Comparative studies on molecular induced aggregation of hepta-imidazoliumyl-β-cyclodextrin towards anionic surfactants

Article abstract of DOI:10.1016/j.cclet.2014.11.028

A β-cyclodextrin derivative bearing seven cationic arms and its singly charged analogue, i.e., per-6-deoxy-6-(1-methylimidazol-3-ium-3-yl)-β-cyclodextrin (3) and mono-6-deoxy-6-(1-methylimidazol-3-ium-3-yl)-β-cyclodextrin (4) were synthesized and fully characterized. Their induced aggregation behaviours towards two anionic surfactant, that is, sodium dodecyl sulfonate (SDS) and dioctyl sodium sulfosuccinate (Aerosol OT, AOT), were investigated by UV-vis, NMR, Zeta-potential, dynamic light scattering (DLS), and transmission electron microscopy. The results revealed that host 3 can induce the molecular aggregation of anionic surfactant at concentration far lower than its original CAC, leading to the larger diameter, the narrower size distribution and the higher thermal stability of the induced aggregate towards the anionic surfactant possessing more hydrophobic tails.

Full text of DOI:10.1016/j.cclet.2014.11.028

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CCLET-3162; No. of Pages 5
Chinese Chemical Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Original article
Comparative studies on molecular induced aggregation of
hepta-imidazoliumyl- -cyclodextrin towards anionic surfactants
b
Di Zhao, Yong Chen, Yu Liu *
Department of Chemistry, State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Collaborative Innovation Center of Chemical Science and
Engineering (Tianjin), Tianjin 300071, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A
b
-cyclodextrin derivative bearing seven cationic arms and its singly charged analogue, i.e., per-6-
deoxy-6-(1-methylimidazol-3-ium-3-yl)- -cyclodextrin (3) and mono-6-deoxy-6-(1-methylimidazol-
-ium-3-yl)- -cyclodextrin (4) were synthesized and fully characterized. Their induced aggregation
Received 29 September 2014
Received in revised form 29 October 2014
Accepted 3 November 2014
Available online xxx
b
3
b
behaviours towards two anionic surfactant, that is, sodium dodecyl sulfonate (SDS) and dioctyl sodium
sulfosuccinate (Aerosol OT, AOT), were investigated by UV–vis, NMR, Zeta-potential, dynamic light
scattering (DLS), and transmission electron microscopy. The results revealed that host 3 can induce the
molecular aggregation of anionic surfactant at concentration far lower than its original CAC, leading to
the larger diameter, the narrower size distribution and the higher thermal stability of the induced
aggregate towards the anionic surfactant possessing more hydrophobic tails.
Keywords:
Cyclodextrin
Surfactant
Molecular assembly
Induced aggregation
ß 2014 Yu Liu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
1
. Introduction
In recent years, the construction of structurally well-defined
two anionic surfactants possessing the same hydrophilic head and
the different hydrophobic tails (Scheme 2) by UV–vis, NMR, Zeta-
potential, dynamic light scattering (DLS), and transmission electron
microscopy. It is our particular interest to explore the governing
factors on the molecular assembly of amphiphilic substrates
induced by modified CDs and to open a new approach for the CD-
based supramolecular assembly.
architectures with good functions employing supramolecular
amphiphiles has received increasing attention in chemistry, biology,
materials and many other fields [1]. As compared with the
traditional amphiphiles, supramolecular amphiphiles possess the
advantages of the easy preparation, adjustive aggregation behaviour
and multistimuli responsive properties towards light, temperature,
redox, pH and enzyme [2]. Recently, various macrocyclic receptors,
including calixarene [3], pillararene [4], and cucurbituril [5] were
widely reported to induce the supramolecular amphiphilic aggre-
gation of guest molecules. However, the related works based on
cyclodextrins (CDs) are still rare, despite that CDs are regarded as
one of the best building blocks for the construction of functional
molecular assemblies [6]. Zhang et al. [2a] reported a photo-
controlled reversible supramolecular assembly of an azobenzene-
containingsurfactantwith
the formation and physicochemical properties of noncovalent
amphiphiles between -CD and aliphatic guests. In the present
work, we synthesized a two positively charged -CD derivatives
bearing one or seven methylimidazoliumyl arms (Scheme 1) and
compared their molecular induced aggregation behaviours towards
2. Experimental
All solvents and reagents were commercially available used
without further purification unless otherwise noted. Anhydrous
N,N-dimethylfomamide (DMF) was dried and distilled over
calcium hydride (CaH
solutions were prepared from distilled water.
2
) under reduced pressure. All aqueous
-CD of reagent
b
grade (Shanghai Reagent Factory) was recrystallized twice from
1
water and dried in vacuo at 95 8C for 24 h prior to use. H NMR and
a-CD.Lauth-deViguerieetal. [7]reported
1
3
2
C NMR spectra were recorded in D O on a Bruker AV 400
spectrometer. Mass spectra were performed on a Varian 7.0 T
FTICR-MS (MALDI). The elemental analysis (C, H and N) was
performed using a Vario EL Cube elemental analyzer (Elementar
Ltd. Corp., Germany). Turbidity measurements were carried out
with a Shimadzu UV-3600 spectrophotometer equipped with a
PTC-384WI temperature controller. Each sample for TEM mea-
b
b
*
Corresponding author.
surements was prepared by dropping a 50
mL of sample solution
E-mail address: yuliu@nankai.edu.cn (Y. Liu).
on a copper grid. The grids were then air-dried, and the samples
http://dx.doi.org/10.1016/j.cclet.2014.11.028
001-8417/ß 2014 Yu Liu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
1
Please cite this article in press as: D. Zhao, et al., Comparative studies on molecular induced aggregation of hepta-imidazoliumyl-
b-
cyclodextrin towards anionic surfactants, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.11.028

