A Novel zwitterionic imidazolium-based ionic liquid surfactant: 1-carboxymethyl-3-dodecylimidazolium inner salt

Article abstract of DOI:10.1007/s11743-011-1254-7

A novel zwitterionic imidazolium-based ionic liquid (IL) surfactant, 1-carboxymethyl-3-dodecylimidazolium inner salt, was synthesized. The molecule structure was confirmed by means of electrospray ionization mass spectrometry, ¹H nuclear magnetic resonance and elemental analysis. The isoelectric point (pI) is 3.8 ± 0.1 at 35 ± 0.1 °C. The other important physicochemical parameters such as the critical micelle concentration (CMC), the surface tension at CMC (γ_CMC), the adsorption efficiency (pC ₂₀), the surface pressure at CMC (Π_CMC), the maximum surface excess (Γ_m), the minimum molecular cross-sectional area (A _min), the value of CMC/C ₂₀ and the average number of aggregation (N _m) were determined by surface tension and steady-state fluorescence probe methods, respectively. AOCS 2011.

Full text of DOI:10.1007/s11743-011-1254-7

J Surfact Deterg (2011) 14:497–504
DOI 10.1007/s11743-011-1254-7
ORIGINAL ARTICLE
A Novel Zwitterionic Imidazolium-Based Ionic Liquid Surfactant:
1
-Carboxymethyl-3-Dodecylimidazolium Inner Salt
Xue-feng Liu
•
Li-li Dong Yun Fang
•
Received: 23 September 2010 / Accepted: 29 January 2011 / Published online: 9 March 2011
Ó AOCS 2011
Abstract A novel zwitterionic imidazolium-based ionic
liquid (IL) surfactant, 1-carboxymethyl-3-dodecylimida-
zolium inner salt, was synthesized. The molecule structure
was confirmed by means of electrospray ionization mass
counter-ion, respectively. This particular aspect allows
people to consider some long-chain ionic liquids (when
dispersed in aqueous solutions), in fact, as new types of
cationic surfactant, namely an IL surfactant [2, 6, 7, 23],
surfactant-like ILs [14], and surface active ILs [18].
IL surfactants with single long-chain and Gemini IL
surfactants have been reported [4, 24, 25]. Compared
with single-chain IL surfactants, the IL-type Gemini ones
have better surface activity and dispersion ability [26].
More recently, the hydroxyl-functionalized chiral IL
surfactant [27] and novel IL surfactants, for instance,
1
spectrometry, H nuclear magnetic resonance and ele-
mental analysis. The isoelectric point (pI) is 3.8 ± 0.1 at
3
5 ± 0.1 °C. The other important physicochemical
parameters such as the critical micelle concentration
CMC), the surface tension at CMC (cCMC), the adsorption
efficiency (pC ), the surface pressure at CMC (PCMC), the
(
2
0
maximum surface excess (C ), the minimum molecular
m
cross-sectional area (Amin), the value of CMC/C20 and the
average number of aggregation (N ) were determined by
[C mim][C H
n
SO ] [28] have been synthesized. From
2m?1 3
m
the electric charge characteristic of the ionic head groups,
all the aforementioned IL surfactants are cationic. Inter-
estingly, apart from a very recent paper [29], reports on
the synthesis and the surface activity for zwitterionic
imidazolium-based IL surfactants, by far, have seldom
been reported yet.
m
surface tension and steady-state fluorescence probe meth-
ods, respectively.
