Amino acid based low-molecular-weight tris(bis-amido) organogelators

Article abstract of DOI:10.1039/c0sm01293a

Two new amino acid based low-molecular-mass organic gelators were designed, synthesized, and examined for their ability to gelate various organic solvents. The gelator molecules were found to aggregate via N-H...O hydrogen bonding to form an interwinding 3D network which immobilized a large number of organic solvents. Among the different organic solvents used in the gelation study mesitylene is found to be the best solvent. The microstructure of organogels was studied with FESEM and optical micrographs. The involvement of hydrogen bonding in the aggregation of the gelator molecule was studied using temperature dependent ¹H NMR. The obtained organogels were found to exhibit significant mechanical strength.

Full text of DOI:10.1039/c0sm01293a

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PAPER
Amino acid based low-molecular-weight tris(bis-amido) organogelators†
*
Suman Samai, Joykrishna Dey and Kumar Biradha
*
Received 10th November 2010, Accepted 6th December 2010
DOI: 10.1039/c0sm01293a
Two new amino acid based low-molecular-mass organic gelators were designed, synthesized, and
examined for their ability to gelate various organic solvents. The gelator molecules were found to
aggregate via N–H/O hydrogen bonding to form an interwinding 3D network which immobilized
a large number of organic solvents. Among the different organic solvents used in the gelation study
mesitylene is found to be the best solvent. The microstructure of organogels was studied with FESEM
and optical micrographs. The involvement of hydrogen bonding in the aggregation of the gelator
molecule was studied using temperature dependent 1H NMR. The obtained organogels were found to
exhibit significant mechanical strength.
that a chiral tripodal urea is an excellent LMOG to form elegant
Introduction
helical gel fibers from aqueous methanol or from aqueous
DMSO solvent.8 The helicity of the fibers was explained on the
basis of their intermolecular aggregation in the crystal structure.
Further, several groups have reported that the amino acid
containing straight chain amide molecules can also act as
gelators.2b,2d,9
Gels have potential applications in drug delivery, cosmetics,
cleaning agents, sensors, polymers, tissue engineering, enzyme-
immobilization matrices, as well as in phase selective gelation
and water purification by dye absorption.1,2 A very wide variety
of low-molecular-mass organic gelators (LMOGs) are known in
the literature.3,4 The common forces which are responsible for
supramolecular gel formations are some specific or noncovalent
interactions, such as electrostatic, dipole–dipole, hydrogen
bonding (H-bonding), p–p stacking, and van der Waals inter-
actions. The new molecular gelators with predefined properties
can be designed by the proper understanding of molecular
geometry as well as the various intermolecular forces. About two
decades ago Weiss and Lin serendipitously discovered the gelling
ability of a steroid derivative.5 Currently several different
approaches exist in the literature depending on the nature of
gelators. Several versatile molecules were shown to act as gela-
tors viz. fatty acid derivatives, steroid derivatives, anthryl
derivatives, molecules containing steroidal and condensed
aromatic rings, dendrimers n-alkanes, oligo(p-phenyl-
enevinylene), dipyridylurea–carboxylic acid combination,
diamine linked dendron, etc.6 To design a new gelator molecule
the understanding of molecular geometry as well as the various
intermolecular forces is an essential requirement. These inputs
can be readily availed from the wealth of information that exists
in the solid-state chemistry.7 For example, recently, in several
cases, the structure of gels was established by studying and
understanding the crystal structures of gelator molecules. In one
of the recent examples reported by Steed et al., it has been shown
The use of amino acid containing compounds as gelators
(LMOGs) is very important due to their inherent biocompati-
bility and easy functionalization for tuning the properties. These
qualities make LMOGs containing amino acids as excellent
candidates for the application in biomedicinal area.10 In the
present paper we would like to explore the self-assembly
behavior of a new tripodal chiral ligand (Scheme 1), linked with
amino acid units, particularly in their gel state and deduce the
structure and properties by various spectroscopic and other
studies. The molecules 3a (G-1) and 3b (G-2) were chosen for our
studies due to the following reasons. The presence of six amide
moieties can drive the aggregate formation through amide-to-
amide N–H/O hydrogen bonding. The nature of the
Department of Chemistry, Indian Institute of Technology, Kharagpur,
721302, India. E-mail: kbiradha@chem.iitkgp.ernet.in; Fax: +91-3222-
282252; Tel: +91-3222-283346
Scheme 1 Synthetic route to compounds G-1 and G-2. (i) N-Hydroxy
succinimide; DCC, dry THF, 0 ꢀC, 5 h; (ii) (a) tris(2-aminoethyl)amine,
DME, 0 ꢀC, 18 h; (b) reflux, 60 ꢀC, 6 h.
