Nonaqueous lyotropic liquid-crystalline phases formed by gemini surfactants in a protic ionic liquid

Article abstract of DOI:10.1021/la204489v

The aggregation behaviors of three Gemini surfactants [(C_sH _2s-α,ω-(Me₂N⁺C_mH _2m+1Br^-)₂, s = 2, m = 10, 12, 14] in a protic ionic liquid, ethylammonium nitrate (EAN), have been investigated. The polarized optical microscopy and small-angle X-ray scattering (SAXS) measurements are used to explore the lyotropic liquid crystal (LLC) formation. Compared to the LLCs formed in aqueous environment, the normal hexagonal and lamellar phases disappear. However, with increasing the surfactant concentration, a new reverse hexagonal phase (H_II) can be mapped over a large temperature range except for other ordered aggregates including the isotropic solution phase and a two-phase coexistence region. The structural parameters of the H_II are calculated from the corresponding SAXS patterns, showing the influence of surfactant amount, alkyl chain length, and temperature. Meanwhile, the rheological profiles indicate a typical Maxwell behavior of the LLC phases formed in EAN.

Full text of DOI:10.1021/la204489v

Article
pubs.acs.org/Langmuir
Nonaqueous Lyotropic Liquid-Crystalline Phases Formed by Gemini
Surfactants in a Protic Ionic Liquid
†
,†
†
†
†
‡
Xudong Wang, Xiao Chen,* Yurong Zhao, Xiu Yue, Qiuhong Li, and Zhihong Li
†Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, China
‡Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China
*
S Supporting Information
ABSTRACT: The aggregation behaviors of three Gemini
+
−
surfactants [(C H -α,ω-(Me N C H Br ) , s = 2, m = 10,
s
2s
2
m
2m+1
2
1
2, 14] in a protic ionic liquid, ethylammonium nitrate (EAN),
have been investigated. The polarized optical microscopy and
small-angle X-ray scattering (SAXS) measurements are used to
explore the lyotropic liquid crystal (LLC) formation.
Compared to the LLCs formed in aqueous environment, the
normal hexagonal and lamellar phases disappear. However, with increasing the surfactant concentration, a new reverse hexagonal
phase (H ) can be mapped over a large temperature range except for other ordered aggregates including the isotropic solution
II
phase and a two-phase coexistence region. The structural parameters of the H are calculated from the corresponding SAXS
II
patterns, showing the influence of surfactant amount, alkyl chain length, and temperature. Meanwhile, the rheological profiles
indicate a typical Maxwell behavior of the LLC phases formed in EAN.
1
7,18
1
. INTRODUCTION
biomembranes.
A nematic phase at the boundary between
the micelle (L ) and normal hexagonal (H ) phases in the 12-2-
12/water binary system was observed, whereas a bicontinuous
cubic phase at high temperature in the 12-3-12/water system
was formed. The concentration range for the lyotropic
mesophases in 12-s-12/water binary system contracts as s is
increased and completely disappears for s = 10 or 12. By
increasing the alkyl chains, a richer lyotropic behavior for 15-s-
5 than 12-s-12 in water was mapped, due to the stronger
The Gemini (dimeric) surfactants are made up of two
1
I
1
,2
surfactant-like moieties connected by a spacer group. Much
interest has been generated for their lower critical micelle
concentration, higher surface activity, lower Krafft temperature,
better wetting ability, and richer aggregate structures, compared
to those properties of the corresponding monomeric counter-
parts. The structural character also makes it possible to greatly
modify their aggregation behaviors in aqueous solution by
modulating the length and the nature of both the spacer group
and the alkyl chains.
the Gemini surfactants have been attracting great attention in
both academic and industrial fields and widely used in various
areas, such as food industry, gene and drug delivery, synthesis
of nanostructured materials, phase transfer catalysts, and oil
15
11
16
1
hydrophobic effect. It was also reported that the formation of
lyotropic phases for the m-6-m (m = 8, 10, 12)/water system
was determined by the concentration, temperature, and length
3
−5
Because of their excellent properties,
19
of alkyl chains.
