Thermotropic liquid crystals of a non-mesogenic group bearing surfactant-encapsulated polyoxometalate complexes

Article abstract of DOI:10.1021/la101928k

A tri(ethylene oxide)octadecyldimethylammonium p-toluenesulfonate (C ₁₈NEO₃Ts) amphiphile was employed to encapsulate Keggin-type polyoxometalates by ion metathesis reactions with phosphotungstic acid, silicotungstic acid, pentapotas

Full text of DOI:10.1021/la101928k

pubs.acs.org/Langmuir
© 2010 American Chemical Society
Thermotropic Liquid Crystals of a Non-Mesogenic Group Bearing
Surfactant-Encapsulated Polyoxometalate Complexes
Xiankun Lin, Wen Li, Jing Zhang, Hang Sun, Yi Yan, and Lixin Wu*
State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012,
People’s Republic of China
Received May 14, 2010. Revised Manuscript Received June 28, 2010
A tri(ethylene oxide)octadecyldimethylammonium p-toluenesulfonate (C18NEO3 Ts) amphiphile was employed to
3
encapsulate Keggin-type polyoxometalates by ion metathesis reactions with phosphotungstic acid, silicotungstic acid,
pentapotassium dodecatungstoborate(III), and phosphomolybdic acid, giving the surfactant-encapsulated polyoxo-
metalates (SEPs) in SEP-P, SEP-Si, SEP-B, and SEP-PMo, respectively. Meanwhile, a C18NEO3 PF6 amphiphile was
3
prepared by substituting the p-toluenesulfonate of C18NEO3 Ts with hexafluorophosphate. The chemical composition
3
of all SEPs and amphiphiles was characterized through 1H NMR, infrared spectroscopy, mass spectroscopy, elemental
analysis, and thermogravimetric analysis. The thermal properties of the SEPs and the amphiphiles were investigated by
differential scanning calorimetry, polarized optical microscopy, and variable-temperature X-ray diffraction. The types
of anions have a remarkable influence on the thermal properties of the prepared compounds or complexes. C18NEO3 Ts
3
displays a smectic A phase with low transition temperatures, and C18NEO3 PF6 is a near-room-temperature ionic
3
liquid. In contrast to the fact that SEP-Si and SEP-PMo decompose upon just reaching or before reaching the isotropic
liquid state, both SEP-P and SEP-B reveal smectic B phase structures. The present results indicate that the combination
of proper cationic amphiphiles and polyoxometalates can impart typical thermotropic liquid-crystalline behavior to
SEPs, although no mesogenic groups are introduced into the complex systems.
Introduction
hand, LCs can afford an ordered microenvironment for the
organization and arrangement of nanoobjects as well as the opti-
mization of manifold properties.16 On the other hand, the intro-
duced nanoobjects can provide new properties to LCs through
realizing the synergy with LCs toward functional soft materials.17
Among these numerous nanoobjects, a special one is metal
oxides or polyoxometalates (PMs). As a kind of multiply charged
nanoscale inorganic cluster, PMs possess structure and property
advantages as well as promising applications in many areas
including catalysis, optics, electronics, and magnetics and are
considered to be fascinating building blocks for supramolecular
architectures and functional hybrid materials.18-20 Pioneering
research results with the aim of incorporating PMs into thermo-
tropic liquid crystals through the encapsulation of PMs with
organic surfactants have been obtained in recent years. Among
the designed surfactant-encapsulated PM (SEP) building blocks
for LCs, modification of the surfactants with mesogenic groups
such as azobenzene or biphenyl groups or precursors for hydro-
gen-bonding dimers was found to be necessary to endow the
obtained SEPs with tunable thermotropic liquid-crystalline
properties.21-25
Liquid crystals (LCs), as a fascinating class of supramolecular
soft matter, have attracted considerable attention because of fun-
damental interest and potential applications. As a consequence of
possessing fluidity like that of isotropic liquids and anisotropic
structures like those of crystals, LCs are able to respond to ex-
ternal stimuli reversibly and exhibit functionalities such as aniso-
tropic optical, electrical, and conductive properties, leading to
various applications such as displays, ion-conductive materials,
and sensors.1-7 In recent years, one of the hot points in LC
research is organic-inorganic hybrid liquid crystals, especially in
the introduction of inorganic functional nanoobjects such as
gold,8-10 anisotropic TiO2,11 and Fe2O3 nanoparticles,12 silsesqui-
oxanes,13 and inorganic clusters14,15 into liquid crystals, owing to
the diverse advantages provided by such a combination. On one
*To whom correspondence should be addressed. E-mail: wulx@jlu.edu.cn.
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A. W.; Sia, S.; Cosquer, G.; Lee, S. E.; Raynes, E. P. Angew. Chem., Int. Ed. 2008,
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~
Langmuir 2010, 26(16), 13201–13209
Published on Web 07/15/2010
DOI: 10.1021/la101928k 13201

Article
Lin et al.