Keywords Zwitterionic surfactant ꢀ Imidazolium-based
ionic liquid ꢀ Surface activity
Here, we synthesized a novel zwitterionic imidazoli-
um-based IL surfactant, namely 1-carboxymethyl-3-
dodecylimidazolium inner salt (CMDim, Scheme 1). The
molecule structure was confirmed by means of electro-
Introduction
1
The inherent amphiphilic imidazolium-based ([C mim]X)
n
spray ionization mass spectrometry (ESI–MS), H nuclear
1
magnetic resonance ( H NMR) and elemental analysis. In
ionic liquids (ILs) has been one of the attractive aspects of
the surface active ILs in recent years [1–22], here, C , m,
n
addition to the Krafft temperature (T ) and the isoelectric
K
im, and X stands for alkyl chain, methyl, imidazolium, and
point (pI), many important physicochemical parameters,
such as the critical micelle concentration (CMC), the
surface tension at the CMC (cCMC), the adsorption effi-
ciency (pC ), the surface pressure at the CMC (PCMC),
2
0
the maximum surface excess (C ), the minimum molec-
m
ular cross-sectional area (Amin), the value of the CMC/
C , the average number of aggregation (N ) and the
X. Liu (&) ꢀ L. Dong ꢀ Y. Fang
School of Chemical and Material Engineering,
Jiangnan University, 214122 Wuxi,
People’s Republic of China
2
0
m
micellar microenvironment polarity were determined by
using surface tension and fluorescence probe methods,
respectively.
e-mail: xfliu@jiangnan.edu.cn
1
23

4
98
J Surfact Deterg (2011) 14:497–504
Scheme 1 The synthesis routes
of CMDim
Step 1
Step 2
CH OH
CH CH CN
2 2
3
N
NH
N
N
+
CH =CHCN
2
5
0 oC
-
Br
Isopropanol
N+ CH (CH ) CH Br
N
2
NCCH CH2
N
N
N
3
2 10
2
NCCH CH2
80 oC
CH (CH ) CH
2 2 10 3
2
Step 3
Step 4
-
NaOH (aq)
15% (w/w)
Br
(CH ) CH
2 11 3
N
N
N
2
NCCH CH2
CH (CH ) CH
2 2 10 3
+ H2O + NaBr +CH =CHCN
2
O
(
CH ) CH
O
Ethanol/water
pH 8-9, 75 oC
2
11
3
OCCH2
N
N
(CH ) CH
3
2
11
+
ClCH2CONa
N
N
Experimental Section
purified by recrystallization in deionized water at 3–4 °C
and then dried under a vacuum for 3 days (yield 73.2%).
The corresponding CMDim is white acicular crystal pow-
der at room temperature.
Materials
Imidazole, 1-bromododecane, sodium chloroacetate, acry-
The melting point of CMDim is 69.0 ± 0.3 °C. By the
definition [30], the organic salts are called ILs when the
melting point is lower than 100 °C, so we can also call
CMDim an IL.
lonitrile, D O and other reagents were purchased from
2
Sinopharm 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).
1
The Measurements of ESI–MS, H NMR
and Elemental Analysis
The Synthesis of CMDim
The ESI–MS measurement was carried out using a Maldi
Synapt Q-TOF MS (waters) under a positive ion mode (see
Fig. 1).
The synthesis routes of CMDim are shown in Scheme 1.
For a general 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 °C for 8 h under
nitrogen. The solvent methanol and the unreacted acrylo-
nitrile 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 °C
for 18 h under nitrogen. The isopropanol was removed
under vacuum when the reaction was completed. The res-
idue was dissolved in a 15 wt% NaOH aqueous solution
1
The H-NMR spectrum was obtained with an Avance III
400 MHz digital NMR spectrometer (Bruker) using D O as
2
solvent. And the NMR peak of D O (d = 4.710 ppm) was
2
used as the reference in determining the chemical shifts of
protons in CMDim (see Fig. 2).
The elemental analysis measurements were conducted
on a Vario EL III elementar analyzer (Elementar). All the
experiments were performed at room temperature (about
25 °C).