† Electronic supplementary information (ESI) available: Temperature
dependent 1H NMR. See DOI: 10.1039/c0sm01293a
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aggregation is predictable to some extent as there exists enor-
mous literature on the amide recognition patterns, especially for
the long alkyl substituents, bis- and tris(urea) LMOG deriva-
tives, which have proved to be highly effective gelators.11 The
hydrophobic attachments on the arms of the molecule can be
varied according to the requirements of the organic solvents. The
presence of L-amino acid moiety relates to the biologically
important peptide derivatives. Another interesting feature of
these molecules is the chirality, which is expected to transfer into
the supramolecular aggregation.
Fig. 2 Gels formed by G-1: (1) o-xylene; (2) toluene; (3) benzonitrile; (4)
chlorobenzene; (5) bromobenzene; (6) iodobenzene; (7) m-xylene; (8)
ethyl benzene; (9) p-xylene; (10) nitrobenzene and (11) benzyl chloride.
substitutions on the aromatic ring of a solvent decreased the
MGC values for both G-1 and G-2. In other words, G-1 or G-2 is
a good gelator for mesitylene than xylenes or toluene. The studies
with o-xylene and p-xylene indicate the position of methyl does
not have significant effect on MGC. However, a marginal
increase in MGC value was observed when the methyl group
(toluene) was changed to ethyl group (ethyl benzene). Among all
the solvents studied mesitylene is found to be the best gel-forming
solvent. This could be due to the match in the symmetries of
gelator and mesitylene. In the case of halo-substituted aromatic
solvents chlorobenzene is found to be the better gel-forming
solvent than bromobenzene or iodobenzene. This result reflects
in the better intermolecular interactions (due to polarization) of
iodo derivative with the gelator, thereby reducing the gelation
capability. Similarly, the reduction of gelation capability was
also observed in the case of nitrobenzene or benzonitrile both of
which have an excellent capability of hydrogen bonding to
amides of G-1 and G-2.
Result and discussion
Gelation behavior
Our initial efforts to crystallize these tripodal compounds G-1
and G-2 for establishing their aggregation in the solid state were
unsuccessful, despite several trials in different solvents and
conditions. Interestingly, on the other hand, we have found
a series of solvents in which the molecules G-1 and G-2 form
transparent, stiff and thermo-reversible gels (Fig. 1). The gel
formation in different solvents is confirmed by ‘inverted vial’
method (Fig. 2). The gelation behavior of G-1 and G-2 has been
summarized in Table 1. These two gelator molecules exhibit gel
formation ability with a large number of organic solvents. The
organogels were found to be stable at room temperature for six
to eight months. In all the cases, the gels show thermoreversi-
bility since the aggregation occurs through the weak interactions.
To compare gelation abilities, MGC (Minimum Gelator
Concentration) values (Table 1), which is defined as the
minimum amount of gelator required to gelate a given volume of
solvent, have been determined for both gelators G-1 and G-2.
The aromatic solvents containing alkyl or halogen group
attached to it were found to be preferably trapped by the gelator
G-1 or G-2. This could be due to increased self-aggregation of
G-1 or G-2 due to the hydrophobic nature of aromatic solvents
towards the hydrophilic functional groups, such as amides.
