Aggregation behavior of zwitterionic surface active ionic liquids with different counterions, cations, and alkyl chains

Article abstract of DOI:10.1039/c6ra02986h

A group of zwitterionic surface active ionic liquids (SAILs) with different counterions, cations and alkyl chains, 3-(1-alkyl-3-imidazolio)propanesulfonate β-naphthalene sulfonate, (C_nIPS-Nsa, n = 12, 14), 3-(1-dodecyl-3-imidazolio)propanesulfonate benzenesulfonate (C₁₂IPS-Bsa), and dodecyl-N,N-dimethylammonio-3-propane sulfonate β-naphthalene sulfonate (SB-12-Nsa), were synthesized. Their aggregation behaviors in aqueous solutions were systematically investigated by surface tension, dynamic light scattering (DLS) and ¹H NMR spectroscopy. Surface tension and DLS results illustrated that the surface properties, micelle size, and micellization behavior of zwitterionic SAILs in aqueous solutions are significantly affected by the anion type, anionic structure and the hydrophobicity of the alkyl chain. The SAILs with more hydrophobic anions and long alkyl chains are expected to favor the micellization. The steric hindrance and hydrophobicity of the cations, as well as the binding strength of the cations with the anions, also play important roles in the aggregation of zwitterionic SAILs. Additionally, the micelle formation mechanism was acquired by detailed analysis of the ¹H NMR spectra. The existence of π-π stacking between imidazolium and counterions was proved. The enhanced π-π stacking and hydrophobic effect of Nsa^- can promote the aggregation of zwitterionic SAILs. Density functional theory (DFT) calculations illustrated that the negative interaction energy of the complexes were C₁₂IPS-Bsa/H₂O > SB-12-Nsa/H₂O > C₁₂IPS-Nsa/H₂O > C₁₄IPS-Nsa/H₂O. It is more difficult to form micelles in complexes with more negative interaction energy. The counterion electronegativity of Nsa^- is smaller than that of Bsa^-, which favors the formation of micelles.

Full text of DOI:10.1039/c6ra02986h

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PAPER
Aggregation behavior of zwitterionic surface active
ionic liquids with different counterions, cations,
and alkyl chains†
Cite this: RSC Adv., 2016, 6, 27370
a
b
a
a
*
Panpan Sun, Lijuan Shi, Fei Lu and Liqiang Zheng
A group of zwitterionic surface active ionic liquids (SAILs) with different counterions, cations and alkyl
chains, 3-(1-alkyl-3-imidazolio)propanesulfonate b-naphthalene sulfonate, (CnIPS-Nsa, n ¼ 12, 14), 3-(1-
dodecyl-3-imidazolio)propanesulfonate
benzenesulfonate
(C12IPS-Bsa),
and
dodecyl-N,N-
dimethylammonio-3-propane sulfonate b-naphthalene sulfonate (SB-12-Nsa), were synthesized. Their
aggregation behaviors in aqueous solutions were systematically investigated by surface tension, dynamic
light scattering (DLS) and 1H NMR spectroscopy. Surface tension and DLS results illustrated that the
surface properties, micelle size, and micellization behavior of zwitterionic SAILs in aqueous solutions are
significantly affected by the anion type, anionic structure and the hydrophobicity of the alkyl chain. The
SAILs with more hydrophobic anions and long alkyl chains are expected to favor the micellization. The
steric hindrance and hydrophobicity of the cations, as well as the binding strength of the cations with the
anions, also play important roles in the aggregation of zwitterionic SAILs. Additionally, the micelle
formation mechanism was acquired by detailed analysis of the 1H NMR spectra. The existence of p–p
stacking between imidazolium and counterions was proved. The enhanced p–p stacking and hydrophobic
effect of Nsaꢀ can promote the aggregation of zwitterionic SAILs. Density functional theory (DFT)
calculations illustrated that the negative interaction energy of the complexes were C12IPS-Bsa/H2O >
SB-12-Nsa/H2O > C12IPS-Nsa/H2O > C14IPS-Nsa/H2O. It is more difficult to form micelles in complexes
with more negative interaction energy. The counterion electronegativity of Nsaꢀ is smaller than that of
Bsaꢀ, which favors the formation of micelles.
Received 1st February 2016
Accepted 8th March 2016
DOI: 10.1039/c6ra02986h
www.rsc.org/advances
unique chemical and physical properties. Many new SAILs can
be created by the combination of different cation and anion.
