New dicholesteryl-based gelators: Gelling ability and selective gelation of organic solvents from their mixtures with water at room temperature

Article abstract of DOI:10.1039/b807576j

Four new diacid amides of dicholesteryl l-phenylalaninate were designed and prepared. The compounds with spacers containing three, four, five, or six carbon atoms are denoted as 1-4, respectively. Gelation tests showed that the four compounds are versatile organogelators, and a subtle change in the length of the spacer can produce a dramatic change in the gelation behaviors of the compounds, as well as the micro-structures of the gel networks as revealed by SEM and XRD measurements. Within the 77 gel systems studied, at least seven of them, including 2/kerosene, 2/toluene, 2/xylene, 3/kerosene, 3/1-pentanol, 3/1-hexanol and 3/1-heptanol, gel spontaneously at room temperature. Furthermore, 2 and 3 can be used for the selective gelation of xylene or kerosene from their mixtures with water. Importantly, heating and cooling cycle or addition of a co-solvent is not necessary for the selective gelation. Furthermore, the gels of 2/xylene are mechanically strong enough for separation, and thereby it is believed that 2 is a strong candidate for the practical separation of xylene from its mixture with water. FT-IR and temperature- dependent ¹H NMR measurements demonstrated that inter-molecular hydrogen bonding plays an important role for the formation and maintenance of the gel networks. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2008.

Full text of DOI:10.1039/b807576j

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www.rsc.org/njc | New Journal of Chemistry
New dicholesteryl-based gelators: gelling ability and selective gelation
of organic solvents from their mixtures with water at room temperaturew
Junxia Peng, Kaiqiang Liu, Xufei Liu, Huiyun Xia, Jing Liu and Yu Fang*
Received (in Durham, UK) 6th May 2008, Accepted 1st July 2008
First published as an Advance Article on the web 1st September 2008
DOI: 10.1039/b807576j
Four new diacid amides of dicholesteryl L-phenylalaninate were designed and prepared. The
compounds with spacers containing three, four, five, or six carbon atoms are denoted as 1–4,
respectively. Gelation tests showed that the four compounds are versatile organogelators, and a
subtle change in the length of the spacer can produce a dramatic change in the gelation behaviors
of the compounds, as well as the micro-structures of the gel networks as revealed by SEM and
XRD measurements. Within the 77 gel systems studied, at least seven of them, including
2/kerosene, 2/toluene, 2/xylene, 3/kerosene, 3/1-pentanol, 3/1-hexanol and 3/1-heptanol, gel
spontaneously at room temperature. Furthermore, 2 and 3 can be used for the selective gelation
of xylene or kerosene from their mixtures with water. Importantly, heating and cooling cycle or
addition of a co-solvent is not necessary for the selective gelation. Furthermore, the gels of
2/xylene are mechanically strong enough for separation, and thereby it is believed that 2 is a
strong candidate for the practical separation of xylene from its mixture with water. FT-IR and
1
temperature-dependent H NMR measurements demonstrated that inter-molecular hydrogen
bonding plays an important role for the formation and maintenance of the gel networks.
hindering gelation. A few examples for the selective gelation
of oil from an oil/water mixture have been reported,5 but the
phase selective gelation could only be achieved by heating and
Introduction
Recent years have witnessed an intense investigation on the
design and preparation of new low molecular mass gelators
cooling the mixtures, or by addition of another co-solvent into
(LMGs) for the purpose of creating functional nanostructured
the systems, which must limit the practical applications of the
soft materials with properties that are tunable by external
findings. In addition, LMGs for selective gelation of aromatic
solvents such as toluene and xylene from their mixtures with
stimuli,1 such as light, temperature, sonication and chemical
additives. It is believed that these materials are important for
water have not been reported. Separation of them from water
is crucial because these solvents are widely used, are toxic, and
forward-looking applications of them,2 such as confined reac-
very difficult to be decomposed naturally.6
tion media, templates for well-defined inorganic materials or
polymers, light-harvesting systems, sensors, biomaterials and
In that connection, from both fundamental and practical
optoelectronic devices. However, structural requirements for
standpoints, development of these LMGs is of great impor-
LMGs are not well understood, and a rational correlation of
tance, particularly, when the structures of the LMGs are
molecular structure and gelation ability in a given solvent still
simple, and can be synthesized from naturally available, and
cheap starting materials. L-Amino acid- and cholesterol-based
remains a challenge.3 Therefore, intensive effort has been
expended to understand the structure–property relationships
gelators are some of the best gelators according to the follow-
to tailor LMGs, and has resulted in the discovery of a variety
of small molecules which are efficient gelators.4
Up to now, gelators, which can gelate one solvent in
methodologies have been already established.7 Our group has
preference to another from a given mixture, particularly, if
one of the solvents in such a mixture is water, have still been
ing concepts: they are safe for the environment and living
organisms, most L-amino acids and cholesterol are commer-
cially available and relatively cheap, and the relevant synthetic
developed some efficient LMGs through combination of the
two components.8 In this study, we describe the synthesis of
rarely reported. The reason might be that water competes for
the hydrogen bonding sites in the gelator molecules, and
thereby disrupts the self-association of the gelator and so
new LMGs (Scheme 1) based on L-phenylalanine and
Key Laboratory of Applied Surface and Colloid Chemistry (Shaanxi
Normal University), Ministry of Education, School of Chemistry and
Materials Science, Shaanxi Normal University, Xi’an, 710062,
P. R. China. E-mail: yang@snnu.edu.cn; Fax: +86-29-85310097;
Tel: +86-29-85310081
w Electronic supplementary information (ESI) available: XRD pat-
terns of powder samples 1–4 and of xerogels of compounds 1–4 from
cyclohexane. SEM images of xerogels of 1/cyclohexane. See DOI:
Scheme 1 Molecular structures of the diacid amides of dicholesteryl
L-phenylalaninate.
