Instantaneous low temperature gelation by a multicomponent organogelator liquid system based on ammonium salts

Article abstract of DOI:10.1021/ja8002777

A new synergistic multicomponent organogelator liquid system (MOGLS) was discovered during the standard protocol of tartaric acid-mediated racemic resolution of (±)-trans-1,2-diaminocyclohexane. The MOGLS is formed by a 0.126 M methanolic solution of (1R,2R)-(+)-1,2-diaminocyclohexane L-tartrate and 1 equiv of concentrated hydrochloric acid. Nonreversible gelation of oxygenated and nitrogenated solvents occurs efficiently at low temperature. Several features make this system unique: (1) it is a multicomponent solution where each of the five components is required for the organogelation property; (2) the multicomponent organogelator liquid system (MOGLS) is formed by simple, small, and commercially available chiral building blocks dissolved in a well-defined solvent system (MeOH/HCl/H₂O); (3) the chiral building blocks are easily amenable for further modifications in structure-property relationship studies; (4) the gelation phenomenon takes place efficiently at low temperature upon warming up the isotropic solution, conversely to the typical gel preparation protocol (gel formation upon cooling down the isotropic solution); (5) the formed organic gels are not thermoreversible in spite of the noncovalent interactions that hold the 3D-fibrillar network together.

Full text of DOI:10.1021/ja8002777

Published on Web 06/03/2008
Instantaneous Low Temperature Gelation by a
Multicomponent Organogelator Liquid System Based on
Ammonium Salts
Daniel Garc´ıa Vela´zquez, David D´ıaz D´ıaz,*,† Angel Gutie´rrez Ravelo, and
´
Jose´ Juan Marrero Tellado*
Instituto UniVersitario de Bio-Orga´nica “Antonio Gonza´lez”, UniVersity of La Laguna,
Astrof´ısico Francisco Sa´nchez 2, 38206 La Laguna, Tenerife, Spain
Received January 16, 2008; Revised Manuscript Received April 15, 2008; E-mail: DDiaz-Diaz@dow.com; jtellado@ull.es
Abstract: A new synergistic multicomponent organogelator liquid system (MOGLS) was discovered during
the standard protocol of tartaric acid-mediated racemic resolution of (()-trans-1,2-diaminocyclohexane.
The MOGLS is formed by a 0.126 M methanolic solution of (1R,2R)-(+)-1,2-diaminocyclohexane L-tartrate
and 1 equiv of concentrated hydrochloric acid. Nonreversible gelation of oxygenated and nitrogenated
solvents occurs efficiently at low temperature. Several features make this system unique: (1) it is a
multicomponent solution where each of the five components is required for the organogelation property;
(2) the multicomponent organogelator liquid system (MOGLS) is formed by simple, small, and commercially
available chiral building blocks dissolved in a well-defined solvent system (MeOH/HCl/H2O); (3) the chiral
building blocks are easily amenable for further modifications in structure-property relationship studies; (4)
the gelation phenomenon takes place efficiently at low temperature upon warming up the isotropic solution,
conversely to the typical gel preparation protocol (gel formation upon cooling down the isotropic solution);
(5) the formed organic gels are not thermoreversible in spite of the noncovalent interactions that hold the
3D-fibrillar network together.
Introduction
nogels, are thermoreversible, viscoelastic materials consisting
of an organic liquid and a low molecular-mass compound
(usually <5 wt%) self-assembled into gel fibers, often of
micrometer scale lengths and nanometer scale diameters.7a Thus,
the gel architecture strongly reflects the molecular shape of unit
components. The aggregation of such low molecular-mass
organic gelators (LMOGs) typically occurs spontaneously upon
cooling down their isotropic solutions, being driven by multiple,
weak interactions such as dipole-dipole, van der Walls (long
alkyl, aromatic groups, etc.), and hydrogen-bonding interactions
(amide, urea, hydroxyl, etc.).1 It is widely accepted that the
entanglement of such microheterogeneous fibrillar phases gives
complex nanoscale three-dimensional (3D) networks through
new “junction zones”, which immobilizes a large volume of
organic liquid into the network compartments primarily by
surface tension and capillary forces.8 Despite the extensive and
remarkable achievements of supramolecular chemistry in many
In a fascinating journey from serendipity to rational design,
materials made by means of gelation of organic solvents have
received increasing attention over the last 10-15 years,1 because
of their unique supramolecular architectures and potential
applications2 as functional soft materials in the fabrication of
sensors,3 liquid crystallines,4 electrophoretic and electrically
conductive matrices,5 templates for cell growth or the growth
of sol-gel structures,6 and in many other industrial fields such
as cosmetics, oils, and foods.1 These systems, so-called orga-
† Current address: The Dow Chemical Company, Dow Europe GmbH,
Bachtobelstrasse, 3, CH 8810 Horgen, Switzerland.
(1) (a) Abdallah, D. J.; Weiss, R. G. AdV. Mater. 2000, 12, 1237–1247.
