Liquid crystalline phases of the amphiphilic ionic liquid n -hexadecyl- n -methylpyrrolidinium bromide formed in the ionic liquid ethylammonium nitrate and in water

Article abstract of DOI:10.1021/jp103728x

The phase behavior of a surfactant-like ionic liquid, N-hexadecyl-N- methylpyrrolidinium bromide (C₁₆MPB), was studied in both water and a room temperature ionic liquid, ethylammonium nitrate (EAN). Polarized optical microscopy (POM) and small-angle X-ray scattering (SAXS) measurements were employed to investigate the phase behavior of the two systems and to determine which lyotropic liquid crystalline (LC) phases were formed. With increasing C₁₆MPB concentration, an isotropic solution phase, a hexagonal (H₁) phase, and a cubic phase (V₂) are all present in either EAN or H₂O. The structural parameters of the H₁ phase were calculated from SAXS patterns, which show the structural changes as a function of the amount of C₁₆MPB. The rheological results reveal that the H₁ phase constructed by C₁₆MPB in EAN displays a typical Maxwell behavior, whereas the H₁ phase formed by C ₁₆MPB in water shows a gel-like behavior, unlike traditional cationic surfactants. POM and differential scanning calorimetry (DSC) results demonstrate that the lyotropic LC phase in EAN has a higher thermal stability than that formed in H₂O, which may be important to extend the applications of the LC phase.

Full text of DOI:10.1021/jp103728x

1
1382
J. Phys. Chem. B 2010, 114, 11382–11389
Liquid Crystalline Phases of the Amphiphilic Ionic Liquid
N-Hexadecyl-N-methylpyrrolidinium Bromide Formed in the Ionic Liquid Ethylammonium
Nitrate and in Water
Mingwei Zhao, Yanan Gao, and Liqiang Zheng*
Key Laboratory of Colloid and Interface Chemistry, Shandong UniVersity, Ministry of Education,
Jinan, Shandong, 250100, People’s Republic of China
ReceiVed: April 26, 2010; ReVised Manuscript ReceiVed: July 1, 2010
The phase behavior of a surfactant-like ionic liquid, N-hexadecyl-N-methylpyrrolidinium bromide (C16MPB),
was studied in both water and a room temperature ionic liquid, ethylammonium nitrate (EAN). Polarized
optical microscopy (POM) and small-angle X-ray scattering (SAXS) measurements were employed to
investigate the phase behavior of the two systems and to determine which lyotropic liquid crystalline (LC)
phases were formed. With increasing C16MPB concentration, an isotropic solution phase, a hexagonal (H
phase, and a cubic phase (V ) are all present in either EAN or H O. The structural parameters of the H phase
were calculated from SAXS patterns, which show the structural changes as a function of the amount of
16MPB. The rheological results reveal that the H phase constructed by C16MPB in EAN displays a typical
Maxwell behavior, whereas the H phase formed by C16MPB in water shows a gel-like behavior, unlike
traditional cationic surfactants. POM and differential scanning calorimetry (DSC) results demonstrate that
1
)
2
2
1
C
1
1
the lyotropic LC phase in EAN has a higher thermal stability than that formed in H
to extend the applications of the LC phase.
2
O, which may be important
1
. Introduction
destroyed. Exploring thermodynamically stable lyotropic LCs
remains a great challenge.