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D. Zhao et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
Scheme 1. Synthesis route of hosts 3 and 4.
were examined by a Philips EM400st transmission electron
microscope. The sample solution for the dynamic light scattering
measurements was prepared by filtering the solution through a
filtration and then recrystallized from water to give a white powder
1
2
(382.2 mg, 72%). H NMR (400 MHz, D O): d 3.21–4.05 (m, 45H),
4.93–5.03 (m, 7H), 7.38–7.42 (s, 1H), 7.45–7.49 (s, 1H). HRMS
+
4
50 nm Millipore filter into a clean scintillation vial. The samples
(MALDAI, m/z) [MÀI] 1199.4201, found 1199.4203.
were examined by a laser light scattering spectrometer (BI-200SM)
equipped with a digital correlator (Turbo Corr.) at 532 nm and a
3. Results and discussion
scattering angle of 908. Per-6-deoxy-6-iodo-
b
-CD (1) [8] and
mono-6-deoxy-6-iodo-
literature procedure.
b
-CD (2) [9] are prepared according to the
Possessing the a hydrophilic sulfonate head and one or two
hydrophobic tails, SDS and AOT are both able to form self-
aggregate in water, and their critical aggregation concentrations
(CACs) at 25 8C are 9.7 mmol/L and 2.7 mmol/L, respectively [10].
However, with the addition of host 3, the CAC of guest surfactants
greatly decreased, which can be quantitatively investigated by
monitoring the dependence of the optical transmittance around
450 nm on the concentration of anionic surfactants in the absence
and presence of host 3. In the absence of host 3, the optical
transmittance of guest surfactant at 450 nm showed no appreci-
able changes in the experimental concentration range, indicating
that the guest surfactant could not aggregate at these concentra-
tions. As seen in Fig. 1, the optical transmittance of guest
surfactant gradually decreased in the presence of host 3, and the
host-induced CAC values were measured to be 0.14 mmol/L at
0.04 mmol/L 3 and 0.017 mmol/L at 0.04 mmol/L 3 for SDS and
AOT, respectively. Significantly, these CACs were 69 and 158 times
lower than the corresponding values of free SDS and AOT,
respectively. These results demonstrated that host 3 could induce
the aggregation of anionic surfactants at a concentration far lower
than its original CAC, especially for the anionic surfactant with a
more hydrophobic tail.
2.1. Synthesis of per-6-deoxy-6-(1-methylimidazol-3-ium-3-yl)-
b-
CD (3)
1
(500 mg, 0.26 mmol) was dissolved in 1-methylimidazole
(3.0 mL, 45.0 mmol), and the reaction mixture was stirred at 80 8C
under argon atmosphere for 48 h. The resultant solution was poured
into acetone (100 mL). The precipitate formed was collected by
filtration and then recrystallized from water to give a translucence
1
flaky solid (355.2 mg, 55%). H NMR (400 MHz, D
2
O):
d
3.30–3.38 (t,
1
4
H, J = 9.2 Hz), 3.50–3.56 (d, 1H, J = 9.9 Hz), 3.75–3.82 (s, 3H), 3.93–
.01 (t, 1H, J = 9.3 Hz), 4.08–4.17 (dd, 1H, J = 14.5, 5.6 Hz), 4.43–4.54
1
3
(m, 2H), 5.02–5.08 (s, 1H), 7.41–7.45 (s, 1H) 7.49–7.53 (s, 1H).
C
2
NMR (100 MHz, D O): d 36.49, 50.05, 69.20, 71.52, 72.04, 81.98,
2
+
1
1
6
3
01.92, 123.38, 124.17, 137.46. HRMS (MALDAI, m/z) [MÀ2I] /2
3+
112.1223, found 1112.1244, [MÀ3I] /3 699.1134, found
4+
5+
99.1126, [MÀ4I] /4 492.6089, found 492.6081, [MÀ5I] /5
2+
68.7062, found 368.7058, [MÀ7I] /7 227.1032, found 227.1032.
28Á8H O, C 32.05, H 4.65,
Elemental Anal. Calcd. for 3: C70
N 7.48; Found C 32.29, H 4.92, N 7.27.
105 7 14
H I N O
2
In the molecular aggregate induced by host 3, there exist two
possible interactions between host and guest. One is the inclusion
2.2. Synthesis of mono-6-deoxy-6-(1-methylimidazol-3-ium-3-yl)-
b
-CD (4)
complexation of b-CD cavity with guest surfactant, and the other is
the electrostatic interactions between seven positively charged
methylimidazoliumyl groups and the negatively charged sulfate
head of guest surfactant. In order to figure out the contribution of
these two interactions, host 4, which also possesses a hydrophobic
cavity but only one positively charged methylimidazoliumyl
2
(500 mg, 0.40 mmol) was dissolved in 1-methylimidazole
(
2.0 mL, 30.0 mmol), and the reaction mixture was stirred at 80 8C
under argon atmosphere for 24 h. The resultant solution was poured
into acetone (100 mL). The precipitate formed was collected by
Scheme 2. Structures of guests SDS and AOT.
Please cite this article in press as: D. Zhao, et al., Comparative studies on molecular induced aggregation of hepta-imidazoliumyl-
b-
cyclodextrin towards anionic surfactants, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.11.028