Introduction
Such as, numerous SAILs with different headgroups like imi-
dazolium, pyridinium, pyrrolidinium cations and different
Ionic liquids (ILs) are a class of organic electrolytes whose
melting points are generally below 373 K.1 They have been
exploited in many elds, including novel nanomaterials
synthesis, catalysis, organic synthesis, and electrochemistry,2–6
due to their extraordinary physicochemical properties, such as
no signicant vapor pressures, non-ammability, high ionic
conductivity, high thermal stability, and easy recyclability.7–9 An
interesting class of ILs, which consists of long hydrophobic
chains, can self-assemble to form various aggregates in aqueous
solutions. These ILs have been generally named as surface
active ionic liquids (SAILs).10–20
anions (e.g. Clꢀ, Brꢀ, NO3ꢀ, Iꢀ, BF4ꢀ, CF3SO3ꢀ, CH3COOꢀ,
ꢀ
ClO4
) were investigated. The aggregation behaviors of
imidazolium-based SAILs in aqueous solutions have been
intensively explored. Watson et al. synthesized a series of closely
related ionic liquids [Cnmim][X] (n ¼ 4, 8, 12).10 They found that
only [C8mim][X] and [C12mim][X] can form micelles, revealing
the effect of alkyl chain length on the micellization of ILs. The
role of anionic types in aggregation behaviors of 1-decyl-3-
methlimidazolium
chloride
([C10mim]Cl),
1-decyl-3-
methlimidazolium hexauorophosphate ([C10mim]PF6) and
1-decyl-3-methlimidazolium bis(triuoromethane) sulfonamide
([C10mim]NTF2) in aqueous solutions has been explored by
Rebelo and Lopes et al.11 It was observed that no aggregates was
detected when Clꢀ was replaced by BF4ꢀor NTF2ꢀ. The aggre-
gation behaviors of [C8mim]Br, 1-octyl-4-methylpyridinium
bromide (4-m-[C8pyr]-Br), and 1-methyl-1-octylpyrrolidinium
([C8mpyrr]Br) has been investigated by Wang et al.12 They re-
ported that the hydrophobicity and steric hindrance of the
cations play important roles in the aggregation of these SAILs.
Recently, SAILs have been extensively studied and widely
used in the eld of colloid and interface science due to their
aKey Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of
Education, Jinan 250100, China. E-mail: lqzheng@sdu.edu.cn; Fax: +86-531-
88564750; Tel: +86-531-88366062
bKey Laboratory of Coal Science and Technology of Ministry of Education and Shanxi
Province, Taiyuan University of Technology, Taiyuan 030024, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra02986h
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Scheme 1 Chemical structures of the zwitterionic surface active ILs used in this study.
Above the studies indicated that the aggregation behaviors of the formate anion shows an almost pure salt effect. Addition-
SAILs can be regulated by changing the structures of SAILs, such ally, Yu et al. investigated the surface adsorption and micelle
as alkyl chain lengths, cationic structures and anionic types. formation of imidazolium-based zwitterionic surface active
However, the ion exchange, which is incomplete and time- ionic liquids in aqueous solution.26 These results showed that
consuming, is one of the biggest troubles of modulating SAIL the alkyl chain length affects the micellar properties remarkably
structures.
and while temperature, pH, and low salt concentration have
To overcome this problem, zwitterions with a positive and a slight effect. However, the effect of structure changes on the
negative charge chemically bonded intramolecularly are the zwitterionic SAILs self-assembly have not been systematic
good candidates. The molecular structures of zwitterionic SAILs studied. The intermolecular interactions among cations,
can be manipulated by the simple mixing of zwitterions and counterions, and solvent molecules during micellization are
ionic salts.21–40 The self-assemblies formed by zwitterionic SAILs still unclear.
consisting of imidazolium cations and anionic sites such as
Herein, we synthesized a group of imidazolium-based zwit-
sulfonate, sulfonamide, or carboxylic groups have been widely terionic SAILs containing sulfonate group, N-alkyl-N0-propane-
studied. Faruk et al. rstly synthesized the zwitterionic surfac- sulfonate imidazolium inner salts (Scheme 1). The effect of
tant containing an imidazolium cation and evaluated the counterions, cations and alkyl chains on their aggregation
chameleon-like behaviors of zwitterionic micelles.21 It was behaviors was systematically investigated by surface tension, 1H
found that the molecular structure of zwitterionic surfactant NMR, and DLS. The intermolecular interactions during aggre-
headgroup play important roles in anion binding and control of gation are revealed and discussed.