(
100 mL) and extracted by chloroform (50 mL). The
295.2
100
chloroform layer was washed several times with deionized
water. The residual solvent was removed under vacuum to
give n-dodecylimidazole 29.7 g (90% yields). The n-dod-
ecylimidazole (0.09 mol, 21.2 g) and sodium chloroacetate
(
0.09 mol, 10.5 g) were dissolved in ethanol–water (vol-
ume ratio 1:1), and was stirred at 75 °C for 8 h under
nitrogen. The pH was maintained in the range of 8–9. The
solvent was removed under reduced pressure when the
reaction was completed. The product was dissolved in
anhydrous ethanol and the inorganic salt was eliminated by
filtration. And the unreacted n-dodecylimidazole was
washed away by acetone. The objective compound was
2
96.3
346.4
258.3
0
60
80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400
m/z
Fig. 1 The ESI–MS spectrum of CMDim with positive ion mode
1
23

J Surfact Deterg (2011) 14:497–504
499
capillary was 0.66 mm. During the procedure of c mea-
surements, the sufficient aging time is necessary for the
surfaces of the pendant drops formed at the tip of a cap-
illary to reach the equilibrium state. Here, the aging time of
the drop surface for the lowest sample solution was
determined by the plot of c versus the drop detachment
time t (in s) (Fig. 4). Obviously, the c of the solution
decreased with the drop detachment time increasing with a
B
F
G
D
-
N
N
COO
A
C
K
E
B
breakpoint at t & 200 s. The change of c was less than
E
D2O
-1
A
±
0.1 mN m in the region of t [ 300 s. Under this con-
dition, the c could be considered as an equilibrium surface
tension. The lower concentration sample solution needs
longer aging time for the drop surface to reach an equi-
librium state. Thus, in order to make the drop surfaces
reach the equilibrium state, all the drops of the surfactant
solutions were aged for at least 10 min. Finally, the drop
volume was calibrated by The surface activity parameters,
such as the critical micelle concentration (CMC), the c at
K
GF
D
C
1
Fig. 2 The H-NMR spectrum of CMDim
The Measurements of Isoelectric Point, Surface
Tension, and Fluorescence Emission Spectra
CMC (cCMC), the adsorption efficiency (pC ), the surface
pressure at the CMC (PCMC), the maximum surface
20
excess (C ), the minimum molecular cross-sectional area
m
The isoelectric point (pI) of CMDim was determined by
(Amin) and the value of CMC/C20 were obtained from the
c-C curves (Fig. 5).
means of a Zeta potentiometer (Malvern) at 25 ± 0.1 °C,
the electric field intensity was 15 V cm . The concen-
-
1
The fluorescence emission spectral measurements
were carried out on a RF5301PC spectrofluorophotometer
(Shimadzu, Japan). Pyrene was used as the fluores-
cence probe. The concentration of pyrene was fixed at
-
3
tration of the sample solutions was constant at 3.4 9 10
-
1
mol L which is about 2.7 times of the CMC of the sur-
factant CMDim. The pHs of the sample solutions were
adjusted by adding definite amount of HCl and NaOH,
respectively. And the pI was determined by the plot of the
Zeta potential versus pH (see Fig. 3).
-
7
-1
1.0 9 10 mol L . The excitation wavelength was
335 nm.
The microenvironment polarity of the surfactant
micelles and the value of CMC with pH = 7 were deter-
mined by means of the fluorescence emission spectral
method [11, 31] (see Fig. 6). The value of Nm was
The surface tension (c) measurements of CMDim
aqueous solutions were conducted on a drop volume ten-
siometer at 35 ± 0.1 °C. The outer radius of the glass
4
3
2
1
0
0
0
0
0
64.6
o
2
5 C
6
6
4.4
4.2
pI
-10
-20
-30
-40
64.0
6
6
3.8
3.6
1
2
3
4
5
6
7
8
0
50
100
150
200
250
300
350
400
Fig. 3 The relationship between Zeta potential and pH for CMDim in
-
aqueous solutions, the surfactant concentration is 3.4 9 10
mol L , and the pH was adjusted by adding HCl and NaOH,
respectively
3
Fig. 4 The aging curve of CMDim aqueous solution drop surface at
35 °C at a pH of 7, the concentration of the surfactant was
2.5 9 10 mol L
-1
-
3
-1
1
23

5
00
J Surfact Deterg (2011) 14:497–504
binding with one mole of protons, was detected (Fig. 1).
