Synthesis and self-aggregation of a hydroxyl-functionalized imidazolium-based ionic liquid surfactant in aqueous solution

Article abstract of DOI:10.1007/s11743-010-1234-3

This paper deals with the synthesis and self-aggregation of a hydroxyl-functionalized imidazoliumbased ionic liquid (IL) surfactant, namely 1-hydroxyethyl-3-dodecylimidazolium chloride ([C₂OHC ₁₂im]Cl). The molecular structure was confirmed by means of electrospray ionization mass spectrometry (ESI-MS), ¹H nuclear magnetic resonance (¹H NMR) and elemental analysis. Many important physicochemical parameters, such as the critical micelle concentration (CMC), the surface tension at CMC _(γCMC), the adsorption efficiency (pC₂₀), the surface pressure at CMC _(ΠCMC), the maximum surface excess (Γ_m), the minimum molecular cross-sectional area (A_min), the value of CMC/C₂₀, the average number of aggregation (N_m) and the micellar microenvironment polarity were determined by surface tension-concentration curves, fluorescence spectra, and electrical conductivity. The phenomena of the second CMC, the concentration dependence of N_m, and the critical average aggregation number (N _m,c) of imidazolium-based IL surfactants are reported for the first time in this paper. AOCS 2010.

Full text of DOI:10.1007/s11743-010-1234-3

J Surfact Deterg (2011) 14:203–210
DOI 10.1007/s11743-010-1234-3
ORIGINAL ARTICLE
Synthesis and Self-Aggregation of a Hydroxyl-Functionalized
Imidazolium-Based Ionic Liquid Surfactant in Aqueous Solution
•
Xue-feng Liu Li-li Dong Yun Fang
•
Received: 29 May 2010 / Accepted: 8 September 2010 / Published online: 29 September 2010
Ó AOCS 2010
Abstract This paper deals with the synthesis and
self-aggregation of a hydroxyl-functionalized imidazolium-
based ionic liquid (IL) surfactant, namely 1-hydroxyethyl-
3-dodecylimidazolium chloride ([C2OHC12im]Cl). The
molecular structure was confirmed by means of electro-
Introduction
According to the generally accepted definition, ionic
liquids (ILs) are liquid organic salts at, or below 100 °C
[1]. ILs have drawn increasing interest for their promising
roles as alternative media in a variety of catalytic [2],
separation [3, 4], and electrochemical processes [5] as a
result of their unique chemical and physical properties [6].
In recent years, one of the attractive aspects of ILs, for
instance imidazolium-based ILs ([Cnmim]X), is their
inherent amphiphilic character as surfactants in some
conditions [7–28]. Here, Cn, m, im, and X stand for alkyl
chain, methyl, imidazolium, and counter-ion, respectively.
This particular aspect allows us to consider some long-
chain ionic liquids (when dispersed in aqueous solutions),
in fact, as a new type of cationic surfactants, namely IL
surfactants [6, 8, 12, 13], surfactant-like ILs [20], and
surface active ILs [24]. Therefore, IL surfactants would
form self-organized aggregates in aqueous solutions (or the
solutions containing water), e.g., the adsorbed layer at
water–air interfaces, micelles and wormlike ones [29],
microemulsions [30–33], and lyotropic liquid crystals [34,
35], etc.
1
spray ionization mass spectrometry (ESI–MS), H nuclear
magnetic resonance (1H NMR) and elemental analysis.
Many important physicochemical parameters, such as the
critical micelle concentration (CMC), the surface tension at
CMC (cCMC), the adsorption efficiency (pC20), the surface
pressure at CMC (PCMC), the maximum surface excess
(Cm), the minimum molecular cross-sectional area (Amin),
the value of CMC/C20, the average number of aggregation
(Nm) and the micellar microenvironment polarity were
determined by surface tension-concentration curves,
fluorescence spectra, and electrical conductivity. The
phenomena of the second CMC, the concentration depen-
dence of Nm, and the critical average aggregation number
(Nm,c) of imidazolium-based IL surfactants are reported for
the first time in this paper.
Keywords Hydroxyl-functionalized ꢀ Imidazolium-based ꢀ
Ionic liquid surfactant ꢀ The second CMC ꢀ The critical
aggregation number
Surveying the recent literature, two classes of IL
surfactants can be identified, namely single-chain IL
surfactants and IL-type Gemini surfactants [10, 36, 37].
In the light of the hydrophilic head group, four families
of single-chain IL surfactants, i.e. pyridinium-, piperid-
inium-, pyrrolidinium-, and imidazolium-based surfac-
tants have been synthesized and investigated widely.
The effects of the length of the hydrocarbon chain,
counter ions, temperature, and co-existing neutral inor-
ganic salts on the surface activities of IL surfactant have
been explored in detail. Compared with single-chain
imidazolium-based IL surfactants, the Gemini surfactants
Electronic supplementary material The online version of this
article (doi:10.1007/s11743-010-1234-3) contains supplementary
material, which is available to authorized users.
X. Liu (&) ꢀ L. Dong ꢀ Y. Fang
School of Chemical and Materials Engineering,
Jiangnan University, Wuxi 214122,
People’s Republic of China
e-mail: xfliu@jiangnan.edu.cn
Y. Fang
Jiangsu Key Laboratory of Fine Chemicals, Jiangsu, China
123

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J Surfact Deterg (2011) 14:203–210
have better surface activities and dispersion ability [38].