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D. Zhao et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
3
Fig. 1. Dependence of the optical transmittance of (a) SDS and (b) AOT at 450 nm in the presence of 0.04 mmol/L 3 at 25 8C.
3
increases. This phenomenon may be attributed to that the
electrostatic interactions between imidazolium cations and
sulfonate anions decreased the electronegativity of sulfate anions
a
and caused the high field shift of H atom, and the low field shifts of
H
b
, H , H and H atoms and peaks broadening can be attributed to
c
d
e
the Van der Waals effect and the polarity change of ambient
environment which was caused by amphiphilic aggregation of SDS.
The optical transmittance spectral experiments showed that, in the
measured concentration range, the optical transmittance of guest
surfactant kept unchanged in the presence of host 4 (Fig. S3 in
Supporting information). In addition, as shown in the plot of
optical transmittance (Fig. 3) of SDS (0.20 mmol/L) at 450 nm as a
function of the concentration of 3 showed that the plot of optical
transmittance of SDS the transmittance decreased rapidly and then
gradually increased thereafter to a quasi-plateau with increasing
the concentration of 3, and the minimum was reached at a SDS/3
ratio of 7, i.e., a sulfonate anion/imidazolium cation ratio of 1. The
rapid decrease of the optical transmittance indicated that a large
assembly was formed between 3 and SDS. Then, the addition of an
excess amount of 3 led to the formation of a simple 1:1 inclusion
complex, resulting in the decrease of turbidity. Similar phenome-
non was also observed in the case of 3/AOT system (Fig. 3), and the
minimum was also reached at an AOT/3 ratio of 7. In the control
experiment, the optical transmittance of SDS or AOT at 450 nm was
nearly unchanged with the addition of 4 (Fig S4 in Supporting
information). These results jointly indicated that the electrostatic
interactions between imidazolium cations and sulfonate anions,
1
Fig. 2. Partial
H NMR spectra of SDS (1.0 mmol/L) with the increasing
concentration of 3 in D
.4 Â 10À4, 2.0 Â 10À4, and 3.3 Â 10 mol/L, respectively.
2
O at 25 8C. From 1 to 6, [3] = 0, 1.4 Â 10À5, 1.0 Â 10À4
,
À4
1
group, was synthesized as a reference compound, and the
molecular aggregation behaviours of guest surfactants induced
by host 4 were investigated and compared with those induced by
1
host 3.
H NMR of SDS (1.0 mmol/L) with the increasing
concentration of 3 (Fig. 2) showed that the resonance of the H
atom on SDS was slightly shifted to the higher field, the resonance
of the H , H , H and H atoms were shifted to the lower field, and
all of the peaks gradually broadened as the concentration of
a
rather than the inclusion complexation of
predominant between host and guest.
The information about the structure and stability of the
molecular induced aggregate comes from the Tyndall effect,
b-CD cavity, are
b
c
d
e
Fig. 3. Dependence of the optical transmittance (a) SDS (0.20 mmol/L) and (b) AOT (0.03 mmol/L) at 450 nm with increasing the concentration of 3 at 25 8C.
Please cite this article in press as: D. Zhao, et al., Comparative studies on molecular induced aggregation of hepta-imidazoliumyl-
b-
cyclodextrin towards anionic surfactants, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.11.028