the micellar zeta potential. Aerwards, they also investigated
the effect of cations binding on the chameleon-like nature of
Experimental section
Materials and synthesis
zwitterionic micelles.23 They reported that the micellar surface
potential are strongly dependence on the cation valence. Sarkar
et al. studied the addition of ionic liquids (dimethylethanol
ammonium hexanoate-DAH and dimethylethanol ammonium
formate-DAF) induced the changes of zwitterionic micelles.25 It
was observed that the alkyl chain present on the anion of DAH
leads to its partitioning into the CTAB–DAH interface and
behavior as a cosurfactant. The DAF having no alkyl chain on
1-Bromododecane (98%), 1-bromotetradecane (98%), sodium
benzenesulfonate (98%), 1,3-propanesultone (99%), and imid-
azole (99%) were purchased from J&K Scientic Ltd. b-Naph-
thalenesulfonic acid sodium salt (Nsa) (98%) and dodecyl-N,N-
dimethylammonio-3-propane sulfonate were the products of
TCI. CDCl3 (99.96%) and D2O (99.96%) were obtained from
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Sigma-Aldrich. Acetone, NaOH and tetrahydrofuran (THF) were the scattering angle of 180). The temperature was controlled
obtained from Shanghai Chemical Co. All the materials above with a thermostat (F31C, Julabo) with an accuracy of 25 ꢂ 0.1 ꢁC.
were used without further purication. Triply distilled water
1H NMR spectra were operated on a Bruker Advance 400
was used throughout the whole experiments.
spectrometer equipped with a pulse eld gradient module
Synthesis of 3-(1-alkyl-3-imidazolio)propanesulfonate b- (Z axis) using a 5 mm BBO probe. The instrument was run at
naphthalene sulfonate (C14IPS-Nsa).21,34 1-Bromotetradecane a frequency of 400.13 MHz at 25 ꢂ 0.1 ꢁC. The obtained
(10 g, 36.1 mmol) in THF was slowly added to the solution of chemical shis (dobs) of the discrete protons were examined as
imidazole (2.95 g, 43.2 mmol) in NaOH (50%) aqueous solution a function of concentration below and above CMC.
(4.33 g, 54.1 mmol). The reaction mixture was reuxed for
3 days. Aer the reaction, THF was removed by rotary evapo-
ration. The residue was extracted with CH2Cl2/water for three
times. The produced 1-tetradecylimidazole was dried under
Results and discussion
Surface properties and micellization parameters
a vacuum at room temperature to acquire clear yellow oil Surface tension measurements were carried out to evaluate the
(9.20 g, 93.8% yield). 1-Dodecylimidazole was synthesized aggregation behavior of SAILs in aqueous solution. Fig. 1
following the synthetic procedure for 1-tetradecylimidazole.
demonstrates the surface tension (g) versus concentration (C)
1-Tetradecylimidazole (9.0 g, 34.1 mmol) was dissolved in plots for the solutions of zwitterionic SAILs at 25 ꢁC. The surface
80 mL acetone. Then an equimolar amount of 1,3-propane- tension gradually decreases with increasing IL concentrations
sultone (4.16 g, 34.1 mmol) in acetone (40 mL) was added until to a plateau region, indicating the formation of micelle.
ꢁ
dropwise into a solution at 0 C under a nitrogen atmosphere. The critical micelle concentration (CMC) can be obtained from
Then reaction mixture was stirred for 5 days at room tempera- the distinct break point and the corresponding gcmc (surface
ture. Aer the reaction, a white powder was obtained following tension at CMC) are listed in the Table 1.
purication by acetone twice. The 3-(1-alkyl-3-imidazolio)
Surface activity expresses that a compound has the capacity
propanesulfonate (C14IPS) was dried under vacuum at room to reduce the surface tension of solution. As can be seen from
temperature and a white powder was acquired (7.63 g, 58% Fig. 1a, it is obvious that C12IPS-Nsa can form micelles easily at
yield). The C14IPS-Nsa was obtained from the mixture of C14IPS lower concentration than SB-12-Nsa. The gcmc of C12IPS-Nsa is
and Nsa with equal mole. The C12IPS-Nsa was synthesized smaller than SB-12-Nsa, indicating the C12IPS-Nsa possesses
following the synthetic procedure for C14IPS-Nsa.
a greater ability to reduce the surface tension of water. It is
1H NMR spectrum (CDCl3, 400 MHz), d (relative to TMS), probably due to the difference in their cation structures. The
(C14IPS) ppm: 9.67 (s 1H), 7.52 (s 1H), 7.19 (s 1H), 4.57 (t 2H), delocalized positive charge and steric effect generating by rigid
4.25 (t 2H), 2.85 (t 2H), 2.39 (t 2H), 1.99 (m 2H), 1.31 (m 22H), and bulky imidazolium rings are benet to the micellization.26
0.89 (t 3H).