6
6
5
5
4
4
3
3
5
0
5
0
5
0
5
0
o
3
5 C
pH=2
pH=7
pH=12
?
The mass-to-charge ratio (m/z) of [CMDim ? H] is
295.2, which matches well with the theoretical calculation
result (294.4 ? 1) of the molecule ion weight for
?
CMDim ? H] . The found results of elemental analysis
[
for CMDim are C 69.26%, H 10.36%, O 9.56%, and N
0.82%, which matches well with the theoretical calcula-
tion compositions (C 69.40%, H 10.20%, O 9.52%, and N
1
10.88%).
In order to get the molecule structural information in
1
detail, the H-NMR spectrum of CMDim was obtained.
1
The interpreting results of the H-NMR spectrum using the
signal of solvent D O as a reference are also presented in
2
-
4
-3
-2
1
0
10
10
Fig. 2. All of the results of ESI–MS, elemental analysis and
1
H NMR indicate that the CMDim presents a sufficient
-
1
C (mol L )
purity.
Fig. 5 The concentration dependence of the surface tension for
CMDim in aqueous solutions at 35 °C
The Isoelectric Point of CMDim
The isoelectric point (pI), sometimes abbreviated to IEP, is
the pH at which a particular molecule or surface carries no
net electrical charge. Amphoteric molecules called zwit-
terions contain both positive and negative charges
depending on the functional groups present in the mole-
cule. The net charge on the molecule is affected by pH of
their surrounding environment and can become more pos-
itively or negatively charged due to the loss or gain of
pH=7
1
1
1
1
1
1
.8
.7
.6
.5
.4
.3
o
2
3
5 C
o
5 C
?
protons (H ). The pH dependence of the Zeta potential for
CMDim in aqueous solutions is shown in Fig. 3. And the pI
of the zwitterionic CMDim is 3.8 ± 0.1 at 25 ± 0.1 °C.
The pI value can affect the solubility of a molecule at a
given pH. Such molecules have minimum solubility in
water or salt solutions at the pH which corresponds to their
pI and often precipitate out of the solution. Based on this
particular aspect, the objective CMDim was purified by
recrystallization in water with adjusting the pH of the
aqueous solutions in the temperature range of 3–4 °C.
CMC
0
.000
0.001
0.002
0.003
0.004
-1
C (mol L )
Fig. 6 The I
5 °C, pH = 7, the additional curves with minima are the first order
derivatives of I /I -C, and the minima correspond to the CMC of
1 3
/I versus the concentration of CMDim at 25 °C and
3
1
3
CMDim at 25 °C and 35 °C, respectively
determined using a steady-state fluorescence quenching
method [32, 33]. All the fluorescence spectral measure-
ments were conducted at 25 ± 0.1 and 35 ± 0.1 °C,
respectively.
The Surface Activity Parameters of CMDim
At a pH below their pI, zwitterions carry a net positive
charge; above their pI they carry a net negative charge. The
concentration dependence of the c for CMDim in aqueous
solutions at various pHs are shown in Fig. 5. One can see
-
1
Results and Discussion
that, the c (in mN m ) gradually decreases to a plateau
region with the increase of CMDim concentration
-
1
Structural Characterization of CMDim
(C, mol L ). A decrease of surface tension indicates that
the surfactant molecules adsorbed at the air-solution
interface. The break point appears in the c-C curves sug-
gests the formation of micelles in aqueous solutions. Here, it
is worth mentioning that the purity of CMDim was also
confirmed from the observed sharp break point in the
The ESI–MS technique was employed to measure the
molecule weight of the objective product CMDim in this
work. Under the positive ion mode, the cationic
?
CMDim ? H] , i.e. one mole of CMDim inner salts were
[
1
23

J Surfact Deterg (2011) 14:497–504
501
-
3
c-C curves. The values of the CMC and cCMC for the sur-
factant at various pHs were obtained from the c-C curves.
It is recognized that the pC20 value can measure the
efficiency of surfactant adsorbing at the air-solution
interfaces.