More recently, a series of complex IL surfactants
[Cnmim][CmH2m?1SO3] [39] and a chiral IL surfactant
[40] have been synthesized.
Experimental Section
Material
Hydroxyl-functionalized imidazolium-based ILs exhibit
an excellent ability to dissolve HgCl2 and LaCl3 [41].
Moreover, hydroxyl group(s) in the side chain of ILs may
play a unique role in the control of metal nanoparticles size
and distribution [42, 43]. Thus, we expect that hydroxyl-
functionalized imidazolium-based IL surfactants and their
aggregates may have potential applications in the fields of
the controllable synthesis for metal nanoparticles. The
surface activity and aggregation behavior of such surfac-
tants are essential. However, except for a very recent report
[40], the surface activity and aggregation behavior of
hydroxyl-functionalized IL surfactants have rarely been
reported.
Imidazole, 1-bromododecane, 2-chloroethanol, acryloni-
trile, D2O and other reagents were purchased from Sinop-
harm Chemical Reagent Company (Shanghai, China), and
were used without further purification. Deionized water
was obtained from a Millipore Milli-Q water purification
system (Millipore, USA).
The Synthesis of [C2OHC12im]Cl
The synthesis route of [C2OHC12im]Cl is shown in
Scheme 1. For a typical procedure, imidazole (0.2 mol,
13.6 g) and acrylonitrile (0.27 mol, 14.3 g) were dissolved
in 20 mL methanol. The mixture was stirred at 50 1 °C
for 8 h under nitrogen. The solvent methanol and the
unreacted acrylonitrile were subsequently removed under
vacuum. After that, 1-bromododecane (0.14 mol, 34.9 g)
was added with isopropanol (40 mL) and the mixture was
refluxed at 80 1 °C for 18 h under nitrogen. After the
reaction was completed, isopropanol was removed under
vacuum. The residue was dissolved in a mixture solution of
15% NaOH (w/w, 100 mL)-chloroform (50 mL), and then
was stirred at room temperature until an interface between
organic and aqueous phases was observed. The aqueous
phase was removed, and the resulting chloroform layer was
washed several times with deionized water. The residual
solvent was removed under vacuum to give n-dodecylim-
idazole 29.7 g (90% yields). Using 30 mL isopropanol as
solvent, n-dodecylimidazole (0.13 mol, 29.7 g) and
2-chloroethanol (0.7 mol, 56.4 g) were stirred at 70 °C for
24 h under nitrogen. The solvent and the unreacted
2-chloroethanol were removed under reduced pressure. The
product was purified several times by recrystallization in
acetone at -15 °C and then dried under vacuum for 2 days.
Here, we prepared a hydroxyl-functionalized imidazo-
lium-based IL surfactant, namely1-hydroxyethyl-3-dode-
cylimidazolium chloride ([C2OHC12im]Cl, Scheme 1).
The molecular structure was confirmed by means of
1
electrospray ionization mass spectrometry (ESI–MS), H
nuclear magnetic resonance (1H NMR) and elemental
analysis. Many important physicochemical parameters,
such as the critical micelle concentration (CMC), the
surface tension at CMC (cCMC), the adsorption efficiency
(pC20), the surface pressure at the CMC (PCMC), the
maximum surface excess (Cm), the minimum molecular
cross-sectional area (Amin), and the value of CMC/C20, the
average number of aggregation (Nm) and the micellar
microenvironment polarity were determined by surface
tension-concentration curves, fluorescence spectra, and
electrical conductivity. The phenomena of the second
CMC, the concentration dependence of Nm, and the
critical average aggregation number (Nm,c) of imidazoli-
um-based IL surfactants are reported for the first time in
this paper.
Scheme 1 The synthesis routes
of [C2OHC12im]Cl
Step 1
Step 2
CH3OH
50 oC
CH2CH2CN
N
NH
N
N
+ CH2=CHCN
-
Br
Isopropanol
N+ CH3(CH2)10CH2Br
N
N
N
N
NCCH2CH2
NCCH2CH2
NCCH2CH2
CH2(CH2)10CH3
°
80 C
Step 3
Step 4
-
NaOH (aq)
Br
(CH2)11CH3
N
N
N
15% (w/w)
CH2(CH2)10CH3
H O
+
+ NaBr
+CH2=CHCN
2
(CH2)11CH3
HOCH2CH2
(CH2)11CH3
Isopropanol
-
N
N
Cl
N
N
ClCH CH OH
+
2
2
°
70 C
123

J Surfact Deterg (2011) 14:203–210
205
281.2
The corresponding [C2OHC12im]Cl is a pale yellow solid
at -15 °C (ESM Fig. 1). The melting point of
[C2OHC12im]Cl is 29.5 0.5 °C. By definition, the
organic salts are called ILs when the melting point is lower
than 100 °C [1], so we can also call [C2OHC12im]Cl as IL.
