Calix[4]arene-based supramolecular gels with unprecedented rheological properties

Article abstract of DOI:10.1039/c2sm07251c

A novel calix[4]arene-based dimeric-cholesteryl derivative was synthesized, and its gelation behaviour in thirty organic solvents was investigated. It has been shown that the compound cannot gel any of the pure solvents tested. However, it gels a mixture of solvents n-decane and acetonitrile efficiently, provided the volume ratio of the two solvents in a mixture is within 9:1 and 3:2. AFM and SEM measurements revealed that the molecules of the compound aggregate into micro-/nano-rods first, then fine fibers, and then thick fibers, and finally networked structures in the mixture solvents. Interestingly, the gel with a composition of 1 to 1 (V_n-Decane:V_Acetonitrile) and 2.5% (w/v) of the compound exhibits super-smart and fully reversible thixotropic properties, a phenomenon never reported before. Furthermore, the mechanical strength of the gel could be easily adjusted by altering the concentration of the gelator and the composition of the mixture solvents. Further interrogation of the gel revealed that structurally the gel is a gel-emulsion with acetonitrile dispersed in n-decane, a rarely found O/O (oil in oil) gel-emulsion which may find uses in the templated preparation of low-density materials with complicated internal structures.

Full text of DOI:10.1039/c2sm07251c

As featured in:
Showcasing research from Fang lab, Shaanxi Normal
University, China.
Title: Calix[4]arene-based supramolecular gels with
unprecedented rheological properties
A mixture of n-decane and acetonitrile can be efficiently gelled by
introduction of a small amount of a specific calix[4]arene-based
dimeric-cholesteryl derivative as designed and synthesized in the
present work. The gel exhibits super-smart thixotropic property and
adopts an unusual O/O gel-emulsion structure.
See Yu Fang et al.,
Soft Matter, 2012, 8, 3756.
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PAPER
Calix[4]arene-based supramolecular gels with unprecedented rheological
properties†
a
a
a
a
a
a
Xiuqin Cai,ab Kaiqiang Liu, Junlin Yan, Helan Zhang, Xiaoyu Hou, Zhang Liu and Yu Fang
*
Received 25th November 2011, Accepted 11th January 2012
DOI: 10.1039/c2sm07251c
A novel calix[4]arene-based dimeric-cholesteryl derivative was synthesized, and its gelation behaviour
in thirty organic solvents was investigated. It has been shown that the compound cannot gel any of the
pure solvents tested. However, it gels a mixture of solvents n-decane and acetonitrile efficiently,
provided the volume ratio of the two solvents in a mixture is within 9 : 1 and 3 : 2. AFM and SEM
measurements revealed that the molecules of the compound aggregate into micro-/nano-rods first, then
fine fibers, and then thick fibers, and finally networked structures in the mixture solvents. Interestingly,
the gel with a composition of 1 to 1 (Vn-Decane:VAcetonitrile) and 2.5% (w/v) of the compound exhibits
super-smart and fully reversible thixotropic properties, a phenomenon never reported before.
Furthermore, the mechanical strength of the gel could be easily adjusted by altering the concentration
of the gelator and the composition of the mixture solvents. Further interrogation of the gel revealed
that structurally the gel is a gel-emulsion with acetonitrile dispersed in n-decane, a rarely found O/O (oil
in oil) gel-emulsion which may find uses in the templated preparation of low-density materials with
complicated internal structures.
it is anticipated that combining them into LMMGs may bring
molecular gels new functionalities.