The (g–C) plots for the zwitterionic SAILs with different coun-
1H NMR spectrum (CDCl3, 400 MHz), d (relative to TMS), terions, C12IPS-Nsa and C12IPS-Bsa in aqueous solution at 25 ꢁC
(C12IPS) ppm: 9.71 (s 1H), 7.57 (s 1H), 7.21 (s 1H), 4.58 (t 2H), are shown in Fig. 1b. Obviously, compared with C12IPS-Bsa, the
4.25 (t 2H), 2.85 (t 2H), 2.38 (t 2H), 2.05 (m 2H), 1.32 (m 22H), smaller value of CMC for C12IPS-Nsa is obtained. The counter-
0.88 (t 3H).
ions of Nsaꢀ can promote the formation of micelles than that of
Bsaꢀ. Meanwhile, a lower gcmc value for C12IPS-Nsa was also
obtained as compared with C12IPS-Bsa, which suggests that
Sample preparation
C
12IPS-Nsa possesses a greater ability to reduce the surface
Samples were prepared by weighing the designed compositions
of ionic liquid in stoppered glass vials, and then the H2O was
added to the ionic liquid. The samples were mixed using
a vortex mixer and they were stored in a thermostat for at least
1 week for equilibration before further investigations.
tension of water. It is possible that naphthalene ring can
dramatically increase the hydrophobicity of zwitterionic SAILs.
Other possibility was proposed that the existence of p–p
stacking among Nsaꢀ moieties with imidazolium facilitated the
micellization. The (g–C) plots for zwitterionic SAILs (CnIPS-Nsa
n ¼ 12, 14) with different alkyl chain length are shown in Fig. 1c.
It can be seen that with the increasing length of the hydro-
Characterization
Surface tension measurements were carried out on a model carbon chain, the CMC values decrease due to the enhanced
JYW-200B tensiometer (Chengde Dahua Instrument Co., Ltd., hydrophobicity.
accuracy ꢂ0.1 mN mꢀ1) using the ring method. The tempera-
Applying the Gibbs adsorption isotherm to the surface
ture was controlled by a thermostatic bath with an accuracy of tension plots, the maximum excess surface concentration (Gmax
)
ꢁ
25 ꢂ 0.1 C. Each sample was equilibrated for 15 min and all and the area occupied by a single amphiphile molecule at the
41
measurements were repeated at least twice until the values were air–liquid interface (Amin) were estimated.
G
and Amin both
max
reproducible.
reect the arrangement situation of amphiphile molecules at
The size distributions of the micelles were determined by the air–liquid interface.20 A greater value of Gmax or a smaller
dynamic light scattering (DLS) using a Nanotrac Particle Size value of Amin means a denser arrangement of surfactant mole-
Analyzer (Nanotrac NPA 250) and the microtracFLEX applica- cules at the surface of solution. As shown in Table 1, the Gmax
tion soware program. All measurements were made with value of C14IPS-Nsa is lower than C12IPS-Nsa, indicating that
a laser diode (780 nm wavelength, 3 m Wnominal, Class IIIB at
C
14IPS-Nsa molecules at the air–water interface are arranged
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Table 1 Surface properties and micellization parameters of CnIPS-Nsa
(n ¼ 12, 14), C12IPS-Bsa and SB-12-Nsa in aqueous solutions at 25 ꢁ
C
CMC
gcmc
(mN mꢀ1
Gmax
(mmol mꢀ2
Amin
(A )
2
˚
(mmol Lꢀ1
)
)
)
SB-12-Nsa
12IPS-Bsa
C12IPS-Nsa
C14IPS-Nsa
1.6
1.8
1.0
0.2
40.6
40.5
39.7
38.9
2.75
2.90
3.09
3.13
60.4
57.3
53.8
53.1
C
The micelle sizes and size distributions of zwitterionic SAILs
DLS was utilized to further investigate the size distributions of
the aggregates formed by zwitterionic SAILs. The micelle sizes
of 10 mM zwitterionic SAILs aqueous solution are shown in
Fig. 2. When the counterions varies from benzene sulfonic
anions to naphthalene sulfonic anions, the hydrodynamic radii
(R) of the micelles increases from 1.8 to 2.3 nm. It can be
explained by the more hydrophobic and steric effect of naph-
thalene ring than benzene ring.42 As shown in Fig. 2b, compared
with the SB-12-Nsa, the imidazole ring of C12IPS-Nsa induced
the size of aggregate from 1.7 to 2.3 nm. This is probably due to
the higher hydrophobic zwitterionic group of C12IPS-Nsa, which
has less reorganizing structure effects of water than ammonium
cations.26 Compared with C12IPS-Nsa (R ¼ 2.3 nm), the larger
values of R for C14IPS-Nsa (R ¼ 2.5 nm) is ascribed to the
increased hydrophobicity resulting from the extension of the
hydrocarbon chain.