Finding environmentally benign solvents for industrial use
Due to their negligible vapor pressure, ILs provide a chance
to design such thermodynamically stable lyotropic LCs. ILs are
usually employed as solvents, and some recent investigations
have concentrated on the lyotropic LCs. Friberg et al. first
investigated the phase behavior of lyotropic LCs composed of
is a great challenge to the scientific community. Ionic liquids
ILs) have emerged in the last two decades and show great
(
1
-3
potential to solve this puzzle. ILs are compounds which are
liquid below 100 °C and are composed of ions. In recent years,
ILs have received considerable attention due to their unique
properties such as low melting temperature, nonflammability,
high ionic conductivity, negligible vapor pressure, wide elec-
trochemical window, and good catalytic properties. Thus, ILs
have been employed widely in the areas of catalysis, preparation
of novel nanomaterials, organic synthesis, electrochemistry, and
water, nonionic surfactant C12EO
zolium hexafluorophosphate (BmimPF
4
, and 1-butyl-3-methylimida-
21
6
). Small-angle X-ray
diffraction results showed that a lamellar liquid crystal phase
appeared and the location of IL in the LC phase was also
clarified. After that, Warr et al. studied the phase behavior of
LC phases formed by a number of polyoxyethylene nonionic
4
-8
liquid/liquid extraction.
22
surfactants in ethylammonium nitrate (EAN). In addition, some
It is well-known that amphiphilic molecules play an important
role in technological applications because of their unique ability
to self-assemble both at surfaces and in bulk solutions. In
general, surfactant molecules can form thermodynamically stable
ILs with long alkyl chains have also been designed, which
23-30
demonstrated good amphiphilic behavior.
Firestone and
Dzielawa described the formation of an LC phase composed of
28
1
-decyl-3-methylimidazolium bromide and water. Our group
also investigated the phase behavior of ternary mixtures of
16mimBr/p-xylene/water by SAXS, POM, and rheology mea-
9
10
self-organized structures such as micelles, microemulsions,
vesicles,11 and liquid crystals (LCs). Among these, LCs have
12
C
become the most important subject, due to their wide applica-
29
surements.
1 R
H and L phases are constructed, and the rheo-
tions in chemical reactions, material science, and pharmaceutical
logical properties were found to be related to the structures of
the LCs. Although there are some investigations into LC phases
constructed with ILs, the organic components or water in the
LC systems still limit their applications. Up to now, there have
been few reports about the construction of LCs with high
vehicles.1
3-16
Through characterization by polarized optical
microscopy (POM), small-angle X-ray scattering (SAXS), and
differential scanning calorimetry (DSC), hexagonal (H ), lamel-
lar (L ), and cubic (V ) phases of LCs can be determined. Due
to their rich phase behavior, LCs have been widely investigated
1
R
2
30
thermal stability. Thus, in this work, our aim is to develop a
in the scientific field.1
7-20
Nevertheless, the low thermodynamic
new LC system with higher thermal stability.
stability of LCs greatly limits their applications. This is because
the traditional lyotropic LCs generally contain low-boiling-point
solvents, like water and organic solvents, which can be rapidly
evaporated at high temperature, and the structure of LCs is easily
In the present study, we investigate two LC systems:
N-hexadecyl-N-methylpyrrolidium bromide (C16MPB)/EAN and
C
2
16MPB/H O. Through the investigation of the phase behavior
and comparison of these two systems, the solvent effect on the
phase diagrams, structural parameters, and rheological properties
of LC is clarified. We expect this work to not only enrich our
*
To whom correspondence should be addressed. E-mail: lqzheng@
sdu.edu.cn. Fax: +86-531-88564750. Phone: +86-531-88366062.
1
0.1021/jp103728x  2010 American Chemical Society
Published on Web 08/17/2010

Liquid Crystalline Phases of C16MPB
J. Phys. Chem. B, Vol. 114, No. 35, 2010 11383
knowledge of the phase behavior of surface active ILs but also
provide a new way to prepare thermodynamically stable LCs.
sensor was used with 20 mm diameter and 1° cone angle. The
cone-plate distance was adjusted to 52 µm for all measure-
ments. The temperature was kept at 25 ( 0.1 °C. Each sample
was inserted onto the top of the cone template, and then, the
plate was slowly elevated to its measuring position at a constant
velocity. Any excess sample squeezed out from the sensor
system was gently removed. Dynamic oscillation-shear mea-
surements were employed, in which the stress (σ) was varied
while the frequency was kept at 1.0 Hz. Once the linear
viscoelastic region was determined, frequency sweep measure-
ments were performed as a function of frequency at a constant
stress.