3
6
As we know, the solvent is another important factor to
govern the phase behavior except for the amphiphile molecular
7
8
20
9
structure. Exploration on the aggregation behaviors of the
Gemini surfactant in nonaqueous solvents may prvode a better
understanding of amphiphile self-assembly. Furthermore, such
studies are also useful to improve the low thermodynamic
stability of LLCs constructed in aqueous media. This is because
the water evaporation at high temperature can destroy the
structure of LLCs easily and limit their applications. Therefore,
searching the suitable nonaqueous solvent may be an effective
way to fabricate LLCs stable at high temperature, which is
significant to extend the applications of both LLCs and Gemini
surfactants.
recovery.
Among so many Gemini surfactants, the cationic quaternary
ammonium salt type molecules deserve to be mentioned for
their interesting biological properties. Zana and co-workers
have carried out many pioneering works on the synthesis of
such kinds of Gemini surfactants with a general formula C H -
α,ω-(Me N C H Br ) , referred to as m-s-m, and on the
aggregate formation in water.
the lyotropic mesophase behaviors of them have been
reviewed. Becasue they are amphiphilic molecules with
special structures, the Gemini surfactants have the ability to
create organized structures including micelles, microemul-
sions, vesicles, and lyotropic liquid crystals (LLCs) by
self-assembling themselves either at surfaces or in bulk
solutions. Thus, the LLC phases have been well documented
and widely used as templates or media for chemical reactions
and materials synthesis or as models to mimic the
6
s
2s
+
−
2
m
2m+1
2
1
0,11
Both the thermotropic and
1
2
13
As a novel nonaqueous media, ionic liquids (ILs) have
currently attracted much attention in the areas of organic
1
4
15
16
Received: November 14, 2011
Revised: December 20, 2011
Published: December 21, 2011
©
2011 American Chemical Society
2476
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Article
synthesis and catalysis, biochemical engineering, materials
science, electrochemistry, carbohydrate chemistry, inorganic
21−26
nanomaterials, and separation techniques
for their unique
characteristics of low melting temperature, negligible vapor
pressure, wide electrochemical window, nonflammability, good
catalytic properties, high thermal stability, and ionic con-
Figure 1. Structures of the studied m-2-m type Gemini surfactants (m
= 10, 12, 14).
1
2
1,27
vacuum conditions for 48 h. The product purity was ascertained by H
ductivity.
Ionic liquids can be generally divided into two
NMR (nuclear magnetic resonance) spectrum in CDCl3 and the
broad categories: the protic (PILs) and the aprotic (AILs).
Because of the presence of the proton donor and acceptor sites,
the PILs can form a hydrogen-bonded network which makes
elementary analysis. The ionic liquid EAN was prepared according to
4
2
the procedures reported previously. In a typical synthesis, a portion
of ∼3 M nitric acid was slowly added to the ethylamine solution while
stirring and cooling in an ice bath. The water in product solution was
first removed with a rotary evaporator and then with a lyophilizer
28
them good candidates as nonaqueous self-assembling media
and makes it possible to design LLCs with high thermo-
stabililty.
(
MartinchristerALPHA1-2). The residual water content of the final
product was determined by Karl Fischer titration to be 0.5 wt %, and
The research on self-assemblies in PILs primarily started with
its melting point was about 12 °C. The purity of EAN was ascertained
29
ethylammonium nitrate (EAN), the first truly protic ionic
liquid showing more analogous properties than other PILs to
1
by the H NMR spectrum in D O.
2
2.2. Sample Preparation and Phase Diagram Mapping. The
3
0,31
water.
Evans and co-workers reported the lamellar liquid
process for mapping the phase diagram has been described
32,33
39,43
crystals formation by lipids in EAN.
The nonionic
elsewhere.
All samples were prepared by mixing the m-2-m
polyoxyethylene alkyl ether surfactants were found by Warr
et al. to form LLCs easily in EAN due to the hydrogen-
surfactants and EAN with designed compositions (in weight percent,
wt %, thereinafter). These mixtures were homogenized by repeat
mixing and centrifugation. Then they were equilibrated for at least 3
months before further investigations. The phase change was detected
by ocular observation and visual inspection through the crossed
polarizers. The liquid crystal types were determined by the polarized
optical microscopy and small-angle X-ray scattering techniques. The
composition interval was first selected as 5% for a rough mapping and
then 2% for the determination of the phase boundaries.