10.1039/b807576j
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cholesterol, and their gelation properties. To our delight, we
observed firstly the selective gelation of kerosene, toluene or
xylene from their mixtures with water without the need of
heating or addition of a co-solvent. More interestingly, the gels
of the organic solvents possess salt resistant properties, adding
value for the practical uses of the systems.
Table 2 The critical gelation concentrations (CGC) of the gels of
different gelators in cyclohexane
Gelator
1
2
3
4
CGC (%, w/v)
2.00
1.17
1.11
0.28
and there is no appreciable viscosity difference between the
solvents and the solutions.
Results and discussion
Further investigation reveals that the critical gelation con-
centrations (CGC) are also spacer length dependent. As
examples, Table 2 provides the CGC values of the gelators
in cyclohexane. Clearly, CGC decreases gradually with
increasing the spacer length of the gelators. For gelator 4,
the value is only 0.28% (w/v), which is quite low if compared
with those of other cholesterol-based gelators.
Gelation behavior of the compounds
The gelation properties of compounds 1–4 were examined in
various organic solvents at a concentration of 2.5% (w/v) by
the ‘‘stable to inversion of a test tube’’ method and the results
are listed in Table 1. With reference to the data shown in
Table 1, it can be seen that 1, 2, 3 and 4 are versatile LMGs
and can gelate 20, 20, 19 and 18 of the 29 solvents tested,
respectively.
It is somewhat surprising to find that seven of the gels listed
in Table 1 could be formed at room temperature without
sonication, or heating. For example, dissolving 2 in kerosene,
toluene or xylene at room temperature, and then keeping the
solution without agitation resulted in gelation spontaneously.
A similar behavior was also observed for 3/1-pentanol,
3/1-hexanol, 3/1-heptanol and 3/kerosene. The kinetics of
the gel formation processes differ from each other. At 25 1C,
the gel of 3/kerosene formed within 10 min. For other systems,
however, the gels formed 1–12 h later after dissolution of
the gelators in the solvents. It is of note that variation of
temperature produces little effect upon the formation of the
gels of 2 in toluene and xylene even down to 0 1C. In contrast,
for other systems decreasing temperature prohibits gelation.
Specifically, 12 h at 15 1C leads to no indication of gelation.
These findings demonstrate clearly that the length of the
spacer connecting the two cholesteryl units has a pronounced
effect upon the gelling performance of the compounds studied.
This is not difficult to understand because the length of the
spacer must affect the conformational and assembling beha-
viors of the gelators, and thereby result in different supra-
molecular structures in the solvents.
Although the numbers of solvents gelated by each of the
four gelators are almost the same, differences between them
still exist. For example, unlike others, 4 can gelate only a few
of the alcohols tested in the experiment, and similarly 1 can
gelate ethyl acetate, DMF and acetic acid, but others cannot.
Another interesting comparison is among benzene, toluene,
xylene and CCl4, 2 and 4 show better gelation ability even
though the systems of 2/CCl4, 4/benzene, and 4/CCl4 are
marked as ‘‘s’’, but they are viscous solutions. By contrast,
dissolution of 1 and 3 in the solvents is a spontaneous process,
Table 1 Gelation performances of some diacid amides of dicholes-
teryl L-phenylalaninate in various organic solventsa
Solvents
1
2
3
4
Methanol
Ethanol
I
I
I
PG
S
I
PG
P
PG
G
P
P
P
P
P
G
PG
G
G
G
G
G
S
G
G
G
G
G
G
G
G
G
G
I
G
G
G
G
P
PG
PG
PG
G
G
G
G
G
G
G
1-Propanol
1-Butanol
1-Pentanol
1-Hexanol
1-Heptanol
1-Octanol
1-Nonanol
1-Decanol
Cyclohexane
n-Hexane
n-Heptane
n-Octane
n-Nonane
n-Decane
Kerosene
Benzene
PG
G*
G*
G*
G
G
G
G
G
G
G
G
G
G*
S
Scanning electron microscopy (SEM) studies
The gel systems of cyclohexane were adopted as examples and
investigated in order to see how the network structures of the
gels were influenced by the lengths of the spacers of the
gelators. Xerogels were prepared by freeze-drying of each of
the gels (3%, w/v). Fig. 1 shows the SEM images of the
xerogels. With reference to the images, it can be seen that
the network structures are very different from each other
although they are all well developed, a distinct spacer length
effect.