(b) Terech, P.; Weiss, R. G. Chem. ReV. 1997, 97, 3133–3159. (c)
van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263–
2266. (d) Gronwald, O.; Shinkai, S. Chem. Eur. J. 2001, 7, 4328–
4334. (e) Hanabusa, K. Springer Ser. Mater. Sci. 2004, 78, 118–137.
(f) Weiss, R. G. Terech, P. Molecular Gels: Materials with Self-
Assembled Fibrillar Networks; Springer: New York, 2006. (g) George,
M.; Weiss, R. G. Acc. Chem. Res. 2006, 39, 489–497, and references
therein.
(2) (a) Vintiloui, A.; Leroux, J.-C. J. Controlled Rel. 2008, 125, 179–
192. (b) Jung, J. H.; Shinkai, S. Top. Curr. Chem. 2004, 248, 223–
260. (c) Sangeetha, N. M.; Maitra, U. Chem. Soc. ReV. 2005, 34, 821–
836.
(6) (a) Sanchez, C.; Llusar, M. Chem. Mater. 2008, 20, 782–820. (b) van
Bommel, K. J. C.; Friggeri, A.; Shinkai, S. Angew. Chem., Int. Ed.
2003, 42, 980–999, and references therein. (c) Friggeri, A.; van
Bommel, K. J. C.; Shinkai, S. In Molecular Gels: Materials with Self-
Assembled Fibrillar Networks; Springer: New York 2006; pp 857-
893.
(3) Li, S.; John, V. T.; Irvin, G. C.; Bachakonda, S. H.; McPherson, G. L.;
O’Connor, C. J. J. Appl. Phys. 1999, 85, 5965–5972.
(4) Kato, T. Science 2002, 295, 2414–2418, and references therein.
(5) (a) Puigmarti-Luis, J.; Laukhin, V.; del Pino, A. P.; Vidal-Gancedo,
J.; Rovira, C.; Laukhina, E.; Amabilino, D. B. Angew. Chem., Int.
Ed. 2007, 46, 238–241. (b) Kubo, W.; Kambe, S.; Nakade, S.;
Kitamura, T.; Hanabusa, K.; Wada, Y.; Yanagida, S. J. Phys. Chem.
B 2003, 107, 4374–4381.
(7) (a) Ihara, H.; Takafuji, M.; Sakurai, T. Encyclopedia of Nanoscience
and Nanotechnology 2004, 9, 473. (b) Low Molecular Mass Gelators
ed. Fages, F. in Topics in Current Chemistry, vol. 256, Springer-Verlag,
Berlin, Heidelberg, 2005.
(8) Huang, X.; Terech, P.; Raghavan, S. R.; Weiss, R. G. J. Am. Chem.
Soc. 2005, 127, 4336–4344.
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2008 American Chemical Society
J. AM. CHEM. SOC. 2008, 130, 7967–7973 7967

A R T I C L E S
Vela´zquez et al.
solution, conversely to the typical gel preparation protocol (gel
formation upon cooling down the isotropic solution); (5) the
formed organic gels are not thermoreversible in spite of the
noncovalent interactions that hold the 3D-fibrillar network
together.
Results and Discussion
Discovery of the Multicomponent Organogelator Liquid
System (MOGLS). The serendipitous discovery of this MOGLS
took place when we were using the standard protocol of tartaric
acid-mediated racemic resolution of (()-trans-1,2-diaminocy-
clohexane14 for other purposes. During the cleaning of the
glassware used during the experimental work, we were fasci-
nated to observe the instantaneous formation of a transparent
jellylike material upon addition of acetone to wash out any
remained substance. The unexpected phenomenon took place,
based on the experimental protocol, only in the beaker contain-
ing (1R,2R)-(+)-1,2-diaminocyclohexane L-tartrate, MeOH, and
aqueous HCl.
The stunning discovery was followed by a rational design of
experiments in order to disclose the nature of the jellylike
material and the required components for its formation, as well
as their possible synergistic effect. Such comprehensive study
(see below) led to a 0.126 M methanolic solution of (1R,2R)-
(+)-1,2-diaminocyclohexane L-tartrate and 1 equiv of concen-
trated hydrochloric acid (37% HCl) as the optimum multicom-
ponent organogelator solution, which corresponds to a molar
ratio of diaminocyclohexane:tartaric acid:MeOH:HCl:H2O )
1:1:195:1:3.25. As it will be discused later, the presence of each
component, including the water,15 was found to be crucial for
the gelation event.
We were delighted to observe that the addition of tiny
amounts of this solution to a variety of organic solvents
instantaneously induced their gelation at room temperature.