2.3. Characterization. 2.3.1. Small-Angle X-ray Scattering.
The obtained LC phases were characterized by an HMBG-SAX X-ray
small-angle scattering system (Austria) with a Ni-filtered Cu Kα
radiation (0.154 nm) operating at 50 kV and 40 mA. The distance
between the sample and detector was 27.8 cm. For comparison, one
sample exhibiting the two-phase coexistence was determined with the
SAXS beamline (1W2A) of the Beijing Synchrotron Radiation Facility
3
4
bonding. Drummond et al. have compared the solvophobic
effect dominated LLCs formation by hexadecyltrimethyl-
35
ammonium bromide (CTAB) in 20 PILs. They also observed
the LLCs formed by a monoolein and a phytantriol in a series
36
of PILs, including EAN. The nonionic block copolymer
EO PO EO (P123) can also exhibit liquid crystal behaviors
20
70
20
37
in EAN as studied from our group. The normal micellar
cubic, hexagonal, lamellar, and reverse bicontinuous cubic
phases have been identified. We have also investigated the
aggregation behaviors of a nonionic oleyl polyoxyethylene (10)
ether (Brij 97) and a cationic 1-hexadecyl-3-methylimidazolium
38,39
chloride (C mimCl) in EAN.
Only a hexagonal phase in
16
the Brij 97/EAN system was mapped, but a series of ordered
aggregates including micelles, normal hexagonal, lamellar, and
reverse bicontinuous cubic liquid crystalline phases could be
observed in C mimCl/EAN mixture.
(
BSRF) in China under the experimental conditions of 0.154 nm
incident X-ray wavelength, 1850 mm distance between the sample
chamber and the detector, and a 400 s data accumulation time. Except
for the enhanced intensity, no major difference in the scattering
profiles could be found between these two X-ray sources.
16
Motivated by these studies, it is natural to consider the phase
behaviors of Gemini surfactants in PILs, expecting for the
special aggregate structures and eminent properties. Unfortu-
nately, such research is rarely reported. To our best knowledge,
only Gin’s group reported that the LLCs could be formed by
the imidazolium-based Gemini surfactants in ionic liquids with
2
.3.2. Polarized Optical Microscopy. Photographs of samples with
birefringence were taken by a Motic B2 polarizing optical microscope
POM) with a CCD camera (Panasonic Super Dynamic II WV-
CP460).
(
2
.3.3. Rheological Measurement. The rheological measurements
were carried out with a Haake Rheostress 6000 rheometer using a
Rotor C35/1 system at 40.0 ± 0.1 °C. Frequency sweep measure-
ments were performed in the linear viscoelastic region which was
determined from the strain sweep measurement with the stress varying
at a constant frequency of 1.0 Hz.
−
−
the [BF ] anion. The reason is that BF can form hydrogen
4
4
bonds in all directions with the protons present on imidazolium
4
0
cations. To explore more about the aggregation properties of
Gemini surfactant in PILs, we here choose the Gemini
molecules of the well-known type of m-2-m (m = 10, 12, 14)
to investigate their phase behaviors in EAN over a whole
concentration range. The effects of Gemini molecule amount,
temperature, and alkyl chain length on the formed LLC
structures and thermodynamic stability are clarified. The
obtained results could be expected to not only provide us
further knowledge on the self-assembled structures and
properties of Gemini surfactants in nonaqueous solvents but
also extend the application fields of LLCs.
3. RESULTS AND DISCUSSION
EAN molecules can form a three-dimensional hydrogen-bond
network, which seemed to be an essential feature in its self-
31,44−46
assembly.
The aggregation of amphiphiles in EAN has
been interpreted in terms of a solvophobic driving force
analogous to the hydrophobic effect in water, which is
35,47
dominated by the entropic contribution.
There is a strong
solvent−solvent interaction between EAN molecules and a
weak van der Waals bonding between the EAN molecules and
the amphiphile hydrocarbon chains. Upon the amphiphile
molecules aggregation, the EAN-hydrocarbon interaction is
reduced and the solvent is released from the ordered structure,
resulting in an increase in entropy. The EAN mediated Gemini
molecules self-assembly here should progress in a similar
manner.
2. EXPERIMENTAL SECTION
2.1. Materials. The structures of studied m-2-m type Gemini
surfactants are shown in Figure 1. They were prepared according to
1
0,41
the procedures reported previously.