Scheme 1. Synthesis Route to C18NEO3 Ts and C18NEO3 PF6 and Polyhedron Structure of Keggin-Type PMs
3
3
A logic problem from a fundamental research point of view is
whether SEPs exhibiting typical thermotropic liquid-crystalline
behavior can be obtained through surfactants without mesogenic
groups because, in some cases, aromatic mesogenic groups have
intrinsic disadvantages. For example, they can disturb the specific
properties of the inorganic clusters, such as the luminescence of
PMs.25 To obtain functional SEPs with thermotropic LC beha-
vior, some efforts have been made by using simple surfactants
without mesogenic groups to coordinate with the PMs, but the
suggested LC behavioris oftennot typical and cannot beobserved
completely and clearly because of the decomposition of the
complexes before reaching melting or clearing points.26-28 Al-
though surfactants without mesogenic groups have been used to
assembly PMs into liquid-crystalline-like arrangements,28 the
SEPs possessing no mesogenic groups but exhibiting typical
thermotropic LC properties have not yet, to the best of our
knowledge, been reported.
Consequently, to endue SEPs with typical LC properties suc-
cessfully requires the elaborate design of surfactants, the suitable
selection of PMs, and the perfect combination. It is well known
that the lengths of alkyl chains, the types of ions, and the types of
heterocycles in ionic heads can affect the thermal properties of the
ionic liquid crystals.29,30 Here, we propose a new strategyto adjust
the thermal properties of SEPs through regarding them as a class
of special ionic complexes. Different from the common anions,
PM polyanions possess a nanoscaled size, which provides large
hydrophilic and ionic domains in the hydrophobic organized
structures. Therefore, it is possible to alter the thermal properties
of SEPs through introducing hydrophilic moieties (i.e., short
poly(ethylene oxide) chains) into the ionic domains. Compared
with the SEPs prepared using the simple surfactants, the intro-
duction of hydrophilic moieties will increase the relative volume
of hydrophilic domains in SEPs and could affect the packing
efficiency of ions, which may result in depressed transition tem-
peratures of SEPs. Moreover, SEPs could hold thermal stability
when reaching the isotropic liquid state, which will make it
possible for SEPs to show typical, reversible LC behavior without
employing mesogenic groups.
(TEO) chain and a relatively long alkyl chain, as shown in Scheme 1.
We expected that the TEO chain could serve as a hydrophilic
moiety to affect the self-organized properties and thermal beha-
vior of the obtained SEPs. The following results supported the
proposed motivation and confirmed some new understanding of
the PM-based LCs. Herein, we reported that certain SEPs, which
do not contain mesogenic groups or precursors for hydrogen-
bonding dimers, can maintain thermal stability when reaching the
isotopic liquid state and exhibit typical thermotropic liquid
crystals for the first time. We also found that introducing the
TEO chains into SEPs is an efficient strategy for influencing the
thermal properties of SEPs.
Experimental Section
Materials. Unless otherwise noted, the chemicals were all
obtained from commercial suppliers and used without furt-
her purification. Triethylene glycol monomethyl ether CH3-
(OCH2CH2)3OH with a purity of 97% was purchased from
Fluka, and N,N-dimethyloctadecylamine with a purity of 89%
and ammonium hexafluorophosphate (NH4PF6) with a purity of
99.5% were the product of Acros Organics. p-Toluenesulfonyl
chloride (TsCl), phosphotungstic acid (H3PW12O40, denoted as
HPW), and silicotungstic acid (H4SiW12O40, denoted as HSiW)
were purchased from Sinopharm Chemical Reagent Co. Ltd.
Phosphomolybdic acid (H3PMo12O40, denoted as HPMo) was
purchased from Alfa Aesar. Pentapotassium dodecatungsto-
borate(III) (K5BW12O40, denoted as KBW) was synthesized
according to a procedure in the literature.31
Synthesis of Oligo(oxyethylene)octadecyldimethylamm-
onium 4-Methylbenzenesulfonate (C18NEO3 Ts). The amp-
3
hiphilic diblock molecule C18NEO3 Ts was synthesized accord-
3
ing to a modified process.32 Beginning with the tosylation of
triethylene glycol monomethyl ether, the obtained product was
allowed to react with N,N-dimethyloctadecylamine, giving the
target product (Scheme 1).
To the mixture of CH3(OCH2CH2)3OH (8.24 g, 50.2 mmol)
and 16% sodium hydroxide aqueous solution (17 mL) in an
ice-salt bath was added dropwise the THF solution of TsCl
(11.97g, 62.8mmol) withcontinuousstirringfor7 h. The resulting
mixture wasstirred atroomtemperatureforanother27hand then
poured into ice water. The aqueous solution was extracted with
three portions of chloroform. The combined organic extract was
dried over anhydrous MgSO4, filtered, and concentrated under
reduced pressure to give product p-toluenesulfonyl tri(ethylene
oxide) monomethyl ether (EO3Ts) (15.40 g) in a yield of 96.8% as
a colorless liquid. 1H NMR (500 MHz, CDCl3, δ): 2.42 (s, 3H),
3.34(s, 3H), 3.59-3.49(m, 8H), 3.67(t, J = 5 Hz, 2H), 4.14(t, J =
5 Hz, 2H), 7.33 (d, J = 8 Hz, 2H), 7.78 (d, J = 8 Hz, 2H).
In this study, we designed and synthesized amphiphilic diblock
molecule C18NEO3 Ts, which bears a short tri(ethylene oxide)
3
(26) Zhang, T. R.; Spitz, C.; Antonietti, M.; Faul, C. F. J. Chem.;Eur. J. 2005,
11, 1001–1009.