Further, this analogy was supported by the gelation studies with
solvents containing hydroxyl groups which have not resulted in
the gel formation in spite of good solubility of the gelators in
those solvents. This proves that the aggregation of the gelator
molecules was not possible if solvents contain functional groups
capable of forming hydrogen bonds with gelators.
Thermal behaviour
The thermal stability of the organogels was investigated by
measuring the gel–sol transition temperature (Tgel). It was
Table 1 Gelation properties of G-1 and G-2a
Gelation
behavior
MGC/mg mLꢁ1
(Tgel(ꢂ1 ꢀC))
Solvent used
G-1
G-2
G-1
G-2
Methanol
Ethanol
CL
CL
CL
CL
CL
Ya
Ya
Ya
Ya
Ya
Y
Y
Y
Y
Y
Y
Y
–
CL
CL
CL
CL
CL
Yb
Y
CVL
Ya
Ya
Y
—
—
—
—
Chloroform
Dichloromethane
Acetonitrile
Nitrobenzene
Benzonitrile
Benzylchloride
Chlorobenzene
Bromobenzene
Iodobenzene
Toluene
o-Xylene
m-Xylene
p-Xylene
Ethylbenzene
Mesitylene
Some important conclusions can be drawn from the results
summarized in Table 1. The increase of the number of
—
16.1 (80)
33.3 (64)
40.0
14.0 (88)
20.0
22.2 (73)
7.6 (78)
7.1 (83)
6.3 (86)
7.7 (86)
6.0 (83)
4.6 (98)
—
—
25.0 (82)
—
16.0 (85)
20.0 (98)
21.0 (87)
7.2 (82)
7.1 (97)
7.1 (90)
7.1 (90)
7.0 (91)
5.8 (95)
12.5 (80)
15.0
Y
Y
Y
Y
Y
Y
Y
Acetophenone
Cyclohexanone
Hexane
CVL
INS
Ya
INS
—
—
a
INS ¼ insoluble, Y ¼ forming gel within 30 min, Ya ¼ it takes several
hours to form gel, Yb ¼ gel formed after some days, CVL ¼ clear viscous
liquid, CL ¼ clear liquid, ‘—’ ¼ suitable data were not found.
Fig. 1 Illustrations depicting the thermoreversibility of the gelators (in
mesitylene) by inverted vial method.
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Fig. 3 FESEM images of organogels. Left image—G-1, dried gel from
mesitylene; and right image—G-2, dried gel from mesitylene.
determined by heating the sample vial in the temperature-
controlled water bath until tube inversion showed that the gel
had melted. The higher thermal stability of the organogels in
different solvents is shown by the high Tgel values (Table 1) and
can be attributed to the stronger H-bonding interaction among
gelator molecules. Mesitylene was found to be the best solvent
for the gelation which led to the formation of very tightly bound
networks through strong intermolecular H-bonding interaction.
Consequently, the organogel in mesitylene showed higher
thermal stability compared to the organogels obtained from
other solvents.
Fig. 5 POM image of G-2 in 50ꢄ magnification.
1
Temperature dependent H NMR study
It is well known that H-bonding is considered to be one of the
major driving forces for the supramolecular aggregation of
LMOGs.1a Recently a cyclohexane based amide-containing
tripodal LMOG has been reported where the molecules stacked
through the formation of a triple chain of intermolecular
hydrogen bonds.12a From the view point of molecular structure
the gelators G-1 and G-2 are expected to assemble in a number of
ways via N–H/O hydrogen bonds. Two of these possible
aggregations have been shown in Scheme 2. To get a quantitative
idea about the assembly of gelator molecules via H-bonding
interactions, a systematic temperature-dependent 1H NMR
study was performed by using the saturated solution of G-1 and
G-2 in CDCl3. 1H NMR measurements of these compounds
indicate that the amide protons are sensitive to changes in
temperature. At 20 ꢀC two amide protons of gelator G-1
appeared at d ¼ 6.095 and d ¼ 7.538 ppm, respectively. With the
gradual increase in temperature (in steps of 10 ꢀC) these two
protons experienced a notably upfield shifts and at 60 ꢀC the two
amide protons appeared at d ¼ 5.89 ppm and d ¼ 7.256 ppm,
respectively (Fig. S1, see ESI†). Similarly for gelator G-2 at 20 ꢀC
two amide protons appeared at d ¼ 6.064 ppm and d ¼ 7.722
ppm, respectively, which shifted to d ¼ 5.93 ppm and d ¼ 7.551
Microstructural study
The microstructure of organogels was examined by a field-
emission scanning electron microscope (FESEM). The FESEM
image (Fig. 3) of xerogel (dried gel) (G-1) from mesitylene shows
a 3D network of intertwined ribbons with a width of ꢃ100 nm.