100
282.2
%
1
The Measurements of ESI–MS, H NMR
0
and Elemental Analysis
100
150
200
250
300
350
400
m/z
The ESI–MS spectra were obtained using a Maldi Synapt
Q-TOF MS (Waters) under positive ion mode. The
1H-NMR spectra were obtained with an Avance III
400 MHz digital NMR spectrometer (Bruker) using D2O as
solvent. And the NMR peak of D2O (d = 4.710 ppm) was
used as the reference in determining the chemical shifts of
protons in [C2OHC12im]Cl. The elemental analysis
measurements were conducted on a Vario EL III elementar
analyzer (Elementar). In the elemental analysis measure-
ments, the chloride ion of [C2OHC12im]Cl was exchanged
into nitrate ion by means of ion-exchange technique.
Fig. 1 The ESI–MS spectrum of [C2OHC12im]Cl at positive ion
mode
B
K
G
E
Cl−
D
N
N
A
OH
C
F
L
B
D2O
A
The Measurements of Surface Tension, Electrical
Conductivity and Fluorescence Spectra
G
D
FE
K
L
C
The surface tension measurements of [C2OHC12im]Cl
aqueous solutions were conducted on a surface tensiometer
with a drop volume method (the accuracy of temperature
controlling and volume measuring were 0.1 °C and
0.001 mL, respectively). The specific conductivity, j (lS
cm-1), of the surfactant solutions was measured as a
function of concentration with a low-frequency conduc-
tivity analyzer (Model DDS-307, Shanghai Precision &
Scientific Instrument Co., Ltd, accuracy of 1%).
ppm
9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5
1
Fig. 2 The H-NMR spectrum of [C2OHC12im]Cl
in this work. Under the positive ion mode, only the cationic
[C2OHC12im]? should be detected (Fig. 1). The mass-to-
charge ratio (m/z) of [C2OHC12im]? is 281.2, which
matches well with the theoretical calculation result (281.4)
of the molecule ion weight for [C2OHC12im]?. The results
of elemental analysis for [C2OHC12im]NO3 (C 59.41%, H
9.69%, O 18.64%, and N 12.26%) match well with the
theoretical calculation compositions (C 59.45%, H 9.68%,
O 18.63%, and N 12.24%). In order to get the molecule
The fluorescence spectral measurements were carried
out on a RF5301PC spectrofluorophotometer (Shimadzu,
Japan). The fluorescence emission spectra of surfactant
micelle-solubilized pyrene were employed for the deter-
minations of CMC, microenvironment polarity [44] and
Nm. The concentration of pyrene was fixed at 1.0 9 10-7
mol L-1. The excitation wavelength is 335 nm. The Nm
was determined from benzophenone-mediated steady-state
fluorescence quenching of micelle-solubilized pyrene. All
the measurements of surface tension, electrical conductiv-
ity and fluorescence spectra were conducted at both
25 0.1 and 35 0.1 °C.
1
structural information in detail, the H-NMR spectrum of
[C2OHC12im]Cl was obtained (Fig. 2). The interpretation
1
results of the H-NMR spectrum are also shown in Fig. 2.
The NMR signal of the OH– is overlapped with that of the
solvent D2O. All of the results of ESI–MS, elemental
1
analysis and H NMR indicate that the [C2OHC12im]Cl as
prepared presents a sufficient purity.
The Surface Properties of [C2OHC12im]Cl Aqueous
Solution
Results and Discussion
Structural Characterization of [C2OHC12im]Cl
The concentration dependence of the surface tension for
[C2OHC12im]Cl in aqueous solutions at 25 and 35 °C are
shown in Fig. 3. The surface tension (c, mN m-1) gradually
The ESI–MS technique was employed to measure the
molecular weight of the objective product [C2OHC12im]Cl
123

206
J Surfact Deterg (2011) 14:203–210
70
65
60
55
50
45
40
35
1.70
1.68
1.66
1.64
1.62
25 oC
35 oC
25 oC
35 oC
1.29
1.28
1.27
0.0016
0.0024
0.012
C (mol L-1)
0.014
0.016
0.018
1E-3
0.01
-1
C
mol L
Fig. 5 The variation of I1/I3 with the concentration of
[C2OHC12im]Cl in aqueous solution at 25 and 35 °C
Fig. 3 The concentration dependence of the surface tension (c) for
[C2OHC12im]Cl aqueous solution at 25 and 35 °C
It is recognized that the pC20 value can measure the
efficiency of surfactant adsorbing at the air/solution
interface.
1200
1000
800
600
400
200
0
25 oC
35 oC
pC20 ¼ ꢁ log C20
ð1Þ
where C20 is defined as the surfactant concentration at
which the surface tension of pure solvent is reduced by 20
mN m-1. The larger the value of pC20, the higher the
adsorption efficiency of the surfactant is.