(27) Zhang, T. R.; Liu, S. Q.; Kurth, D. G.; Faul, C. F. J. Adv. Funct. Mater.
2009, 19, 642–652.
(28) Polarz, S.; Smarsly, B.; Antonietti, M. ChemPhysChem 2001, 2, 457–461.
(29) Goossens, K.; Lava, K.; Nockemann, P.; Hecke, K. V.; Meervelt, L. V.;
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Driesen, K.; Gorller-Walrand, C.; Binnemans, K.; Cardinaels, T. Chem.;Eur. J.
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(31) Rocchiccioli-Deltcheff, C.; Fournier, M.; Franck, R.; Thouvenot, R. Inorg.
Chem. 1983, 22, 207–216.
(30) Goossens, K.; Lava, K.; Nockemann, P.; Hecke, K. V.; Meervelt, L. V.;
Pattison, P.; Binnemans, K.; Cardinaels, T. Langmuir 2009, 25, 5881–5897.
(32) Lin, X. K.; Wang, Y. L.; Wu, L. X. Langmuir 2009, 25, 6081–6087.
13202 DOI: 10.1021/la101928k
Langmuir 2010, 26(16), 13201–13209

Lin et al.
EO3Ts (6.52 g, 20.5 mmol) and N,N-dimethyloctadecylamine
(6.86 g, 20.5 mmol) were dissolved in anhydrous acetonitrile. The
mixture was refluxed for 27 h. Solvent was removed through
evaporation, and the crude product was purified by silica gel
column chromatography with chloroform/methanol (10:1 v/v) as
Article
initial molar ratio of C18NEO3 Ts to KBW was controlled at 5:1.
3
The complex was obtained as a sticky white material (0.19 g) in a
yield of 74.5%. 1H NMR (500 MHz, CDCl3, δ): 0.88 (t, J = 7 Hz,
3H), 1.26 (m, 30H), 1.77 (m, 2H), 3.40 (s, 6H), 3.36 (s, 3H), 3.46
(m, 2H), 3.54-3.72(m, 8H), 3.94 (m, 2H), 4.27(m, 2H). IR(KBr):
3442, 2956, 2923, 2852, 1468, 1353, 1293, 1260, 1201, 1107, 951,
899, 816 cm-1. Anal. Calcd for SEP-B (C135H290N5O55BW12,
5080.64): C, 31.91; H, 5.75; N, 1.38. Found: C, 32.41; H, 5.70;
N, 1.15. SEP-B should correspond to the formula (C18NEO3)5-
BW12O40.
the eluent to givethe compound C18NEO3 Ts (3.80 g, 30.1%) as a
3
white solid. 1H NMR (500 MHz, CDCl3, δ): 7.78 (d, J = 8 Hz,
2H), 7.15 (d, J = 8 Hz, 2H), 3.96 (t, J = 5 Hz, 2H), 3.82 (t, J =
5 Hz, 2H), 3.64-3.50 (m, 8H), 3.46 (m, 2H), 3.35 (s, 3H), 3.30 (s,
6H), 2.33 (s, 3H), 1.70 (m, 2H), 1.26 (m, 30H), 0.88 (t, J = 7 Hz,
3H). IR (KBr): 3467, 3031, 2954, 2923, 2852, 1635, 1468, 1353,
1303, 1196, 1122, 1035, 1014, 816, 717, 681 cm-1. Matrix-assisted
laser desorption/ionization time-of-flight mass spectrometry
(MALDI-TOF MS) (m/z): 444.4 [M - Ts]þ, 171.0 [Ts]-.
SEP-PMo. This compound complex was prepared by follow-
ing a procedure similar to that for SEP-P by using HPMo instead.
The initial molar ratio of C18NEO3 Ts to HPMo was controlled
3
at 3:1. The complex was obtained as a yellow needlelike material
(0.15 g) in a yield of 67.8%. 1H NMR (500 MHz, CDCl3, δ): 0.88
(t, J = 7 Hz, 3H), 1.26 (m, 30H), 1.82(m, 2H), 3.34(s, 6H), 3.38(s,
3H), 3.46 (m, 2H), 3.55-3.73 (m, 10H), 4.13 (m, 2H). IR (KBr):
3427, 2956, 2923, 2852, 1467, 1352, 1296, 1261, 1199, 1107, 1062,
Synthesis of Oligo(oxyethylene)octadecyldimethylamm-
onium Hexafluorophosphate (C18NEO3 PF6). The diblock
3
molecule C18NEO3 PF6 was prepared through a metathesis re-
3
action in water; that is, the p-toluenesulfonate in C18NEO3 Ts
was exchanged for hexafluorophosphate (Scheme 1).
C18NEO3 Ts (0.28 g, 0.46 mmol) in water (5 mL) was stirred at
3
955, 879, 801 cm-1. Anal. Calcd for SEP-PMo (C81H174
-
N3O49PMo12, 3156.49): C, 30.82; H, 5.56; N, 1.33. Found: C,
30.56; H, 5.46; N, 1.16. SEP-PMo should correspond to the
formula (C18NEO3)3PMo12O40.