N,N,N′,N′-Tetramethylethyle-
nediamine (20 mmol) and the alkyl bromide (5 equiv) were refluxed
in dry EtOAc or CH CN (250 mL) for 2 days. After evaporation, the
3
residue was recrystallized for at least three times and then dried under
2
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The phase behaviors of m-2-m type Gemini molecules in
binary systems with the ionic liquid EAN have been
investigated. On the basis of the POM textures and the
SAXS patterns, their phase diagrams, especially the lyotropic
liquid crystal phase regions, were identified.
sample at 25 °C, which are correlated with the H phase
characteristic relative positions of 1:√3:2, (i.e., d100, d110,
49
d200, ...). When the 12-2-12 concentration locates between
64 and 70%, a two-phase coexisting region appears as indicated
by the POM image and SAXS pattern as shown in Figure 3b. A
noncontinuous bright striated texture (Figure 3b, top), like a
diluted fanlike one, can be seen, and their SAXS curves behave
mostly like that for the micelle solution except with a small
sharp peak overlapped (Figure 3b, bottom). The q value of this
peak is 2.18 nm− and very close to that of the first peak for the
H phase. The reliability of such a small peak has also been
shown by the synchrotron SAXS measurement in BSRF as seen
in the Supporting Information. Therefore, in this concentration
range, it is reasonable to consider that the coexisting phases
3
.1. Phase Diagrams of the 12-2-12/EAN Binary
System. The mapped temperature−composition (T−X)
phase diagram of 12-2-12/EAN binary system is shown in
Figure 2. The identified phases include the isotropic solution
1
belong to L and H.
1
As for the formed H phase, it is necessary to clarify its
structural model: whether the surfactant alkyl chains in the
cylinder will stretch inside (normal hexagonal phase: H ) or
I
11
outside (reverse hexagonal phase: HII). Usually, the H and
I
H phases appear respectively on the solvent-rich and solvent-
II
50
poor sides of the lamellar (Lα) phase. However, in our
studied system now, no Lα phase is identified.
To solve this problem, the structural parameters of the
formed H phase are analyzed based on the SAXS pattern shown
in Figure 3c. The lattice parameter, D (i.e., the distance
between two centers of the neighboring cylinders), can be
calculated from the scattering factor corresponding to the first
Figure 2. Phase diagram of 12-2-12/EAN binary system. L and HII
denote respectively the normal micelle solution, and the reverse
hexagonal phase.
1
Bragg peak with the formula of 4π/√3q , and the calculated D
1
value is 34.7 Å for 71.0% 12-2-12 concentration at 25 °C. From
the D value, the area per molecule of amphiphile at the
(
L1), the two-phase coexisting region and the reverse hexagonal
(
H ). The system behaves as a homogeneous, transparent, and
II
hydrophilic/hydrophobic interface (S) for an assumed H phase
I
isotropic solution at lower 12-2-12 concentrations (less than
4%). This phase is considered to be a normal micelle solution
L ) because only one weak and broad scattering peak was
(
S ) can be obtained according to eqs S1−S2 (Supporting
I
6
2
Information), which is 111.6 Å for the sample of 71.0% 12-2-
(
1
1
2 concentration at 25 °C. In addition, the effective headgroup
48
observed in its SAXS curve (Figure 3a, bottom). When the
2-2-12 concentration is between 70 and 80%, a large area of
area (a ) of the 12-2-12 molecule at the critical micelle
concentration in EAN (25 °C) is about 81.8 Å , which is
obtained from its adsorption at the air/EAN interface
0
1
2
the hexagonal (H) phase is found, as confirmed by the typical
fanlike texture under POM observation and the SAXS pattern
(
Supporting Information). Usually, the S value decreases with
12,20,51
(
Figure 3c, top and bottom). The scattering factor (q) values
increasing the surfactant concentration.
However, the
corresponding to the first, second, and third Bragg peaks are
respectively 2.09, 3.60, and 4.16 nm for the 71.0% 12-2-12
value of S is bigger than that of a , which exclude the possibility
for a normal hexagonal phase. Then, such S data for an assumed
I
0
−1
Figure 3. POM images (top) and SAXS curves (bottom) at 25 °C for different 12-2-12 concentrations (wt %): (a) 58.8, (b) 65.4, and (c) 71.0.