Such a supramolecular network clearly depicts the better
entrapment of the solvent molecule. In the case of dried xerogel
G-2 from the same solvent, a lamellar morphology was observed.
This could be due to the presence of different hydrophobic group
in the side arm of the gelator molecule.
The visual image of the supramolecular aggregation of orga-
nogels G-1 and G-2 in mesitylene was also investigated by using
a polarised optical microscope (POM). Fig. 4 and 5 show the
POM images of the organogels of G-1 and G-2, respectively.
That the elongated fibers form a gel network are clearly seen in
the POM images.
ꢀ
ppm at 60 C (Fig. S2, see ESI†). The change in d value (N–H
proton of amino acid part each from G-1 and G-2) with
temperature is shown in Fig. 6. These results suggested that in the
gel state at room temperature intermolecular N–H/O hydrogen
bonding was present between the neighboring amide groups. The
observed upfield shift of amide protons with an increase in
Scheme
2 Possible aggregation of gelator molecule via N–H/O
Fig. 4 POM image of G-1 in 50ꢄ magnification.
hydrogen bonds.
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Fig. 7 Variation of storage modulus (G0) and loss modulus (G00) with
frequency of organogel G-1.
Fig. 6 Temperature-dependent chemical shift of amide protons of G-1
and G-2.
temperature is due to the disruption of intermolecular hydrogen
bonds between the amide functionalities leading to the phase
transition from gel to sol. From the above experimental evidence
it is very much clear that both the amide protons from each arm
of the gelator molecules were involved in the formation of N–
H/O hydrogen bond which is considered to be one of the most
significant driving forces for the supramolecular aggregation of
gelator molecule that immobilized the large volume of solvents.
Extensive research works have demonstrated the self-assem-
bling nature of the amide-containing C3 symmetrical molecules
via N–H/O hydrogen bond, particularly in their gel state and
solid state.8,12 Based on the plethora of existing literature and on
our own studies7,12d of the related molecules in the solid state, we
would like to propose the columnar or lamellar aggregation of
the gelator molecules as the possible models (Scheme 3) for the
gelator structures of G-1 and G-2.
Fig. 8 Variation of storage modulus (G0) and loss modulus (G00) with
frequency of organogel G-2.
supramolecular aggregation of gelator molecules. The rheolog-
ical analysis provides a detailed information about the structure,
consistency and phase transition process. For characterization of
the general rheological behavior and to provide information
about the stiffness of aggregation we have examined the orga-
nogels of G-1 and G-2 in mesitylene at room temperature.
Frequency sweep rheometry measurement of the organogels
(Fig. 7 and 8) indicates that the changes in elastic modulus,
Rheology
The rheological behavior is an important factor for the potential
applications of the gels. It is strongly influenced by the
Scheme 3 Schematic model for the possible way of aggregation of gelator molecules to form the gel network. (A) Columnar packing and (B) layered
arrangement of the gelator molecule.
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rich centre and showing the structural proximity with that of the
gelator molecules were proved to be the better solvent for gela-
tion. The gelation process was observed to be thermo-reversible.
It is interesting to note here that the urea related gels reported by
Steed et al. are stable even in the presence of water unlike the
present systems.8 This could be due to more hydrophobic nature
of the G-1 and G-2 compared to the urea based gelators. The
FESEM image suggests the different nature of aggregation mode
that depends on the structure of the gelator molecule. The
temperature-dependent 1H NMR study confirmed the impor-
tance of intermolecular H-bonding between amide linkages in the
gel-forming process.