3
room temperature, and then NH4PF6 (0.39 g, 2.39 mmol) dis-
solved in water (10 mL) was added dropwise. A white precipitate
formed immediately, and the reaction mixture was stirred for
30 min. The sticky precipitate was separated by pouring out the
supernatant, and thentheprecipitate wasdissolved inchloroform.
The organic phase was washed with three portions of water, dried
over anhydrous MgSO4, filtered, and concentrated under reduced
pressure to give the product as a white solid (0.23 g) in a yield of
85.4%. 1H NMR (500 MHz, CDCl3, δ): 3.93 (t, 2H), 3.68-3.51
(m, 10H), 3.36 (s, 3H), 3.34 (m, 2H), 3.14 (s, 6H), 1.72 (m, 2H),
1.26 (m, 30H), 0.88 (t, J = 7 Hz, 3H). IR (KBr): 3426, 2953, 2919,
2851, 1623, 1471, 1355, 1299, 1252, 1200, 1122, 1029, 835, 721
cm-1. MALDI-TOF MS (m/z): 444.4 [M - PF6]þ, 145.0 [PF6]-.
Characterization. 1H NMR spectra were recorded on a
Bruker Avance 500 instrument using CDCl3 as the solvent and
TMSasthe internalreference. Infrared(IR) spectrawere recorded
on a Bruker Optics VERTEX 80v FT-IR spectrometer equipped
with a DTGS detector (32 scans) with a resolution of 4 cm-1 from
pressed KBr pellets. Elemental analysis (C, H, N) was performed
on a Vario EL elemental analyzer from Elementar Analysensys-
teme GmbH. Thermogravimetric analysis (TGA) was conducted
with a PerkinElmer Diamond TG/DTA instrument with a heat-
ing rate of 10 °C/min under flowing air or N2. MALDI-TOF MS
spectra were recorded on an autoflex III smartbeam mass spectro-
meter (Bruker Daltonics Inc.) with CHCl3 as the solvent and
dithranol (DIT) as the matrix or without a matrix. Polarized
optical microscopy (POM) was performed using an Axioskop 40
polarized optical microscope (Carl Zeiss Light Microscopy,
Germany) equipped with a LINKAM THMS 600 hot stage and
a LINKAM CI 94 temperature controller. The pictures were
captured through a ProgRes CT3 camera (JENOPTIK Laser,
Optik, Systeme GmbH). Differential scanning calorimetry (DSC)
measurements were performed on a Netzsch DSC 204 with
scanning rate of 10 °C/min. The samples were sealed in aluminum
capsules in air, and the atmosphere of the holder was sustained
under dry nitrogen. For variable-temperature X-ray diffraction
(XRD) experiments, a Bruker AXS D8 ADVANCE X-ray
diffractometer using Cu KR radiation with a wavelength of
Anal. Calcd for C18NEO3 PF6 (C27H58F6NO3P, 589.72): C,
3
54.99; H, 9.91; N, 2.38. Found: C, 55.12; H, 9.89; N, 2.03.
Preparation of SEPs. The diblock molecule C18NEO3 Ts
3
was employed to prepare complexes SEP-P, SEP-Si, SEP-B, and
SEP-PMo through the metathesis reaction with HPW, HSiW,
HBW, and HPMo, respectively. The preparation processes for all
of the SEPs are similar. Taking the preparation of SEP-P as an
example, its detailed procedure is as follows.
SEP-P. C18NEO3 Ts (0.15 g, 0.24 mmol) in water (6 mL) was
3
stirred at room temperature, and then HPW (0.23 g, 0.08 mmol)
dissolved inwater(4 mL) was added dropwise. A white precipitate
formed immediately and was collected by filtration and washed
with a bit of water. The obtained crude product was purified
further through recrystallization from chloroform/methanol, giv-
ing the complex as a white needlelike material (0.26 g) in a yield of
77.8%. 1H NMR (500 MHz, CDCl3, δ): 0.88 (t, J = 7 Hz, 3H),
1.26 (m, 30H), 1.81 (m, 2H), 3.32 (s, 6H), 3.37 (s, 3H), 3.45 (m,
2H), 3.54-3.74 (m, 10H), 4.15 (m, 2H). IR (KBr): 2956, 2925,
2852, 1468, 1350, 1293, 1253, 1200, 1111, 1079, 977, 896, 811
cm-1. Anal. Calcd for SEP-P (C81H174N3O49PW12, 4211.29): C,
23.10; H, 4.17; N, 1.00. Found: C, 22.99; H, 4.56; N, 0.73. SEP-P
should correspond to the formula (C18NEO3)3PW12O40.
˚
€
1.5418 A with an mri Physikalische Gerate GmbH TC-Basic
temperature chamber was used. All samples were prepared by
spreading the SEP solids on a thin, cleaned Si wafer, and the
thermal history of the solids was eliminated before measurements.
Results and Discussion
Preparation of SEPs. For the preparation of the SEPs, the
C18NEO3 Ts amphiphile has been employed. The designed
C18NEO3 Ts amphiphile is a substitute for octadecyltrimethyla-
mmonium p-toluenesulfonate, in which one of the methyl groups
has been replaced by a tri(ethylene oxide) chain (Scheme 1).