I
G
G
G
G
G*
TG
TG*
TG*
S
S
S
S
S
Toluene
Xylene
CCl4
S
S
S
TG
G
S
Ethyl acetate
Acetone
Ether
Triethylamine
Pyridine
DMSO
G
G
I
G
S
G
G
G
P
I
G
G
S
P
P
P
P
PG
G
PG
G
PG
S
Further examination of the SEM images reveals: (1) the
morphologies of the gel networks in this investigation varied
from ribbons (Fig. 1(a)) to sheets (Fig. 1(b) and (c)) and fibers
(Fig. 1(d)); (2) unlike 1 and 2 in the gels, the aggregate of 3 in
the solvent adopted beautiful and nearly perfect feather-like
structures, which is rarely reported in the literature; (3) the
morphology of the xerogel of 4/cyclohexane is characterized
by routine fibrous networks; (4) a further difference among the
aggregates is the flexibility of the aggregates. Clearly, the
aggregates of 1 and 2 are more brittle than those of 3 and 4,
G
G
G
S
G
P
DMF
Acetic acid
P
PG
P
a
Concentration of gelator: 2.5%, w/v; G: turbid gel; PG: partial gel;
TG: transparent gel; P: precipitate; I: insoluble; S: solution; G*: gels
forming at room temperature.
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Fig. 2. With reference to the spectrum of the control, it can be
observed that the two typical bands of 4 corresponding to the
stretching vibrations of NH and CQO in the amide, appeared
at 3321 and 1650 cmꢁ1, respectively. Upon gelation, the bands
shifted to 3311 and 1644 cmꢁ1, respectively, suggesting the
participation of the amide NH and CQO groups in hydrogen
bonding.9 Similar spectral shifts were also found for other gel
systems (Table 3). It is somewhat surprising that the direction
of the spectral shifts of the systems containing 2 is different
from other gel systems (Table 3), indicating that a different
aggregation mode might be adopted by this system.10 It is to
be noted that in contrast, the characteristic absorptions of the
ester CQO groups did not change very much during the
gelation processes (Fig. 2 and Table 3), an evidence of their
non-participation in the aggregation processes.
1H NMR spectroscopy studies
The aggregation behavior of the gelators in gel and solution
state was studied in detail by conducting temperature- depen-
Fig. 1 SEM images of the gel networks of the gelators of different
spacer length n in cyclohexane: (a) n = 1; (b) n = 2 (inset, high-
magnification image); (c) n = 3 (inset, high-magnification image);
(d) n = 4.
1
dent H NMR measurements of 2 and of 4 in benzene-d6 in
order to obtain further information on the formation mecha-
nism of the gel networks. The results are shown in Fig. 3 and 4,
respectively. It can be seen that change in temperature has a
distinct effect upon the chemical shifts of the N–H protons of
the gelators. For the system of 2 in benzene-d6, the signal was a
broad band and appeared at 6.55 ppm at 25 1C, but it
gradually shifted to up-field (6.37–6.40 ppm) and separated
into two peaks at 65 1C. As mentioned already, 4 can not
gelate benzene under the experimental conditions, but dis-
solving it into the solvent can increase the viscosity of the
solution significantly, an evidence of association of the gelator
molecules in the system.11 This association was confirmed by
an obvious up-field shift of the 1H NMR signal of N–H upon
increasing the temperature of the solution (Fig. 4). Again, the
NH group of 4 plays a role in the self-assembly of the gelator
molecules, in support of the tentative conclusion from FTIR
studies.
as revealed by the existence of the breakpoints in the xerogels
of 1/cyclohexane and 2/cyclohexane (Fig. 1(a) and (b)). These
observations demonstrate that only one CH2 difference can
result in a great difference in the structures of the gel networks.