Fig. 9 Variation of storage modulus (G0) and loss modulus (G00) with
shear stress (s) of organogel G-1.
Experimental details
Synthesis
N-Hydroxysuccinimide esters of N-Cbz-L-amino acids (2a,b)
were synthesized by following a literature procedure.13
General procedure for the synthesis of G-1 (3a). The N-
hydroxysuccinimide ester of N-Cbz-L-phenylalanine (2.0 g,
5.0mmol) was dissolved in dry DME (40 mL) cooled in an ice
bath. Tris(2-aminoethyl)amine (0.246 g, 1.6 mmol) dissolved in
dry DME (15 mL) was added as several small portions to the
above solution. The reaction mixture was stirred at room
temperature for 18 h and then warmed for 6 h at 40–50 ꢀC. The
white solid was filtered off and washed with cold water and cold
methanol. Yield (3.80 g, 76%); [a]D25 (1%, CH3OH) ¼ ꢁ1.28; mp
ꢀ
1
167–170 C; H NMR (200 MHz, CDCl3) d 1.26–1.32 (m, 3H),
1.70 (br, 3H), 2.20–2.60 (m, 9H), 2.95–3.07 (m, 6H), 4.78 (d, 3H,
J ¼ 12.6), 4.91 (d, 3H, J ¼ 12.6), 6.04 (br s, 3H), 7.15–7.23 (m,
30H), 7.54(br s, 3H); 13C NMR (50 MHz, CDCl3) d 33.97, 39.12,
54.91, 56.20, 66.74, 126.68, 127.62, 127.97, 128.36, 128.45,
129.47, 136.37, 136.91, 156.63, 172.39; ESI-MS for C57H63N7O9
(G-1) [M], [M + H]+ ¼ 990.5064. Anal. Calcd for C57H63N7O9: C,
69.16%; H, 6.37%; N, 9.91%; found: C, 68.66%; H, 6.35%; N,
9.93%.
Fig. 10 Variation of storage modulus (G0) and loss modulus (G00) with
shear stress (s) of organogel G-2.
G0, and viscous modulus, G00, with frequency follow the same
trend, and at any given frequency G0 values were much higher
than G00, indicating more elastic nature of the gels like solids.
Moreover, both G0 and G00 are frequency independent in the
linear viscoelastic regime. Such a linear viscoelastic nature is in
good agreement with classical gel and supports the belief that
these materials undergo a transition to a true gel state. The
difference between G0 and G00 can be regarded as the indication of
rigidity of the gel sample. The (G0 ꢁ G00) values were 2.28 ꢄ 105
Pa and 1.77 ꢄ 105 Pa for G-1 and G-2, respectively, indicating the
firmness and rigidness of the organogels. Fig. 9 and 10 show the
plots of G0 and G00 against shear stress (s) at a constant frequency
of 1 Hz. It can be observed that above a critical stress value both
G0 and G00 abruptly fall to a very low value, indicating flow of the
organogel. This critical stress value is referred to as yield stress
(sy). The sy value of G-2 as obtained from the breakpoint of the
respective plot is 1481 Pa and that for G-1 is 316 Pa. The sy
values are quite large, suggesting high mechanical strength of the
organogels. The higher sy value of the organogel of G-2
compared to that of G-1 suggests its greater mechanical strength.