2
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HII lyotropic phase (S ) and other structural parameters
according to eqs S3−S4 (Supporting Information) are
calculated and listed in Table 1. It is an intuitive judgment
CPP > 1. For a straight alkyl chain, the length l and volume v
II
c
of the solvophobic part can be obtained from the following
5
3,54
Tanford equations:
v = (27.4 + 26.9N) Å3
(1)
(2)
Table 1. Structural Parameters of H Phases in the 12-2-12/
EAN System
II
a
lc = (1.5 + 1.265N) Å
temperature 12-2-12 concentration (ω,
RII
dII
(Å)
S
II
2
where N represents the number of carbon atoms of the alkyl
chain which is incorporated in the hydrophobic core. The value
of v obtained from the formula above is usually for amphiphiles
with a single hydrocarbon chain. For the Gemini molecules
(
°C)
wt %)
D (Å) (Å)
(Å )
6
6
6
5
4
2
0
0
0
0
0
5
78.5
73.5
71.0
71.0
71.0
71.0
33.9
33.3
32.9
33.3
34.1
34.7
8.5
9.3
16.9
14.7
13.7
13.9
14.3
14.5
58.9
71.8
78.9
77.7
75.4
74.5
9.6
5
3
studied here, it is just enlarged by a factor of 2.
9.7
With the molecules of 12-2-12, the values of v and l are
c
9.9
3
calculated to be 700.4 Å and 16.68 Å, respectively. The areas a
0
10.1
obtained from the adsorption of 12-2-12 at the air/solvent
interface are respectively 102 (at air/H O interface) and 81.8
Å (at air/EAN interface). Such a reduction of a in EAN is due
a
RII is the radius of the reversed cylinder-like aggregates; dII is the
55
2
thickness of hydrocarbon domains.
2
0
to its ionic nature, which causes a large degree of charge
screening and therefore leads to a more densely packing of the
Gemini head groups. Thus, the CPP values of 12-2-12 are
that the formed hexagonal phase should be a reverse one
2
because the data of S (74.5 Å , 71.0%, 25 °C) is smaller than
36
II
2
that of a (81.8 Å ).
0
calculated as 0.41 in H O and 0.51 in EAN, which therefore
2
To understand more about the EAN solvent effects, it is
better to compare them with the phase behavior of 12-2-12/
H O binary system. As shown in Figure 4, compared to the
2
explains why the normal hexagonal and lamellar phases could
be obtained in the 12-2-12/H O system but the normal
2
hexagonal phase disappears in 12-2-12/EAN mixtures.
For the same reason, the EAN molecules can easily partition
into the interface regions with the 12-2-12 molecules due to the
36
charge screening. The ethyl group of EAN molecules here
may squeeze into the 12-2-12 aggregates, which causes a net
increase in the average volume of the 12-2-12 and thus an
increase in the CPP. The affinity for solvent of the alkyl chains
of Gemini molecules is also different. The higher affinity for
EAN than for water might increase the volume of the
3
6,37,39
solvophobic part,
which also leads to a higher CPP and
then results in the formation of a reverse phase. Moreover, the
formation of a reverse phase has been confirmed by the
calculated molecular packing parameter (MPP= V/al > 1)
based on the structure parameters of a hexagonal phase (71.0%,
2
5 °C). Such a value of MPP is 1.30 calculated with V = 700.4,
the real headgroup area (a = S ), and the length of the
II
solvophobic part (l = d /2) adopted from Table 1. All of these
II
local molecular packing changes lead to a different phase
behavior between EAN and water.
Figure 4. Phase diagrams for 12-2-12 binary systems with H O and
2
EAN at 40 °C. Symbol notification: L (normal micelle solution), HI
1
3
.2. Effect of 12-2-12 Concentration on Reverse
(normal hexagonal phase), Lα (lamellar phase), and HII (reverse
Hexagonal Liquid Crystals. The influences of the 12-2-12
concentration on the reverse hexagonal liquid crystal have been
also explored. Because the phase region is narrow at 25 °C,
such an effect is investigated at a higher temperature of 60 °C.
Figure 5 shows the SAXS patterns of the HII phase
constructed at three concentrations of 12-2-12 in EAN at 60
hexagonal phase).