Nevertheless, a closer inspection of the formed jellylike materi-
als revealed a nonhomogeneous gel phase, which often leaked
a small amount of solvent after turning the test tube upside-
down. In order to prepare clear homogeneous and stable gels,
we found that the organic solvents must be cooled down close
to their freezing point prior to addition of an appropriate volume
of the MOGLS. Then, the homogeneous mixture was allowed
to warm up to promote formation of the gels. Interestingly, both
the solvent-MOGLS mixture and the MOGLS solution remain
as clear solutions at temperatures as low as -100 °C for any
period of time. As far as we are aware, this is the first example
where the gelation phenomenon efficiently takes places upon
warming up instead of cooling down the isotropic solution of
the gelator.16 The only example that might be considered rather
similar is the uncommon gelation of organic fluids at room
temperature without the heating/cooling cycle.11b It is worth
noting that the formation of higher aggregates upon raising the
temperature is also observed in nature. Living organisms have
developed an impressive molecular machinery to achieve motion
and other biological functions by free energy transductions of
any chemical event (protein folding/unfolding or self-assembling
Figure 1. New multicomponent organogelator liquid system (MOGLS),
in which the presence and stoichiometry of each component is critical for
the gelation of organic fluids. Molar ratio of diaminocyclohexane:tartaric
acid:MeOH:HCl:H2O ) 1:1:195:1:3.25.
controlled self-assembly processes,9 most of LMOGs have been
found by serendipity rather than rational design. Thus, the
control of gelation phenomena and the design of new LMOGs
are still challenging taks,10 in particular when multiple dynamic
noncovalent interactions are taken into account.
One of the main attributes of the gelling phenomenon is the
one-dimensional kinetic growth of the superstructure that leads
to the formation of a fiber.1g In this regard, the discovery of
new organogelators based on organic ammonium salts has
become a scientific topic of great interest. This is primarily due
to their flexible and versatile hydrogen bond pattern, which
allows control of the dimensionality of the network.11 On the
other hand, organic ammonium salts have been recognized as
structurally simple supramolecular synthons,12 which could
permit the fine-tunning of the noncovalent molecular aggregation
mode by the incorporation of selected functional groups. In
comparison with the conventional one-component gelators, the
two-component ammonium-based systems provide additional
interest because multicomponent systems allow a further
complexity of the hierarchical self-assembling process to form
higher aggregates, which are responsible for the microphase
separation in the gel state.7b,13
In this communication, we report on the serendipitous
discovery of a new synergistic multicomponent organogelator
liquid system, which shows intriguing organogelation properties.
The MOGLS is formed by a methanolic solution of (1R,2R)-
(+)-1,2-diaminocyclohexane L-tartrate and hydrochloric acid
(Figure 1).
Several features make this system unique in comparison with
other organogelators, specially those based on ammonium
carboxylates:11 (1) it is a multicomponent solution where each
of the five components is required for the organogelation
property; (2) the multicomponent organogelator liquid system
(MOGLS) is formed by simple, small, and commercially
available chiral building blocks dissolved in a well-defined
solvent system (MeOH/HCl/H2O); (3) the chiral building blocks
are easily amenable for further modifications in structure-property
relationship studies; (4) the gelation phenomenon takes place
efficiently at low temperature upon warming up the isotropic
(9) (a) Serpe, M. J.; Craig, S. L. Langmuir 2007, 23, 1626–1634. (b)
Diederich, F. Angew. Chem., Int. Ed. 2007, 46, 68–69. (c) Gale, P. A.
Chem. Soc. ReV. 2007, 36, 141–142. (Special issue on Supramolecular
Chemistry.)
(10) De Loos, M.; Feringa, B. L.; van Esch, J. H. Eur. J. Org. Chem. 2005,
3615–3631.
(14) (a) Gaslbøl, F.; Steenbøl, P.; Sørensen, B. S. Acta Chem. Scand. 1972,
26, 3605–3611. (b) Walsh, P. J.; Smith, D. K.; Castello, C. J. Chem.
Educ. 1998, 75, 1459–1462.
(11) (a) Sada, K.; Tani, T.; Shinkai, S. Synlett 2006, 15, 2364–2374. (b)
Trivedi, D. R.; Dastidar, P. Chem. Mater. 2006, 18, 1470–1478, and
references cited therein. (c) Ayabe, M.; Kishida, T.; Fujita, N.; Sada,
K.; Shinkai, S. Org. Biomol. Chem. 2003, 1, 2744–2747.
(12) Desiraju, G. R. Angew. Chem., Int. Ed. 1995, 34, 2311–2327.
(13) Hirst, A. R.; Smith, D. K. Chem. Eur. J. 2005, 11, 5496–5508.
(15) (a) Tamaru, S.-i.; Luboradzki, R.; Shinkai, S. Chem. Lett. 2001, 30,
336–337. (b) Oda, R.; Huc, I.; Candau, S. J. Angew. Chem., Int. Ed.
1998, 37, 2689–22691. (c) Berthier, D.; Buffeteau, T.; Le´ger, J.-M.;
Oda, R.; Huc, I. J. Am. Chem. Soc. 2002, 124, 13486–13494.
(16) Urry, D. W. Angew. Chem. Int. Ed. Engl. 1993, 32, 819–841. We do
thank a referee for this valuable reference.