FT-IR spectroscopy studies
Considering the structures of the LMGs prepared in this work,
and the well-known fact that hydrogen bonding is one of the
main driving forces for the self-assembly of cholesterol-based
LMGs, it is expected that intermolecular hydrogen bonding
should exist in the gel systems studied. Accordingly, FT-IR
measurements were conducted due to their usefulness in
revealing such intermolecular interaction. During the mea-
surement, 4/cyclohexane was adopted as a representative
system, and the solution of 4 in CDCl3 was chosen as a
reference. The FTIR spectra of both systems are depicted in
XRD studies
The packing structures of the gelator molecules in gels with
cyclohexane as a solvent and in the solid state were investi-
gated using XRD technique (see ESIw). The major peaks
observed are listed in Table 4. It can be seen that the diffrac-
tion pattern of the powder sample of compounds 1 and 2 are
characterized by four obvious reflection peaks, and the ratio of
the d values is 1 : 1/2 : 1/3 : 1/4, suggesting a perfect lamellar
organization.12 Further examination of the XRD data of the
xerogels reveals that compound 1 adopted a similar structure
via the aggregation of the gelator 1 in the gel state, even
though peak 3 did not appear in its XRD pattern. However
the aggregated structure of gelator 2 in the gel state is different
from that of 2 in the solid state. As for compounds 3 and 4, it
was impossible to deduce detailed structural information as a
result of the limited data available in the XRD patterns.
The interlayer distance of the lamellar structure, the d
spacing, just equals the molecular length (4.11 nm) of the
gelator 1, indicating that the advanced structures of the
aggregates of 1 in the gel state might be based on this
Fig. 2 FT-IR spectra of (a) a gel of 4 in cyclohexane and (b) a
solution of 4 in CDCl3.
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Table 3 FT-IR data for gelators in CDCl3 (solution) and in cyclohexane (gel)
n~max/cmꢁ1
NH stretch
CQO ester
CQO amide
NH bend
1
2
3
Solution
Gel
Solution
Gel
Solution
Gel
3446
3320
3296
3300
3300
3284
1738
1736
1737
1737
1736
1740
1648
1652
1646
1646
1651
1648
1540
1525
1539
1533
1539
1536
fundamental structure (Fig. 5). By reference to the model
shown in Fig. 5, it can be seen that it is characterized by
hydrogen bonds between the spacers, as already demonstrated
by FTIR and 1H NMR studies, and the aggregation of the
cholesteryl units of the gelators. Furthermore, the molecules of
the gelator are arranged in parallel to form spherical aggre-
gates. The aggregates are further amalgamated into a 1D
fibrous structure, and eventually these elemental fibers form
ribbons via the above-mentioned interactions. These hypo-
theses have been proved by observing the SEM images of the
aggregates of 1 in the solvent at different concentrations (see
ESIw).
Selective gelation of xylene from its mixture with water
Selective gelation of organic solvents, such as commercial oil
and aromatic solvents, from their mixtures with water is very
important in view of its applications in oil extraction, collec-
tion of leaked oil, and purification of water contaminated by
oil.5 Interestingly, 2 displays a remarkable ability to gelate
xylene selectively from its mixture with water without the need
of heating or addition of another co-solvent. 3 gelates
kerosene from its mixture with water in a similar way. To
our knowledge, 2 and 3 are the first examples of this kind of
gelators even though other selective gelators have been
reported. Unlike 2 and 3, the gelators as reported in the
literature can only show selective gelation with the aid of
heating or addition of a co-solvent.
Fig. 3 Temperature-dependent 1H NMR spectra of the gel of 2 in
benzene-d6 at a concentration of 3% (w/v).
Fig. 4 Temperature-dependent 1H NMR spectra of the solution of 4
in benzene-d6 at a concentration of 3% (w/v).
Table 4 Major peaks in the XRD patterns for compounds 1–4
d-Spacing/nm
1
2
3
4
Solid: 4.21, 2.12, 1.43, 1.07; gel: 4.05, 2.10
Solid: 4.58, 2.29, 1.62, 1.09; gel: 2.79
Solid: 1.33, 0.99; gel: 2.61
Fig. 5 Schematic representation of the possible aggregation mode of
1 in cyclohexane.
Solid: 1.38, 1.02; gel: 4.14, 2.2
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Fig. 6 Selective gelation of xylene from its mixture with water.
Fig. 6 shows a typical result of the selective gelation of
xylene by 2 from a xylene/water mixture. In the experiment,
10 mL of water was mixed with 0.1 mL of xylene in a 25 mL
weighing bottle, then a amount of 2 was added, and then the
bottle was shaken gently until 2 was dissolved. After some time
an organogel formed. Importantly, the selective gelation still
works at temperatures as low as 0 1C, which is believed to be
very crucial for practical uses. In contrast, selective gelation of
kerosene by 3 from its mixture with water only works at a
temperature range between 25 and 30 1C.
with increasing the frequency, specifically, from 100 Pa for G0
and 40 Pa for G00 at 0.628 rad sꢁ1 to 600 Pa for G0 and 200 Pa
for G00 at 300 rad sꢁ1, a typical viscoelastic behavior, suggest-
ing that the gel has a good tolerance to external forces. With
further increasing the frequency, however, the G0 and G00 of the
gel of 2 in xylene become erratic (e.g. f 4 300 rad sꢁ1), an
indication of breaking of the gel. These results demonstrate
that this system is a very competitive candidate for gentle
separation of xylene from its mixture with water.