Differential Scanning Calorimetry. DSC was carried out on
an SDT Q600 Simultaneous DSC-TGA (TA Instruments). The
linear heating rate was always 10 °C/min, and the samples were
heated under a nitrogen atmosphere at a flow rate of 50 mL/
min.
2
. Experimental Section
.1. Materials. N-Methylpyrrolidine (97%) and 1-bromo-
2
hexadecane (97%) were products of Sigma and used as received.
Ethylamine (70%) was purchased from Alfa Aesar. Toluene
(
99%), diethyl ether (99%), and nitric acid (65%) were
purchased from Beijing Chemical Reagent Company. Deionized
water was used through the experiment.
Synthesis of C16MPB. C16MPB was synthesized according
3
1
to the procedures reported previously. N-Methylpyrrolidine
(
(
0.1 mol, 26.0 g) and an excess amount of 1-bromohexadecane
0.11 mol, 32.2 g) were mixed in toluene (200 mL) in a 500
mL round-bottom flask and then refluxed under a nitrogen
atmosphere at about 80 °C for 48 h. Then, the product was
cooled to room temperature and purified by recrystallization in
fresh diethyl ether at least four times. Finally, the product was
dried under a vacuum for 48 h. The purity of C16MPB was
1
3. Results and Discussion
ascertained by the H NMR spectrum in CDCl
3
.
3.1. Phase Diagrams of the C16MPB/EAN and C
16
MPB/
Synthesis of EAN. EAN was synthesized according to the
3
2
procedures presented by Evans et al. In a typical synthesis, a
portion of nitric acid (50 mL) was slowly added to ethylamine
solution (83 mL) dropwise while stirring in an ice bath. Then,
water in the resulting mixture was removed with a rotary
H O Systems. The phase diagram of C16MPB/EAN in the
2
temperature range 25-70 °C is shown in Figure 1, together
with the corresponding POM images and SAXS patterns. As
shown in Figure 1a, the single-phase region consists of the
1
evaporator. The purity of EAN was ascertained by the H NMR
isotropic solution phase, anisotropic H
1
phase, and isotropic V
2
spectrum in D
2
O.
phase. At low C16MPB content (less than 50 wt %), the
homogeneous and isotropic solution phase appears. With the
2
.2. Phase Diagram. The process of mapping the phase
27,28,33
phases appear. When
the C16MPB content is between 50 and 83 wt %, a large area
of H phase is found, which is confirmed by the POM image
and SAXS pattern shown in Figure 1b. The fanlike texture
Figure 1b (top)) is the typical character of the H phase, and
the SAXS pattern (Figure 1b (bottom)) also confirms the
existence of the H phase. Three scattering peaks are observed
in the SAXS pattern, and the q values corresponding to the
scattering peaks are in the ratio q :q :q
) 1:ꢀ3:2, which are
characteristic for the H phase is found
phase.20 An isotropic V
diagram has been described elsewhere.
The samples were
1 2
increase of C16MPB content, the H and V
prepared by weighing the selected amount of components in
stoppered glass vials to avoid evaporation. The mixtures were
homogenized and equilibrated by repeated vortex mixing and
centrifuging. Then, they were kept in a thermostat at least 1
month for equilibration. The phase diagram was established
between 25 and 70 °C over the whole water-surfactant
composition range. The optical nature of the samples (nonbi-
refringent or birefringent) was checked by crossed polarizers.
In the case of LCs, polarized microscopy and SAXS were
usually used to identify the type of LCs. The composition
interval was first selected as 5 wt % for a rough mapping, while
smaller intervals (2 wt %) were selected in the vicinity of phase
boundaries.