Mechanism
NMR analysis is a sensitive and accurate technique to investi-
gate the intermolecular interaction of classical surfactants.43–50
The protons of zwitterionic SAILs have different chemical shis
(dobs) below and above the CMC, and the observed chemical
shis (ESI Table S1†) can reect the microenvironment of
protons in the aggregates. Before the formation of micelle, no
signicant change in chemical shis is observed, since the
zwitterionic SAILs molecules are only dissolved in D2O as
monomer. However, upon micellization, the chemical shis of
protons change obviously. The change of the chemical shi, Dd
(¼dobs ꢀ dmon), as a function of concentration for various zwit-
terionic SAILs, is shown in Fig. 3.
In general, the chemical shis of protons on the surfactants
move downeld during aggregation in the absence of any other
specic interaction.37,42 As can be seen from Fig. 3a, b and d, the
protons on hydrocarbon chains of zwitterionic SAILs show an
upeld shi upon aggregation except the terminal methyl group
protons (H7) of C14IPS-Nsa, C12IPS-Nsa and SB-12-Nsa. This
observed trend can be deduced that the naphthalene sulfonic
anion would penetrate into the hydrophobic regions. The
insertion of naphthalene ring brings a more signicant
shielding effect to the protons on the alkyl chain due to the
circular current effect. The upeld shis of terminal methyl
group protons can be attributed to the shielding effect. Mean-
while, the insertion of naphthalene ring restrains the confor-
mation change of alkyl chain from trans to gauche
Fig. 1 Surface tension as a function of concentration of (a) C12IPS-
Nsa, SB-12-Nsa; (b) C12IPS-Nsa, C12IPS–Bsa, and (c) C12IPS-Nsa,
C
14IPS-Nsa in aqueous solutions at 25 ꢁC.
more densely than C12IPS-Nsa. It is also clearly seen that C12IPS-
Nsa has a smaller Gmax value than C12IPS-Bsa and SB-12-Nsa,
although C12IPS-Nsa have a much larger headgroup. Except
the enhanced hydrophobicity of C12IPS-Nsa, it is may be likely
that the enhanced p–p stacking between the cations and anions
of C12IPS-Nsa could attract the molecules to pack more
compactly at the air–water interface.
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benzene sulfonic anion is adsorpted at the surface of the
micelles. This suppose of conformation is conrmed by the
optimized the structures from DFT calculations.
The protons at C-3 of the imidazolium ring have no signals
due to the vicinity of electronegative nitrogen, and are available
for hydrogen bonding with water.41 When the counterion is
naphthalene sulfonic acid anions (see Fig. 3a, b and d), the
protons of naphthalene ring and headgroup are upeld shi.
This is mainly because of the existence of p–p stacking among
adjacent naphthalene moieties and the p–p stacking between
naphthalene ring and imidazolium ring. The resulting circular
current effect makes the protons around the headgroup shi
upeld due to the shielding effect.13,46,48,49 Additionally, the
shis of the protons on the head group of C12IPS-Nsa are more
obvious than SB-12-Nsa, as shown in Fig. 4a and d. This result is
possibly due to the enhanced shielding effect by the p–p
stacking of imidazolium ring with naphthalene ring. With the
results shown in Fig. 4c and a conclusion is that the protons of
aromatic ring near the sulfonate (H11, H15) shi upeld, and
other protons on aromatic ring (H12, H13, H14) shi downeld.
The imidazolium ring and phenyl were combined together
through the cooperative effect of p–p stacking and electrostatic
attraction. The circular current effect leads the protons (H11
,
H15) to shi a higher eld. In addition, this circular current
effect has quite rigorous geometrical restriction; only the
protons localized in the eld of the ring are substantially shif-
ted.48,49 Other protons, H12, H13, H14, were adsorpted at the
surface of the micelle and cannot be affected by the shielding
effect, and thus the protons shi downeld. According to the
analysis above, the possible packing structures of molecules in
the micelle are depicted and shown in Scheme 2.