3
3
SEP-Si. This compound complex was prepared by following a
procedure similar to that for SEP-P by using HSiW instead. The
initial molar ratio of C18NEO3 Ts to HSiW was controlled at 4:1.
3
The complex was obtained as a white flakelike material (0.14 g) in
a yield of 60.3%. 1H NMR (500 MHz, CDCl3, δ): 0.88 (t, J = 7
Hz, 3H), 1.26 (m, 30H), 1.79 (m, 2H), 3.36 (s, 6H), 3.37 (s, 3H),
3.46 (m, 2H), 3.55-3.73 (m, 8H), 3.81 (m, 2H), 4.19 (m, 2H). IR
(KBr): 3440, 2956, 2923, 2852, 1630, 1467, 1351, 1300, 1256, 1200,
1110, 1012, 973, 918, 884, 791 cm-1. Anal. Calcd for SEP-Si
(C108H232N4O52SiW12, 4653.16): C, 27.88; H, 5.03; N, 1.20.
Found: C, 27.83; H, 5.04; N, 0.96. SEP-Si should correspond to
the formula (C18NEO3)4SiW12O40.
Because of the high solubility of C18NEO3 Ts in water, the
3
common phase-transfer methods33 are not applicable to encap-
sulating PMs, and the ion metathesis reaction in water was used in
the present work. In addition, Keggin-type PMs, which possess
pseudospherical structures, were chosen to prepare SEPs. In
detail, HPW, HSiW, and KBW are polyoxotungsates with dif-
ferent central atoms, whereas HPMo possesses a structure that is
SEP-B. This compound complex was prepared by following a
procedure similar to that for SEP-P by using KBW instead. The
€
€
(33) Kurth, D. G.; Lehmann, P.; Volkmer, D.; Colfen, H.; Koop, M. J.; Muller,
A.; Chesne, A. D. Chem.;Eur. J. 2000, 6, 385–393.
Langmuir 2010, 26(16), 13201–13209
DOI: 10.1021/la101928k 13203

Article
Lin et al.
Table 1. Summary of the Assignment of Mesophases, Phase-Transi-
tion Temperatures (°C), and Enthalpies (kJ/mol) of C18NEO3 Ts,
3
C
18NEO3 PF6, and All of the SEPs
3
first cooling
second heating
transitiona T (°C) ΔH (kJ/mol) T (°C) ΔH (kJ/mol)
C18NEO3 Ts S1-S2
-22.4
21.1
58.4
14.9
37.9
-1.3
-42.9
-0.7
-2.4
35.6
60.9
20.3
47.9
-11.0
189.5
219.4
-6.3
200.9
216.7
-30.8
72.0
2.2
39.3
0.7
14.7
29.8
25.5
5.3
5.9
63.0
2.7
3
S2-SmA
SmA-Iso
C18NEO3 PF6 S1-S2
-14.3
-29.7
-17.0
-5.1
3
S2-Iso
S1-S2
SEP-P
SEP-B
SEP-Si
-16.2
S2-SmB 181.4
SmB-Iso 214.9
-5.6
S-X
-12.9
163.9
-62.9
-3.4
X-SmB
SmB-Iso 210.6
S1-S2
S2-S3b
S1-S2c
-2.8
3.5
-33.9
55.1
-22.2
-13.1
-12.5
-17.6
12.5
12.5
19.9
SEP-PMo
-13.7
a S, SmA, SmB, Iso, and X indicate solid, smectic A, smectic B,
isotropic liquid, and an unknown phase, respectively. b Decomposse
when the sample reaches the isotopic liquid state. c Decomposes before
melting.
Figure 1. 1H NMR spectra of (a) C18NEO3 Ts, (b) SEP-PMo,
3
(c) SEP-P, (d) SEP-Si, and (e) SEP-B in the range of 0-5 ppm (A)
and in the highlighted range of 3.25-3.45 ppm (B). The peak
assignments have been marked with Arabic numbers. The peaks
marked with stars correspond to the H2O signal, and the symbol #
represents the signal of the methyl group of p-toluenesulfonate.
similar to that of HPW except for different peripheral atoms. All
of the employed PMs have the same volume. The four Keggin-
type PMs, including HPW, HSiW, KBW, and HPMo, were
complexed with C18NEO3 Ts to give SEP-P, SEP-Si, SEP-B,
3
and SEP-PMo as the precipitates, respectively. Direct precipita-
tion from water usually provides the resulting electrostatic com-
plexes with high-enough purity in the ionic assembly process.34
Figure 2. DSC curves of (a) C18NEO3 Ts and (b) C18NEO3 PF6
3
3
on their first cooling and second heating processes.
1
However, H NMR spectra of unpurified precipitates indicate
chains vary. Meanwhile, among the SEPs, these signals are also
different. The variation confirms the influence of anions on TEO
chains and implies that the TEO chains are close to the PMs
although the organic cations are in a more free state in solution
than in solid. In addition, the chemical shifts of most methyl or
methylene protons near the nitrogen atom vary linearly as a
function of the charge number of PMs in SEPs (Figure S10).