1
(
1
1
1
2
3
1
2
when the C16MPB content is between 83 and 87 wt % in the
temperature range 35-70 °C. The POM image shown in Figure
c (top) displays only background. Its SAXS pattern (Figure
c (bottom)) gives two distinct Bragg peaks with relative ratios
1
1
of ꢀ3:ꢀ4, which are assigned to the (211) and (220) reflections
2
.3. Characterization. Polarized Optical Microscopy. A
of the Ia3d structure of the cubic phase.17
polarized optical microscope equipped with a charge-coupled
device camera (Panasonic Super Dynamic II WV-CP460) was
used to observe the LC phase. Birefringent textures from the
optical microscope allow the determination of particular LCs.
The phase diagram of the C16MPB/H
temperature range 25-70 °C is shown in Figure 2a. At low
16MPB content (less than 25 wt %), the mixture is an isotropic,
clear, and transparent solution. With increasing C16MPB content,
the system remains transparent but becomes more viscous. H
and V phases were observed in the SAXS patterns. For instance,
a H phase forms when the C16MPB content is in the range
25-70 wt % (Figure 2b). Three sharp scattering peaks are also
detected which satisfy the ratios q :q :q
) 1:ꢀ3:2, showing
the existence of the H phase. A further increase in C16MPB
phase.
2
O mixture in the
C
In general, the H
while the L phase shows a distinct woven structure or a Maltese
cross texture.
phase displays a fanlike and angular texture,33
1
R
1
34,35
The isotropic cubic LC is nonbirefringent and
2
shows only background.
1
Small-Angle X-ray Scattering. The SAXS technique was
used to characterize the LC phase in an HMBG-SAX X-ray
small-angle scattering system (Austria) with Cu KR radiation
operating at 2 kW (50 kV and 40 mA). To remove the Kꢀ
radiation, a 10 µm thick nickel filter was employed and a 2
mm tungsten filter was used to protect the detector from the
primary beam. The distance between the sample and detector
was about 278 mm. All samples were held in a vacuum steel
holder to provide good thermal contact to the computer-
controlled Peltier heating system (Hecus MBraun Austria). The
structural parameters of the LC phase could be deduced from
the Bragg peaks of the SAXS patterns.3
1
2
3
1
content (70-85 wt %) leads to the appearance of the V
2
The three Bragg peaks in the SAXS spectrum (Figure 2c) can
be indexed as the (211), (220), and (321) reflections of the Ia3d
structure, with the ratio q
Through comparison of the phase diagrams of the two LC
systems, some differences become clear, which may be due to
1
:q
2
:q
3
) ꢀ3:ꢀ4:ꢀ7.
the solvent effect between EAN and H
2
O. As previously
reported, nonaqueous polar solvents reduce the LC region in
6,37
38,39
comparison to aqueous mixtures.
In our study, the C16MPB/
Rheological Measurement. Rheology measurements were
performed with a HAAKE RS 75 rheometer. A cone-plate
EAN system occupies a narrower LC region originating from
55 wt % C16MPB, while it begins from 25 wt % C16MPB in

1
1384 J. Phys. Chem. B, Vol. 114, No. 35, 2010
Zhao et al.
Figure 1. Phase diagram (a) for the C16MPB/EAN binary mixture, in which H
1
and V
2
denote the normal hexagonal and reverse bicontinuous
1 2
phase. POM images (top, upon cooling from the isotropic phase) and SAXS curves (bottom) for the typical liquid crystal phases: (b) H and (c) V .
The scale bar is 200 µm.
2 1 2
Figure 2. Phase diagram for (a) the C16MPB/H O binary mixture and typical SAXS curves for (b) H and (c) V .
the C16MPB/H
3 wt % C16MPB dissolved in EAN, while it appears from 70
wt % C16MPB in the C16MPB/H O mixture. This means that it
2
O system. The V
2
phase is constructed above
2
is easier to construct the LC phase in H O than in EAN. In
general, the solvent effect on the formation of LC phases
includes two relationships. The first relationship was provided
8
2

Liquid Crystalline Phases of C16MPB
J. Phys. Chem. B, Vol. 114, No. 35, 2010 11385
by W a¨ rnheim and J o¨ nsson,40,41 who showed that the size of the
LC regions was correlated to the solvent/hydrocarbon interfacial
tension. A high interfacial tension suggests a considerable
solvophobic interaction which favors extensive surfactant ag-
gregation, and such a higher interfacial tension is always caused
4
2
by the stronger polarity. It is well-known that the polarity of
O is stronger than other solvents, so H O forms LC phases
H
2
2
more easily than EAN. The second relationship is that the size
of the LC regions depends on the ability of the solvent to
4
3,44
participate in hydrogen bonding.