12
phase diagram of the 12-2-12/H O system, the H and Lα
2
I
phases are found to disappear with a H phase instead in the
II
12-2-12/EAN system. It is known that the solvophobic effect is
the main driving force to decide whether or not a surfactant
could form certain self-assembly. Meanwhile, for a specific
amphiphile molecule, its possibly formed liquid crystalline
mesophases are dependent upon both the local molecular and
°
C. As described above, the first scattering peak in the SAXS
profile originates from the Bragg reflection of the (100) plane,
which can help us to obtain the lattice parameter D. The HII
phase consists of infinitely long cylinder-like aggregates packed
in a hexagonal array and separated by a continuous hydro-
3
5,36
the global aggregate packing constraints.
Usually, the
critical packing parameter (CPP) is adopted as a powerful
and simple tool to interpret the possible self-assembled
51
carbon domains, where the solvophobic tails are located
outside the cylinders, and the solvophilic heads are solvated by
structures. CPP is defined as v/(a l ), where v, a , and l are
0
c
0
c
EAN molecules. According to the model of the H phase, D
II
the average volume of the amphiphile, the effective headgroup
can be divided into two parts: the twice radius of the EAN core
52
area, and the effective length in the molten state, respectively.
in cylinder-like aggregate (R ) and the thickness of the outside
II
The general relationships between the organized aggregates and
CPP are as follows: the spherical micelles for CPP < 1/3, the
rod shaped micelles for 1/3 < CPP < 1/2, the bilayers and
vesicles for 1/2 < CPP < 1, and the reverse structures when
hydrocarbon domains (d ) in between, They are in accord with
the following equation:
II
D = 2R + d
(3)
II
II
2
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(
Figure 6a ). When the concentration is increased to 78.5%, the
2
LLC phase structure could survive to a temperature of 110 °C,
as shown in Figure 6c . All of these results denote a more
1
densely and ordered molecular packing in the H phase at
II
higher 12-2-12 content. A detailed discussion about the
temperature effect on the reverse hexagonal phase is shown
in the following text.
3
.3. Temperature Effect on Reverse Hexagonal Liquid
Crystals. To explore the relationship between the molecule
arrangement and the thermal stability of the formed reverse
hexagonal mesophase (H ), the effect of temperature is
II
investigated. From the phase diagram shown in Figure 2, the
region of H can be seen to cover the 12-2-12 concentration
II
range from 70 to 80% and may go through a temperature range
from 25 to 115 °C. By increasing the temperature for a typical
sample at 71.0% 12-2-12 concentration, the gradual changes of
fanlike textures can be seen in Figure 7. The typical fan-like
textures are clearly present at 25−60 °C (Figure 7a−d). At
higher temperatures, such textures become gradually dimmed
or broken and finally lose any birefringence when it is turned
into an isotropic phase (Figure 7e−g). The SAXS patterns for
the samples with the typical hexagonal phase texture at four
different temperatures (25, 40, 50, and 60 °C) are shown in
Figure 8. Three Bragg peaks with the relative positions
corresponding to a ratio of 1:√3:2 indicate definitely a reverse
hexagonal structure. Besides, increasing temperature leads the
first SAXS peak position to a higher q value, corresponding to a
smaller repeat lattice spacing. The corresponding phase
structural parameters are also calculated and listed in Table 1.
The reduction of the repeat lattice spacing can be considered
from the temperature induced conformational fluctuation of the
hydrophobic chains. The increased fluctuations at higher
temperatures may shorten the average length of the alkyl
chain and lead to the decrease of repeat lattice spacing.
Meanwhile, large conformational fluctuations of the hydro-
phobic chains may also expand the average chain volume. Both
factors may result in a thinner hydrocarbon domain with a
Figure 5. SAXS curves for the 12-2-12/EAN system as a function of
concentration at 60 °C. Insets are the corresponding POM images.
The measured D values are 32.9, 33.3, and 33.9 Å respectively
for 71.0, 73.5, and 78.5% 12-2-12 concentrations, indicating a
gradual increase with the concentration. From these D values,
more structural parameters can be obtained according to eqs
S3−S4 (Supporting Information), the values which are listed in
Table 1. With the concentration of 12-2-12 increasing from
71.0 to 78.5%, the RII value decreases from 9.6 to 8.5 Å,
whereas the d increases from 13.7 to 16.9 Å at 60 °C. Both
II
results suggest that the radius of the EAN solvent core inside
the cylinder becomes contracted in the case of more 12-2-12,
whereas the hydrocarbon layer of the cylinder expands.