Conclusion
Rheological properties of the gels
The mechanical properties of the gels above are important for
practical separation of them from water, and thereby their
storage modulus G0, associated with the energy storage, and
loss modulus G00, associated with the loss of energy, were
measured as functions of shear stress at room temperature by
using 2/xylene (3.0%, w/v) as an example. The results are
shown in Fig. 7. By reference to Fig. 7(a), it is seen that at low
stress values (below 10 Pa), G0 is about one order of magnitude
greater than its corresponding G00, indicating that the gel is
dominated by elastic character.13 Furthermore, both G0 and
G00 remain roughly constant below a critical stress value of
around 50 Pa, known as the yield stress value. Frequency
sweep is a method to detect the tolerance performance of a
material to external forces. Accordingly, this test was also
conducted for the sample gel, and the results are shown in
Fig. 7(b). Clearly, both G0 and G00 of the gel increased slightly
It has been demonstrated that the diacid amides of dicholes-
teryl L-phenylalaninate described in this article can act as
versatile gelators. Both the aggregation mode, the morpho-
logies of the gel networks, and the gelation abilities of the
compounds are very dependent upon the length of the spacer
connecting the two cholesteryl moieties. Within the gel systems
studied, at least seven of them, including 2/kerosene,
2/toluene, 2/xylene, 3/kerosene, 3/1-petanol, 3/1-hexanol and
3/1-heptanol, gel spontaneously at room temperature. Impor-
tantly, 2 can gel xylene and 3 can gel kerosene selectively from
mixtures of the two solvents with water, respectively. More
importantly, heating and cooling cycle or addition of a
co-solvent, which is recognized as an essential procedure for
selective gelation, is unnecessary for the systems discovered.
Furthermore, rheological studies revealed that the gels of
2/xylene are soft, but mechanically strong enough for collection,
Fig. 7 (a) Determination of the linear regime. Measurement of the evolution of G0 and G00 as a function of the applied shear stress, and the
frequency of the measurement is f = 1 Hz. (b) Evolution of G0 and G00 as a function of the frequency. The applied shear stress is equal to 0.5 Pa,
and the experiment is performed in the linear regime. The sample is a gel of 2 (c = 3.0%, w/v) in xylene, and the experiments were performed
at 25 1C.
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and thereby 2 is potentially applicable for the separation of the
solvent from its mixture with water. FTIR and temperature-
calc. (%) for C76H112O6N2 (1148.85): C, 79.39; H, 9.82; N,
2.44; Found: C, 79.18; H, 10.02; N, 2.24.
3: Yield: 55%. IR (KBr) n/cmꢁ1: 3306 (nN–H, amide), 1738
(nCQO, ester), 1649 (nCQO, amide I), 1539 (dN–H, amide II).
1H NMR (300 MHz, CDCl3, TMS, 25 1C): d 7.71–7.74
(d, J = 8.2 Hz, CONH, 2H), 7.26 (m, benzyl, 10H), 5.38
(s, alkenyl, 2H), 4.80–4.82 (d, CH(CH3)COO, 2H), 4.64
(s, oxycyclohexyl, 2H), 3.09–3.10 (d, –CH2(C6H5), 4H),
2.91–2.87 (m, COCH2, –CH2CO, 4H), 2.00–2.28 (m, –CH2–,
2H), 0.69–1.93 (m, cholesteryl protons, 86H); elemental ana-
lysis: calc. (%) for C77H114O6N2 (1162.87): C, 79.47; H, 9.87;
N, 2.41; Found: C, 78.77; H, 10.05; N, 2.24.
1
dependent H NMR studies revealed that hydrogen bonding
plays an important role for the formation and maintenance of
the gel networks.
Experimental
Materials
Cholesteryl, L-phenylalaninate, dicyclohexylcarbodimide
(DCC), N,N-dimethylaminopyridine (DMAP) and diacid were
supplied from the Sinopharm Chemical Reagent Co., Ltd, and
used without further purification. All solvents used in the
syntheses were purified, dried, or freshly distilled as required.
4: Yield: 63%. IR (KBr) n/cmꢁ1: 3315 (nN–H, amide), 1740
(nCQO, ester), 1651 (nCQO, amide I), 1537 (dN–H, amide
1
II). H NMR (300 MHz, CDCl3, TMS, 25 1C): d 7.15–7.29
(m, benzyl, 10H), 6.19 (s, CONH, 2H), 5.36 (s, alkenyl, 2H),
4.82–4.84 (d, CH(CH3)COO, 2H), 4.60 (s, oxycyclohexyl, 2H),
3.10 (s, –CH2(C6H5), 4H), 2.18–2.23 (m, COCH2, –CH2CO,
4H), 0.68–1.99 (m, –CH2CH2–, cholesteryl protons,
90H); elemental analysis: calc. (%) for C78H116O6N2
(1176.88): C, 79.54; H, 9.93; N, 2.38; Found: C, 78.93; H,
10.18; N, 2.28.