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Multicomponent Organogelator Liquid System
A R T I C L E S
Table 1. Organogelation Ability of the Optimal MOGLSa
entry
solventb
mgvc (mL)
mgcd (%)
Tmix (°C)
Tfe (°C)
Tdf (°C)
Tdg (°C)
1
2
3
4
5
6
7
8
acetone
ethyl acetate
0.10
0.15
0.10
0.14
0.10
0.18
0.08
0.07
0.06
0.25
0.08
M
M
M
I
I
I
M
M
M
0.40
0.50
0.50
0.60
0.40
0.90
0.30
0.30
0.03
0.90
0.30
--
--
--
--
--
--
--
--
--
-78
-78
-78
-78
-60
-100
-20
-80
-40
-10
15
--
--
--
--
--
--
--
--
-35
-38
-44
-40
-40
-30
-6
-45
-26
-1
15
--
--
--
--
--
--
--
--
--
74
98
71
69
75
40
66
42
76
50
--h
--
--
--
--
--
--
--
--
--
74
83
70
69
76
45
64
52
78
49
--h
--
--
--
--
--
--
--
--
--
tetrahydrofuran
1,2-dimethoxyethane
2-methoxyethyl ether
diethyl ether
cyclohexanone
3-methyl-2-butanone
acetonitrile
benzonitrile
1,4-dioxane
dichloromethane
chloroform
1,2-dichloroethane
carbon tetrachloride
chlorobenzene
1,2-dichlorobenzene
dimethylformamide
dimethylsulfoxide
ethanol
n-hexane
pyridine
isooctane
benzene
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
--
--
--
--
--
--
--
P
M
M
I
I
P
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
toluene
methyl tert-butyl ether
a Abbreviations: M ) miscible, I ) immiscible, P ) precipitated. b 1 mL of solvent was used to prepare the gels using (1R,2R)-1,2-
diaminocyclohexane L-tartrate complex 0.126 M in MeOH and 1 equiv of 37% HCl (solution AL). Transparent or slightly turbid gels were formed in
each case, except for diethyl ether and ethyl acetate for which opaque and translucent gels were attained, respectively. c Minimum gelation volume.
d Minimum gelation concentration referred to HCl-diamine-tartaric salt complex. e Gel formation temperature determined by setting a digital
thermocouple (Ø ) 2 mm) into the solution. f Gel destruction temperature determined by the “dropping ball method” (ramping rate >2 °C, average of
three independent measurements). g Maximum endo transition observed by DSC. h Partial gel formation. The estimated error for the Td obtained by the
“dropping ball method” and DSC was ( 1 °C and ( 3 °C, respectively.
processes). This chemical phenomenon is referred to as inVerse
temperature transition and has been studied in detail by Urry
and co-workers.16 At this stage, we believe that the origin for
this transition might be the same for our system.
the family of the most efficient organogelator systems so far
reported, acting as supergelator17 for several organic solvents.
Preliminary oscillatory rheological measurements19 were
reproducible from batch to batch and confirmed the gel state
(storage modulus G′ > loss modulus G′′) of the samples within
the linear viscoelastic regime. The material rigidity as indicated
by G′ followed no uniform trend with respect to Td. In general,
dynamic frequency sweep (DFS) experiments covering a
pulsation (ω) range from 0.1 to 60 rad/s highlighted the
viscoelastic nature of the gels. On the other hand, dynamic strain
sweep (DSS) experiments in the range 0.01-100% strain
showed that these gels fracture at ca. 5-10% strain, confiming
their brittle nature. Taking into account the DFS and DSS results,
further dynamic time sweep (DTS) experiments at strain and
frequency constant (1% and 6 rad/s, respectively) showed that
the storage moduli was always 1 order of magnitude greater
than the respective loss moduli, indicating that the gels were
quite rigid. As could be expected for these types of materials,
an increase in the organogelator system concentration usually
causes a decrease in the tan δ (G′′/G′) value, which is a measure
of the internal resistance of the material, suggesting an increase
in the mechanical damping properties.
In 1998, Huc and co-workers15b reported a series of LMOGs
based on gemini surfactants consisting of dimers of nonchiral
cetyltrimethylammonium (CTA) ions with chiral L- or D-tartrate
as counterions, which self-organize, locating the polar groups
at the core of the aggregate and the nonpolar long hydrocarbon
chains in contact with the solvent. In contrast to the MOGLS
reported herein, (CTA)2-tartrate were reported to gel in mostly
chlorinated solvents and water at concentrations of about 10-30
mM upon the heating/cooling process. These gels are disrupted
easily by the addition of small amounts of alcohols, and only
the pure enantiomers yield gels.