DFT calculations
To better understanding the aggregation behavior mechanism
of zwitterion SAILs, the interactions of zwitterion SAILs with
H2O were investigated. Density functional theory (DFT) calcu-
lations using the Gaussian 09 package with the hybrid B3LYP
functional and the 6-31G (d,p) basis set were performed. The
structures of the isolated zwitterion SAILs (C12IPS-Nsa, C14IPS-
Nsa, C12IPS-Bsa, SB-12-Nsa), H2O, and the (1 : 1) binary
complex of zwitterion SAILs with H2O were rst optimized. The
most stable optimized structures are presented in Fig. 4. Their
interaction energies (Eint), which is the energy difference
between complexes and the monomers, are listed in the Table 2.
As shown in Table 2, the order of the calculated negative
interaction energy of the complexes were C12IPS-Bsa/H2O > SB-
Fig. 2 Size and size distributions of the aggregations formed by 10 mM
zwitterionic SAILs at 25 ꢁC.
12-Nsa/H2O
>
C12IPS-Nsa/H2O
>
C14IPS-Nsa/H2O. The
complexes with more negative interaction energy are more
difficult to form micelles,51–55 resulting in a higher CMC value.
This is consistent with the surface tension measurement results
(Table 1). Furthermore, the electrostatic potentials at the 0.001 e
bohrꢀ3 isodensity surfaces of Nsaꢀ and Bsaꢀ counterions were
shown in the Fig. 5. It is clearly that both counterions are
electronegative and the electronegative of Nsaꢀ is smaller than
Bsaꢀ. This phenomenon means that the electrostatic repulsion
of Nsaꢀ is weaker than that of Bsaꢀ anion, which primarily
conformation, leading to the upeld shi of the protons on
terminal methyl group. It is also interesting to nd that the
terminal methyl group protons (H7) of C12IPS-Bsa shi down-
eld and the protons (H9) on the C-9 between imidazolium ring
and sulfonic acid are upeld shi in the Fig. 3c. The decreasing
polarity of microenvironment lead the terminal methyl group
protons (H7) downeld shi. The shielding effect may be
contributed to the upeld shi of H9. We proposed that the
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Fig. 3 Variation in chemical shifts for protons of zwitterionic SAILs at different concentration at 25 ꢁC; (a) C12IPS-Nsa, (b) C14IPS-Nsa, (c) C12IPS-
Bsa, and (d) SB-12-Nsa.
Fig. 4 Geometries optimized at the B3LYP/6-31G (d, p) level: (a) C12IPS-Nsa, (b) C14IPS-Nsa, (c) C12IPS-Bsa, and (d) SB-12-Nsa. Color code for
atoms: blue, nitrogen; red, oxygen; dark gray, carbon; light gray, hydrogen.
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is smaller than that of Bsaꢀ, which is also favor the micelliza-
tion. The obtained results can help us to regulate the self-
assembly of zwitterionic SAILs and lay a foundation for their
application in various elds.
Acknowledgements
The authors are grateful to the National Natural Science Foun-
dation of China (No. 21573132), the National Basic Research
Program (2013CB834505), the Specialized Research Fund for
the Doctoral Program of Higher Education of China (No.
20120131130003) and the National Natural Science Foundation
of China (No. 21403151).
Scheme 2 Schematic illustration of the proposed structure of micelle
formed by zwitterionic SAILs (C12IPS-Bsa, C12IPS-Nsa as examples) in
aqueous solution.
References
1 R. D. Rogers and K. R. Seddon, Science, 2003, 302, 792–793.
2 N. Jain, A. Kumar and S. Chauhan, Tetrahedron, 2005, 61,
1015–1060.
Table 2 Interaction energy (ꢀEint) of zwitterion SAILs with H2O
Zwitterion SAILs
C14IPS-Nsa C12IPS-Nsa C12IPS-Bsa SB-12-Nsa
8.26 30.10 17.39
3 J. Muzart, Adv. Synth. Catal., 2006, 348, 275–295.
4 V. I. Parvulescu and C. Hardacre, Chem. Rev., 2007, 107,
2615–2665.
Interaction energy 5.77
(ꢀEint/kJ molꢀ1
)
5 C. P. Mehnert, R. A. Cook, N. C. Dispenziere and M. Afeworki,
J. Am. Chem. Soc., 2002, 124, 12932–12933.
6 C. F. Poole, S. K. Poole, S. Patel, K. Dehring and H. Workman,
J. Chromatogr. A, 2004, 1037, 49–82.
7 A. B. McEwen, H. L Ngo, K. Lecompte and J. L. Goldman, J.
Electrochem. Soc., 1999, 146, 1687–1695.
8 T. Welton, Chem. Soc. Rev., 1999, 99, 2071–2084.
9 J. L. Anthony, E. J. Maginn and J. F. Brennecke, J. Phys. Chem.
B, 2002, 106, 7315–7320.
