Aggregation behavior of long-chain piperidinium ionic liquids in ethylammonium nitrate

Article abstract of DOI:10.3390/molecules191220157

Micelles formed by the long-chain piperidinium ionic liquids (ILs) N-alkyl-N-methylpiperidinium bromide of general formula CnPDB (n = 12, 14, 16) in ethylammonium nitrate (EAN) were investigated through surface tension and dissipative particle dynamics (DPD) simulations. Through surface tension measurements, the critical micelle concentration (cmc), the effectiveness of surface tension reduction (Π_cmc), the maximum excess surface concentration (Λ_max) and the minimum area occupied per surfactant molecule (A_min) can be obtained. A series of thermodynamic parameters (ΔG⁰_m, ΔH⁰_m and ΔS⁰_m) of micellization can be calculated and the results showed that the micellization was entropy-driven. In addition, the DPD simulation was performed to simulate the whole aggregation process behavior to better reveal the micelle formation process.

Full text of DOI:10.3390/molecules191220157

Molecules 2014, 19, 20157-20169; doi:10.3390/molecules191220157
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Aggregation Behavior of Long-Chain Piperidinium Ionic
Liquids in Ethylammonium Nitrate
Caili Dai, Mingyong Du, Yifei Liu, Shilu Wang, Jianhui Zhao, Ang Chen, Dongxu Peng
and Mingwei Zhao *
State Key Laboratory of Heavy Oil Processing, School of Petroleum Engineering,
China University of Petroleum (Huadong), Qingdao 266580, Shandong, China;
E-Mails: daicl306@163.com (C.D.); upcdmy@163.com (M.D.); zgsydxliuyifei@163.com (Y.L.);
wangshilu2009@126.com (S.W.); jessica_zjh@hotmail.com (J.Z.); chenang1016@126.com (A.C.);
pengdx731@163.com (D.P.)
*
Author to whom correspondence should be addressed; E-Mail: zhaomingwei@upc.edu.cn;
Tel.: +86-532-8698-1183; Fax: +86-532-8698-1161.
External Editor: Derek J. McPhee
Received: 13 October 2014; in revised form: 7 November 2014 / Accepted: 21 November 2014 /
Published: 2 December 2014
Abstract: Micelles formed by the long-chain piperidinium ionic liquids (ILs)
N-alkyl-N-methylpiperidinium bromide of general formula C PDB (n = 12, 14, 16) in
n
ethylammonium nitrate (EAN) were investigated through surface tension and dissipative
particle dynamics (DPD) simulations. Through surface tension measurements, the critical
micelle concentration (cmc), the effectiveness of surface tension reduction (Πcmc), the
maximum excess surface concentration (Гmax) and the minimum area occupied per
0
m
surfactant molecule (Amin) can be obtained. A series of thermodynamic parameters (ΔG
,
0
0
ΔH
m
and ΔS
m
) of micellization can be calculated and the results showed that the
micellization was entropy-driven. In addition, the DPD simulation was performed to simulate
the whole aggregation process behavior to better reveal the micelle formation process.
Keywords: aggregation behavior; long-chain piperidinium ionic liquid; surface tension;
dissipative particle dynamics