For all of the SEPs, no weight loss below 150 °C was observed
according to the TGA curves (Figure S13). Thus, no crystalline
water is considered in the resulting formulas of SEPs. According
to elemental analysis, the formulas of SEPs are (C18NEO3)3-
PW12O40 for SEP-P, (C18NEO3)4SiW12O40 for SEP-Si, (C18NEO3)5-
BW12O40 for SEP-B, and (C18NEO3)3PMo12O40 for SEP-PMo.
Here, the organic cations are denoted as C18NEO3 for concise
illustration. It should be noted that the protons in PMs have been
totally exchanged in SEPs, which also implies that the interaction
between the TEO chains and PMs bridged by oxonium ions is
almost nonexistent in SEPs.32,36
some compounds with p-toluenesulfonate, which may be derived
from the absorbed C18NEO3 Ts remaining in the yielded pre-
3
cipitates. The impurity cannot be removed by washing. Fortu-
nately, recrystallization from chloroform/methanol provides pure
SEPs.
All of the SEPs were characterized through IR, 1H NMR,
TGA, and elemental analysis, as shown in the Supporting
Information and the Experimental Section. In the IR spectra of
SEPs, the bands attributed to PMs remain but have obvious
shifts, indicating that the structure of the PMs are maintained,
and implying the interaction of cationic C18NEO3 with PMs
through electrostatic attraction.31 Compared with the resonance
signals of C18NEO3 Ts alone, 1H NMR spectra of SEPs (Figure
3
1A,B) show the following points: (1) The peaks corresponding
to p-toluenesulfonate totally disappear, indicating that organic
cation C18NEO3 is anchored firmly onto the polyanions and all of
the SEPs are highly pure without the adsorbed C18NEO3 Ts or
3
p-toluenesulfonate acid or salts. (2) The chemical structure of
cationic C18NEO3 is also well maintained in the complexes. (3)
The signals of the N-methyl proton in SEPs are shifted to low
field. The signals in SEPs with the dioctadecyldimethylammo-
nium cation are usually shifted to high field in contrast to that of
surfactant dioctadecyldimethylammonium bromide.35 The differ-
ence in the two situations originates from the different anions in
the reference amphiphile (p-toluenesulfonate vs bromide). Dif-
ferent anions should have an influence on the electron cloud of the
ammonium group. (4) Most signals that correspond to the TEO
Thermal Properties of Amphiphiles. The thermal properties
of C18NEO3 Ts and C18NEO3 PF6 were investigated by DSC,
3
3
POM, and XRD. Table 1 summarizes the phase-transition
temperatures, enthalpies, and assignments of both amphiphiles.
According to the TGA results (Figure S12), both amphiphiles
show very little weight loss below 180 °C, which is indicative of the
thermal stability below that temperature. Figure 2a shows the
DSC traces of C18NEO3 Ts during its first cooling and second
3
heating processes. During the first cooling process, C18NEO3 Ts
3
reveals three exothermic transitions at 58.4, 21.1, and -22.4 °C.
The enthalpy value for the phase transition at high temperature
(34) Faul, C. F. J.; Antonietti, M. Chem.;Eur. J. 2002, 8, 2764–2768.
(35) Bu, W. F.; Li, H. L.; Li, W.; Wu, L. X.; Zhai, C. X.; Wu, Y. Q. J. Phys.
Chem. B 2004, 108, 12776–12782.
(36) Neumann, R.; Assael, I. J. Chem. Soc., Chem. Commun. 1989, 547–548.
13204 DOI: 10.1021/la101928k
Langmuir 2010, 26(16), 13201–13209

Lin et al.
Article
Figure 3. Polarized optical micrographs of C18NEO3 Ts at (a) 57 and (b) 54 °C, (c) C18NEO3 PF6 at 22 °C, SEP-P at (d) 222 and (e) 210 °C,
3
3
and (f) SEP-B at 197 °C. Image d was captured during the first heating process, whereas the other images were captured during their first
cooling runs.
phase. The phase transition at 21.1 °C shows a sharp exothermic
peak and a large enthalpy (ca. -42.9 kJ/mol), suggesting a
SmA-solid phase transition. In addition, the change at -22.4 °C
is a solid-solid phase transition.
On the basis of the XRD results, a possible aggregation struc-
ture is assumed in the SmA phase, where the alkyl and TEO
chains of individual C18NEO3 Ts compound are distributed at
3
the opposite sides of the quaternary ammonium group because of
microphase segregation. As a result of the quaternary ammonium
group interacting with p-toluenesulfonate through electrostatic
interaction, the cationic C18NEO3 should penetrate and be
normal to the ionic sublayer consisting of p-toluenesulfonate ani-
ons. Because the measured layer spacing is smaller than the ideal
molecular length of C18NEO3 with an all trans conformation of
about 3.5 nm as calculated by the MM2 force field method, the
conformation distortion can be imagined. Triethylene glycol
monomethyl ether has a melting point below room temperatures.