300
250
200
150
100
50
The PCMC is defined as
PCMC ¼ c0 ꢁ cCMC
ð2Þ
where c0 is the surface tension of pure solvent. The value of
Cm (in mol cm-2) was estimated by applying the Gibbs
adsorption isotherm to the surface tension data,
0.001
0.002
0.003
0.004
C
mol L-1
0.00
0.01
0.02
C (mol L-1)
0.03
0.04
ꢀ
ꢁ
1
oc
Cm ¼ ꢁ
ð3Þ
Fig. 4 The specific conductivity (j) of [C2OHC12im]Cl in aqueous
solution at 25 and 35 °C
4:606RT o lg C
T
where R is the gas constant (8.314 9 107 ergs mol-1 K-1),
ꢂ
ꢃ
decreases to a plateau region with the [C2OHC12im]
Cl concentration (C, mol L-1) increasing. A decrease in
surface tension indicates that the surfactant molecules
adsorbed at the air/solution interface. The break points
appear in the c-C curves suggests the formation of micelles
in aqueous solutions. Here, it is worth mentioning that the
purity of [C2OHC12im]Cl was also confirmed from the
observed sharp break points in the c-C curves. The results
of the CMC and cCMC for the surfactant were obtained
from the c-C curves. The values of CMC at 25 and 35 °C
were also determined by means of electrical conductivity
(Fig. 4) and fluorescence spectra (Fig. 5). From the surface
tension data, several surface property parameters, such as
the adsorption efficiency (pC20), the surface pressure at the
CMC (PCMC), the maximum surface excess (Cm), the
minimum molecular cross-sectional area (Amin), and the
value of CMC/C20 may be obtained.
oc
o lg C
T is the experiment temperature (K), and the
is the
T
slope of the linear portion of the graph of surface tension
against logarithmic concentration at a fixed temperature.
The Amin (in square Angstroms) was calculated using
Eq. 4, where NA is Avogadro’s number.
1016
Am
¼
ð4Þ
NA ꢂ Cm
The value of CMC/C20 ratio is correlated with structural
factors in the micellization and adsorption processes. The
surfactant with larger CMC/C20 ratio has the greater
tendency to adsorb at the interfaces than the tendency to
form micelles. All aforementioned physicochemical
parameters are listed in Table 1.
The CMC of [C2OHC12im]Cl increases with tempera-
ture increasing. At 25 °C, the value of CMC for
123

J Surfact Deterg (2011) 14:203–210
207
Table 1 The physicochemical parameters of [C2OHC12im]Cl and the other types of surfactants from references [11, 17, 44]
[C2OHC12im]Cl
[C12mim]Cl
[C14mim]Cl
[C14mim]Br
35 °C
25 °C
CMC (10-3 mol L-1
)
2.82a
2.86b
2.67c
38.8
31.6
1.51
2.05
0.32
110.2
1.63
2.31a
2.69b
2.29c
38.6
33.4
1.82
2.36
0.53
91.2
1.64
13.17 [17]a
13.47 [17]b
14.0 [17]c
2.98 [17]a
3.68 [17]b
3.65 [17]c
2.80 [44]a
2.60 [11]b
2.90 [44]c
39.2 [44]
33.8 [44]
1.96 [44]
3.33 [44]
5.99d
cCMC (mN m-1
PCMC (mN m-1
)
42.0 [17]
45.0 [11]
)
–
–
–
–
–
–
–
Cm (10-10mol cm-2
)
–
pC20
–
CMC/C20
2
Am (A )
–
˚
72 [17]
–
84.7 [44]
1.25 [44]
I1/I3
a
From surface tension methods
b
c
d
From electrical conductivity methods
From fluorescence methods
Calculated from the data in [44]
[C2OHC12im]Cl is lower than that of [C12mim]Cl [13, 17]
and compares well with that of [C14mim]Cl [11, 13, 17]
and [C14mim]Br [11, 45, 46]. This can be attributed to the
longer side chain (hydroxyethyl group) in [C2OHC12im]Cl,
i.e. the hydrophobicity of IL surfactants probably increases
with the increase of the side chain length. On the other
hand, the hydroxyl group will lower the CMC of
[C2OHC12im]Cl [47]. In addition, the side chain length
increase will enlarge the cationic head group of
[C2OHC12im]Cl surfactant. Thus, it is reasonable that Am
of [C2OHC12im]Cl is slightly larger than that of
[C14mim]Br [45].
conductivity and fluorescence spectra, respectively.). The
temperature dependence of the CMC2 is in accordance with
that of dodecyl dimethyl benzyl ammonium bromide in
aqueous solutions [52]. The values of CMC2/CMC1 are 4.94
(electrical conductivity) and 6.45 (fluorescence spectra) at
25 °C, 4.82 (electrical conductivity) and 5.93 (fluorescence
spectra) at 35 °C, respectively. The phenomena of the
second CMC should deepen the understanding of the unique
properties for IL surfactants.
In order to investigate the polarity of the microenvi-
ronment for the micelles, pyrene was used as a fluorescence
probe. It is known that pyrene preferentially dissolves into
hydrophobic regions. The emission spectrum of pyrene
presents five vibration bands. The first band (I1, around
373 nm) may be enhanced in a polar microenvironment,
while the third band (I3, around 384 nm) is not sensitive to
the surrounding environment. Thus, the ratio of I1/I3 may
not only probe the micropolarity of the surfactant aggre-
gates, but is also used to obtain the CMC of the surfactant
in aqueous solution [53].
The Second CMC of [C2OHC12im]Cl
Generally, the spherical micelles are formed at the first
CMC (CMC1). And the appearance of the second CMC
(CMC2) indicates the structural transition, for instance,
from spherical to rod-like ones [48]. Strong evidence for
the existence of the CMC2 within traditional surfactants
has been proved [49–52]. For the traditional ionic surfac-
tants, the value of CMC2/CMC1 ratio is in the range of 2–9
[49–52]. However, no reports on the CMC2 for IL surfac-
tants have been reported yet.