Cm calculation are within the regions of C \ 10
-1
mol L . Thus, the concentration of the remaining HCl in
solutions is, at least, one order of magnitude larger than the
cationic surfactant. One can consider the condition is a
state with the constant ion strength. Due to the analogous
reasons discussed above, the prefactor is also 1 under the
conditions of pH = 12.
pC20 ¼ ꢁ log C20 ð1Þ
where C20 is defined as the surfactant concentration at
The Amin (in square angstroms) was calculated using
Eq. 4, where N is Avogadro’s number.
which the surface tension of pure solvent is reduced by
1
A
-
2
0 mN m . The larger the value of pC , the higher the
20
016
1
adsorption efficiency of the surfactant is.
Amin
¼
ð4Þ
NA ꢂ Cm
The PCMC is defined as
where NA is Avogadro constant. The value of CMC/C20
ratio is correlated with structural factors in the micelliza-
tion 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 afore-
mentioned parameters may be obtained and are listed in
Table 1.
PCMC ¼ c ꢁ cCMC
ð2Þ
0
where c is the surface tension of pure solvent. The values
0
-
2
of C (in mol cm ) under the conditions of pH = 2, 7 and
m
1
2 were estimated by applying Eq. 3 [34] to the surface
tension data in the surfactant concentration regions of
-
3
-1
C \ 10 mol L . Thus, the activity of the surfactant can
be replaced by the concentration in the procedure of the Cm
The CMC of CMDim increases with pH decreasing due
to the characteristic zwitterionic head group in CMDim
molecule. This tendency is similar to that of traditional
zwitterionic surfactants, for instance, carboxybetaines. The
CMC of CMDim is higher in acid solutions than in alkaline
solutions, because the CMDim is at least partly in the
cationic form at low pH, and it is in the form of the anions
in the case of high pH. As the pH lowering, the amount of
the zwitterionic and then cationic form in solution increa-
ses, and the resulting CMCs of the anionic/zwitterionic,
zwitterionic, zwitterionic/cationic, and then cationic sur-
factants increase. On the other hand, increasing the con-
centration of acid (here HCl was used) also increases the
ionic strength of the solutions, which tends to lower the
CMCs. However, compared with that of the pH, the effect
of the ionic strength on the CMC of zwitterionic CMDim
seems to be small (see Fig. 5; Table 1). It is attributed to
that the pH can change the type of hydrophilic groups in
CMDim (i.e. from the cationic to the neutral inner salt then
to the anionic when the pH changes from the basic to the
acidic); but the ionic strength does not change the hydro-
philic group type of the surfactant.
calculation.
ꢀ
ꢁ
C
b
c ꢁ c ¼ RTC ln
þ 1
ð3Þ
0
m
7
is the gas constant (8.314 9 10 ergs -
where
-
mol
R
1
-1
K ), T is the experiment temperature (K), and b is
the empirical constant. Equation 3 is a derivate form from
the Gibbs adsorption equation. Thus, the prefactor in the
Gibbs adsorption equation must be considered carefully.
Here, the prefactor is 1 for the surfactant CMDim at
pH = 7, because there is no counter ions binding with
CMDim. Under the condition of pH = 2, the prefactor can
be considered as 1. The reason is as following. The
?
-
concentrations of both H and Cl (come from HCl,
-
because we used HCl for pH adjustments) are 10
2
-
1
mol L . Due to the chemical characteristic of carboxyl
?
groups in CMDim, the H will bind with the carboxyl
groups. Then, the CMDim presents as a cationic surfactant
?
CMDim Cl ). The remaining HCl should act as a coex-
-
(
isting electrolyte with the same counter ions as cationic
?
surfactant CMDim Cl . The surface tension data used for
-
Table 1 The physicochemical
?
