Morphology tunable organogels based on dihydrazide derivatives

Article abstract of DOI:10.1016/j.molliq.2012.07.001

We report on the organogels of a new series of compounds containing a dihydrazide unit in the rigid core and three long alkoxy chains. SEM images revealed that the morphologies of the xerogels strongly depend on the polarity of the gelling solvent. Textur

Full text of DOI:10.1016/j.molliq.2012.07.001

Journal of Molecular Liquids 173 (2012) 108–112
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Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Morphology tunable organogels based on dihydrazide derivatives
b
b
a
Binglian Bai a,b, , Jue Wei , Jian Zhao , Haitao Wang , Min Li
⁎
a,⁎⁎
a
Key Laboratory for Automobile Materials (JLU), Ministry of Education, Institute of Materials Science and Engineering, Jilin University, Changchun 130012, PR China
College of Physics, Jilin University, Changchun 130012, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
We report on the organogels of a new series of compounds containing a dihydrazide unit in the rigid core and
three long alkoxy chains. SEM images revealed that the morphologies of the xerogels strongly depend on the
polarity of the gelling solvent. Textured spheroid-like morphologies were observed in the xerogels from low
polar aromatic solvents such as benzene, whereas fibrous morphologies from ethanol. Further detailed
analysis of their aggregation modes was conducted by FT-IR and X-ray diffraction measurement. The cooperation
of hydrogen bonds, hydrophobic interactions and the van der Waals interactions should to be the key point of the
self-assembly.
Received 21 February 2012
Received in revised form 17 May 2012
Accepted 2 July 2012
Available online 11 July 2012
Keywords:
Organogels
Dihydrazide derivatives
Hydrogen bonding
Supramolecular chains
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
connected by thin ribbons in 1-butanol, while homogeneous cabbage
like spherical structures in xylene.
The organogels are a class of nanostructured materials composed
of a self-assembled superstructure of the low molecular weight
organogelators through specific noncovalent intermolecular interac-
tions including hydrogen bonds, hydrophobic interactions, π–π inter-
actions, or Van der Waals forces and a large volume of organic liquid
immobilized therein [1]. In order for the formation of organogels, it is
important to control the noncovalent intermolecular interactions in
such a way to avoid transformation to isotropic solution or crystalline
state.
Among the noncovalent interactions, hydrogen bonding was the
most commonly used to direct the self-assembly process. Well-known
organogelators such as derivatives of carbohydrates [2,3], amino acids
[4,5], and urea [6,7], have been revealed to self-assemble through
hydrogen bonding interactions into long fibrous structures, which in
turn form an entangled network and thus immobile solvents therein.
The morphologies of the gels depend on many factors such as molec-
ular structures of gelators, the ratio of the gelators in two-component sys-
tems, or external stimulus (temperature, ultrasound etc.) [8]. However,
the report on solvents impacting on the morphologies of the gels is rela-
tively rare. Recently, John et al. [9] found that the morphology of gels was
different depending on the solvents, and both fibrous networks and
sheets were observed in different solvents. Huang et al. [10] reported
the morphology of an azobenzene derivative exhibited regular flakes
Recently, we reported the synthesis and mesophase from non-
symmetrical tapered hydrazide derivatives, N-(3,4,5-trialkoxylbenzoyl)-
N′-(4′-aminobenzoyl) hydrazine (Dn) [11] as shown in Scheme 1,
which formed stable columnar mesophase driven by N\H…O_C
intermolecular double H-bonding. In this work, we report gelation
behaviors of Dn (n=6 and 16), and the compound N-(3,4,5-
trihexadecyloxybenzoyl)-N′-(4′-aminobenzoyl) hydrazine (D16)
showed strong gelation ability in some organic solvents. Notably,
the morphologies of the organogels are tunable by the polarity of
the gelling solvent.
2. Experimental
2.1. Gelation experiments
The weighted powder compound and solvent were sealed in a lit-
tle glass tube, and were then gently heated to make the gelator
completely dissolved. The resulting solution was cooled in air at
about 4 °C (or 20 °C) for several minutes, and then the gelation was
checked visually. When upon inversion of the test tube no fluid ran
down the walls of the tube, we judge its “gelation”. The xerogels
were obtained through slowly evaporating the solvents from
organogels, which were kept in air at about 4 °C.
2.2. Characterization
⁎
Correspondence to: B. Bai, Key Laboratory for Automobile Materials (JLU), Minis-
try of Education, Institute of Materials Science and Engineering, Jilin University, Chang-
chun 130012, PR China. Tel.: +86 431 85168254.
⁎⁎ Corresponding author. Tel.: +86 431 85168254.
FT-IR spectra were recorded with a Perkin‐Elmer spectrometer
(Spectrum One B). The sample was pressed tablet with KBr. X-ray dif-
fraction was carried out with a Bruker Avance D8 X-ray diffractometer.
E-mail addresses: baibinglian@jlu.edu.cn (B. Bai), minli@mail.jlu.edu.cn (M. Li).
0167-7322/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.molliq.2012.07.001