Synthesis of G-2 (3b). This compound was synthesized as
described above starting from the N-hydroxysuccinimide ester of
N-Cbz-L-isoleucine and tris(2-aminoethyl)amine. Yield (3.18 g,
25
ꢀ
1
65%); [a]D (1%, CH3OH) ¼ ꢁ4.11; mp 192–194 C; H NMR
(200 MHz, CDCl3) d 0.853–0.967 (m, 18H), 1.18–1.25 (m, 3H),
1.59–1.79 (m, 6H), 2.37–2.59 (m, 9H), 3.26 (br s, 3H), 4.20–4.28
(m, 3H), 4.91 (d, 3H, J ¼ 12.4), 5.02 (d, 3H, J ¼ 12.4), 6.03–6.07
(m, 3H), 7.24–7.26 (m, 15H), 7.72 (br s, 3H); 13C NMR (50 MHz,
CDCl3) d 10.67, 15.31, 24.97, 37.22, 38.74, 55.12, 59.24, 66.73,
127.60, 127.97, 128.49, 136.58, 157.06, 172.95; ESI-MS for
C48H69N7O9 (G-2) [M], [M + H]+ ¼ 888.5341. Anal. Calcd for
C48H69N7O9: C, 64.93%; H, 7.78%; N, 11.04%; found: C,
65.66%; H, 7.81%; N, 10.98%.
Methods
The melting point of solid compounds was measured using
Fisher Scientific melting point apparatus. Optical rotations were
measured with a JASCO (Model P-1020) digital polarimeter.
Elemental analysis was carried out in PE 2400 series II CHN
Conclusions
This study presented a new class of organogelators that form
stable gels of significant mechanical strength in aromatic
solvents. The solvents such as mesitylene having more electron
1
analyzer. The H NMR spectra were recorded on an AVANCE
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K. Y. Lee and D. J. Mooney, Chem. Rev., 2001, 101, 1869; (d)
A. M. Bieser and J. C. Tiller, Chem. Commun., 2005, 3942.
DAX-400 (Bruker, Sweden) 200 MHz NMR spectrometer in
CDCl3 as solvent. All measurements were done at room
temperature (298 K) unless otherwise mentioned.
4 (a) M. George and R. G. Weiss, Acc. Chem. Res., 2006, 39, 489; (b)
N. M. Sangeetha and U. Maitra, Chem. Soc. Rev., 2005, 34, 821; (c)
A. Ajayaghosh and V. K. Praveen, Acc. Chem. Res., 2007, 40, 644;
(d) J. Becerril, M. I. Burguete, B. Escuder, S. V. Luis, J. F. Miravet
and M. Querol, Chem. Commun., 2002, 738; (e) P. Dastidar, Chem.
Soc. Rev., 2008, 37, 2699.
A weighed amount of solid gelators taken in screw caped vials
was dissolved in different organic solvents by heating and
subsequently allowed to stand at 25 ꢀC in a thermostatted water
bath to obtain a stable gel as confirmed by the resistance to flow
under gravity upon inversion of the vial.
5 Y.-c. Lin and R. G. Weiss, Macromolecules, 1987, 20, 414.
6 (a) K. Murata, M. Aoki, T. Susuki, T. Harada, H. Kawabata,
T. Komori, F. Ohseto, K. Ueda and S. Shinkai, J. Am. Chem. Soc.,
1994, 116, 6664; (b) A. Ajayaghosh and S. J. George, J. Am. Chem.
Soc., 2001, 123, 5148; (c) N. Pilpel, Chem. Rev., 1963, 63, 221; (d)
V. J. Bujanowski, D. E. Katsoulis and M. J. Ziemelis, J. Mater.
Chem., 1994, 4, 1181; (e) A. R. Hirst and D. K. Smith, Chem.–Eur.
J., 2005, 11, 5496; (f) A. Ajayaghosh, S. J. George and
V. K. Parveen, Angew. Chem., Int. Ed., 2003, 42, 332; (g)
D. J. Abdallah and R. G. Weiss, Langmuir, 2000, 16, 352; (h)
T. Nakashima and N. Kimizuka, Adv. Mater., 2002, 14, 1113; (i)
T. Brotin, R. Utermohlen, F. Fages, H. Bouas-Laurent and
J. P. Desvergne, J. Chem. Soc., Chem. Commun., 1991, 416; (j)
D. K. Smith, Adv. Mater., 2006, 18, 2773; (k) M.-
O. M. Piepenbrock, G. O. Lloyd, N. Clarke and J. W. Steed, Chem.
Rev., 2010, 110, 1960; (l) J. W. Steed, Chem. Soc. Rev., 2010, 39,
3686; (m) N. N. Adarsh, D. K. Kumar and P. Dastidar,
Tetrahedron, 2007, 63, 7386; (n) F. Placin, J.-P. Desvergne and J.-
C. Lassgues, Chem. Mater., 2001, 13, 117; (o) K. Hanabusa,
T. Miki, Y. Taguchi, T. Koyama and H. Shirai, J. Chem. Soc.,
Chem. Commun., 1993, 1382.