Molecules 2014, 19
. Introduction
Ionic liquids (ILs) are a class of organic salts that are liquids at or near room temperature. They
20158
1
have attracted much attention because of their special properties, such as low volatility,
nonflammability, high thermal stability and high ionic conductivity [1–12]. These characteristics make
ILs attractive alternatives to traditional organic solvents [13–15]. There is now extensive literature
reporting the successive synthesis and investigation of a large number of ILs. ILs are based on
imidazolium, pyrrolidinium, pyridinium, piperidinium and quaternary ammonium cations. The anions
−, BF −
, (CF
SO
3
)
N and CF
−
SO −
. These materials are widely
may vary, for example, halides, PF
6
4
3
2
3
3
used in organic synthesis, catalysis and preparation of nanostructured matters [16–19]. ILs with long
alkyl chains can be regarded as a novel kind of amphiphilic molecule. In recent years, numerous
papers have reported the aggregation behavior of IL-type surfactants in aqueous solution [20–26].
In this context, piperidinium-based ILs have been investigated recently [27–30]. The Chen group
studied the phase behavior of a series of piperidinium ILs using Polarized Optical Microscopy (POM),
Small-Angle X-Ray Scattering (SAXS) and rheology measurements [31]. Milioto and his co-workers
investigated the thermodynamic properties of a series of long-chain piperidinium salts in water [32].
Zhao et al., investigated the micelle behavior of piperidinium ILs N-alkyl-N-methylpiperidinium
bromide C PDB (n = 12, 14, 16) through surface tension, electrical conductivity and steady-state
n
fluorescence measurements [33].
Ethylammonium nitrate (EAN) is a room-temperature ionic liquid (RTIL) discovered in 1914 [34].
EAN has been widely investigated and used in many fields. Dielectric spectroscopy studies were
carried out in order to study the dielectric behavior of EAN [35]. In protein chemistry, EAN has many
potential applications, for example, it can be used as an additive, a detergent, a precipitating agent or to
deliver ligands to protein crystals [36,37]. Phase behaviors of surfactants and lipids in EAN were studied
over 20 years ago [38–43]. EAN is a protic ionic liquid and has the ability to form a three-dimensional
hydrogen-bond network, which is a characteristic supporting self-assembly of a surfactant [44]. Zheng
group studied the aggregation behavior of some 1-alkyl-3-methylimidazolium bromides (C
n = 12, 14, 16) in EAN. They concluded that C mimBr can form micelles in EAN, then investigated
the solvophobic interactions between the hydrocarbon chains of C mimBr and EAN molecules [45].
The aggregation behavior and micelle formation mechanism of N-alkyl-N-methylpyrrolidinium
bromide (C MPB n = 12, 14, 16) in EAN were investigated through surface tension measurement and
n
mimBr,
n
n
n
1H-NMR spectrometry by Shi and coworkers [46]. The Drummond group has studied the
self-assembly of hexadecyltrimethylammonium bromide (CTAB), myverol 18–99 K and phytantriol in
many protic ILs, including EAN [47,48]. Recently, research has focused on the aggregation behavior
of surface active ILs in RTIL, including in EAN [49,50].
In the present work, we prepared a series of piperidinium ILs with different alkyl chain lengths,
n
C PDB (n = 12, 14, 16). The aggregation behaviors of these ILs in EAN have been investigated by
surface tension measurements and dissipative particle dynamics (DPD) simulations. Our aim was to
examine the influence of alkyl chain length on the aggregation behavior, so that we can offer a systematic
study of the mechanism of formation of aggregations formed by surface active ILs in RTILs.

Molecules 2014, 19
20159
2
. Results and Discussion
.1. Surface Tension of C PDB in EAN
2
n
Figure 1 shows the surface tension of C
98 K. The surface tension of the C
PDB concentrations increase further, the surface tension decreases gradually. Finally,
n
PDB (n = 12, 14, 16) in EAN at various concentrations at
2
n
PDB solution decreases sharply at the beginning compared with
pure EAN. As C
n
the surface tension remains constant above the critical micelle concentration (cmc). The cmc values are
listed in Table 1. The cmc value of a surfactant reflects its surface properties, a smaller cmc value
means better surface activity. The value of cmc declines with the increase of hydrocarbon chain length,
which is similar to their aggregation behavior in aqueous solution. The result suggests that there exist
solvophobic interactions between the hydrocarbon chain and EAN, similar to the hydrophobic interactions
in water. The obtained cmc values of C
values of C PDBs in EAN are smaller than those of C
and C MPB (0.097, 0.026 and 0.0078 mol·L ) for the same alkyl chain length [45,46]. The cations of
mimBr and C MPB have a great impact on that. There are two main two reasons, head groups have
n
PDB are higher than those in aqueous solution [32]. The cmc
−1
n
n
mimBr (0.139, 0.0350 and 0.00913 mol·L )
−1
n
C
n
n
opposing tendencies to keep close to minimize hydrocarbon-solvent contacts and to repel as a result of
electrostatic repulsion, solvation and steric hindrance [51]. The reason might be the lower
hydrophilicity of the C
n
PDB head groups than that of C
n
mimBr and C
n
MPB.
Figure 1. Surface tension as a function of C
n
PDB concentration at 298.15 K.
44
42
40
38
36
34
C PDB
12
C PDB
14
C PDB
16
1E-3
0.01
C(mol/L)
0.1
Table 1. Surface properties of C PDB (n = 12, 14, 16) in EAN at 298.15K.
n
3
max (μmol/m2)
0.372
min (Å2)
446.5
138.3
119.4
ILs
cmc (×10 mol/L)
13.7 ± 0.5
γ
cmc (mN/m)
Π
cmc (mN/m)
Г
A
C
C
C
12PDB
14PDB
16PDB
36.412 ± 0.001
35.112 ± 0.001
34.610 ± 0.001
12.935 ± 0.001
14.351 ± 0.001
14.737 ± 0.001
4.7 ± 0.03
1.201
2.8 ± 0.03
1.390
Figure 2 shows the relationship between the number of carbon atoms in the hydrocarbon chain of
C
n
PDB and lgcmc. Figure 2 shows that lgcmc decreases with the increase of alkyl chain length and the
plot is almost liner. The rule can be expressed by the empirical formula:

Molecules 2014, 19
20160
lgcmc = A − BNc
(1)
In this formula, A and B are constants. A stands for the ability of forming micelles of a surfactant
and B stands for the average contribution to the micelle formation by the methylene in the hydrophobic
chain. The value of A is obtained by extrapolation of the straight line and A and B were calculated to
be 0.1807 and 0.1736, respectively. The value of B for C
mimBr (0.30). The value of A for C MPB and C mimBr in EAN are 2.25, and 2.10, respectively.
The lower value of A for C PDB indicates that C PDB is easier to form micelle in EAN, which is in
n
n
PDB is similar with C MPB (0.28) and
C
n
n
n
n
n
accordance with the result from the comparison of surface tension. This phenomenon is resulted from
the special interactions between the different head groups and EAN.
The effectiveness of surface tension reduction (Πcmc) can be obtained using the following formula:
Πcmc = γ
0
− γcmc
(2)
where γ
0
is the surface tension of pure solvent and γcmc is the surface tension of the solvent. The values
are listed in Table 1. The results indicate that Πcmc decreases with the increase of the length of
hydrocarbon chain, and when n = 16, the IL behaves best in reducing the surface tension, which
indicates the ILs with longer hydrocarbon chains can reduce surface tension easier.
Figure 2. Plot of logarithmic cmc versus the number of carbon atoms in the hydrocarbon
chain of C PDB at 298.15 K.
n
-2.0
-2.2
-2.4
-2.6
12
14
16
Nc
The maximum excess surface concentration (Гmax) and the minimum area occupied per surfactant
molecule (Amin) at the air/liquid surface can be obtained from the Gibbs adsorption isotherm:
1
ꢇꢈ
Γꢀꢁꢂ = −
ꢆ
ꢊ
(3)
(4)
ꢃꢄꢅ ꢇlnꢉ
1
ꢋ
ꢌꢀꢍꢎ
−1
=
ꢏ ꢑ
ꢐ ꢀꢁꢂ
−
1
where R is the gas constant (8.314 J·mol ·K ), T is the absolute temperature and the value of n is
taken as 2 [52], dγ/d(lnC) is the slope of γ versus lnC dependence while the concentration is near cmc,
2
3
−1
N is Avogadro’s number (6.022 × 10 mol ).
A