As shown in Fig. 5, the I1/I3 decreases with the surfactant
concentration increasing. When the surfactant concentration
reaches the CMC1, the I1/I3 is about 1.64, which is higher
than that of [C12mim]Br [45], [C14mim]Br [45], and
[C14hpim]Br [40]. It is also probably attributed the existence
of the side-chain (the hydroxylethyl group) in the head group
of [C2OHC12im]Cl. This side-chain probably produces a
larger steric hindrance effect between the head groups of
[C2OHC12im]Cl, which leads to a relative looser packing
mode within the formed micelles. That the relative small
aggregation number (discussed in the next section) of the
micelles in the range of CMC1 to 8.53 mmol L-1 will
probably be the evidence, at least in part, to the
Figures 4 and 5 are the results of CMC measurements by
means of electrical conductivity and fluorescence spectra,
respectively. In addition to the traditional breakpoints
(corresponding to the CMC1), the second breakpoints (cor-
responding to the CMC2) were observed in the relative
higher concentration regions. The CMC2 (mmol L-1) of
[C2OHC12im]Cl were obtained (25 °C: 13.29 and 14.78,
from electrical conductivity and fluorescence spectra,
respectively; 35 °C: 13.79 and 15.83, from electrical
123

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J Surfact Deterg (2011) 14:203–210
Nm ¼ 16:9C0:13 ðR2 ¼ 0:9985Þ
ð6Þ
ð7Þ
aforementioned conjecture. As the concentration of the
surfactant higher than the CMC2, the value of I1/I3 further
decreases to about 1.26, which is comparable with the results
in the literature [40, 45]. In this case, the smaller result of
I1/I3 is in accordance with the relative large aggregation
number (discussed in the next section).
Nm ¼ 71:6C0:03 ðR2 ¼ 0:9982Þ
One can see that an obvious increment for Nm in the range
of 8.53–19.70 mmol L-1. This is probably due to a struc-
tural transition of the micelles as the concentration
increases [48, 54]. Similarly, the sharp increment of Nm in
the range of 70.94–84.98 mmol L-1 suggests a newer
structural change for the micelles. In addition, the experi-
mental results of Nm in the range of 3.81–5.19 mmol L-1
do not obey the empirical power law. This may be the
experimental deviation resulting from the relative small
micelle concentration [55] in the range of 3.81–5.19
mmol L-1. However, the aforementioned structural tran-
sition should be investigated by means of cryo transmission
electron microscopy [56], and this part of work is under
way.
The Concentration Dependence of Nm
for [C2OHC12im]Cl
The aggregation number (Nm) of [C2OHC12im]Cl in
aqueous solutions at 35 °C were determined by a steady-
state fluorescence quenching technique. Pyrene and ben-
zophenone were used as the probe and the quencher,
respectively. The Nm was obtained by applying the Eq. 5 to
the fluorescence data.
ꢀ ꢁ
I0
I
Nm ꢂ CQ
ln
¼
ð5Þ
Here, we can use the packing parameter, VH/lCAm, for
predicting the micelle shape of [C2OHC12im]Cl in aqueous
solutions [57], the VH (the volume of the hydrophobic long
C ꢁ CMC
where I0 and I are the fluorescence intensities of pyrene in
the absence and presence of the quencher at a specific
wavelength, CQ and C are the concentrations of the quencher
and [C2OHC12im]Cl, respectively. And the CMC is the
CMC1 when the surfactant concentration is in the range of
CMC1 to CMC2. When the surfactant concentration is larger
than CMC2, the CMC in Eq. 5 should be the CMC2. The plot
of Nm versus the surfactant concentration at 35 °C is shown
in Fig. 6. The Nm increases, in the mass, with the
concentration of [C2OHC12im]Cl increasing. The Nm
grows as an empirical power law [54] of the total
surfactant concentration in the ranges of 5.82–8.53 and
26.7–70.9 mmol L-1, respectively. The relevant mathematic
equations can be written as (the coefficient constants are
listed in the parentheses)
3
˚
carbon chain) is 27.4 ? 26.9 Cn A [58], lC (the length of the
long carbon chain) is 1.5 ? 1.265 Cn [58], where Cn is the
number of carbons in the long-chain. The calculated value of
VH/lCAm is 0.19 at 35 °C. Based upon the geometry
relationship between the micelle shape and the surfactant
molecule structure [57], the calculated value of VH/lCAm
allows us to consider that the shape of [C2OHC12im]Cl
micelles is spherical in aqueous media in the range of CMC1
to 8.53 mmol L-1
.
In order to estimate the reasonableness of the experi-
mental results for Nm, we used a simple theoretical model
[57, 58] for calculating the possible maximum Nm in a
spherical micelle of [C2OHC12im]Cl. In this simple model,
we tacitly assumed the surfactant micelle is spherical with a
hydrophobic core radius of lC. Thus, the volume of the
hydrophobic core for the [C2OHC12im]Cl micelles is
100
3
Nm 9 VH which equals to 43 pðlCÞ [57]. The calculated result
-1
CMC2=15.83 mmol L
80
60
for the possible maximum Nm is 55.5 at 35 °C, which
matches well with the results in the case of sodium dodecyl
sulfate [55, 58]. Our experimental results of Nm in the range
of CMC1 to 8.53 mmol L-1 is smaller than that of the
calculated value (55.5). This may be attributed to the larger
‘‘rebelling’’ effects between the head groups of the surfac-
tants in micelles. Thus, the micelles present a very loose
packing mode, which can be demonstrated, on the other
hand, by the relative larger I1/I3 in the range of CMC1 to
15
10
5
-1
CMC1=2.67 mmol L
8.53 mmol L-1
.