Gelation Studies and Gel Characterization. The gelation
ability of the MOGLS was assayed for 25 organic solvents, and
the gelation was visually monitored using the tube inversion
technique. Interestingly, the MOGLS was found to be selective
for the gelation of oxygenated and nitrogenated organic solvents
(Table 1). The gels were stables for months when stored at 12
°C. For halogenated, hydrocarbon, or aromatic solvents, com-
plete miscibility or precipitate formation was observed upon
addition of the MOGLS. The minimum gelation volume (mgv)
was established in the range 60-250 µL, which corresponds to
a minimum gelation concentration (mgc) range of 0.03-0.9 wt%
of gelator based on the tartaric-diamine-HCl complex. For
instance, this means that the MOGLS can entrap approximately
20 000 molecules of acetonitrile per gelator (tartaric-diamine-
HCl complex) particle. These values position this MOGLS in
Thermal Studies. Thermal characterization of gels was first
carried out by the “dropping ball method”18 to determine the
temperature at which the gels are destroyed, Td (Table 1). As
(17) (a) Valkonen, A.; Lahtinen, M.; Virtanen, E.; Kaikkonen, S.; Kole-
hmainen, E. Biosens. Bioelectron. 2004, 20, 1233–1241. (b) Lubo-
radzki, R.; Gronwald, O.; Ikeda, A.; Shinkai, S. Chem. Lett. 2000,
1148–1149.
(18) Takahashi, A.; Sakai, M.; Kato, T. Polym. J. 1980, 12, 335–341.
(19) See Supporting Information.
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A R T I C L E S
Vela´zquez et al.
salt (1R,2R)-(+)-1,2-diaminocyclohexane L-tartrate was evident
from these studies. Thus, a broadband appears at 3250-2350
cm-1 assignable to the ammonium stretching vibration, and three
intense bands at 1560, 1530, and 1378 cm-1 assignable to the
carboxylate carbonyl overlapped with the ammonium deforma-
tion bands. A very intense band at ca. 1710-1749 cm-1
assignable to the carbonyl stretching vibration of the carboxylic
group was identified for the liquid MOGLS, gel, and xerogel
states. Strong hydrogen bond association bands at ca. 3600-3200
cm-1, overlapping the alkane stretching bands, were consistently
found for all gels and xerogels. The intensity of this broadband
is dramatically reduced for the xerogel state as it might be
expected once the aggregate gelator-solvent20 interactions
disappear. The liquid MOGLS showed also a characteristic
intense band at ca. 3516 cm-1 assignable to nonassociated
hydroxycarbonyl groups. According to the FT-IR data, the
molecular species responsible for the separation-phase phenom-
enon presents the same ionization state for both the gel and
xerogel state but clearly different from that of tartrate ammonium
salt.
Microscopy Studies. In order to gain better insight into the
molecular organization the gel specimens were analyzed by
transmission (TEM) and scanning (SEM) electron microscopy.
Images obtained from both TEM and SEM observations were
complementary. The xerogels consist of 3D highly entangled
fiberlike aggregates through numerous junction zones21 (Figures
3a-d). Most of the straight fibers presented relatively uniform
diameters of 180 ( 30 nm and lengths on the micron scale.
The high aspect ratios of the gel fibers plainly point out that
the intergelator interactions are highly anisotropic. However,
no remarkable changes in general appearance were observed
on the fibers morphology for specimens prepared at different
concentrations (Figures 3e-j).
Figure 2. Phase diagram for THF-gel (red line), CH3CN-gel (blue line),
and acetone-gel (green line) from the critical mgv to 4 × mgv. Inset:
representative digital photographs of inverted test tubes containing THF
and acetone-gels. Inset: digital photographs of the organogels. Collapse of
the gels started to become evident at concentrations higher than the
maximum values outlined in the graphic (e.g., only 60% of the total volume
remained gelled at 6 × mgv).
Table 2. Thermodynamic Parameters of the Gels
entry
solvent
weight of gel (mg) Tda (°C)
Tdb (°C)
∆Hc (J/g)
1
2
3
4
5
6
7
8
9
ethyl acetate
tetrahydrofuran
acetonitrile
benzonitrile
cyclohexanone
acetone
2-methoxyethyl ether
1,2-dimethoxyethane
3-methyl-2-butanone
7.1
6.5
9.0
98
71
76
50
66
74
75
69
42
40
82.2-83.1 0.61
69.4-70.2 0.73
77.4-78.3 0.82
47.9-48.9 0.54
62.5-64.4 1.93
73.4-73.6 2.26
75.4-75.7 2.20
67.5-69.1 1.47
51.6-52.5 0.082
43.1-45.0 5.93
5.4
12.9
10.0
22.8
18.0
10.0
19.0
10 diethyl ether
Initial atomic force microscopy (AFM) images showed a
densely entangled network in good agreement with TEM and
SEM images (Figure 4a). Dimensions of 1.8 µm width and 700
nm height was often found for largest fibers with the fiber unit
of about 2.5 nm height × 25 nm width (Figure 4b). The
entanglement of four unit fibers forms bundles (Figure 4c) with
dimensions of ca. 10 nm height × 100 nm width.
a Gel destruction temperature determined by the “dropping ball
method” (ramping rate <2 °C). b The first value corresponds to the
onset of the endothermic peak observed by DSC and the second value to
the maximum. c Enthalpic change. The estimated error for the Td
obtained by the “dropping ball method” and DSC were ( 1 °C and ( 3
°C, respectively.