Molecules 2014, 19
20161
The value of Гmax and Amin obtained from the Gibbs adsorption isotherm reflects the molecule
arrangement of ILs at the air/liquid interface [53] and they are listed in Table 1. With the increase of
alkyl chain length, Гmax increases but Amin decreases, which means the longer alkyl chain can make the
C
n
PDB molecules packing more closely. Compared with the values of C
n
PDB with the same alkyl
chain length in water, Гmax is larger but Amin is smaller in EAN than that of in water. [31] This indicate
that less C
n
PDB molecules would aggregate in air/EAN interface. The estimated Гmax values for
2
C
n
MPB in EAN are 0.96, 1.36 and 1.85 μmol/m , the Amin of that are 173, 122 and 89 Å. This means
C
n
MPB molecules have a higher packing density at the air/EAN interface.
2
.2. Temperature Dependence of cmc
n
Plots of surface tensions against C PDB concentrations at various temperatures are shown in Figure 3.
The values of cmc for C PDB at various temperatures are listed in Table 2. Figure 4 gives the
n
correlations between cmc and temperature. It indicates that the value of cmc decreases with the
temperature increase in a trend of U-shape and fits with a second-order polynomial. This trend is
similar to the other ILs in EAN [46,47].
Figure 3. Surface tensions versus concentration at different temperatures of C12PDB (a);
C14PDB (b); C16PDB (c) in EAN.
44
42
40
38
36
34
42
4
2
a
b
c
2
98K
303K
08K
313K
18K
298K
303K
305K
308K
313K
298K
40
40
38
36
34
303K
3
308K
313K
318K
3
8
6
3
3
34
γ
γ
γ
32
-3
10
-2
-1
-3
10
-2
-1
10
-3
10
-2
-1
10
10
C (mol/L)
10
10
C (mol/L)
10
C (mol/L)
Table 2. Critical micelle concentration (cmc) and thermodynamic parameters of
micellization for C PDB (n =12, 14, 16) in EAN at various temperatures.
n
3
ꢕ
−1
ꢕ
−1
ꢕ
−1
ILs
T (K) cmc (×10 mol·L) ꢒꢓ (kJ·mol ) ∆ꢖ (kJ·mol ) −ꢗ∆ꢘ (kJ·mol )
ꢔ
ꢔ
ꢔ
2
3
98
03
13.7 ± 0.05
12.5 ± 0.06
12.0 ± 0.03
11.9 ± 0.02
11.7 ± 0.04
4.7 ± 0.03
3.5 ± 0.02
2.9 ± 0.02
2.7 ± 0.04
2.6 ± 0.01
−21.77 ± 0.009
10.73 ± 0.161
−32.50 ± 0.152
−22.30 ± 0.012
−22.83 ± 0.006
−23.28 ± 0.004
−23.76 ± 0.009
−24.49 ± 0.017
−25.89 ± 0.016
−26.48 ± 0.018
−27.24 ± 0.037
−27.68 ± 0.010
9.172 ± 0.106
7.667 ± 0.053
6.210 ± 0.001
4.799 ± 0.049
52.94 ± 0.590
36.47 ± 0.296
20.54 ± 0.011
5.109 ± 0.265
−9.83 ± 0.532
−31.47 ± 0.094
−30.49 ± 0.046
−29.49 ± 0.003
−28.56 ± 0.058
−77.43 ± 0.607
−62.36 ± 0.312
−47.02 ± 0.028
−32.35 ± 0.228
−17.85 ± 0.522
C12PDB
308
3
3
2
3
13
18
98
03
C
14PDB
308
3
3
13
18

Molecules 2014, 19
20162
Table 2. Cont.
3
ꢕ
−1
ꢕ
−1
ꢕ
−1
ILs
T (K) cmc (×10 mol·L) ꢒꢓ (kJ·mol ) ∆ꢖ (kJ·mol ) −ꢗ∆ꢘ (kJ·mol )
ꢔ
ꢔ
ꢔ
2
3
98
03
2.8 ± 0.03
2.6 ± 0.02
2.5 ± 0.01
2.3 ± 0.02
2.2 ± 0.02
−25.66 ± 0.027
15.47 ± 0.970
−41.13 ± 0.943
−26.36 ± 0.020
−26.99 ± 0.010
−27.49 ± 0.023
−28.09 ± 0.035
12.63 ± 0.417
9.887 ± 0.119
7.231 ± 0.637
4.658 ± 0.139
−38.99 ± 0.397
−36.87 ± 0.129
−34.72 ± 0.660
−32.75 ± 0.174
C16PDB
308
3
3
13
18
Figure 4. Plots of cmc versus temperature of C12MDB (a), C14MDB (b), C16MDB (c).
0.014
0.013
0.012
0.005
0.004
0.003
0.0028
b
a
c
0.0026
0.0024
0.0022
300
310
T (K)
320
300
310
T (K)
320
300
310
T (K)
320
2
.3. Thermodynamic Analysis on the Micelle Formation of C PDB in EAN
n
As is shown in Figure 4, the temperature has a significant relationship with the micelle formation of
C
n
PDB in EAN. The standard Gibbs free energy of micelle formation is given as follows:
ꢛ
ꢙꢚ = 2ꢄꢅlnꢜ
(5)
is
ꢀ
ꢝ
ꢛ
where ꢙꢚ is the standard Gibbs free energy; R is the gas constant; T is the absolute temperature; X
S
ꢀ
the mole fraction of surfactant monomer coexisting with the micelle.
Then, the enthalpy of aggregation formation can be calculated by the Gibbs-Helmholtz Equation:
ꢛ
∆
ꢚ
ꢀ
∂
ꢆ
ꢊ
ꢅ
1
ꢛ
∆
ꢞ = ꢟ
ꢢ
(6)
ꢀ
∂
ꢠ ꢡ
ꢅ
ꢛ
ꢛ
ꢛ
On the basis of ꢙꢚ and ∆ꢞ , ∆ꢣ can be derived as the following equation:
ꢀ
ꢀ
ꢀ
ꢛ
ꢛ
∆
ꢞ − ∆ꢚ
ꢀ ꢀ
ꢛ
∆
ꢣ =
(7)
ꢀ
ꢅ
ꢛ
The value of ꢙꢚ at different temperatures can be calculated based on Equation (5). As is shown in
ꢀ
ꢛ
Figure 5, the value of ꢙꢚ /ꢅ increases along with the increase of 1/T. The plot fits with a second-order
ꢀ
ꢛ
ꢛ
polynomial and the quadratic equations can be obtained. The values of ∆ꢞ and −ꢅ∆ꢣ of C12PDB,
ꢀ
ꢀ
C
14PDB and C16PDB at different temperatures can be calculated according to Equations (6) and (7).
ꢛ
ꢛ
ꢛ
Figure 6 shows the plots of ꢙꢚ , ∆ꢞ and −ꢅ∆ꢣ versus temperature of C
ꢙꢚ is negative and decreases with the increase of temperature which is similar to the other ILs in
EAN. From 298 K to 318 K, −ꢅ∆ꢣ increases with the temperature while the value of ∆ꢞ decreases.
The figure indicates that the negative ꢙꢚ is mainly contributed by the large negative −ꢅ∆ꢣ . Thus,
n
PDB (n = 12, 14, 16).
ꢀ
ꢀ
ꢀ
ꢛ
ꢀ
ꢛ
ꢛ
ꢀ
ꢀ
ꢛ
ꢛ
ꢀ
ꢀ
the micelle formation of C
n
PDB (n = 12, 14, 16) in EAN is an entropy-driven process.