Introduction
Supramolecular gels based on low-molecular-mass gelators
(LMMGs) have attracted great attention during the last two
decades.1–6 This is because, on one hand, it offers unique features
to create various superstructures with respect to self-assembly
phenomena,7–12 and on the other hand it has potential for
creating new soft materials, which may find use in oil recovery,
controlled release, bioactivity maintenance, protein separation,
regenerative medicine, tissue engineering, and template prepa-
ration of micro-/nano- materials etc.13–19 To date, several classes
of low-molecular mass compounds from simple alkanes to crown
ethers, cyclodextrins and cholesteryl derivatives have been shown
to be efficient LMMGs for water or/and organic solvents.20–26
However, from the view point of practical uses of LMMGs-
based supramolecular gels there haven’t been many reported. In
particular, LMMGs containing macrocyclic components, such
as crown-ethers, cyclodextrins, calixarenes and cucurbiturils are
very limited.20–26 These components are important because they
are rich in host–guest and supramolecular chemistry, and thereby
Among the macrocyclic compounds known till now, calixar-
enes are unique because of their ability to be modified, and their
‘‘unlimited possibilities’’.27 It is these advantages that make them
have a wide variety of applications in molecular recognition,
sensing, selective binding and carrying, etc.28 However, calixar-
enes have been rarely studied as a component of LMMGs. To the
best of our knowledge, only 7 reports have been published till
now, of which 5 were published recently.27,29–35 In 1993, Shinkai
and coworkers presented the first report on the gelation prop-
erties of calix[4]arene derivatives, which bear long alkyl groups at
their lower rims. It was demonstrated that the calix[4]arene
derivatives could serve as a gelator for some apolar solvents,
including cyclohexane, hexane, and carbon disulfide.29,30 To
mimic the uptake of alkanes by bacteria from the aqueous phase,
Xu and coworkers created a stable DMSO gel by reacting
3-pyridine-azo-calix[4] arene with [Pd(en)(H2O)2](NO3)2
(en ¼ ethylenediamine) in situ. Model system studies demon-
strated that the organogel ‘‘uptakes’’ toluene from an aqueous
solution selectively and completely, and the absorbing process
follows first order kinetics.31,32 Ogden and coworkers designed
and prepared a proline-functionalized calix[4]arene, which forms
hydrogels when triggered by the presence of specific anions. The
properties of the gels could be modified by the associated cations,
and furthermore, the gels could be disassembled by increasing
the pH above 7. It was believed that a new kind of LMMGs-
based stimulus-responsive hydrogel was found.33 Zheng and
coworkers reported a series of calix[4]arene derivatives, which
aKey Laboratory of Applied Surface and Colloid Chemistry of Ministry of
Education, School of Chemistry and Chemical Engineering, Shaanxi
Normal University, Xi’an 710062, P. R. China. E-mail: yfang@snnu.edu.
cn; Fax: +0086-29-85310081; Tel: +0086-29-81530788
bCollege of Chemistry and Life Science, Weinan Normal University,
Weinan 714000, P. R. China
† Electronic supplementary information (ESI) available: Gelation
behaviours of the compound 4; Microscopy images; Molecular
structure of the fluorescent probe and rheological studies. See DOI:
10.1039/c2sm07251c
3756 | Soft Matter, 2012, 8, 3756–3761
This journal is ª The Royal Society of Chemistry 2012

bear long tertiary alkyl groups at the upper rim and S-1-phenyl
ethylamine groups at the lower rim.34 Gelation and aggregation
studies showed that in the presence of D-2,3-dibenzoyltartaric
acid, the compounds gelate cyclohexane when heated or aggre-
gate into egg-like vesicles, which is the first example of heat-set
gels resulting from a difference in interactions between two
component gelators. Recently, the same group prepared another
calix[4]arene derivative, which bears L-2,3-dibenzoyltartaric acid
groups at the lower rim.35 This compound could enantio-selec-
tively self-assemble and result in a gel or suspension only with
one enantiomer of a number of chiral amines, as revealed by
SEM. The results from Zheng and coworkers’ research demon-
strated clearly that molecular recognition could be utilized, via
careful selection of two recognition units, for the construction of
supramolecular gels, a strategy rarely employed and reported in
the literature. Very recently, Liu and coworkers employed
a similar strategy to create a series of supramolecular binary
hydrogels.27 This calix[4]arene derivative was prepared by
binding four proline structures at the upper rim of the macro-
cyclic compound. A combination of this calix[4]arene derivative
with arginine, histidine or lysine gels water efficiently. The
structures of the gel networks are dependent upon the nature of
amino acid employed.
equilibrate for some time. Finally, the test tube was inverted to
observe if the solution inside could still flow. The systems
which lose fluidity are denoted as ‘‘G’’. Those that remained as
solutions are denoted ‘‘S’’. However, systems with precipitates or
crystals are denoted as ‘‘P’’.