Synergism of the Components in the Organogelling System. The
importance of each component in the MOGLS was studied by
means of a systematic set of experiments where each of the
putative components responsible for the gelation phenomenon
was individually, or in combination, assayed for gelling proper-
ties. Thus, different solutions were prepared and their gelling
abilities determined for acetone, 1,2-DME, and THF, using the
tube inversion method (Table 3). However, all experiments were
unsuccessful in producing stable organogels, except when an
exact amount of each component was utilized: 0.126 M of
(1R,2R)-(+)-1,2-diaminocyclohexane L-tartrate in MeOH, and
1 equiv of 37% HCl (entry 9). Hence, it is concluded that there
is an exquisite stoichiometric balance between all components
in the mixture, the amount of water15 (exactly the amount
might be expected, Td was found to rise as the concentration of
the organogelator system increases (Figure 2).
The gel formation temperature was ascertained between -45
and -1 °C. A particular situation was observed for 1,4-dioxane,
since at its freezing point (12 °C) the gelation process is still
too fast to enable the complete gelation of the solvent. Despite
the noncovalent nature of the supramolecular aggregate, the
metastable gel state in these materials was found to be
nonreversible. Once the destruction of the gels is induced either
upon heating or by mechanical stress (1R,2R)-(()-1,2-diami-
nocyclohexane dihydrochloride is formed as a white precipitate,
which is fully insoluble in the solvent medium and unable to
get back into the solution. Differential scanning calorimetry
(DSC) analyses showed an acceptable correlation, within the
experimental error, between the observed maximum endothermic
peak with the Td determined by the “dropping ball method”
(Table 1). Thermodynamic data obtained through DSC mea-
surements showed endothermic transitions characterized as
relatively narrow peaks (∆T ∼ 1-8 °C) with modest enthalpic
changes (∆H ∼ 0.5-5.9 J/g) (Table 2).19
(20) (a) Zhu, J.; Shi, C. Chem. Mater. 2007, 19, 2392–2394. (b) Zhu, G.;
Dordick, J. S. Chem. Mater. 2006, 18, 5988–5995. (c) Jeong, Y.;
Hanabusa, K.; Masunaga, H.; Akiba, I.; Miyoshi, K.; Sakurai, S.;
Sakurai, K. Langmuir 2005, 21, 586–594. (d) Hirst, A. R.; Smith,
D. K. Langmuir 2004, 20, 10851–10857. (e) Wang, R.; Geiger, C.;
Chen, L.; Swanson, B.; Whitten, D. G. J. Am. Chem. Soc. 2000, 122,
2399–2400.
FT-IR Studies. FT-IR was used in order to ascertain how
the complex aggregated into a supramolecular structure. The
difference of the spectra between the gel state and the solid
(21) (a) Shin, J. H.; Gardel, M. L.; Mahadeva, L.; Matsudaira, P.; Weitz,
D. A. Proc. Nat. Acad. Sci. 2004, 101, 9636–9641. (b) Terech, P.;
Furman, I.; Weiss, R. G. J. Phys. Chem. 1995, 99, 9558–9566.
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Multicomponent Organogelator Liquid System
A R T I C L E S
Figure 3. Electron micrograph images of self-assembled fibrils: (a) TEM for CH3CN-gel, scale bar ) 500 nm; (b) TEM for THF-gel, scale bar ) 500 nm; (c) SEM for
CH3CN-gel, scale bar ) 500 nm; (d) SEM for THF-gel, scale bar ) 2 µm; (e) TEM of the gel made of solution ALin 1,2-DME, scale bar ) 500 nm; (f) TEM of the gel
made of solution AL in acetone-gel, scale bar ) 200 nm; (g) TEM of the gel made of solution AL in CH3CN-gel prepared at 2 × mgv, scale bar ) 500 nm; (h) TEM of
the gel made of solution AL in CH3CN-gel prepared at 3 × mgv, scale bar ) 500 nm; (i) TEM of the gel made of solution AL in THF-gel prepared at 2 × mgv, scale bar
) 500 nm; (j) TEM of the gel made of solution AL in THF-gel prepared at 3 × mgv, scale bar ) 500 nm.
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J. AM. CHEM. SOC. VOL. 130, NO. 25, 2008 7971

A R T I C L E S
Vela´zquez et al.