B. Bai et al. / Journal of Molecular Liquids 173 (2012) 108–112
109
Scheme 1. The synthesis of compounds Dn.
Fig. 1. Concentration-dependent Tgel of D16 gels in ethanol.
SEM observations were taken with SSX-550 and JSM-6700F apparatus.
Samples for SEM study were xerogels. Transmission electron microsco-
py (TEM) morphology was obtained with JEM 2010 apparatus. Samples
for TEM study were prepared by dropping a small amount of solution
onto a 400-mesh copper grid followed by evaporating the solvent at
ambient temperature. Thermal gravimetry (TG) curves were measured
on a TA SDTQ 600 instrument.
a
c
b
3. Results and discussion
3.1. Thermal behavior of the organogels
Interestingly, only D16 with long alkyl chains showed strong gelation
ability in organic solvents such as ethanol, benzene, 1,2-dichloroethane
et al. (Table 1), while D6 with short alkyl chains cannot gel any of the
above solvents. D16 exhibits strong gelation ability in ethanol and the
gelation occurs even at 0.14 wt.%. Whereas the minimum gel concentra-
tion of D16 in benzene appears at 1.3 wt.% suggesting its weak gelation
ability. Fig. 1 shows the gel–sol transition temperature (Tgel) of D16
gel in ethanol as a function of concentration. The Tgel increases from
52 °C to 67 °C as the concentration of D16 increases from 0.65 wt.% to
2.98 wt.%.
d
e
g
f
3.2. Morphological properties of organogelator D16
The morphologies of the corresponding xerogels were investigated
by scanning electron microscopy (SEM). Fig. 2 shows the SEM images
of the organogels in different solvents. Interestingly, the morphologies
of the xerogels are different in different solvents. For example, the
D16 xerogels from benzene showed the surface-patterned, clumped,
and textured spheroids (Fig. 2a, b, c), with the diameters of 5–10 μm.
The diameters of the textured spheroids remain almost unchanged,
h
Table 1
Gelation properties of D16 (G: gel. S: solution. PG: partial gel. CGC: values mean critical
gelation concentration necessary for gelation).
Solvents
State
CGC
4 °C
20 °C
0.6%
Tetrahydrofuran
Ethanol
Methanol
Chloroform
1,2-Dichloroethane
Benzene
S
G
PG
S
G
G
G
G
0.14%
0.87%
1.3%
2.5%
1.1%
3.4%
2.7%
S
Fig. 2. SEM images of D16 xerogels from (a) benzene (1.30 wt.%), (b) benzene (3.39 wt.%),
(c) benzene (3.39 wt.%), (d) toluene (11.5 wt.%), (e) dimethylbenzene (3.79 wt.%),
(f) methanol (0.83 wt.%), (g) ethanol (0.56 wt.%), and (h) ethanol (1.11 wt.%).
Toluene
Dimethylbenzene
3.79%