General procedure for the synthesis of compound 1
Cholesteryl L-phenylalaninate primary amine was synthesised
according to a previous report. malonic acid (0.208 g, 2 mmol)
was dissolved in 20 mL tetrahydrofuran (THF), and then
40 mL of THF solution containing 1.83 g (4 mmol) of
cholesteryl phenylalaninate primary amine and 0.824
g
(4 mmol) of dicyclohexylcarbodimide (DCC) was added drop-
wise to the above solution with stirring. The reaction mixture
was stirred at 50 1C for 10 h. After the reaction, the mixture
was filtered, the filtrate was evaporated to dryness, and the
resulting solid was washed with hot methanol three times, and
dried in vacuum to give the desired product as a white or
yellowish powder. Satisfactory results were obtained.
1: Yield: 30%. IR (KBr) n/cmꢁ1: 3305 (nN–H, amide), 1739
(nCQO, ester), 1658 (nCQO, amide I), 1525 (dN–H, amide
II); 1H NMR (300 MHz, CDCl3, TMS, 25 1C) d 7.14–7.25
(m, benzyl, CONH, 12H), 5.36 (s, alkenyl, 2H), 4.77–4.79
(d, CH(CH3)COO, 2H), 4.58 (s, oxycyclohexyl, 2H), 3.14
(s, –CH2(C6H5), 4H), 3.09 (s, COCH2CO, 2H), 0.68–2.47
(m, cholesteryl protons, 86H); elemental analysis: calc. (%)
for C75H110N2O6 (1134.84): C, 79.32; H, 9.76; N, 2.47; Found:
C, 78.64; H, 9.85; N, 2.29.
General methods
Measurements. The elemental analyses were performed
using CHNS/O analyzer Vario EL III. Scanning electron
micrograph (SEM) images of the xerogel were taken on a
Quanta 200 scanning electron microscopy spectrometer
(Philips-FEI). The accelerating voltage was 15 kV, and the
emission was 10 mA. Rheological measurements were per-
formed using a stress-controlled rheometer (TA instrument
AR-G2) equipped with aluminum-coated parallel-plate
geometry (40 mm diameter, 0.400 mm gap between the two
plates). A solvent-trapping device was placed above the plate
to avoid evaporation. The FT-IR spectra were recorded by a
Brucher EQUINX55 spectrometer in an attenuated total
reflection (ATR) way with ZnSe as sample slot. 1H NMR
data of samples were collected on Bruker AVANCE 300 MHz
spectrometer. X-Ray diffraction (XRD) data of samples were
collected on a D/Max-2550/PC with Cu-Ka X-ray radiation
generated under a voltage of 40 kV and a current of 40 mA.
General procedure for the synthesis of compounds 2, 3 and 4
Cholesteryl L-phenylalaninate primary amine (1.83 g, 4 mmol)
and succinic acid (0.24 g, 2 mmol) were dissolved in 50 mL
tetrahydrofuran, and the mixture was stirred at 0 1C. To the
system, 0.824 g (4 mmol) of dicyclohexylcarbodimide (DCC)
and 0.048 g (0.4 mmol) of N,N-dimethylaminopyridine
(DMAP) were added, and the mixture stirred at 0 1C for
4 h. After the reaction, the mixture was filtered, the filtrate was
evaporated to dryness, and the resulting solid was washed with
hot methanol for three times, and dried in vacuum to give a
desired product as a white powder. The procedures used for
the preparation of 3 and 4 are similar to that for 2. Satisfactory
results were obtained.
The scan rate was 0.51 minꢁ1
.
Gelation test. A weighed amount of potential gelator and a
measured volume of selected pure organic solvent were placed
into a sealed glass tube (10 mm i.d.), and the system was
heated in an oil or water bath until all solid materials were
dissolved. The solution was cooled to room temperature
(B25 1C) in the air, and finally, the test tube was turned
upside down to observe if the solution inside could still flow. A
positive test is obtained if the flow test is negative. It is to be
noted that some of the gels obtained are turbid (G), but some
are transparent (TG). In some cases, solution and gel may
coexist within a system and they are referred to as ‘‘partial
gels’’ (PG). Systems in which only solution remained until the
end of the tests are referred to as solution (S). Systems that
are clear solutions when they are hot but in which precipita-
tion or crystallization occurs when they are cooled down to
room temperature are denoted by P (precipitation) and R
2: Yield: 48%. IR (KBr) n/cmꢁ1: 3255 (nN–H, amide), 1739
(nCQO, ester), 1640 (nCQO, amide I), 1540 (dN–H, amide
1
II). H NMR (300 MHz, CDCl3, TMS, 25 1C): d 7.15–7.26
(m, benzyl, 10H), 6.43 (s, CONH, 2H), 5.36 (s, alkenyl, 2H),
4.79 (s, CH(CH3)COO, 2H), 4.59 (s, oxycyclohexyl, 2H), 3.08
(s, –CH2(C6H5), 4H), 2.29–2.49 (d, COCH2, –CH2CO, 4H),
0.68–1.99 (m, cholesteryl protons, 86H); elemental analysis:
ꢀc
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View Article Online
(recrystallization), respectively. Insoluble systems, in which
the potential gelator could not be dissolved even at the boiling
point of the solvent, were also found, and they are labeled as
insoluble (I).