Comparing the structures
O exhibits a tetrahedral
of the two solvents used in our study, H
2
Figure 3. SAXS curves of different amounts of C16MPB in the H
phase of the C16MPB/EAN system at 25 °C.
1
H-bond network structure, while EAN only forms a three-
dimensional network structure. The values of negative enthalpy
and entropy changes for EAN are only half as large as those of
1
TABLE 1: Structural Parameters of H Phases in the
32,45,46
C MPB/EAN and C MPB/H O Systems at 25 °C
H
2
O.
conclude that LC phases are formed more easily in H
in EAN and the size of the LC regions is larger in H
Furthermore, large H phase regions exist in both systems,
According to these two relationships, we can
O than
O.
16
16
2
2
sample
0 L W H s
(wt %) R (Å) ꢁ d (Å) d (Å) a
(Å2)
2
1
C
C
16MPB/EAN H
(298 K)
55
60
47.28 0.55 10.48
18.40 49.76
1
1
which may be due to the molecular structure of C16MPB. The
critical packing parameter (CPP) is a very useful tool to interpret
the self-assembled structure for a particular amphiphile.47 The
CPP is determined as P ) V/(a·l), where P is the abbreviation
of CPP, V is the volume of the solvophobic chains of surfactant
molecules, a is the effective headgroup area of the surfac-
tant molecules, and l is the effective chain length of surfactant
molecules. In general, the values of CPP are as follows: for
spherical micelles, P e 1/3; for cylindrical aggregates, 1/3 < P
e 1/2; for bilayers, vesicles, and bicontinuous cubic LC phase,
46.56 0.60 8.680 18.94 48.34
45.87 0.65 6.650 19.61 46.69
45.57 0.70 5.600 20.25 45.21
61.14 0.31 25.38
63.24 0.36 23.38
58.41 0.41 19.11
55.92 0.45 16.50
53.32 0.50 13.70
52.41 0.56 11.21
6
7
5
0
16MPB/H
2
O
H
(298 K)
35
40
17.88 51.21
19.93 45.94
19.65 46.60
19.71 46.45
19.81 46.22
20.60 44.45
4
5
5
6
5
0
5
0
65
51.24 0.61 9.200 21.02 43.56
49.58 0.66 7.260 21.16 43.27
7
0
1
/2 < P e 1; and for inverted structures, P > 1. For C16MPB
of the solvent layer. From the SAXS patterns shown in Figure
3, it is clear that the positions of the first peak shift right with
increasing concentration of C MPB. Through calculation, the
molecules, the hydrocarbon chain length l and volume of the
solvophobic chains V can be obtained from the following
Tanford equations:48
16
R values are assigned to be 47.28, 46.56, and 46.10 Å for 55,
0
6
0, and 70 wt % C16MPB, respectively, which means that R
0
decreases with the increase in C16MPB concentration. The
increased amount of C16MPB causes a decrease of the distance
between the adjacent cylinders. To explain this phenomenon,
V ) 27.4 + 26.9N
l ) 1.5 + 1.265N
(1)
(2)
we should first make clear the structure of the H
1
phase. The
H
1
phase consists of infinitely long cylinder-like aggregates
where N is the number of carbon atoms in the hydrocarbon
chain. N ) 16 for C16MPB. From the equations above, V and l
packed in a hexagonal array and separated by a continuous
solvent region,38 where the solvophobic tails are located in the
interior part of the cylinder-like aggregates and the solvophilic
heads are solvated by EAN molecules. According to the model
of the H
the cylinder-like aggregates (d
3
are determined to be 457.8 Å and 21.74 Å. The area a is
obtained from the adsorption of C16MPB at the air/water
2
interface, and the value is about 45.2 Å (Supporting Informa-
1
phase, R
0
can be divided into two parts: the radius of
) and the thickness of the solvent
tion). Thus, P of C16MPB is calculated to be 0.46. The result
shows a strong preference for C16MPB molecules to pack in a
H
layer (d
W
). They accord well with the following equation:
hexagonal array. Thus, a large region of H
both the C16MPB/EAN and C16MPB/H O systems. In the
following section, we will focus on the effect of the amount of
16MPB on the structure of the H phase in the two systems
and their different thermal stabilities.