0
From the relationship between Nm and the surfactant
concentration, one of the most important parameters, the
critical average aggregation number (Nm,c), can be obtained
by reversely extending the linear Eqs. 6 and 7 to CMC.
Here, the parameter Nm,c describes the surfactant monomer
0.004
0.008
0.04
0.08
-1
C mol L
Fig. 6 The concentration dependence of Nm for [C2OHC12im]Cl in
aqueous solution at 35 °C
123

J Surfact Deterg (2011) 14:203–210
209
16. Inoue T, Ebina H, Dong B, Zheng L (2007) Electrical conduc-
tivity study on micelle formation of long-chain imidazolium ionic
liquids in aqueous solution. J Colloid Interface Sci 314:236–241
17. El Seoud OA, Pires PAR, Abdel-Moghny T, Bastos E (2007)
Synthesis and micellar properties of surface-active ionic liquids:
1-alkyl-3-methylimidazolium chlorides. J Colloid Interface Sci
313:296–304
18. Ao M, Xu G, Zhu Y, Bai Y (2008) Synthesis and properties of
ionic liquid-type gemini imidazolium surfactants. J Colloid
Interface Sci 326:490–495
number of the first micelle corresponding to the definite
CMC in solution. This parameter is obtained by the routine
methods only with great difficulty. Corresponding to the
CMC1 and CMC2, the first Nm,c is 8, and the second one is
63, respectively. The former is quite small, which indicates
that the first spherical micelle presents a very loose struc-
ture. And the later is relative larger indicating that the
micelle has a closer packing structure.
19. Thomaier S, Kunz W (2007) Aggregates in mixtures of ionic
liquids. J Mol Liq 130:104–107
20. Zhao Y, Chen X, Wang X (2009) Liquid crystalline phases self-
organized from a surfactant-like ionic liquid C16mimCl in ethy-
lammonium nitrate. J Phys Chem B 113:2024–2030
Acknowledgments This work was financially supported in part by
the Fundamental Research Funds for the Central Universities (JUS-
RP20905) and the youth funds of Jiangnan University (2007LQN14),
and Jiangsu Key Laboratory of Fine Chemicals (KF0903).
21. Blesic M, Lopes A, Melo E, Petrovski Z, Plechkova NV, Lopes
JNC, Seddon KR, Rebelo LPN (2008) On the self-aggregation
and fluorescence quenching aptitude of surfactant ionic liquids.
J Phys Chem B 112:8645–8650
22. Zhao Y, Gao S, Wang J, Tang J (2008) Aggregation of ionic liquids
[Cnmim]Br (n = 4, 6, 8, 10, 12) in D2O: a NMR study. J Phys
Chem B 112:2031–2039
23. Gaillon L, Sirieix-Plenet J, Letellier P (2004) Volumetric study of
binary solvent mixtures constituted by amphiphilic ionic liquids
at room temperature (1-alkyl-3-methylimidazolium bromide) and
water. J Solution Chem 33:1333–1347
24. Dong B, Li N, Zheng L, Yu L, Inoue T (2007) Surface adsorption
and micelle formation of surface active ionic liquids in aqueous
solution. Langmuir 23:4178–4182
25. Wang J, Wang H, Zhang S, Zhang H, Zhao Y (2007) Conduc-
tivities, volumes, fluorescence, and aggregation behavior of ionic
liquids [C4mim][BF4] and [Cnmim]Br (n = 4, 6, 8, 10, 12) in
aqueous solutions. J Phys Chem B 111:6181–6188
26. Pino V, Yao C, Anderson J (2009) Micellization and interfacial
behavior of imidazolium-based ionic liquids in organic solvent-
water mixtures. J Colloid Interface Sci 333:548–556
References
1. Dupont J, Souza RF, Suarez PAZ (2002) Ionic liquid (molten
salt) phase organometallic catalysis. Chem Rev 102:3667–3692
2. Welton T (1999) Room-temperature ionic liquids. Solvents for
synthesis and catalysis. Chem Rev 99:2071–2083
3. Huddleston JG, Willauer HD, Swatloski RP, Visser AE, Rogers
RD (1998) Room temperature ionic liquids as novel media for
‘clean’ liquid-liquid extraction. Chem Commun 16:1765–1766
4. Xia H, Yu J, Jiang Y, Mahmood I, Liu H (2007) Physicochemical
features of ionic liquid solutions in phase separation of Penicillin
(II): Winsor II reversed micelle. Ind Eng Chem Res 46:2112–2116
5. Byme N, Howlett C, MacFarlane R, Forsyth M (2005) The
zwitterion effect in ionic liquids: towards rechargeable lithium-
metal batteries. Adv Mater 17:2497–2501
6. Rebelo LPN, Lopes JNC, Esperanc¸a JMSS, GuedesHJR, Łachwa J,
Najdanovic-Visak V, Visak ZP (2007) Accounting for the unique,