Therefore, in the SmA phase, TEO chains should adopt a con-
tracted configuration,37 which makes the interlayer distance
smaller than the calculated length of C18NEO3. In addition, an
increase of the gauche conformation of the alkyl chains is another
reason for the short interlayer distance.22,24 Thus, a double-layer
structure can be expected in which the alkyl and TEO chains are
fully interdigitated respectively, as illustrated in Figure 4b. It
should be noted that the contraction of TEO chains on heating
could also enlarge the lateral area of every alkyl chain, which leads
Figure 4. (a) XRD patterns of C18NEO3 Ts at 52 °C. (Inset)
3
Corresponding magnification of the partial curve and a wide-angle
XRD pattern. (b) Schematic model of the C18NEO3 Ts organized
3
structure in the SmA phase. Black and red lines and green and
yellow spheres represent alkyl and TEO chains and the quater-
nary ammonium headgroup and p-toluenesulfonate anion of
C18NEO3 Ts, respectively. (c) XRD pattern of C18NEO3 PF6 at
3
3
30 °C.
(0.7 kJ/mol) is relatively small, obviously corresponding to the
transition from an isotropic liquid to a less-ordered mesophase.
The mesophase can be further assigned on the basis of its POM
and XRD results. On cooling from the isotropic liquid state, the
sample shows an all-black appearance (pseudo-isotropic texture)
with some alignment-induced birefringent defects near air bub-
bles (Figure 3a). The focal conic texture appears only when a
slight mechanical deformation was added by shearing the sam-
to a low packing efficiency of ions inC18NEO3 Ts, and as a result,
3
relatively low transition temperatures were found. Therefore,
C18NEO3 Ts represents a new ionic liquid crystal, which exhibits
3
a near-room-temperature melting point (35.6 °C) and a low
clearing point.
In contrast to C18NEO3 Ts, the amphiphile C18NEO3 PF6,
3
3
ple (Figure 3b). The XRD pattern of C18NEO3 Ts (Figure 4a)
which is the product in which the anion of C18NEO3 Ts is
3
3
at 52 °C shows a sharp, intense diffraction peak with two weak
subordinate diffraction peak in the small-angle region with
d-spacings in a ratio of 1:1/2:1/3, suggesting a less-ordered layered
structure with a layer spacing of 2.8 nm. In addition, a diffuse
replaced by hexafluorophosphate, shows no LC properties. As
shown by the DSC results (Figure 2b), the sharp endothermic
peak and the large enthalpy (ca. 29.8 kJ/mol) at 47.9 °C suggest
the melting of the C18NEO3 PF6 solid, which is further proven by
3
˚
scattering halo at about d = 4.5 A emerges in the wide-angle
POM and XRD results. When an isotropic liquid is cooled, the
sample quickly forms spherulites with typical Maltese cross
region, which is attributed to the disordered packing of alkyl
chains. When the above results are combined, the phase between
58.4 and 21.1 °C could definitely be assigned as a smectic A (SmA)
(37) Kim, J.-U.; Lee, J.-C. Macromol. Chem. Phys. 2005, 206, 951–960.
Langmuir 2010, 26(16), 13201–13209
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Article
Lin et al.
extinction patterns (Figure 3c). The diameters of spherulites can
reach several millimeters. Spherulites are often found in polymer
systems, however, they can also appear in small molecules.38 The
XRD curve of C18NEO3 PF6 (Figure 4c) exhibits a crystalline
3
layered structure with a layer spacing of 3.2 nm, as shown by
several sharp equidistant diffraction peaks in the small-angle
region at 30 °C. The irregular variation of the intensity of these
diffraction peaks originates from the specific arrangement of the
layers in the spherulites.39 In the wide-angle region, several sharp
diffraction peaks can be found, suggesting the highly ordered
packing of the alkyl and TEO chains of C18NEO3. Evidently, the
longer layer distance of C18NEO3 PF6 implies a more ordered
3
packing of alkyl and TEO chains than that of C18NEO3 Ts in the
3
SmA phase. Thus, the phase transitions at 47.9 and 20.3 °C can be
assigned to the melting point and the solid-solid phase transition,
respectively.
Normally, the microphase segregation of incompatible parts
and the aggregation of compatible parts afford well-defined
structures, the melting of long alkyl chains endows the meso-
phases with fluidity, and the electrostatic interaction stabilizes the
layer structures and the interlayer bonding in the amphiphilic
ionic liquid crystals, exhibiting smectic phases.40 Moreover, weak
electrostatic interaction is unfavorable to holding the positional
order of mesophases. The electrostatic interaction between
Figure 5. DSC curves of (a) SEP-P, (b) SEP-B, (c) SEP-Si, and
(d) SEP-PMo on their first cooling and second heating processes.
temperatures of decomposition of SEP-P and SEP-B are 309 and
247 °C, respectively. The FT-IR spectra of SEP-P and SEP-B
solids (Figure S14) that have been heated over clearing points are
consistent with those without heat treatment, indicating that both
SEPs are thermally stable when reaching the isotropic liquid state.