SEM pictures of xerogels were taken on a Quanta 200 scanning
electron microscopy spectrometer (Philops-FEI). The acceler-
ating voltage was 20 kV, and the emission was 10 mA. The
xerogel was prepared by freeze-drying a gel of the mixture
solvent at a gelator concentration of 2.5% (w/v). Prior to exam-
ination, the xerogel was attached to a copper holder by using
conductive adhesive tape, and then it was coated with a thin
layer of gold.
AFM measurements were conducted on a SOLVER P47 PRO
system. The sample was prepared by drop casting the dilute
solution of the compound onto a freshly cleaved mica surface.
All FTIR measurements were performed on a Brucher
EQUINX55 spectrometer in an attenuated total reflection
(ATR) mode with ZnSe as a sample slot. The KBr pellets mixed
with samples were measured in transmittance mode.
Rheological measurements were carried out with a stress-
controlled rheometer (TA instrument AR-G2) equipped with
steel-coated parallel-plate geometry (20 mm diameter). The gap
distance was fixed at 1000 mm. A solvent trapping device was
plaꢀced above the plate and measurement temperature was set at
15 C in order to avoid solvent evaporation.
As is well known, calixarenes bear multiple binding sites that
promotes self-assembly into 3-D structures via specific weak
interactions.36–38 Furthermore, the cavity of calixarene could
hold solvent molecules or other guest molecules.39,40 Similarly,
cholesterol is a well studied fragment for building up LMMGs
during the last few decades. It has been shown that cholesterol
self-assembles in a directional way via van der Waals interactions
and forms aggregates with specific structures.41–45 It should be of
interest to see what the gelation behaviours of compounds,
which contain the two components, are. Driving by the curiosity
discussed above, calix[4]arene and cholesterol were chosen as key
components for the first time to construct a novel LMMGs. In
the effort, two cholesteryl residues were successfully attached to
the lower rim of calix[4]arene via flexible linkers. Gelation tests
revealed that the compound is an efficient gelator for mixed
solvents of n-decane and acetonitrile provided the volume ratios
of them are within the range 9 : 1 to 3 : 2 (Vdecane: Vacetonitril).
Interestingly, the gel of a mixed solvent (1 : 1) exhibits a very
smart thixotropic property whereby the sol–gel phase transition
could be reversibly controlled by imposing or removing a simple
shake or shear stress.
For rheological measurements, a stress sweep measurement at
fixed frequency was firstly conducted, which provides informa-
tion about the mechanical strength of the gel sample. And then,
a time sweep was made to observe the recovery property of the
gel. For this measurement, a constant oscillatory shear stress was
applied to destroy the sample, and then, a small constant shear
stress was applied, and the storage modulus G0 and the loss
modulus G00 of the sample were monitored as functions of time.
1
1H NMR and MS measurements: H NMR data of samples
were collected on Bruker AVANCF 300 MHz spectrometer.
MALDI-TOF mass spectra were recorded in a Kratos’ AXIMA-
CFR plus instrument by using 2,5-dihydroxybenzoic acid (DHB)
as a matrix.
Synthetic procedure
Synthesis of the cholesteryl derivative of calix[4]arene is
schematically shown in Scheme 1, and the details of the synthesis
are described below.
Experimental section
Preparation of compound 1. Referencing to a work reported by
Gutsche,46 compound 1 was synthesized by the following
method. 20.5 g (140 mmol) of p-tertbutyphnol, 14.5 mL of 37%
formalin (200.2 mmol), and 0.5 mL of 12.5 mmol mLꢁ1 NaOH
solution were mixed together in a 500 mL four-neck flask. Then,
the mixture was stirred, and heated under a nitrogen atmosphere
for 2 h in an oil bath at 110–120 ꢀC. The mixture became viscous
and took an orange color. Furthermore, water in the system was
almost distilled out completely at this temperature. The system
was then cooled down to room temperature, resulting in a solid-
like mixture. The mixture was suspended by stirring in 200 mL of
diphenyl ether under nitrogen atmosphere. The suspension was
refluxed again for 2 h, and its color became dark red eventually.
Reagents and materials
p-Tertbutyphnol (Aladdin Chemistry Co. Ltd., 98%) used
directly without further purification. THF was distilled over
sodium in the presence of benzophenone under N2 atmosphere
before use. All other reagents were analytically pure.