Figure 4. (a) AFM image of self-assembled fibrils in CH3CN-gel (left) and zoom-in of the black squeared area (right); (b) cross-section of the basic fiber
unit; (c) four fibers bundle (left) and its cross-section (right).
contained in the hydrochloric solution) being crucial for the
stability of gels. Indeed, when the MOGLS was prepared using
anhydrous methanolic HCl (entry 10), very weak gels were
formed.
building blocks, we addressed this subject through the study of
the thermal stability of the gels formed by using salts with an
ee varying in the range 0-100%. Gels of higher thermostability
were obtained when the enantiomerically pure MOGLS based
on (1R,2R)-1,2-diaminocyclohexane L-tatrate was used (Table
5, entry 1). Less stable gels were obtained when MOGLSs of
lower ee were used (Table 5, entries 2 and 3), and no gel was
observed for the racemic MOGLS. Moreover, MOGLS based
on (1R,2R)-1,2-diaminocyclohexane D-tartrate yielded thermally
less stable gels than its diastereomeric counterpart when prepared
under identical experimental conditions (Table 5, entry 4). All
gels were transparent except entries 2-3 for which opaque gels
were attained. As expected, enantiomeric MOGLS showed
identical gelation properties.
The role of the alcoholic component of the gelator system
was also studied. MOGLSs prepared in branched alcohols such
as i-propanol, i-butanol, or isoamilic alcohol did not show any
gelation ability. Nevertheless, solutions prepared in ethanol or
n-butanol were also able to yield gels at the same concentration
than those in methanol, albeit much weaker (Table 4). Interest-
ingly, and independent of the gelled solvent, a dramatic drop
of the Td between 12 and 42 °C was observed when either
ethanol or n-butanol were used as solvent for the organogelator
system in comparison with methanol.
Concerning the role of the mineral acidic component, the use
of 48% HBr instead of 37% HCl remarkably decreased the
gelation ability of the MOGLS affording much weaker gels
(Table 5, entry 5 and 6, respectively). For any other inorganic
acid such as 65% HNO3 or 78% HClO4, no gel formation was
observed.
Chirality doubtless plays a central role in the functional
property of organogels.22 Since our system is formed by chiral
(22) (a) Dzolic, Z.; Wolsperger, K.; Zinic, M. New J. Chem. 2006, 30,
1411–1419. (b) Brizard, A.; Oda, R.; Huc, I. Top. Curr. Chem. 2005,
256, 167–218, and references therein.
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7972 J. AM. CHEM. SOC. VOL. 130, NO. 25, 2008

Multicomponent Organogelator Liquid System
A R T I C L E S
Table 3. Gelation Properties of Aqueous MOGLSa
those reported earlier for the gemini surfactant-based LMOGs.15b
Moreover, no gelation capability was observed whatsoever for
the multicomponent solution if the dicarboxylic component lacks
the two hydroxyl functional groups (e.g., succinic, maleic, or
1,2-benzenedicarboxylic acid). Also, no evidence of gelation
was observed when using either 1,4-diamines, nonconforma-
tional diamines, or tertiary diamines (e.g., ethylenediamine,
benzene-1,2-diamine, N,N,N′,N′-tetramethylethane-1,2-diamine).
MeOH
(mL)
H2Ob
(mL, equiv)
HCl 37%
(equiv)
solution
(M)c
entry (mg, mmol)
phasec
1
2
3
4
5
6
7
8
9
10
50,0.19
50,0.19
50,0.19
50,0.19
50,0.19
50,0.19
50,0.19
50,0.19
50,0.19
163,0.63
1.5
1.5
1.5
1.5
4.5
4.5
1.5
1.5
1.5
4.5
-
-
I
I
N
N
N
WG
N
WG
N
WG
G
0.2, 58.5
0.2, 58.5
0.2, 58.5
-
-
Summary and Conclusions
0.64
0.88
-
1.3
-
0.111
0.111
I
0.042
I
0.126
0.126
0.126
The above-described results indicate that a judicious ther-
modynamic balance flanked by the two main driving forces for
the self-assembling event, hydrogen bonding and electrostatic
forces, seems to be critical for the gelation property shown by
the MOGLS. A comprehensive study on the shelflike aggrega-
tion mode of the (1R,2R)-(+)-1,2-diaminocyclohexane L-tartrate
salt in the crystalline state has already been reported.23
Nevertheless, the ionization state of the species involved in the
phase separation during the gel formation using the MOGLS is
rather different to the one in the crystalline state, as indicated
by the FT-IR studies. The foregoing results including require-
ment of the five components (diamine, tartaric acid, MeOH,
HCl, and H2O) for the gelation property points toward the
existence of an aggregate with at least one nonionized carboxylic
group. In principle, such arrangement would favor the tartrate
carboxyls to form chainlike head-to-tail structures.
-
0.011, 3.26
0.011, 3.26
-
0.9
1.0
0.5 mL HCl/MeOH 1.25
M, 1.0 equiv HCl
WG
a The gelation properties were found to be the same for acetone,
1,2-DME, and THF. b Additional H2O added to the system.
c Abbreviations: I ) insoluble; N ) no gel formation; G ) stable gel;
WG ) weak gel, easily disrupted upon shaking.