.2. The Effect of C16MPB Amount on the Phase Behavior
1
phase appears in
2
R ) 2d + d
W
(3)
0
H
C
1
Then, the following question arises: Which factor plays the
major role in the structural change of the H phase? To answer
this question, we should obtain the values of d and d first,
from eq S3 in the Supporting Information and the above eq 3.
The values of d and d are listed in Table 1. When the amount
of C16MPB is from 55 to 70 wt %, d decreases from 10.48 to
5.600 Å, while d increases from 18.40 to 20.25 Å. The results
suggest that the radius of the cylinder-like aggregates becomes
larger with the increasing amount of C16MPB, while the solvent
layer becomes thinner. In other words, the larger the concentra-
tion of C16MPB in the H
molecules pack in a hexagonal array. These two factors both
lead to the decrease of R as the amount of C16MPB increases.
Furthermore, we can deduce that the more dense packing
certainly causes a decrease of the area per C16MPB molecule
3
1
of the Two LC Systems. 3.2.1. SAXS Measurements. The
SAXS characterization of the LC phase is based on the long-
range order in the LC state, which leads to the Bragg reflections.
The positions of these Bragg reflections are characteristic of
the different types of LC phases, and the relevant structural
parameters can be deduced from the SAXS patterns.
H
W
H
W
W
H
1
Figure 3 shows the SAXS patterns of the H phase constructed
by different amounts of C16MPB in EAN at 25 °C. The first
peak shown in the SAXS patterns originates from the Bragg
reflection of the (100) plane, which can help us to obtain the
1
phase, the more densely the C16MPB
lattice parameter (R
0
) according to eqs S1 and S2 (shown in
0
2
0
the Supporting Information). The lattice parameter R
0
repre-
sents the distance from the center of one cylinder to another,
including the total diameter of the cylinder and the thickness
s 1 s
(a ) in the H phase. To verify this statement, the values of a

1
1386 J. Phys. Chem. B, Vol. 114, No. 35, 2010
Zhao et al.
Figure 4. SAXS curves of different amounts of C16MPB in the H
phase of the C16MPB/H O system at 25 °C.
1
2
were calculated according to eq S4 in the Supporting Informa-
tion and are listed in Table 1. It is clear that a decreases from
9.76 to 45.21 Å with the C16MPB amount from 55 to 70 wt
Figure 5. Schematic diagram of the packing mode of the C16MPB/
H O and C16MPB/EAN systems.
2
s
2
4
%
, which agrees well with the above statement. Thus, in the
phase of the C16MPB/EAN mixture, with the increase of
H
1
C16MPB, the cylinder-like aggregates pack more densely, while
the EAN solvent layer becomes thinner. Investigations into the
BmimPF
system showed similar changes in the structural parameters.
For the C16MPB/H
patterns of the H phase with different amounts of C16MPB.
The first peak for all SAXS patterns also shifts to the right with
the increase of C16MPB concentration. The data show that R
becomes smaller when more C16MPB molecules construct the
phase. Through calculation of the structural parameters (d
, and a ), the structural changes of the H phase are similar
to those of the H phase in the C16MPB/EAN system. With the
increase of the amount of C16MPB, the radius of the cylinder-
like aggregates increases and the area per C16MPB molecule
decreases due to the extension of the hydrophobic chains of
the surfactant. The water layer becomes thinner with the
decrease of the water content.