doubly dual nature of ionic liquids from a molecular thermody-
namic and modeling standpoint. Acc Chem Res 40:1114–1121
7. Dorbritz S, Ruth W, Kragl U (2005) Investigation on aggregate
formation of ionic liquids. Adv Synth Catal 347:1273–1279
8. Merrigan TL, Bates ED, Dorman SC Jr, Davis JH (2000) New
fluorous ionic liquids function as surfactants in conventional
room-temperature ionic liquids. Chem Commun 20:2051–2052
¨
27. Łuczak J, Jungnickel C, Joskowska M, Thoming J, Hupka J
(2009) Thermodynamics of micellization of imidazolium ionic
liquids in aqueous solutions. J Colloid Interface Sci 336:111–116
28. Wang H, Wang J, Zhang S, Xuan X (2008) Structural effects of
anions and cations on the aggregation behavior of ionic liquids in
aqueous solution. J Phys Chem B 112:16682–16689
29. Dong B, Zhang J, Zheng L, Wang S, Li X, Inoue T (2008) Salt-
induced viscoelastic wormlike micelles formed in surface active
ionic liquid aqueous solution. J Colloid Interface Sci 319:338–343
30. Zech O, Thomaier S, Bauduin P, Ru¨ck T, Touraud D, Kunz W (2009)
Microemulsions with an ionic liquidsurfactant and room temperature
ionic liquids as polar pseudo-phase. J Phys Chem B 113:465–473
31. Safavi A, Maleki N, Farjami F (2010) Phase behavior and char-
acterization of ionic liquids based microemulsions. Colloid Surf
A 355:61–66
}
´
¨
9. Miskolczy Z, Sebok-Nagy K, Biczok L, Goktu¨rk S (2004)
Aggregation and micelle formation of ionic liquids in aqueous
solution. Chem Phys Lett 400:296–300
10. Baltazar QQ, Chandawalla J, Sawyer K, Anderson JL (2007)
Interfacial and micellar properties of imidazolium-based mono-
cationic and dicationic ionic liquids. Colloids Surf A 302:150–156
´
11. Jungnickel C, Łuczak J, Ranke J, Fernandez JF (2008) Micelle
formation of imidazolium ionic liquids in aqueous solution.
Colloids Surf A 316:278–284
12. Łuczak J, Hupka J, Thoming J, Jungnickel C (2008) Self-orga-
32. Yan F, Texter J (2006) Surfactant ionic liquid-based micro-
emulsions for polymerization. Chem Commun 25:2696–2698
33. Qiu Z, Texter J (2008) Ionic liquids in microemulsions. Curr
Opin Colloid Interface Sci 13:252–262
¨
nization of imidazolium ionic liquids in aqueous solution. Col-
loids Surf A 329:125–133
34. Firestone MA, Dzielawa JA, Zapol P, Curtiss LA, Seifert S, Dietz
ML (2002) Lyotropic liquid-crystalline gel formation in a room-
temperature ionic liquid. Langmuir 18:7258–7260
35. Li XW, Zhang J, Dong B, Zheng LQ, Tung CH (2009) Charac-
terization of lyotropic liquid crystals formed in the mixtures of
1-alkyl-3-methylimidazolium bromide/p-xylene/water. Colloid
Surf A 335:80–87
13. Blesic M, Marques MH, Plechkova NV, Seddon KR, Rebelo
LPN, Lopes A (2007) Self-aggregation of ionic liquids: micelle
formation in aqueous solution. Green Chem 9:481–490
14. Smirnova NA, Vanin AA, Safonova EA, Pukinsky IB, Anufrikov
YA, Makarov AL (2009) Self-assembly in aqueous solutions of
imidazolium ionic liquids and their mixtures with an anionic
surfactant. J Colloid Interface Sci 336:793–802
ˇ
36. Ao M, Xu G, Pang J, Zhao T (2009) Comparison of aggregation
behaviors between ionic liquid-type imidazolium Gemini sur-
factant [C12–4-C12im]Br2 and its monomer [C12 mim]Br on
silicon wafer. Langmuir 25:9721–9727
15. Goodchild I, Collier L, Millar SL, Prokes I, Lord JCD, Butts CP,
Bowers J, Webster JRP, Heenan RK (2007) Structural studies of the
phase, aggregation and surface behaviour of 1-alkyl-3-methylimida-
zolium halide ? water mixtures. J Colloid Interface Sci 307:455–468
123

210
J Surfact Deterg (2011) 14:203–210
37. Ding YS, Zha M, Zhang J, Wang SS (2007) Synthesis, charac-
terization and properties of geminal imidazolium ionic liquids.
Colloid Surf A 298:201–205
38. Liu Y, Yu L, Zhang S, Yuan J, Shi L, Zheng L (2010) Dispersion of
multiwalled carbon nanotubes by ionic liquid-type gemini imi-
dazolium surfactants in aqueous solution. Colloid Surf A 359:66–70
51. Mu JH, Li GZ, Zhang WC, Wang ZW (2001) Determination of
the second CMCs of dodecyl polyoxyethylene polyoxypropylene
ether by the methods of cloud point, fluorescence, and viscosity.