Furthermore, the area per molecule of 12-2-12 at the
hydrophilic/hydrophobic interface in the reverse hexagonal
2
phase, S , decreases from 74.9 to 58.7 Å with the 12-2-12
II
amount increased reflecting a more dense packing. This can be
supported by the fanlike texture in the images under the POM,
which become more and more perfect upon adding more 12-2-
12 as shown in the insets of Figure 5. Such a concentration
effect is obviously due to the enhanced solvophobic effect.
Meanwhile, the thermal stability of this LLC phase is
improved with more 12-2-12 as observed in Figure 6. The
56
higher inverse curvature in the HII phase. In addition, the
breakage of hydrogen bonds in solvent with temperature
increasing will also make EAN molecules more contracted. It
has been suggested that the curvature of the phase structure is
collapse temperature of the H phase at 71.0% 12-2-12 is about
II
71 °C based on the POM observation with the heating stage
Figure 6. POM images of the 12-2-12/EAN system for 12-2-12 concentrations of 71.0% (a), 73.5% (b), and 78.5% (c) at different temperatures
°C). a : 60; a : 71; b : 100; b : 103; c : 107; c : 109.
(
1
2
1
2
1
2
2
480
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Figure 7. Gradual deteriorated fanlike textures for a typical 12-2-12/EAN sample with 12-2-12 concentration of 71.0% at different temperatures
°C). a, 25; b, 40; c, 50; d, 60; e, 65; f, 71; g, 73.
(
It should be mentioned that the lyotropic phases constructed
in EAN exhibit better thermal stability because of the ionic
liquid nature of EAN. The LLC from 12-2-12/EAN can exist
stably at 60 °C for at least 6 months with the structural
parameters little changed. However, the LLCs from the 12-2-
1
2/water system can not be kept stably for long time at 60 °C.
Only after several days, the water may escape from the
assembly, which will destroy the LLC structure.
3
.4. Effect of the Alkyl Chain Length on Reverse
Hexagonal Liquid Crystals. With the same systematic phase
identification procedures as described in section 3.1, the other
two m-2-m/EAN systems (m = 10, 14) are found to exhibit a
relatively similar phase behavior as seen in Figure 9. Only the
reverse hexagonal LLC phase was identified. For the 10-2-10
system (Figure 9a) the H phase region extends from 71 to
II
9
1% amphiphile concentration at temperatures up to 85 °C.
Such a phase region for the 14-2-14 system (Figure 9b) starts at
a relatively low concentration (61%) and extends the
temperature maximum threshold to 135 °C. Their typical
SAXS patterns and POM textures are shown in Figure 10.
It is noted by comparison between Figures 2 and 9 that the
three phase diagrams exhibit some differences with the change
Figure 8. SAXS patterns for a 71.0% 12-2-12/EAN system at different
temperatures (°C). a, 25; b, 40; c, 50; d, 60.
defined by the solvent channel radius. The smaller of such radii
57
are, the highly curved phase structures could form. From
Table 1, it is clearly seen that the EAN channel radius (71.0%),
RII, decreases from 10.0 to 9.30 Å with the temperature
increasing from 25 to 60 °C, which means a larger curvature
should be obtained. Therefore, a correlation between the
preferred cylinder curvature and the temperature effect seems
reasonable: increasing temperature causes a higher curvature.
of the alkyl chain length. The thermal stabilities of H phases
II
are also enhanced with longer hydrocarbon chains because of
the strengthened solvophobic effect. As shown in Figure 11, the
highest temperatures when the typical POM birefringent
textures of H phase can be observed are proportional to the
II
alkyl chain length. It decreases from 135 °C for 14-2-14, to 110
Figure 9. Phase diagrams of two m-2-m/EAN binary systems, m = 10 (a) and 14(b). L and H denote respectively the normal micelle solution and
1
II
the reverse hexagonal phase.
2
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Figure 10. SAXS curves (a, b) and POM images (a , b ) for the reverse hexagonal phases of the m-2-m/EAN systems at 60 °C (a: 10-2-10, 86.0%
1
1
and b: 14-2-14, 67.5%).
Figure 11. POM images for the reverse hexagonal phases obtained in m-2-m/EAN systems at high temperatures (a: 14−2−14, 78.7%, 130 °C; b: 12-
-12, 78.5%, 105 °C; c: 10−2−10, 85.9%, 82 °C).