7 (a) M. Sarkar and K. Biradha, Cryst. Growth Des., 2006, 6, 202; (b)
L. Rajput, S. Singha and K. Biradha, Cryst. Growth Des., 2007, 7,
2788; (c) L. Rajput and K. Biradha, Cryst. Growth Des., 2009, 9, 40.
8 C. E. Stanley, N. Clarke, K. M. Anderson, J. A. Elder, J. T. Lenthall
and J. W. Steed, Chem. Commun., 2006, 3199.
9 (a) K. Hanabusa, R. Tanaka, M. Suzuki, M. Kimura and H. Shirai,
Adv. Mater., 1997, 9, 1095; (b) T. Patra, A. Pal and J. Dey, J.
Colloid Interface Sci., 2010, 344, 10; (c) A. Pal, S. Shrivastava and
J. Dey, Chem. Commun., 2009, 6997; (d) A. Ghosh and J. dey,
Langmuir, 2009, 25, 8466.
The gel melting temperature (Tgel) was determined by placing
the screw caped glass vials containing gels in a temperature-
controlled water bath (Julabo, Model F 12) and visually
observing the flow upon tilt for every degree rise in temperature.
A Field-Emission Scanning Electron Microscope (FESEM,
Zeiss, Supra-40) operating at 5–10 kV was used to get the
micrograph. For electron micrographs, the hot sample solution
was placed on the aluminium foil, allowed to cool at room
temperature and then dried in a desiccator for 24 h. A layer of
gold was sputtered on top to make conducting surface and finally
the specimen was transferred into the microscope.
For all the gel samples, the rheological measurements were
performed on a Bohlin CVO D100 (Malvern, UK) controlled-
stress rheometer using a 20 mm diameter parallel plate geometry
with a constant tool gap of 200 mm. The organogel was placed on
lower plate and a stress amplitude sweep experiment was carried
out at a constant frequency of 1 Hz at 25 ꢀC to obtain the storage
or elastic modulus, G0, and the loss or viscous modulus, G00. The
frequency sweep measurements were carried out at a constant
stress of 300 Pa in the linear viscoelastic range.
The polarizing optical light micrographs for the samples were
obtained from a LEICA DMLM (Germany) optical microscope
by transmitted light under crossed Nicol and fitted with a JVC-
KY-F550E imaging system. The samples for optical microscopy
contained gelators at a concentration less than the corresponding
minimum gelation concentration. A drop of the liquid was
placed on the microscope slide and placed under microscope.
10 (a) A. Vintiloiu and J.-C. Leroux, J. Controlled Release, 2008, 125,
179; (b) Y. A. Shchipunov, E. V. Shumilina and H. Hoffmann, J.
Colloid Interface Sci., 1998, 199, 218.