-
Parameters
CMDim
12 3 2 2
C H25N (CH ) CH COO
parameters for CMDim from
surface tension method in this
work and the other types of
surfactants with the same
pH
2.0 ± 0.1
7.0 ± 0.2
12.0 ± 0.1
1.52
-
CMC (10 mol L
3
-1
)
2.78
1.71
1.8 (2.0)*
-
1
carbon chain from reference
[
c
CMC (mN m
pC20
CMC (mN m
)
33.8
3.52
36.6
1.74
95.4
9.14
34.3
3.69
36.1
1.41
117.5
8.57
33.8
–
34]
3.77
–
-
1
P
)
36.6
–
-
10
-2
C
m
(10
˚
mol cm
)
1.25
3.2*
2
Amin (A )
133.1
9.01
59.0 (52.0)*
6.5**
CMC/C20
*
At 25 °C; ** at 23 °C
1
23

5
02
J Surfact Deterg (2011) 14:497–504
The value of CMC, pC20 and C for CMDim under the
carbon number of the long-chain. The calculated values of
m
condition of neutral pH are lower than that of
V /l A are in the range of 0.15–0.20 for CMDim under
C min
H
?
C H N (CH ) CH COO [34, 35] without regard to the
-
the various pHs conditions in this work. Based upon the
geometry relationship between the micelle shape and the
surfactant molecule structure [34], the calculated values of
V /l A allow us to consider that the shape of CMDim
1
2
25
3 2
2
effect of experimental temperature. This can be attributed to
the cross-sectional area (Amin) difference between these two
surfactants. As shown in Table 1, the A for CMDim is
m
H C m
?
larger than that of C H N (CH ) CH COO , which can
-
micelle is spherical.
1
2
25
3 2
2
be attributed to the larger bulk of carboxymethyl imidazo-
The average aggregation number, N , of CMDim in
m
lium group in the CMDim molecule. The value of CMC/C
?
aqueous solutions was determined by a steady-state fluo-
rescence quenching method. This method is based on the
procedure proposed by Turro and Yekta [32], and the main
assumptions are that (1) the quenching rate constant is
much larger than the probe decay rate constant and (2) the
random distribution of the probe and quencher over the
micelles results in a Poisson distribution. Here, pyrene and
benzophenone were used as the probe and the quencher,
respectively. Thus the Nm was obtained by applying the
Eq. 5 to the fluorescence data [29],
2
0
for CMDim is larger than that of C H N (CH )
3 2
1
2
25
-
CH COO , which indicates a greater steric effect on
2
micellization than on adsorption at the air-solution interface
for CMDim caused by the larger head groups.
The Micelle Properties of CMDim
In this study, pyrene was used as a fluorescence probe to
monitor the form of micelle for CMDim in aqueous solu-
tion and then to investigate the polarity of the microenvi-
ronment of micelles. The emission spectrum of pyrene
ꢀ
ꢁ
I0
Nm ꢂ CQ
ln
¼
ð5Þ
I
C ꢁ CMC
presents five vibration bands. The first band (I , around
1
3
73 nm) may be enhanced in a polar microenvironment,
where I
absence and presence of the quencher at a specific
wavelength, C and C are the concentration of the
quencher and CMDim, respectively. The plots of N versus
the surfactant concentration at 25 ± 0.1 and 35 ± 0.1 °C
with pH = 7 are shown in Fig. 7. The N increases, in mass,
0
and I are the fluorescence intensities of pyrene in the
while the third band (I , around 384 nm) is not sensitive to
3
the surrounding environment. It is known that pyrene
preferentially dissolves into hydrophobic regions. Thus, the
ratio of I /I may probe the formation of the micelles and
Q
m
1
3
the micropolarity of the surfactant aggregates [31, 32, 36].