6
/H
2
O/Brij-30 (tetraethylene glycol lauryl ether, C12
E
4
4
)
9
2
O system, Figure 4 shows the SAXS
1
0
Figure 6. Storage moduli G′ (solid) and loss moduli G′′ (empty) versus
frequency for the H phase of the C16MPB/EAN system at 25 °C.
1
H
1
H
,
d
W
s
1
for different wt % of C16MPB, while the loss modulus (G′′)
increases with increasing frequency at the beginning and then
remains constant. At low frequencies, the loss modulus G′′ is
larger than the storage modulus G′, showing viscous behavior.
At higher frequencies, G′ is larger than G′′, exhibiting elastic
1
50
behavior. The rheological results mean that the H
viscoelastic behavior, which is consistent with the typical
rheological behavior of H phases.
The frequency dependence of G′ and G′′ when EAN is
substituted by H O is shown in Figure 7. The moduli G′ and
1
phase shows
Comparing the two H
EAN and in water, the effect of C16MPB concentration on the
structural parameters (R , d , d , and a ) is almost the same.
However, the structural parameters (R , d and d ) except a
in the C16MPB/H O mixture are always larger than those of
the C16MPB/EAN system with the same amount of C16MPB.
This result may be due to the density difference of H O (1.0
g/mL) and EAN (1.2 g/mL). The higher density makes the
1
phases constructed by C16MPB in
1
0
H
W
s
2
0
H
W
s
2
2
volume of EAN in the H
results in smaller structural parameters in the H
16MPB/EAN. Furthermore, the value of a
EAN mixture is larger than that in the C16MPB/H
The larger parameter d in the C16MPB/H O mixture results in
the smaller a
, which is consistent with previous results.2
In order to better show the difference between the H phases
1
2
phase smaller than that of H O, which
1
phase of
C
s
in the C16MPB/
O system.
2
W
2
0,29,50
s
1
formed in these two systems, Figure 5 shows the ideal schematic
diagram of the packing of these two lyotropic LC systems. The
Figure 7. Storage moduli G′ (solid) and loss moduli G′′ (empty) versus
frequency for the H phase of the C16MPB/H O system at 25 °C.
1
2
structures of these two H
effect. As shown in Figure 5, the lattice parameter (a
16MPB/H O system is larger, while the area per C16MPB
1
phases are different due to the solvent
0
) of the
C
2
molecule is smaller than in the C16MPB/EAN system. Further-
more, the ideal schematic diagram directly demonstrates that
2
the C16MPB molecules pack more densely in H O than in EAN.
3.2.2. Rheological Measurements. To find more information
about the structure of the H
1
phase, the rheological properties
were also investigated.
For the C16MPB/EAN mixture, the dynamic viscoelastic
behavior is shown in Figure 6. The values of the storage
modulus (G′) increase versus the frequency, with different slopes
Figure 8. Typical curves of storage moduli (G′) and loss moduli (G′′)
versus shear rate for the H phases in the C16MPB/EAN (a) and
16MPB/H O (b) mixtures at a frequency of 1.0 Hz.
1
C
2

Liquid Crystalline Phases of C16MPB
J. Phys. Chem. B, Vol. 114, No. 35, 2010 11387
TABLE 2: Rheological Parameters of H
1
Phases in the
G′′. The H
phase shows more elastic than viscous properties
throughout the whole frequency region. Therefore, the H phase
1
1
C
16MPB/EAN and C16MPB/H
2
O Systems at 25 °C
sample
exhibits elastic gel-like rheograms. In general, it is usual for
the lamellar phase to display such rheograms,51 while for the
(wt %)
σ
c
/Pa G′/Pa /Pa G′′/Pa tan δ
σ
c
H
1
phase the moduli G′ and G′′ increase versus the frequencies
with different slopes, as shown in Figure 6. However, in the
16MPB/H O system, the H phase shows clear elastic gel-like
behavior, not following the classical model of the H
C
16MPB/EAN (298 K)
55
1048
18265 980.0 18457
1.01
0.843
1.00
6
6
0
5
392.0 37484 370.0 31609
790.0 39919 1141
245.8 5649 368.7
180.5 3768 265.3
312.3 7647 470.2 1096
588.0 10990 879.8 1635
765.9 14362 1172
1242
2015
2449
2724
39919
753.4 0.130
625.5 0.166
C
2
1
C16MPB/H
2
O (298 K)
35
52-54
1
phase.