Colloid Surf A 194:1–6
´
´
´
52. Gonzalez-Perez A, Czapkiewicz J, Ruso JM, Rodrıguez JR
(2004) Temperature dependence of second critical micelle con-
centration of dedecyldimethylbenzyl ammonium bromide in
aqueous solution. Colloid Polym Sci 282:1169–1173
53. Kalyansundaram K, Thomas JK (1977) Environmental effects on
vibronic band intensities in pyrene monomer fluorescence and
their application in studies of micellar systems. J Am Chem Soc
99:2039–2044
´
´
39. Blesic M, Swadzba-Kwasny M, Holbrey JD, Lopes JNC, Seddon
KR, Rebelo LPN (2009) New catanionic surfactants based on
1-alkyl-3-methylimidazolium alkylsulfonates, [CnH2n?1mim]
[CmH2m?1SO3]: mesomorphism and aggregation. Phys Chem
Chem Phys 11:4260–4268
40. Li XW, Gao YA, Liu J, Zheng LQ, Chen B, Wu LZ, Tung CH
(2010) Aggregation behavior of a chiral long-chain ionic liquid in
aqueous solution. J Colloid Interface Sci 343:94–101
41. Branco LC, Rosa JN, Ramos JJM, Afonso CAM (2002) Prepa-
ration and characterization of new room temperature ionic liq-
uids. Chem Eur J 8:3671–3677
´
´
54. Bezzobotnov VY, Borbely S, Cser L, Farago B, Gladkih IA,
Ostanevich YM, Vass S (1988) Temperature and concentration
dependence of properties of sodium dodecyl sulfate micelles
determined from small-angle neutron scattering experiments.
J Phys Chem 92:5738–5743
42. Yang X, Yan N, Fei Z, Crespo-Quesada RM, Laurenczy G, Kiwi-
Minsker L, Kou Y, Li Y, Dyson PJ (2009) Biphasic hydrogena-
tion over PVP stabilized Rh nanoparticles in hydroxyl function-
alized ionic liquids. Inorg Chem 47:7444–7446
55. Fang Y, Liu XF, Xia YM, Yang Y, Cai K, Suh JM, Cho HY
(2001) Determination of critical micellar aggregation numbers by
steady-state fluorescence probe method. Acta Phys Chim Sin
17(9):828–831
43. Dorjnamjin D, Ariunaa M, Shim YK (2008) Synthesis of silver
nanoparticles using hydroxyl functionalized ionic liquids and
their antimicrobial activity. Int J Mol Sci 9:807–820
56. Bellare JR, Kaneko T, Evans DF (1988) Seeing micelles. Lang-
muir 4:1066–1067
57. Rosen MJ (2004) Surfactants and interfacial phenomena. Wiley,
New Jersey, pp 108–113
´
44. Aguiar J, Carpena P, Molina-Bolıvar JA, Ruiz CC (2003) On the
determination of the critical micelle concentration by the pyrene
1:3 ratio method. J Colloid Interface Sci 258:116–122
58. Tanford C (1972) Micelle shape and size. J Phys Chem 76(21):
3020–3024
45. Dong B, Zhao X, Zheng L, Zhang J, Li N, Inoue T (2008)
Aggregation behavior of long-chain imidazolium ionic liquids in
aqueous solution: micellization and characterization of micelle
microenvironment. Colloids Surf A 317:666–672
Author Biographies
46. Geng F, Liu J, Zheng L, Yu L, Li Z, Li G, Tung C (2010) Micelle
formation of long-chain imidazolium ionic liquids in aqueous
solution measured by isothermal titration microcalorimetry.
J Chem Eng Data 55:147–151
47. Osanai S, Yamada G, Hidano R, Beppu K, Namiwa K (2010)
Preparation and properties of phosphate surfactants containing
ether and hydroxy groups. J Surfact Deterg 13:41–49
Xue-feng Liu is an associate professor at the Department of Applied
Chemistry, School of Chemical and Materials Engineering, Jiangnan
University. He received his Ph.D. degree in 2005 from Jiangnan
University. His main research interests are synthesis and properties of
novel surfactants, colloidal self-assemblies, and nanofabrications.
Li-li Dong obtained her B.Sc. in Applied Chemistry from Jiangnan
University in 2007. Currently she is an M.Sc. candidate supervised by
Prof. X. Liu.
48. May S, Ben-Shaul A (2001) Molecular theory of the sphere-to-
rod transition and the second CMC in aqueous solutions. J Phys
Chem 105:630–640
49. Romani AP, de Hora Machado AE, Hioka N, Severino D, Baptista
MS, Codognoto L, Rodrigues MR, de Oliveira HPM (2009)
Spectrofluorimetric determination of second critical micellar con-
centration of SDS and SDS/Brij 30 systems. J Fluoresc 19:327–332
Yun Fang is a professor at the Department of Applied Chemistry,
School of Chemical and Materials Engineering, Jiangnan University.
He received his M.Sc. in 1985 from Wuxi Institute of Light Industry.
His main research interests are the fields of colloid and interface
chemistry.
´
´
´
50. Gonzalez-Perez A, Czapkiewicz J, Prieto G, Rodrıguez JR (2003)
Second critical micelle concentration of dedecyldimethylbenzy-
lammonium chloride in aqueous solution at 25 °C. Colloid Polym
Sci 281:1191–1195
123