S. Shinkai, Angew. Chem., Int. Ed., 2003, 42, 980; (c) S. Nayak
and L. A. Lyon, Angew. Chem., Int. Ed., 2005, 44, 7686;
(d) J. H. Jung, J. A. Rim, E. J. Cho, S. J. Lee, I. Y. Jeong,
N. Kameda, M. Masuda and T. Shimizu, Tetrahedron, 2007, 63,
7449; (e) G. Tan, M. Singh, J. He, V. T. John and
G. L. McPherson, Langmuir, 2005, 21, 9322.
SEM. The xerogel was prepared by freezing the gel in liquid
nitrogen, and then evaporated by a vacuum pump for 12–24 h.
Prior to examination, the xerogel was attached to a copper
holder by using conductive adhesive tape, and then it was
coated with a thin layer of gold.
3 (a) P. Terech and R. G. Weiss, Chem. Rev., 1997, 97, 3133;
(b) N. Mohmeyer and H. W. Schmidt, Chem.–Eur. J., 2007, 13,
4499; (c) H. F. Chow and G. X. Wang, Tetrahedron, 2007, 63,
7407; (d) M. George, G. Tan, V. T. John and R. G. Weiss,
Chem.–Eur. J., 2005, 11, 3243; (e) B. Q. Huang, A. R. Hirst,
D. K. Smith, V. Castelletto and I. W. Hamley, J. Am. Chem. Soc.,
2005, 127, 7130; (f) W. G. Weng, J. B. Beck, A. M. Jamieson and
S. J. Rowan, J. Am. Chem. Soc., 2006, 128, 11663; (g) S. Kawano,
N. Fujita and S. Shinkai, Chem.–Eur. J., 2005, 11, 4735.
4 (a) M. de Loos, B. L. Feringa and J. H. van Esch, Eur. J. Org.
Chem., 2005, 3615; (b) N. M. Sangeetha and U. Maitra, Chem. Soc.
Rev., 2005, 34, 821; (c) A. J. Wilson, Soft Matter, 2007, 3, 409;
(d) L. A. Estroff and A. D. Hamilton, Chem. Rev., 2004, 104, 1201;
(e) T. Tu, W. Assenmacher, H. Peterlik, R. Weisbarth, M. Nieger
Rheology. The first step of an experiment consists of deter-
mining the so-called linear regime of the gel. This was done by
measuring the storage modulus G0, associated with the energy
storage, and the loss modulus G00, associated with the loss of
energy, as a function of the stress amplitude. The linear regime
is such that both dynamic moduli are independent of the stress
amplitude and reflect the properties of the unperturbed
network. In that regime, G0 and G00 are measured as functions
of the oscillatory shear stress.
and K. H. Dotz, Angew. Chem., Int. Ed., 2007, 46, 6368;
¨
(f) R. Davis, R. Berger and R. Zentel, Adv. Mater., 2007, 19,
3878; (g) X. Dou, W. Pisula, J. S. Wu, G. J. Bodwell and
K. Mullen, Chem.–Eur. J., 2008, 14, 240; (h) M. Suzuki,
¨
C. Setoguchi, H. Shirai and K. Hanabusa, Chem.–Eur. J., 2007,
13, 8193.
In the second step of the experiment, the modulus as a
function of frequency was measured from 0.1 Hz to 100 Hz at
a constant shear stress of 0.5 Pa in the linear regime.
5 (a) M. Suzuki, T. Sato, H. Shirai and K. Hanabusa, New J. Chem.,
2006, 30, 1184; (b) D. R. Trivedi, A. Ballabh and P. Dastidar,
Chem. Mater., 2003, 15, 3971; (c) S. Bhattacharya and
Y. Krishnan-Ghosh, Chem. Commun., 2001, 185; (d) D. Khatua
and J. Dey, Langmuir, 2005, 21, 109.
´
6 M. J. Huertas, E. Duque, L. Molina, R. Rossello-Mora,
G. Mosqueda, P. Godoy, B. Christensen, S. Molin and
J. L. Ramos, Environ. Sci. Technol., 2000, 34, 3395.
FT-IR. The samples were placed on a glass or a mica slide as
a gel film, then frozen in liquid nitrogen, and finally dried in
vacuum condition for 12–24 h.