General methods
Preparation of gels: in a typical gelation test, a weighed amount
of the potential gelator and a measured volume of the solvent
mixture were placed in a sealed test tube, followed by shaking or
sonicating for a while. And then, the system was left to
This journal is ª The Royal Society of Chemistry 2012
Soft Matter, 2012, 8, 3756–3761 | 3757

After cooling the system down to room temperature, 200 mL of
ethyl acetate was added. Then the system was stirred for another
30 min, and then it was allowed to stand for at least 30 min. Via
filtration, a precipitate that was formed in the mixture was
collected and washed twice with ethyl acetate and acetic acid,
separately. The crude product was purified by re-crystallization
from toluene to give a white product (compound 1) in 50% yield.
For compound 1: 1H NMR (CDCl3/Me4Si, 300 MHz): d ¼ (ppm)
1.21(s, 36H, –C(CH2)3), 3.47 (d, J ¼ 12.0, 4H, –ArCH2Ar–), 4.23
(d, J ¼ 12.0, 4H, –ArCH2Ar–), 7.04 (s, 8H, –ArH), 10.34 (s, 4H,
–OH); MS: m/z calcd for [(M + Na)+]: 671.41, found: 671.41.
twice, and dried under vacuum. In this way, a white powder
(compound 3) was obtained (yield: 91%) For 3: 1H NMR
(CDCl3/Me4Si, 300 MHz): d (ppm) 0.95 (bs, 4H, –NH2), 1.04
(s, 18H, -C(CH2)3), 1.27 (s, 18 H, –C(CH2)3), 3.43 (d, J ¼ 15.0,
4H, –ArCH2Ar–), 4.10 (d, J ¼ 15.0, 4 H, –ArCH2Ar–), 4.64
(s, 4H, –ArOCH2–), 6.92 (s, 4 H, –ArH), 7.04 (s, 4H, –ArH),
7.76 (bs, 2H, –NHCO–), 9.67 (s, 2H, –OH); MS: m/z calcd for
[(M + Na)+]: 814.02, found: 814.02. Anal. calcd for C48H64N4O6:
C 72.70, H 8.14, N 7.06; found: C 72.61, H 8.17, N 7.13.
Preparation of compound 4. Compound 3 (0.790 g, 1 mmol)
and cholesteryl choloformate (0.898 g, 2 mmol) were dissolved in
60 mL of tetrahydrofuran (THF), and the mixture was refluxed
for 5 h. After the reaction, the mixture was evaporated to
dryness. The precursor was re-crystallized with ethanol, and then
dried in vacuum as a white powder (yield: 92%). For 4: 1H
NMR(CDCl3/Me4Si, 300 MHz) d(ppm): 0.68–1.84 (m, 132H,
–CH2CH2–, –C(CH3)3, methyl, cholesteryl protons) 3.39 (d, J ¼
18.0, 4H, –ArCH2Ar–), 4.10 (d, J ¼ 18.0, 4 H, –ArCH2Ar–),
4.58–4.67 (s, 6H, ArOCH2, oxcyclohexyl), 5.39 (s, 2H, –OH),
5.40 (s, 2H, alkenyl), 6.87 (s, 4H, –ArH), 7.08 (d, 4H,–ArH), 7.41
Preparation of compound 2.47 Compound 1 (0.648 g, 1 mmol),
potassium carbonate (0.166 g, 1.2 mmol) and potassium iodide
(0.017 g, 0.1 mmol) were dissolved in 80 mL of acetone, and then
ethyl chloroacetate (0.294 g, 2 mmol) was added slowly with
stirring. The mixture was refluxed for 10 h under nitrogen
atmosphere. The solvent was evaporated, and the residue
obtained was dried. Then, it was dissolved or suspended in
chloroform and then extracted (washed) with 10% hydrochloric
acid for three times. The organic phase was collected and dried
with anhydrous magnesium sulfate, and further evaporated to
dryness. In this way, a crude product was prepared.