110
B. Bai et al. / Journal of Molecular Liquids 173 (2012) 108–112
Fig. 3. TGA curve of D16 xerogel from ethanol (2.22 wt.%).
Fig. 5. FT-IR spectra of the xerogels of D16 from benzene (3.39 wt.%) and ethanol
(1.11 wt.%).
but its density increases obviously with the increase of the concentra-
tion from 1.30% to 3.39%. In contrast, the xerogels of D16 in ethanol
exhibit networks composed of straight bundles of fibers (Fig. 2g, h)
and the fibrous diameters increase obviously with the increase of
the concentration from 0.56% to 1.11%. In the thermal gravimetric
analysis (TGA) on xerogel of D16, no losses of thermogram weight
were detected upon heating to its isotropic point (138 °C), indicating
that there is no ethanol in the xerogel (Fig. 3).
To clarify and confirm the original structure of the gel in benzene, TEM
morphology of D16 in benzene solution was investigated (Fig. 4). Thin
fibers with tens of nanometers in diameter were observed, suggesting
that molecules self-assemble nanofibers in benzene solution.
The formation of elongated fiber-like aggregates indicates that the
self-assembly of D16 is driven by strong directional intermolecular
interactions. To ascertain whether hydrogen bonding plays a role in
the gelation process, infrared spectrum of the organogels was exam-
ined. Fig. 5 shows IR spectra of the xerogels from benzene and ethanol
in the range of 3800–2750 and 1800–1500 cm−1. The observation of
hydrogen bonded N\H stretching bands (3206 cm−1 (benzene gel)
and 3223 cm−1 (ethanol gel)), intense absorption of bonded C_O
stretching vibrations (1627 cm−1 and 1561 cm−1) clearly indicates
that the N\Hs of the central hydrazide group are associated with
the C_O groups via N\H…O_C intermolecular hydrogen bonding in
the gelation process and the intensity of the intermolecular hydrogen
bonding in the xerogels from benzene is stronger than that of ethanol
[12,13], such hydrogen bonding interaction was considered to be the
main driving forces for the formation of the fibers. In addition, the
absorption bands of the antisymmetric (ναs) and symmetric (νs) CH2
stretching vibrational modes are observed at 2918 cm−1 (ναs CH) and
2850 cm−1 (νs CH) in the gel, respectively, indicating that the alkyl
chains are ordered in part and there is a strong organization of the
alkyl groups via van der Waals interaction [14]. The intermolecular
hydrogen bonding of the central hydrazide group as well as the amino
head groups can also be confirmed by concentration-dependent 1H
NMR measurements [11].
Fig. 6 shows the X-ray diffraction patterns of D16 xerogel from
ethanol and benzene. The XRD pattern of D16 xerogel from benzene
exhibited one strong peak (d=59.1 Å) and two weak peaks at 29.4
(200) and 14.7 (400) Å in the low-angle range, suggesting a layer
structure. In the high-angle regions, two diffuse broad peaks with a
maximum at about 4.0 and 4.5 Å, suggesting the coexistence ordered
and disordered feature. The peak at 4.5 Å is characteristic of the
aliphatic chains, while that at 4.0 Å might be attributed to the repeat
distance of hydrazide units in stacking [12d]. The layer spacing from
Fig. 6. X-ray diffraction patterns of the benzene xerogels (3.39 wt.%) (a), and the
Fig. 4. TEM image of D16 in benzene solution (0.18 wt.%).
ethanol xerogels (1.11 wt.%) (b).

B. Bai et al. / Journal of Molecular Liquids 173 (2012) 108–112
111
Scheme 2. Schematic representation of D16 self-assembling to supramolecules (the dashed line illustrates the hydrogen bonding).
XRD is much larger than the calculated full extended molecular
length (about 33.3 Å) and smaller than twice the calculated full
extended molecular length. And considering the results of the 1H NMR
diluting experiment [11] and FTIR spectroscopy, the molecules should
self-assemble into supramolecular chains through intermolecular
hydrogen bonding (Scheme 2) and they have a little tilt angle in layers.
Whereas many sharp diffraction peaks in the low-angle range were
observed for D16 xerogels in ethanol indicating the structure in ethanol
gel was more ordered than that of benzene gel.
new opportunities to one-component low weight molecular (LWM)
materials.
Acknowledgment
The authors are grateful to the National Science Foundation
Committee of China (project Nos. 21072076, 51103057, 51073071), the
Special Foundation for PhD Program in Universities of China Ministry of
Education (Project No. 20090061120021), and Project 985-Automotive
Engineering of Jilin University for their financial support to this work.
Combining the above results, the process of gel formation can be
explained on the assumption that the molecular self-assembled to
form supramolecular chains mainly through intermolecular hydrogen
bonding interactions, namely elemental fibrils, which hierarchically
assembled to fibrous bundles, and farther to textured spheroids (in
benzene gel) or more thicker fibrous bundles (in ethanol gel). Why
the morphologies of xerogels have much difference? These morpholog-
ical features are determined by the extent of the solvent–gelator inter-
actions. In benzene gel, because of the hydrophobic interactions, the
nonpolar benzene solvent is apt to penetrate and interact with the
peripheral nonpolar group, namely the phenyl rings with trialkoxy groups
as well as alkyl chains, the solvent–gelator interaction makes the fibers
interconnect each other and conglutinate to form sheet, thereby loss of
fibrous unidirectional orientation, meanwhile, the strong hydrogen bond-
ing interactions of gelator–gelator as well as the penetration into layer of
benzene solvent make the sheet “curl” and divert from the plane, so the
isotropic aggregation occurs, resulting in complete loss of fibrous struc-
ture and less stable gels. While the polar ethanol solvent is apt to inter-
act with the polar group namely dihydrazide groups, and takes part
in the hydrogen bonding, meanwhile impairs the intermolecular hydro-
gen bonding interactions of gelator–gelator to some extent, thus
the macroscopic morphology maintains fibrous characters, resulting in
high gelation ability and stable gels.
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