10 G. Law and P. R. Watson, Langmuir, 2001, 17, 6138–6141.
11 M. Blesic, M. H. Marques, N. V. Plechkova, K. R. Seddon,
L. P. N. Rebelo and A. Lopes, Green Chem., 2007, 9, 481–490.
12 H. Y. Wang, J. J. Wang, S. B. Zhang and X. P. Xuan, J. Phys.
Chem. B, 2008, 112, 16682–16689.
Fig. 5 B3LYP/6-31G (d,p) electrostatic potentials, in hartrees, at the
0.001 e bohrꢀ3 isodensity surfaces of Bsaꢀ and Nsaꢀ counterions.
13 B. Dong, N. Li, L. Q. Zheng, L. Yu and T. Inoue, Langmuir,
2007, 23, 4178–4182.
accounts for the lower CMC value of C12IPS-Nsa than C12IPS-
Bsa.
14 J. Yang, H. Y. Wang, J. J. Wang, X. J. Guo and Y. Zhang, RSC
Adv., 2015, 5, 96305–96312.
15 J. J. Wang, H. Y. Wang, S. L. Zhang, H. C. Zhang and Y. Zhao,
J. Phys. Chem. B, 2007, 111, 6181–6188.
Conclusion
The effects of different counterions, cations and alkyl chain
lengths on the aggregation behavior of zwitterionic SAILs were 16 K. A. Fletcher and S. Pandey, Langmuir, 2004, 20, 33–36.
investigated. The existence of p–p stacking in the counterions, 17 I.
the increase of alkyl chain length and the larger volume of
cations can reduce the values of CMC and increase the size of
Rodrıguez-Palmeiro,
O. Rodrıguez, A. Arce and A. Soto, RSC Adv., 2015, 5,
I.
Rodrıguez-Escontrela,
37392–37398.
micelles. 1H NMR results illustrate that the imidazolium cation 18 T. Singh, M. Drechsler, A. H. E. Mueller, I. Mukhopadhyay
¨
and aromatic anion are combined together through the coop-
and A. Kumar, Phys. Chem. Chem. Phys., 2010, 12, 11728–
erative effect of electrostatic, p–p stacking and hydrophobic
11735.
interactions. The protons on imidazolium ring and naphtha- 19 S. X. Shi, T. X. Yin, X. Y. Tao and W. G. Shen, RSC Adv., 2015,
lene ring shi upeld due to the shielding effect, which is 5, 75806–75809.
a powerful evidence for the existence of p–p stacking. DFT 20 B. Dong, Y. A. Gao, Y. J. Su, L. Q. Zheng, J. K. Xun and
calculations mainly manifest the negative interaction energy of T. Inoue, J. Phys. Chem. B, 2010, 114, 340–348.
the complexes were C12IPS-Bsa/H2O > SB-12-Nsa/H2O > C12IPS- 21 D. W. Tondo, E. C. Leopoldino, B. S. Souza, G. A. Micke,
Nsa/H2O > C14IPS-Nsa/H2O, revealing C14IPS-Nsa is easier to
form micelles in aqueous solutions. The electronegative of Nsaꢀ
A. C. O. Costa, H. D. Fiedler, C. A. Bunton and F. Nome,
Langmuir, 2010, 26, 15754–15760.
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Paper
RSC Advances
22 J. P. Priebe, M. Lal Satnami, D. W. Tondo, B. S. Souza, 39 L. Marte, R. C. Beber, M. A. Farrukh, G. A. Micke,
J. M. Priebe, G. A. Micke, C. O. Costa, H. i. D. Fiedler,
A. C. A. Bunton and F. Nome, J. Phys. Chem. B, 2008, 112,
14373–14378.
A. C. O. Costa, N. D. Gillitt, C. A. Bunton, P. D. Proo,
G. Savelli and F. Nome, J. Phys. Chem. B, 2007, 111, 9762–
9769.
23 J. P. Priebe, F. D. Souza, M. Silva, D. W. Tondo, J. M. Priebe, 40 J. P. Priebe, B. S. Souza, G. A. Micke, A. C. O. Costa,
G. A. Micke, A. C. O. Costa, C. A. Bunton, F. H. Quina,
H. D. Fiedler and F. Nome, Langmuir, 2012, 28, 1758–1764.
24 K. Behera and S. Pandey, Langmuir, 2008, 24, 6462–6469.
25 V. G. Rao, C. Ghatak, S. Ghosh, R. Pramanik, S. Sarkar,
H. D. Fiedler, C. A. Bunton and F. Nome, Langmuir, 2010,
26, 1008–1012.