XRD. The xerogel was prepared by slow evaporation of the
solvent in the atmosphere of cyclohexane.
7 (a) P. C. Xue, R. Lu, G. J. Chen, Y. Zhang, H. Nomoto,
M. Takafuji and H. Ihara, Chem.–Eur. J., 2007, 13, 8231;
´
(b) M. Zinic, F. Vogtle and F. Fages, Top. Curr. Chem., 2005,
¨
256, 39.
8 (a) Y. G. Li, Y. Fang, J. Liu and M. Z. Wang, J. Chin. Chem. Soc.,
2006, 53, 359; (b) Y. G. Li, K. Q. Liu, J. Liu, J. X. Peng, X. L. Feng
and Y. Fang, Langmuir, 2006, 22, 7016; (c) J. Liu, J. L. Yan,
X. W. Yuan, K. Q. Liu, J. X. Peng and Y. Fang, J. Colloid
Interface Sci., 2008, 318, 397; (d) J. X. Peng, K. Q. Liu, J. Liu,
Q. H. Zhang, X. L. Feng and Y. Yu Fang, Langmuir, 2008, 24,
2992.
9 N. Yamada, T. Imai and E. Koyama, Langmuir, 2001, 17, 961.
10 J. L. Li, X. Y. Liu, C. S. Strom and J. Y. Xiong, Adv. Mater., 2006,
18, 2574.
Acknowledgements
We thank the Natural Science Foundation of China (Nos.
2077308 and 20674048), the Ministry of Science and Tech-
nology of China (No. 2007AA032349) and the Ministry of
Education of China (No. 306015) for financial support.
11 (a) K. A. Haushalter, J. Lau and J. D. Roberts, J. Am. Chem. Soc.,
1996, 118, 8891; (b) F. S. Schoonbeek, J. H. van Esch, R. Hulst,
R. M. Kellogg and B. L. Feringa, Chem.–Eur. J., 2006, 14, 2633;
(c) B. Escuder, S. Martl and J. F. Miravet, Langmuir, 2005, 21,
6776.
12 (a) G. S. Lim, B. M. Jung, S. J. Lee, H. H. Song, C. Kim and
J. Y. Chang, Chem. Mater., 2007, 19, 460; (b) Y. F. Zhou, T. Yi,
T. C. Li, Z. G. Zhou, F. Y. Li, W. Huang and C. H. Huang, Chem.
Mater., 2006, 18, 2974.
13 (a) Z. M. Yang, G. L. Liang and B. Xu, Chem. Commun., 2006,
738; (b) P. Terech and S. Friol, Tetrahedron, 2007, 63, 7366;
(c) M. George, G. P. Funkhouser, P. Terech and R. G. Weiss,
Langmuir, 2006, 22, 7885; (d) S. P. Li, W. G. Hou, D. J. Sun,
P. Z. Guo and C. X. Jia, Langmuir, 2003, 19, 3172;
References
1 (a) S. H. Kang, B. M. Jung and J. Y. Chang, Adv. Mater., 2007, 19,
2780; (b) K. Sugiyasu, N. Fujita and S. Shinkai, Angew. Chem., Int.
Ed., 2004, 43, 1229; (c) N. Koumura, M. Kudo and N. Tamaoki,
Langmuir, 2004, 20, 9897; (d) K. S. Moon, H. J. Kim, E. Lee and
M. Lee, Angew. Chem., Int. Ed., 2007, 46, 6807; (e) K. Isozaki,
H. Takaya and T. Naota, Angew. Chem., Int. Ed., 2007, 46, 2855;
(f) A. Ajayaghosh, C. Vijayakumar, R. Varghese and S. J. George,
Angew. Chem., Int. Ed., 2006, 45, 456; (g) T. Akutagawa,
K. Kakiuchi, T. Hasegawa, S. Noro, T. Nakamura,
H. Hasegawa, S. Mashiko and J. Becher, Angew. Chem., Int. Ed.,
2005, 44, 7283; (h) G. Mieden-Gundert, L. Klein, M. Fischer,
´
F. Vogtle, K. Heuze, J. L. Pozzo, M. Vallier and F. Fages, Angew.
´
¨
Chem., Int. Ed., 2001, 40, 3164.
2 (a) S. Kawano, S. Tamaru, N. Fujita and S. Shinkai, Chem.–Eur.
J., 2004, 10, 343; (b) K. J. C. van Bommel, A. Friggeri and
(e) M. Lescanne, P. Grondin, A. d’Aleo, F. Fages, J. L. Pozzo,
´
O. M. Monval, P. Reinheimer and A. Colin, Langmuir, 2004, 20,
3032.
ꢀc
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2008
2224 | New J. Chem., 2008, 32, 2218–2224