11 (a) F. S. Schoonbeek, J. H. van Esch, R. Hulst, R. M. Kellogg and
B. N. Feringa, Chem.–Eur. J., 2000, 6, 2633; (b) J. van Esch,
F. Schoonbeek, M. de Loos, H. Kooijman, A. L. Spek,
R. M. Kellogg and B. L. Feringa, Chem.–Eur. J., 1999, 5, 937; (c)
Y. Jeong, K. Hanabusa, H. Masunaga, I. Akiba, K. Miyoshi,
S. Sakurai and K. Sakurai, Langmuir, 2005, 21, 586; (d) J. Brinksma,
B. L. Feringa, R. M. Kellogg, R. Vreeker and J. van Esch, Langmuir,
2000, 16, 9249; (e) M. De Loos, A. G. J. Ligtenbarg, J. Van Esch,
H. Kooijman, A. L. Spek, R. Hage, R. M. Kellogg and
B. L. Feringa, Eur. J. Org. Chem., 2000, 3675.
12 (a) K. J. C. van Bommel, C. van der Pol, I. Muizebelt, A. Friggeri,
A. Heeres, A. Meetsma, B. L. Feringa and J. van Esch, Angew.
Chem., Int. Ed., 2004, 43, 1663; (b) M. M. J. Smulders,
A. P. H. J. Schenning and E. W. Meijer, J. Am. Chem. Soc., 2008,
130, 606; (c) P. Jonkheijm, P. van der Schoot, A. P. H. J. Schenning
and E. W. Meijer, Science, 2006, 313, 80; (d) L. Rajput,
V. V. Chernyshev and K. Biradha, Chem. Commun., 2010, 46, 6530;
(e) M. L. Bushey, T. Q. Nguyen, W. Zhang, D. Horoszewski and
C. Nuckolls, Angew. Chem., Int. Ed., 2004, 34, 5446; (f)
M. P. Lightfoot, F. S. Mair, R. G. Pritchard and J. E. Warren,
Chem. Commun., 1999, 1945; (g) J. J. van Gorp,
J. A. J. M. Vekemans and E. W. Meijer, J. Am. Chem. Soc., 2002,
124, 14759; (h) J. van Gestel, A. R. A. Palmans, B. Titulaer,
J. A. J. M. Vekemans and E. W. Meijer, J. Am. Chem. Soc., 2005,
127, 5490; (i) K. Hanabusa, C. Koto, M. Kimura, H. Shirai and
A. Kakehi, Chem. Lett., 1997, 26, 429; (j) A. J. Wilson, J. van
Gestel, R. P. Sijbesma and E. W. Meijer, Chem. Commun., 2006,
4404; (k) M. L. Bushey, A. Hwang, P. W. Stephens and
C. Nuckolls, Angew. Chem., Int. Ed., 2002, 41, 2828.
Acknowledgements
We acknowledge funding from DST for financial support. SS
thanks UGC for a senior research fellowship. We thank Dr S.
Dhara of the School of Medical Science and Technology, IIT
Kharagpur, for assistance with the rheological measurements.
References
1 (a) P. Terech and R. G. Weiss, Chem. Rev., 1997, 97, 3133; (b)
J. C. Tiller, Angew. Chem., Int. Ed., 2003, 42, 3072; (c) S. Kyonaka,
K. Sugiyasu, S. Shinkai and I. Hamachi, J. Am. Chem. Soc., 2002,
124, 10954; (d) S. Bhattacharya and Y. K. Ghosh, Chem. Commun.,
2001, 185; (e) S. Debnath, A. Shome, S. Dutta and P. K. Das,
Chem. –Eur. J., 2008, 14, 6870; (f) R. Denoyel and E. S. Rey,
Langmuir, 1998, 14, 7321.
2 (a) S. Kobayashi, N. Hamasaki, M. Suzuki, M. Kimura, H. Shinkai
and K. Hanabusa, J. Am. Chem. Soc., 2002, 124, 6550; (b)
D. Khatua, R. Maity and J. Dey, Chem. Commun., 2006, 4903; (c)
G. M. Whitesides, J. P. Mathias and C. T. Seto, Science, 1991, 254,
1312; (d) T. Patra, A. Pal and J. Dey, Langmuir, 2010, 26, 7761; (e)
S. Ray, A. K. Das and A. Banerjee, Chem. Mater., 2007, 19, 1633.
3 (a) L. A. Estroff and A. D. Hamilton, Chem. Rev., 2004, 104, 1201; (b)
D. J. Abdallah and R. G. Weiss, Adv. Mater., 2000, 12, 1237; (c)
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13 J. Becerril, M. Bolte, M. I. Burguete, F. Galindo, E. Garcıa-Espana,
S. V. Luis and J. F. Miravet, J. Am. Chem. Soc., 2003, 125, 6677.
2126 | Soft Matter, 2011, 7, 2121–2126
This journal is ª The Royal Society of Chemistry 2011