41 M. J. Jaycock and G. D. Partt, Chemistry of Interfaces, John
Wiley and Sons, New York, 1981.
S. Mandal and N. Sarkar, J. Phys. Chem. B, 2011, 115, 3828– 42 Y. Q. Gu, L. J. Shi, X. Y. Cheng, F. Lu and L. Q. Zheng,
3837. Langmuir, 2013, 29, 6213–6220.
26 X. Q. Wang, J. Liu, L. Yu, J. J. Jiao, R. Wang and L. M. Sun, J. 43 Y. Zhao, S. J. Gao, J. J. Wang and J. M. Tang, J. Phys. Chem. B,
Colloid Interface Sci., 2013, 391, 103–110. 2008, 112, 2031–2039.
27 D. W. Tondo, J. M. Priebe, B. S. Souza, J. P. Priebe, 44 T. Singh and A. Kumar, J. Phys. Chem. B, 2007, 111, 7843–
C. A. Bunton and F. Nome, J. Phys. Chem. B, 2007, 111,
11867–11869.
28 H. Mao, Y. Song, D. M. Qian, D. L. Liu, S. Y. Wu, Y. Zhang,
Y. Hisaed and X. M. Song, RSC Adv., 2015, 5, 91654–91664.
29 J. Z. Zhang, K. Yu, Q. W. Yang, Z. B. Bao, Z. G. Zhang,
7851.
45 S. Shimizu, P. A. R. Pires, H. Fish, T. K. Halstead and O. A. El
Seoud, Phys. Chem. Chem. Phys., 2003, 5, 3489–3497.
46 A. A. McLachlan, K. Singh and D. G. Marangoni, Colloid
Polym. Sci., 2010, 288, 653–663.
Y. W. Yang, Q. L. Ren and H. B. Xing, RSC Adv., 2015, 5, 47 B. O. Persson, T. Drakenberg and B. Lindman, J. Phys. Chem.,
77581–77588. 1976, 80, 2124–2125.
30 J. Lv, W. H. Qiao and C. Q. Xiong, Langmuir, 2014, 30, 8258– 48 X. Huang, Y. C. Han, Y. X. Wang and Y. L. Wang, J. Phys.
8267. Chem. B, 2007, 111, 12439–12446.
31 C. Banerjee, S. Ghosh, S. Mandal, J. Kuchlyan, N. Kundu and 49 S. Shimizu and O. A. El Seoud, Langmuir, 2003, 19, 238–243.
N. Sarkar, J. Phys. Chem. B, 2014, 118, 3669–3681.
50 D. F. Yu, X. Huang, M. L. Deng, Y. Y. Lin, L. X. Jiang,
J. B. Huang and Y. L. Wang, J. Phys. Chem. B, 2010, 114,
14955–14964.
´
32 M. M. Graciani, A. Rodrıguez, M. Munoz and M. L. Moya,
Langmuir, 2005, 21, 7161–7169.
´
33 J. C. Y. Lin, C. J. Huang, Y. T. Lee, K. M. Lee and I. J. B. Lin, J. 51 M. Weimann, M. Farnık, M. Suhm, M. Alikhani and J. Sadlej,
Mater. Chem., 2011, 21, 8110–8121. J. Mol. Struct., 2006, 790, 18–26.
34 H. M. Fan, S. Z. Zhao, B. C. Li, H. J. Fan, W. L. Kang and 52 D. Das, J. Dey, A. Chandra, U. Thapa and K. Ismail, Langmuir,
J. B. Huang, RSC Adv., 2015, 5, 27623–27629. 2012, 28, 15762–15769.
35 B. S. Souza, E. C. Leopoldino, D. W. Tondo, J. Dupont and 53 A. Pan, S. Mati, B. Naskar, S. Bhattacharya and S. Moulik, J.
F. Nome, Langmuir, 2011, 28, 833–840.
36 G. Yang and J. X. Zhao, RSC Adv., 2016, 6, 15694–15700.
37 S. Ghosh, D. Khatua and J. Dey, Langmuir, 2011, 27, 5184–
5192.
38 K. A. Ruzza, F. Nome and D. Zanette, Langmuir, 1996, 11,
2393–2398.
Phys. Chem. B, 2013, 117, 7578–7592.
54 N. Cheng, P. M. Yu, T. Wang, X. Sheng, Y. H. Bi, Y. J Gong
and L. Yu, J. Phys. Chem. B, 2014, 118, 2758–2768.
55 W. W. Xu, Q. W. Yin, Y. A. Gao and L. Yu, Langmuir, 2015, 31,
12644–12652.
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