(s, 2H, –NHCO–), 10.35 (s, 2H, –NHCO-); FTIR, nmax/cmꢁ1
:
3417 (NH), 2955 (CH), 1715 (C]O, –O), 1687 (C]O, NH),
1529 (NH, bending vibration), and 1327 (–C–O). MS: m/z calcd
The crude product was firstly purified by column chromatog-
raphy (silicone, 200–300 mesh; acetone/petroleum ether, v:v ¼
1 : 9), and then by re-crystallization from petroleum ether three
times to give compound 2 as white crystals in a yield of 51%. For
compound 2: 1H NMR (CDCl3/Me4Si, 300 MHz): d (ppm) 1.04
(s, 18H, –C(CH2)3), 1.25 (s, 18H, –C(CH2)3), 1.31(t, 6H, –CH3),
3.29 (d, J ¼ 15.0, 4H, –ArCH2Ar–), 4.26 (q, 4H,–CH2CH3),
4.10 (d, J ¼ 15.0, 4H, –ArCH2Ar–), 4.72 (s, 4H, –ArOCH2), 6.81
(s, 4 H, –ArH), 7.01 (s, 4H, –ArH), 7.09 (s, 2H, –OH); MS: m/z
calcd for [(M + Na)+]: 844.08, found: 844.08.
for [(M
+
Na)+]: 1640.14, found: 1640.14. Anal. calcd
for C104H152N4O10: C 77.18, H 9.47, N 3.46; found C 76.74,
H 9.75, N 3.09.
Results and discussion
Gelation behaviours
The gelation behaviours of the compound were tested in 30
different organic solvents including protic/aprotic and polar/
apolar solvents in a concentration of 2.5% (w/n) (c.f. Table S1,
ESI†). It was found that the compound dissolves in most of the
solvents tested excluding methanol, acetonitrile, and triethyl-
amine (TEA). However, the compound aggregates slowly in n-
decane and it was found, experimentally, that some flocculants
appear 12 h later after the compound was dissolved in the solvent
at room temperature. For this system, gelation occurs at room
temperature when the concentration of the compound exceeds
7% (w/n), the critical gelation concentration (CGC) of the pure
solvent. To decrease the CGC, a poor solvent, acetonitrile was
introduced. It was believed that introduction of this poor solvent
must promote the aggregation of the compound, which
may enhance the establishment of gel networks, a necessity for
gelation. In this way, the addition of 1.0% (w/n) of the compound
at room temperature results in instant gelation ina 1 : 1 mixed
solvent system of n-decane and acetonitrile (in volume) provided
simple shaking or sonication was given, which must bring
Preparation of compound 3. Compound 2 (0.820 g, 1.0 mmol)
and hydrazine hydrate (1.0 mL) were dissolved in a mixed solvent
of toluene and methanol (20 mL, v:v ¼ 1 : 1) under nitrogen
atmosphere. The mixture was refluxed under stirring for 10 h.
Then, the solvent was evaporated under vacuum, and then
a precipitate was produced by addition of a small amount of
water and collected by filtration. Finally, the crude product
obtained was washed, respectively, with methanol and water
Table 1 Gelation behaviours of the compound in n-decane and aceto-
nitrile mixture (2.5%, w/v)
VDecane:VAcetonitrile
Result
VDecane:VAcetonitrile
Result
10 : 0
S
4 : 6
G
9 : 1
G*
3 : 7
P
8 : 2
G*
2 : 8
P
7 : 3
G
1 : 9
P
6 : 4
G
0 : 10
P
5 : 5
G
Scheme 1 Synthesis of a dimeric-cholesteryl derivative bridged by a calix
[4]arene unit.
3758 | Soft Matter, 2012, 8, 3756–3761
This journal is ª The Royal Society of Chemistry 2012

additional advantages for the system to find real-life applica-
tions.48 It is to be noted that mixtures of other volume ratios of
the two solvents can be also gelled in the same way provided their
volume ratios are within the range of 9 : 1 to 2 : 3 (cf. Table 1).
However, not all poor solvents, such as methanol, show the same
effect of acetonitrile.
To further investigate the effect of the compound on the phase
behaviour of the mixed solvent, a simple but illustrative experi-
ment was conducted and the results are shown in Fig. 1. Refer-
ence to the figure reveals: (1) for the system without the
compound, the mixture solvent separated into two layers at once
after sufficient shaking (Fig. 1a); (2) for the systems with the
compound, the phase behaviour of them changes along with
increasing the concentration of the compound. It can be seen that
the introduction of the compound results in some vacuoles at the
interface of the two solvents, and the number and volume of
them increase but they are always in the acetonitrile side (lower
part of the mixture) along with increasing the compound
concentration. Eventually, the system becomes a turbid gel when
the gelator concentration reaches 1% (w/v).