2
°C for 12-2-12, and 85 °C for 10-2-10. Such an effect is also
reflected from the decreased micelle phase region with longer
hydrocarbon chains. The highest m-2-m concentration
boundary of the micelle solution decreases from 66% for 10-
2
-10 to 63% for 12-2-12 and 57% for 14-2-14. Similar
19
phenomena have been also found in m-3-m/water systems.
In addition, from the SAXS patterns shown in Figure 12, it is
clear that the positions of the first peak shift left with longer
alkyl chain for similar concentrations at 60 °C, which represents
the larger lattice parameters (D) of the H phases obtained
II
from m-2-m/EAN system. This should be due to the extension
of the thickness of hydrocarbon domains, which provides the
possibility to control the dimensions of the H phases.
II
3
.5. Rheological Measurements. The viscoelasticity
properties of obtained H phase were characterized by the
II
rheology measurements. Because all of the H samples were
II
Figure 12. SAXS curves for the reverse hexagonal phases of m-2-m/
EAN systems at 60 °C (a: 10−2−10, 80.0%; b: 12-2-12, 78.5%; c: 14−
found to show similar rheological behaviors, the results from
the 12-2-12/EAN system are chosen here as an example. The
LLC phase constructed by the 12-2-12/water system at the
same concentration has also been investigated for comparison.
2
−14, 76.5%).
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The typical steady-shear rheograms (apparent viscosity as a
function of shear rate) are shown in Figure 13. At low shear
water shows bigger storage and loss moduli than those in EAN.
Such a difference may be due to the distinct solvent differences
between H O and EAN, i.e., a tetrahedral H-bond network
2
structure in H O and only a three-dimensional network
2
31,44−46
structure in EAN.
The Gemini molecules exhibit
therefore the stronger solvophobic effect in water than in
EAN, which leads to bigger moduli.
4
. CONCLUSIONS
In summary, the aggregation behaviors of a series of Gemini
surfactants (m-2-m) in EAN have been investigated. Compared
to water, Gemini molecules are expected to pack more densely
in EAN due to charge screening, which causes a higher CPP in
EAN, and thus, a new reverse hexagonal phase (H ) can be
II
formed in these systems. In addition, the thermal stability of the
obtained reverse hexagonal phase is enhanced with increasing
surfactant amount or longer alkyl chain. The obtained results
provide further knowledge on Gemini molecules self-assembly
in room-temperature ionic liquids. Considering its wide use in
Figure 13. Steady-shear rheological data collected at 40 ± 0.1 °C for
the 12-2-12/EAN and 12-2-12/water systems. The inset is a visual
inspection photo on three LLC samples at 12-2-12 concentrations of
18
61
various fields, such as drug delivery, food applications, and
62
material synthesis, the reverse hexagonal phase constructed
by m-2-m dissolved in EAN with high thermal stability could
expand the application area to nonaqueous and high temper-
ature conditions, which will be an important supplement to the
self-assembly behavior of a Gemini surfactant in nonaqueous
solutions.
7
1.0% and 73.5% in EAN and 73.5% in water, from left to right.
rates, the samples whether in EAN or in water exhibit high
apparent viscosity and look like gels, as seen by the inset image
of unflowable phase on the top of inverse tubes. However, from
the frequency sweep measurement data shown in Figure 14, the
ASSOCIATED CONTENT
Supporting Information
The theory for calculation of structural parameters of the liquid
■
*
S
crystalline phase and the calculations of a . This material is
0
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
E-mail: xchen@sdu.edu.cn. Tel: +86-531-88365420. Fax: +86-
31-88564464.
■
*
5
ACKNOWLEDGMENTS
■
We thank Prof. Zhonghua Wu and Dr. Zhihong Li for the kind
help with the synchrotron SAXS measurements. The financial
supports from the National Natural Science Foundation of
China (20773080, 20973104, and 21033005), Shandong
Provincial Science Fund (2009ZRB01147), and the SAXS
station with the Beijing Synchrotron Radiation Facility (BSRF)
in China are acknowledged.
Figure 14. Changes of storage modulus G′ (solid) and loss modulus
G″ (empty) versus frequency for the LLC phase of the 12-2-12/EAN
and 12-2-12/water systems at 40 °C.
dynamic moduli G′ and G″ are found to elevate with increasing
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