4
4
5
5
6
6
7
7
0
5
0
5
0
5
0
5
0.143
0.149
0.152
0.170
0.134
0.249
0.266
This may be due to the strong interactions among the cylinder-
like aggregates, which help to form the network structure of
the H
In addition, through stress sweep measurements, we can
further understand the structures of the H phase constructed
2184
4263
5765
3706
15334
1
phase.
25098 2133
43124 3424
14894 2880
57565 2848
1
by C16MPB dissolved in EAN and water. Figure 8a shows the
typical curves of G′ and G′′ as a function of the stress at a
constant frequency (1.0 Hz) for the C16MPB/EAN mixture. The
moduli G′ and G′′ are nearly the same in the linear viscoelastic
G′′ remain stable in the frequency region and are nearly
frequency-independent. The modulus G′ is always higher than
Figure 9. POM images for the sample with 60 wt % C16MPB in the C16MPB/H
corresponding DSC curves (c). The scale bar is 200 µm.
2
O system at different temperatures, 25 (a) and 70 °C (b), and the
Figure 10. POM images for the sample with 60 wt % C16MPB in the C16MPB/EAN system at 65 °C (a) (after cooling from the liquid phase) and
05 °C (b) and their corresponding DSC curve (c). POM image of the sample with 90 wt % C16MPB at 158 °C (d). The scale bar is 200 µm.
1

1
1388 J. Phys. Chem. B, Vol. 114, No. 35, 2010
Zhao et al.
region, which means that the elastic properties are nearly equal
to the viscous properties. This is also in accordance with the
results listed in Table 2, in which the values of tan δ (Supporting
thermal stability. This may inspire us to design LC samples
with higher thermal stability and other functionality in ionic
liquids, thus enriching the phase behavior of lyotropic LC phases
and opening wide potential applications in catalysis and
nanomaterial synthesis.
Information) are nearly equal to 1. For the C16MPB/H
2
O system,
the typical curves are shown in Figure 8b. The modulus G′ is
always larger than G′′ in the linear viscoelastic region. The H
1
phase exhibits more elastic than viscous behavior and is also
Acknowledgment. The work was supported by the National
Natural Science Foundation of China (Nos. 20773081, 50972080),
National Basic Research Program (2007CB808004, 2009CB-
930101), and Laboratory of Organic Optoelectronic Functional
Materials and Molecular Engineering, TIPC, CAS. The authors
thank Dr. Pamela Holt for proofreading the manuscript.
in agreement with the values of tan δ listed in Table 2. The
difference of tan δ between these two H
the distinct differences between EAN and H
Ludwig studied the hydrogen bonding in EAN and H
They found that the tetrahedral H-bond network only existed
in H O, while three-dimensional network structures appeared
in EAN. Thus, the structural differences between EAN and H
may result in the rheological difference in the two systems.
1
phases may be due to
2
O. Fumino and
4
5,46
2
O.
2
Supporting Information Available: The theory for calcula-
tion of structural parameters of the liquid crystalline phase and
2
O
the calculations of a and tan δ. This material is available free
s
On the basis of the above results, we can see that the
rheological properties are different due to the different solvents.
In addition, through simple comparison, the modulus G′ in the
of charge via the Internet at http://pubs.acs.org.
C
C
the H
16MPB/EAN mixture is always smaller than that in the
16MPB/H O system, suggesting that the network strength of
phase in H O is larger than that in EAN. In accordance
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(

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