Fig. 2 AFM images of the aggregates of the compound in pure n-decane
(a and b; 0.005% and 0.02%, w/v) and mixtures of n-decane and aceto-
nitrile (c and d; 1 : 1, v/v; 0.01% and 0.05%, w/v).
Microscopy studies
by optical and fluorescence microscopy observations (cf. Fig. S2,
ESI†). During the observation, an acetonitrile soluble but
n-decane insoluble fluorescent compound was employed as
a probe.
Gelation of the mixed n-decane and acetonitrile system must be
a result of a gelator network formation during the shaking
process. These kind of networks or gelator aggregates should be
measurable by atomic force microscopy (AFM) and scanning
electron microscopy (SEM) provided the samples are properly
prepared.
From further inspection of the AFM and SEM images, it can
also be seen that the aggregation of the molecules of the
compound starts from micro-/nano-rod like, then thin fibers, and
then thick fibers, and finally networks. Both introduction of the
poor solvent and increasing the concentration of the compound
favors formation of the gel networks. It might be interesting to
note that the immiscible two organic solvents, n-decane and
acetonitrile, form O/O (oil in oil) type gel emulsions in the
presence of the compound, a system rarely reported before.
Fig. 2 shows a series of AFM images of the aggregates of the
compound in pure n-decane and its mixture with acetonitrile. It is
seen that the molecules of the compound start to aggregate at
a very low concentration in its ‘‘good’’ solvent n-decane, and
forms rod-like structure of different sizes. With increasing its
concentration in the solvent, the rodlike structures further
aggregated into network structures (cf. Fig. 2b). Introduction of
a poor solvent, acetonitrile, promotes the aggregation as revealed
by the AFM images shown in Fig. 2c and 2d. In fact, 1% (w/v) of
the compound is not enough to gel pure n-decane, but intro-
duction of acetonitrile changes the phase behaviour as shown in
Table 1. Actually, the gels are two-phase systems, and acetoni-
trile was trapped within the n-decane gel. This is evidenced by the
SEM images of the xerogels which were prepared from the gels of
the mixed solvents (Fig. 3). It is clearly seen that there are a big
number of holes of different sizes, which are believed to be the
sites of acetonitrile. The characteristic structures were supported
Rheological studies
The mechanical performance of a material is extremely impor-
tant for its practical uses. To explore this property of the gel in
detail, rheological measurements were conducted on a series of
gels, which had been prepared by using different concentrations
of the compound and different volume ratios of the mixture
solvents. The storage modulus, G0 and the loss modulus, G00 of
the gels were measured as functions of shear stress at a constant
Fig. 1 Phase behaviour of the system of gelator/n-decane and acetoni-
trile (v:v ¼ 1 : 1) with different gelator concentrations (w/v). (a) 0, (b)
0.05%, (c) 0.1%, (d) 0.5%, (e) 1%.
Fig. 3 SEM images of the gel networks from mixture solvent of
n-decane and acetonitrile (1 : 1, v:v) in the presence of the gelator
(1.0%, 2.5%, w/v; a, b).
This journal is ª The Royal Society of Chemistry 2012
Soft Matter, 2012, 8, 3756–3761 | 3759

gel sample was re-examined by two continuous processes: (1)
deformation: in this process, the gel was sheared at a continuous
stress from 0.1 to 300.0 Pa at a frequency of 1.0 Hz; (2) recovery:
a low shear stress of 1.0 Pa was applied on the destroyed gel at
same frequency, and the recoveries of storage modulus, G0 and
the loss modulus, G00 of the system were monitored as a function
of time. The results are shown in Fig. 5(b). Reference to the figure
reveals that the gel recovers immediately after removing of the
destroyed shear stress. This sol–gel phase transition process
could be repeated for at least six times (cf. Fig. 6). These
results demonstrate that this calix[4]arene-based gel possesses
unprecedented, smart and fully reversible thixotropic property,
an observation never reported before. No doubt, this property
is valuable for some specific applications such as injection
molding and drug delivery etc. To have further understanding of
the sol–gel phase transition behaviour of the system, the same
experiment was also conducted for the systems with gelator
concentration of 1 and 1.5% (w/v), respectively, and the results
are listed in Fig. S2 (ESI†). It can be seen that the reversibility of
the two systems is not as good as the one shown in Fig. 6. For the
system containing 1% (w/v) of the gelator, full recovery of the gel
needs longer time, and for the other the reversibility of the system
is much improved. It should not be difficult to understand that
the full and instant recovery of the gel state is not only dependent
upon the nature of the system, but also the concentration of the
gelator. This may explain why recovery of the gel state of the
system containing 1% (w/v), which is the CGC of the gel, needs
a longer time and the one with higher concentration of the
gelator is improved.