Molecules 2014, 19
Figure 5. Plots of ꢙꢚ /ꢅ against 1/T of C12PDB (a); C14PDB (b); and C16PDB (c).
20163
ꢛ
ꢀ
a
-0.082
c
-0.086
-0.0730
-0.0735
-0.0740
-0.0745
b
-
0.084
0.086
-0.087
-
-0.088
Δ
Δ
Δ
0.0031
0.0032
/T (1/K)
0.0033
0.0031
0.0032
/T (1/K)
0.0033
0.0031
0.0032
1/T (1/K)
0.0033
1
1
ꢛ
ꢛ
ꢛ
Figure 6. Plots of ꢙꢚ , ∆ꢞ and −ꢅ∆ꢣ versus T for C12MDB (a); C14MDB (b); and
ꢀ
ꢀ
ꢀ
C16MDB (c).
c
a 15
0
b
2
0
0
m
ΔG
4
0
0
m
ΔH
0
m
−ΤΔS
0
m
0
0
m
ΔG
ΔG
0
0
m
0
m
ΔH
ΔH
0
m
0
m
−
ΤΔS
−ΤΔS
-
15
30
-
20
40
Δ
-40
-80
−
Τ
Δ
−ΤΔ
−
-
-
Δ
Δ
Δ
300
310
T (K)
320
300
310
T (K)
320
300
310
T (K)
320
Δ
Δ
Δ
2
.4. Dissipative Particle Dynamics (DPD) Simulation on the Micelle Formation of C PDB in EAN
n
The DPD simulation was performed using the Material Studio software. The theory of this
simulation method has been discussed previously [31,54]. In the simulation model, the C PDB
n
molecule is shown in Figure 7 and the amphiphilic molecule is divided into two parts, the hydrophilic
part C and the hydrophobic part H, which are connected by a harmonic spring and the monomer
particle E represents for EAN. The model is simulated in a 10 × 10 × 10 cubic box. The temperature is
kept at 298 K and the step size of the Newton equation for the integration is set to Δt = 0.05.
Figure 7. Simulation model of C
n
PDB in EAN. The C PDB molecule is divided into two
n
parts, alkyl-chain (C) and headgroup (H). Water is represented by (E).
In order to represent dynamic process for the micelle formation, 20% C PDB is used to perform the
n
DPD simulation and the results are shown in Figure 8. At first, the system is unstable and the beads are
unordered, which can be seen from Figure 8a,b. No ordered structure is formed in this step. Then,
Figure 8c indicates that some spherical structures are formed, but not very regular. At last, an ordered