m
The plots of I /I versus the concentration of CMDim at
1
with the concentration of CMDim increasing. The relevant
mathematic equations can be written as (the coefficient
constants are listed in the parentheses)
3
2
5 ± 0.1 and 35 ± 0.1 °C and the corresponding first
order derivatives are shown in Fig. 6. The I /I decreases
with the surfactant concentration increasing. However, the
1
3
3
2
N ¼ 2:44 ꢂ 10 C þ 25:25 R ¼ 0:9589
ð6Þ
ð7Þ
m
dependence of I /I on C is sigmoid; the values of the CMC
3
1
3
2
could be calculated by the inflection point of sigmoid I /I -
1
N ¼ 1:54 ꢂ 10 C þ 33:09 R ¼ 0:9555:
3
m
C curves [11]. The CMCs of CMDim calculated from the
-1
One can see an empirically linear relationship between
the Nm and the CMDim concentration. From the relation-
ship between Nm and the surfactant concentration, one of
-
sigmoid I /I -C curves are 1.74 9 10 mol L
3
1
3
-
3
-1
(
35 ± 0.1 °C) and 1.25 9 10 mol L
(25 ± 0.1 °C),
respectively. Obviously, the value of the CMC at
3
5 ± 0.1 °C from the fluorescence probe method is very
close to that from the surface tension method.
60
o
The variation for I /I with the surfactant concentration
1
25 C
3
3
o
5 C
indicates the formation of the micelles in aqueous solu-
tions. The values of I /I for pyrene are 0.60 and 1.90 under
3
55
1
the conditions of cyclohexane [36] and pure water [37],
respectively. One can see that the values of I /I for the
1
3
5
4
4
0
5
0
micelles are 1.34 and 1.31 at 25 ± 0.1 and 35 ± 0.1 °C,
respectively, which suggests that the polarity of the
CMDim micelle is lower than that of pure water while is
larger than that of pure cyclohexane.
Here, we use the packing parameter, V /l Amin, for
C
H
predicting the micelle shape of CMDim in aqueous solu-
0.006
0.008
0.010
0.012
0.014
0.016
tions, the V (the volume of the hydrophobic long carbon
H
3
chain) is 27.4 ? 26.9C A [34, 38], l (the length of the
˚
n
C
Fig. 7 The concentration dependence of Nm for CMDim with
pH = 7 at 25 °C and 35 °C
long carbon chain) is 1.5 ? 1.265C [38], the C is the
n
n
1
23

J Surfact Deterg (2011) 14:497–504
503
the most important parameters, the critical average aggre-
gation number (Nm,c), can be obtained by reverse extending
the Eqs. 6 and 7 to CMCs, respectively. Here, the param-
eter Nm,c describes the surfactant monomers number of the
first micelle corresponding to the definite CMC in solution
at certain temperature. This parameter is hardly obtained
by other routine methods. Corresponding to the value of
CMCs at 25 ± 0.1 and 35 ± 0.1 °C, the Nm, c is 28.3, and
12. 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
1
3. Thomaier S, Kunz W (2007) Aggregates in mixtures of ionic
liquids. J Mol Liq 130:104–107
14. 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
1
1
5. 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
6. 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
3
5.8, respectively. The former is smaller than that of
?
C H N (CH ) CH COO (about 80–85) [34], which
-
1
2
25
3 2
2
indicates that the CMDim micelle presents a much looser
structure. It is also attributed to the higher steric effects
among the larger bulk head groups in CMDim molecules.
17. 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
Acknowledgments This work was sponsored by the Qing Lan
Project (2010) of Jiangsu Province, the Fundamental Research Funds
for the Central Universities (JUSRP21110), the Jiangsu Key Labo-
ratory of Fine Petrochemicals (KF0903), and the Zhejiang Zanyu
Technology Co.,Ltd Research Funds.
1
8. 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
1
9. 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
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Author Biographies
Xue-feng Liu received his M.Sc. Degree from Wuxi University of
Light Industry, and his Ph.D. degree from Jiangnan University in
China. He is an associate professor at the School of Chemical and
Material Engineering at Jiangnan University in China. His research
field is colloid and interface chemistry.
3
3
Li-li Dong is a graduate student, majoring in Applied Chemistry in
the School of Chemical and Material Engineering at Jiangnan
University in China.
3
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6. 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
Yun Fang is a professor in the School of Chemical and Material
Engineering at Jiangnan University in China. His research field is
colloid and interface chemistry.
9
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of nonionic detergents and of water-soluble polymers. Langmuir
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(2):422–438
1
23