly of strandsas the gels are heatedssome liquids are able to acceleratemelting more than others. Depending on the specificnature of 2, the melting temperature and loss of CO2 canbe linked processes. This phenomenon is observed whenthe 2 exhibit melting temperatures above ca. 80 °C.Comparisons of the heats of melting from the gels andneat ammonium carbamate gelators indicate that theliquid components participate in the dissolution of thegelator networks, but the degree of interaction dependson the ability of the liquid to solubilize the gelatormolecules.

A rather unique aspect of this class of LMOGs is thattheir gels can be formed and destroyed by chemicalinteractions with relatively inert gases, CO2 and N2. Thein situ chemical transformation of solutions/sols or, insome cases, organogels comprised of a 1 and an organicliquid to the corresponding 2-based gels is a new strategyfor organogel formation and transformation. Chemicalreversibility is demonstrated by removal of CO2 from2-based gels upon gentle heating while N2 is used as adisplacing gas. As such, these gel systems are chemire-versible as well as being thermoreversible. Most of thealkylammonium alkylcarbamates examined produce awider variety of gels that are more stable than theircorresponding amines. In fact, several amines are “latent”gelators of some liquids because their solutions or sols areconverted to gels upon bubbling with CO2.

The phenomena described here also occur with othergases capable of being sequestered by amines in such away as to create anionic-cationic surfactant salts thatare analogous to 2. Solutions of amines such as those inScheme 1 react with other triatomic gases (e.g., CS2, SO2,and NO2) to afford organogels,44 and solutions of polymerswith multiple amine groups are gelled by bubbling withCO2.52 In addition, a pH-sensitive system based on latentgelators of carboxylic acids in which the active gas isammonia or a volatile amine may be possible.

Acknowledgment. We thank Dr. Veeradej Chynwatfor assistance in obtaining the NMR spectra, and ProfessorTravis Holman and Dr. Natalie Kaminskaia for helpfuldiscussions. We are grateful to the National Science

(48) Furman, I.; Weiss, R. G. Langmuir 1993, 9, 2084.

(49) (a) Lin, Y.-C. Ph.D. Theis, 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.

(50) (a) Mukkamala, R.; Weiss, R. G. Chem. Commun. 1995, 375. (b)Mukkamala, R.; Weiss, R. G. Langmuir 1996, 12, 1474.

(51) Lu, L.; Weiss, R. G. Langmuir 1995, 11, 3630.(52) Carretti, E.; George, M.; Weiss, R. G. To be published.

Figure 14. X-ray diffraction patterns at room temperature of2e: (a) powder; (b) 5 wt % gel in n-octane; (c) n-octane; and (d)diffractogram b subtracted from diffractogram c.

Figure 15. X-ray diffraction patterns at room temperature of2e: (a) powder; (b) 5 wt % gel in toluene; (c) neat toluene; and(d) diffractogram b subtracted from diffractogram c.


Foundation and the Petroleum Research Fund (admin-istered by the American Chemical Society) for theirsupport of this research.

Supporting Information Available: Melting anddecomposition temperatures by POM and DSC and transitionenthalpies by DSC for the neat LMOGs, TGA and DSC ther-

mograms of 2e powder, comparison of Tg values of silicone oilgels obtained from falling drop method and DSC, 1H and 13CNMR spectra of 2a in chloroform-d, and XRD patterns of severalgels. This material is available free of charge via the Internet athttp://pubs.acs.org.

LA0255424


chemically reversible organogels via “latent” gelators ... · chemically reversible organogels...

Documents