Molecules 2014, 19
20164
structure is finally formed and the structure is more ordered (Figure 8d). The process often happens in
tens of μs and is difficult to observe in a lab experiment, so the simulated result is regarded as an
effective method supplying valuable information about microphase separation.
Figure 8. Simulation of micelle formation of 20% C
different time steps: (a) 3; (b) 10; (c) 100; (d) 20,000. The size of the simulation model is
0 × 10 × 10 in DPD units.
n
PDB in EAN at room temperature at
1
3
. Experimental Section
3.1. Materials
The compounds 1-methylpiperidinium (97%), 1-bromododecane (97%), 1-bromotetradecane (97%),
-bromohexadecane (97%), ethylamine, nitric acid (65%), ethyl ether, 2-butanone (99%), and
1
tetrahydrofuran (THF) were purchased from Aladdin Chemical Reagent Co., Ltd. (Shanghai, China)
3
.1.1. Synthesis of C PDB (n = 12, 14, 16)
n
C
n
PDBs were synthesized according to a previously reported procedure [55]. A solution of
1
-bromoalkane in 2-butanone was added dropwise to a solution of N-methylpiperidine in 2-butanone.
The mixture was refluxed at 75–80 °C under a nitrogen atmosphere for 48 h. After cooling to room
temperature, the 2-butanone was evaporated and the product was recrystallized from fresh
tetrahydrofuran THF at least three times. Then it was dried under vacuum for 48 h at 50 °C. The
1
products were characterized by H-NMR spectroscopy (400 MHz) using CDCl
1H-NMR data were recorded as follows:
3
as solvent. The
C
12PDB δ
H
: 0.881 (t, 3 H), 1.255–1.368 (m, 18 H), 1.727–1.940 (8 H), 3.346 (s, 3 H), 3.615–3.692
m, 4 H), 3.775–3.806 (m, 2 H).
: 0.882 (t, 3 H), 1.255–1.369 (m, 22 H), 1.696–1.934 (m, 8 H), 3.367 (s, 3 H), 3.617–3.660
m, 4 H), 3.821–3.833 (m, 2 H).
(
(
C
14PDB δ
H

Molecules 2014, 19
20165
C16PDB δ
H
: 0.881 (t, 3 H), 1.255–1.368 (m, 26 H), 1.714–1.907 (m, 8 H), 3.362 (s, 3 H),
3
3
.612–3.674 (m, 4 H), 3.824–3.851 (m, 2 H).
.1.2. Synthesis of EAN
EAN was synthesized according to Evans et al. [44]. A portion of nitric acid was added dropwise to
ethylamine solution under stirring and cooling in an ice bath. Then water was removed from the
1
resulting product with a rotary evaporator. EAN was identified by its H-NMR spectrum as follows: δ
H
D
3
-AN use common abbreviation): 1.14 (t, 3H), 2.84 (m, 2H), 7.07 (s, 3H).
3
.2. Apparatus and Procedures
Surface tension measurements were carried out by a model JYW-200B surface tensiometer
Chengde Dahua Testing Instrument Co., Ltd., Chengde, Hebei, China). The temperature was controlled
(
with the help of a thermostatic bath. The surface tension was measured through a single-measurement
method and all tests were repeated at least twice until the results were repeatable.
4
. Conclusions
In summary, the aggregation behavior of long-chain piperidinium ILs C PDB (n = 12, 14, 16) in
n
EAN were investigated in this work. Through surface tension measurements, the cmc, γcmc and Πcmc
can be obtained and thermodynamic parameters related to the micellization can be calculated. Through
an investigation of the effect of the alkyl chain length of the ILs, it can be concluded that the longer the
alkyl chain the better characteristics the ILs possess. By analyzing the thermodynamic parameters at
different temperatures, it can be established that the C PDB micelle formation is an entropy-driven
n
process. The DPD simulation clearly showed the micellization process of C
n
PDB in EAN. We expect
our work will help better understand the aggregation behavior ILs in EAN.
Acknowledgments
The work was supported by the National Science Fund for Distinguished Young Scholars (NO.
1425406), National Natural Science Foundation of China (21303268), Doctoral Fund from National
5
Ministry of Education (No. 20120133110010), China Postdoctoral Science Foundation Funded Project
2013T60689), Shandong Province Postdoctoral Innovation Fund (201203108) and the Fundamental
(
Research Funds for the Central Universities (14CX02041A).
Author Contributions
Caili Dai, Mingwei Zhao conceived and designed the experiments. Mingyong Du performed the
experiments and analyzed the data; Mingyong Du, Caili Dai and Mingwei Zhao wrote and revised the
paper; Yifei Liu, Shilu Wang, Jianhui Zhao, Ang Chen and Dongxu Peng revised the manuscript. All
authors contributed to this study, read and approved the final manuscript.

Molecules 2014, 19
20166
Conflicts of Interest
The authors declare no conflicts of interest.
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Sample Availability: Samples of the compounds are available from the authors.
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