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Fig. 4. Tyndall effect of 3, SDS, AOT, 3/SDS complex, and 3/AOT in water at 25 8C, [3] = 1.4 Â 10À4 mol/L, [SDS] = [AOT] = 1.0 Â 10À3 mol/L.
Fig. 5. TEM images of 3/SDS (left) and 3/AOT (right).
(a)
(b) 100
T% at 450 nm
100
2
5-1
95
90
85
80
75
80
60
40
20
0
25-2
2
2
2
2
5
5
5
5-3
5-4
5-5
5-6
0-1
0-2
0-3
50-4
5
0-5
0-6
5
200
300
400
500
600
700
800
25 50 25 50 25 50 25 50 25 50 25 50
o
Wavelength (nm)
Temperature ( C)
Fig. 6. (a) Optical transmittance of 3/SDS solution at 450 nm with changing the temperature between 25 8C and 50 8C; (b) Dependence of the optical transmittance of 3/SDS
solution at 450 nm on temperatures.
Zeta-potential, dynamic light scattering (DLS), and transmission
electron microscopy. As shown in Fig. 4, the solution of 3/
surfactant exhibits a clear Tyndall effect, indicating the existence of
abundant large particles. No Tyndall effect was observed in the
solution of free 3 or surfactant, revealing that free 3 and surfactant
cannot form nano-scaled aggregates under the same conditions.
The Zeta-potential of 3/SDS or 3/AOT ([3]/[surfactant] = 1:7)
system was measured to be zero in aqueous solution. This
indicated that the charges were packaged inside the aggregate, and
the outside surface of the aggregate is the neutral secondary-side
The thermal stability of [3]/[surfactant] was measured by the
optical transmittance spectra at different temperatures. As shown
in Fig. 6, with increasing the temperature from 25 8C to 50 8C, the
optical transmittance of [3]/[SDS] obviously increased, indicating
the disassembly of the induced aggregate. Then, the increased
optical transmittance could recover to the original level when
decreasing the temperature from 50 8C to 25 8C. These cycles could
be repeated for several times, demonstrating a good temperature-
responsiveness of the induced aggregate. In contrast, [3]/[AOT]
showed the better thermal stability, and the optical transmittance
of [3]/[AOT] exhibited very small changes (less than 2%) when
switching the temperature between 25 8C and 50 8C (Fig S5 in
Supporting information).
of
surfactant with an average diameter of ca. 239 nm for 3/SDS and ca.
85 nm for 3/AOT, accompanied by a comparatively narrow size
b-CD. The DLS results illustrated the large aggregates of 3/
4
distribution. Moreover, no appreciable DLS signals were observed
for free 3 and surfactant under the same condition. In the TEM
images (Fig. 5), 3/SDS showed a number of spherical particles with
an average diameter of ca. 100–200 nm, while the average
diameter of spherical particles formed by 3/AOT was measured
to be ca. 600 nm. The TEM results are consistent with the ones of
DLS that host 3 gave the better induced the aggregation effect
towards the anionic surfactant with more hydrophobic tail.
4. Conclusion
In summary, a new
b-CD derivatives modified with seven
positively charged methylimidazoliumyl groups has been success-
fully synthesized and found able to induced the aggregation of
anionic surfactants to form large nanoparticles. Significantly, this
host-induced aggregation of surfactants was more effective
Please cite this article in press as: D. Zhao, et al., Comparative studies on molecular induced aggregation of hepta-imidazoliumyl-
b-
cyclodextrin towards anionic surfactants, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.11.028

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Appendix A. Supplementary data
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9
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b-
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