Table 4. Influence of the Alcoholic Component of the MOGLS on
the Thermal Stability of Organogels in THF, EtOAc, and 1,2-DME
solventb
Sol ALa MeOH, Tdc (°C) Sol ALa EtOH, Tdc (°C) Sol ALa n-BuOH, Tdc (°C)
THF
EtOAc
1,2-DME
71
98
69
42
56
38
59
63
44
In summary, we have presented the discovery of the first fluid
five-component organogelator system based on chiral am-
monium salts. The described synergistic MOGLS, hidden for
decades behind the classical resolution of the (()-trans-1,2-
diaminocyclohexane with L- or D-tartaric acid, enables the
effective, instantaneous, and non-thermoreversible gelation of
a variety of organic fluids at low temperature. This gift of casual
discovery supports the statement that serendipity frequently
provides a major stage of opportunity for scientific discovery.24
We believe that the finding of this peculiar system opens the
door for the design of new low-cost and efficient liquid
multicomponent organogelators. An evaluation of practical
applications, the influence of the chirality on the morphology
and mechanical properties of these gels, and X-ray scattering
studies to conclusively describe the self-assembled pattern of
the MOGLS in the gel state are currently underway and will be
reported in future publications.
a Sol AL ) (1R,2R)-(+)-1,2-diaminocyclohexane L-tartrate, 0.126 M
in MeOH, EtOH or n-BuOH, 1 equiv of 37% HCl. b Minimum gelation
volumes for THF, EtOAc, and 1,2-DME were 0.10, 0.15, and 0.14 mL,
respectively. c Gel destruction temperature ((1 °C) determined by the
“dropping ball method” (ramping rate <2 °C).
Table 5. Thermostability of THF-Gels Prepared with Different
MOGLSs
entry
MOGLSa
mgvb (mL)
Tdc (°C)
1
2
3
4
5
6
7
8
Sol AL
Sol A9:1
Sol A8:2
Sol AD
Sol BL
Sol CL
Sol DL
Sol EL
0.10
0.12
0.12
0.10
0.10
0.10
0.18
0.13
71
62
44
68
54
66
57
62
a Sol AL: (1R,2R)-1,2-diaminocyclohexane L-tartrate 0.126
M in
MeOH, 1 equiv of 37% HCl; Sol Ax:y: [(1R,2R)-1,2-cyclohexyldiamine
L-tartrate: (1S,2S)-1,2-cyclohexyldiamine D-tartrate, at a molar ratio x:y,
respectively] 0.126 M in MeOH, 1 equiv of 37% HCl; Sol AD: (1R,2R)-1,2-
diaminocyclohexane D-tartrate 0.126 M in MeOH, 1 equiv of 37% HCl;
Sol BL: (1R,2R)-1,2-diaminocyclohexane L-tartrate 0.126 M in MeOH, 1
equiv of dry HCl; Sol CL: (1R,2R)-1,2-diaminocyclohexane L-tartrate
Acknowledgment. This work was supported by the Ministerio
de Educacio´n y Ciencia de Espan˜a (MEC, SAF2003-04200-C02-
02 and SAF2006 06720) and Instituto Canario de Investigacio´n
del Ca´ncer (ICIC, 10/2004). The authors thank Mr. A. Orive and
Prof. A. Creus for AFM pictures, Dr. A. Diego Lozano for
assistance in the DSC measurements, and the reviewers for their
valuable comments. D.G.V. acknowledges Gobierno de Canarias
for a predoctoral fellowship. D.D.D. thanks Profs. M. G. Finn and
Craig J. Hawker, and Dr. Morley, for stimulating discussions.
0.126
M in MeOH, 1.77 equiv 48% HBr; Sol DL: (1R,2R)-1,2-
diaminocyclohexane dibenzoyl-L-tartrate 0.126 M in MeOH, 1 equiv of
37% HCl; Sol EL: (1R,2R)-1,2-diaminocyclohexane L-malate 0.126 M in
MeOH,
1
equiv of 37% HCl. b Minimum gelation volume. c Gel
destruction temperature ((1 °C) determined by the “dropping ball
method” (ramping rate <2 °C). Additional notes: (1) All gels were
transparent except entries 2-3 for which opaque gels were attained. (2)
As expected, enantiomeric MOGLS showed identical gelation properties.
Supporting Information Available: Synthetic procedures,
complete experimental details, 1H NMR spectra, 13C NMR spectra,
FTIR spectra, and DSC thermograms. This material is available
free of charge via the Internet at http://pubs.acs.org.
Structure-property studies revealed the importance of the
hydroxyl groups of the tartaric acid and the architecture of the
diamine in the formation of the gels. Thus, the use of a
dicarboxylic unit lacking one the hydroxyl groups or having
both protected (e.g., L-malic acid, dibenzoyl-L-tartaric acid)
yielded thermally weaker gels (Table 5, entries 7 and 8,
respectively). The above observations are somehow aligned to
JA8002777
(23) (a) Rychlewska, U. J. Mol. Struct. 1999, 474, 235–243. (b) Hanessian,
S.; Simard, M.; Roelens, S. J. Am. Chem. Soc. 1995, 117, 7630–7645.
(24) Stoskopf, M. K. ILAR J. 2005, 46, 332–337.
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J. AM. CHEM. SOC. VOL. 130, NO. 25, 2008 7973