Fig. 4 Evolution of G0 as a function of the applied shear stress at
different concentrations of the compound (a) and at different contents of
acetonitrile in the mixture solvents (volume ratio, b).
frequency of 1.0 Hz at 15 ꢀC (Fig. 4). It can be seen that the value
of G0 increases from 1830 to 63460 Pa along with increasing the
concentration of the compound from 1.5 to 3.5% (w/v), and
correspondingly, the yield stress changes from 54 to 398 Pa,
indicating that both the stability of the gel network and the
elastic property of the gel are well dependent upon the concen-
tration of the gelator in the mixture solvents.
Further reference to Fig. 4b reveals that G0 increases from 9061
to 36 350 Pa along with increasing acetonitrile content from 30%
to 50% (volume to volume), and at the same time, the yield stress
increases from 116 to 215 Pa, suggesting that both the stability
and the elastic property of the gels are also dependent upon the
composition of the mixture solvent. These observations may be
understood by considering the fact that either increase in the
gelator concentration or in the volume ratio of acetonitrile must
enhance the strength of the gel networks.
Interestingly, all these gels exhibit a clear thixotropic property.
In order to learn more about the property, the gel of a mixed
solvent of n-decane and acetonitrile (1 : 1 in volume) with the
compound (2.5%, w/v) was chosen as an example and its thixo-
tropic property was studied. The measurement was conducted at
Conclusions
A calix[4]arene derivative of cholesterol was designed and
synthesized. Gelation tests demonstrated that the compound is
an effective gelator for the mixed solvents, n-decane and aceto-
nitrile. The gels of the mixed solvents form at room temperature
providing a simple shaking or sonication is given. The gels
possess super-smart and fully reversible thixotropic property.
Furthermore, it has been revealed that structurally the gels are
gel-emulsions, implying that acetonitrile is dispersed in the gel of
n-decane. It is believed that both the smart thixotropic property
and the gel-emulsion structure possessed by the mixed solvent gel
guarantee its potential use in a wide variety of areas including
ꢀ
a constant frequency of 1.0 Hz and at 15 C and the results are
shown in Fig. 5(a). It is seen that at the beginning G0 is about five
times greater than G00, indicating the dominant elastic character
of the gel.48–53 The values of the two parameters remain almost
unchanged before the shear stress reaches 100 Pa. However,
a sudden decrease in the two values is observed above this
shear stress, and furthermore the order of the values of the
two parameters is reversed after a critical shear stress, 251 Pa,
indicating the breakup of the gel networks, in other words an
indication of dominated fluidity character.
To further examine the thixotropic property, the recovery of
the destroyed gel system was studied. For this purpose, the same
Fig. 5 Evolution of G0 and G00of the gel as functions of the applied shear
stress (a), and evolution of G0 and G00of the system as functions of
recovery time (b).
Fig. 6 Evolution of G0 as functions of applied shear stress and recovery
time (2.5%, w/v).
3760 | Soft Matter, 2012, 8, 3756–3761
This journal is ª The Royal Society of Chemistry 2012

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Acknowledgements
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We thank the Natural Science Foundation of China (20902055,
20927001, and 91027017) and the 13115 Project of Shaanxi
Province (2010ZDKG-89) for financial support. This work is
also supported by ‘‘Program for Changjiang Scholars and
Innovative Research Team in University’’ of China (IRT1070).
Miss Zhiyan Xu is acknowledged for her help in providing the
fluorophore as a fluorescent probe.
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