In addition, all of the SEPs possess a higher thermal stability than
C18NEO3 Ts and C18NEO3 PF6, which is indicative of the
C18NEO3 and the anion in C18NEO3 PF6 should be weaker than
3
3
3
that in C18NEO3 Ts, similar to the situations in other ionic
dependence of the thermal stability on the anion type.30
3
systems.41 The electrostatic interaction in C18NEO3 PF6 seems
too weak to keep the layered structures and endow C18NEO3 PF6
with LC behavior on heating, unlike the situation in C18NEO3
Ts. The weaker electrostatic forces of C18NEO3 PF6 should also
lead to its lower transition temperature to an isotropic liquid
compared to that of C18NEO3 Ts, although C18NEO3 PF6
forms highly ordered layered structures at near room tempera-
3
The mesophases of SEP-P and SEP-B were further investigated
by POM. The SEP-Psample becomes anisotropic liquid when it is
heated to over ca. 220 °C (Figure 3d). In the following cooling
run, SEP-P exhibits fluidity under shearing and a mosaic texture
at 210 °C (Figure 3e). A mosaic texture is common for ordered
smectic phases (i.e., the smectic B (SmB) phase), especially when a
direct transition from an isotropic liquid occurs.43 Meanwhile,
large homeotropic domains are also observed, suggesting an ort-
hogonal, optically uniaxial mesophase. For SEP-B, a typical
mosaic texture was observed at 197 °C (Figure 3f) upon cooling
from the isotropic liquid state (Figure S15). The high viscosity
also makes the fluidity under shearing not obvious. However, the
organized structure of SEP-B at the mesophase is mobile, and the
appearance under POM observation rapidly changes from a
mosaic texture to a pseudo-isotropic texture by shearing or
decreasing temperature.
To identify the LC phases, we examined the possible packing
structures of SEP-P and SEP-B. Figure 6a shows the variable-
temperature XRD patterns of SEP-P. The reflection peaks cor-
responding to d-spacings of 2.6 and 1.3 nm in the small-angle
region at 210 °C can be indexed as (001) and (002) for a layered
structure with a layer spacing of 2.6 nm. Meanwhile, a single,
moderately strong diffraction peak appears at 2θ ≈ 21°, indicat-
ing that a SmB phase exists.44 Thus, the mesophase can be
definitely assigned as a SmB phase. SEP-P shows a layered struc-
ture with a layer spacing of 2.7 nm at 30 °C. However, more than
one diffraction peak exists in the wide-angle region, suggesting a
more ordered structure. Thus, the phase-transition sequence of
SEP-P can be concluded to be an Iso-SmB-solid2-solid1
process. Like SEP-P, SEP-B shows layered structures with layer
spacings of 2.9 and 3.0 nm at 30 and 200 °C, respectively
(Figure 6b). At 200 °C, a single diffraction peak appears at 2θ
≈ 21° in the wide-angle region, indicating that the mesophase is a
3
3
3
3
3
ture. Here, through a simple ion exchange, although C18NEO3
PF6 loses its LC properties, it reveals a new near-room-tempera-
ture ionic liquid.
3
Thermal Properties of SEPs. Both SEP-P and SEP-B show
typical thermotropic liquid-crystalline properties and belong to
enantiotropic LCs. Upon the first cooling, the DSC trace of SEP-
P exhibits three exothermic transitions at 214.9, 181.4, and -16.2 °C,
respectively (Figure 5a). As shown in Table 1, the correspond-
ing enthalpy values for the first two phase transitions are 5.6 and
5.1 kJ/mol, respectively. This trace is also similar to the DSC trace
on the second heating, except for regular supercooling. SEP-B
also exhibits three exothermic transitions appearing at 210.6,
163.9, and -12.9 °C, respectively (Figure 5b). The enthalpyvalues
for the first two phase transitions are 2.8 and 3.4 kJ/mol, re-
spectively. It should be noted that the phase transition at 163.9 °C
on the first cooling shows strong supercooling (about 37 °C) as
well as a long transition as indicated by the relatively broad peak,
compared with other transitions of SEP-B or SEP-P at high
temperatures. This phenomenon should originate from the high
viscosity of SEP-B, even at high temperatures.42 Table 1 sum-
marizes the phase-transition temperatures, enthalpies, and assign-
ments of all of the SEPs. The calculated enthalpies of SEPs
are attributed to the per molar SEP complex rather than the per
molar organic cation. According to the TGA curves, the onset
(38) Magill, J. H. J. Mater. Sci. 2001, 36, 3143–3164.
(39) Tanakaa, T.; Fujitaa, M.; Takeuchib, A.; Suzukib, Y.; Uesugib, K.; Doia,
Y.; Iwata, T. Polymer 2005, 46, 5673–5679.
(40) Kanazawa, A.; Ikeda, T.; Nagase, Y. Chem. Mater. 2000, 12, 3776–3782.
(41) Bini, R.; Bortolini, O.; Chiappe, C.; Pieraccini, D.; Siciliano, T. J. Phys.
Chem. B 2006, 111, 598–604.
(43) Dierking, I. Textures of Liquid Crystals; Wiley-VCH: Weinheim, Germany,
2003.
(44) Demus, D.; Goodby, J. W.; Gray, G. W.; Spiess, H.-W.; Vill, V. Handbook
(42) Wicklein, A.; Lang, A.; Muth, M.; Thelakkat, M. J. Am. Chem. Soc. 2009,
131, 14442–14453.
of Liquid Crystals; Wiley-VCH: Weinheim, Germany, 1998; Vol. 1.
13206 DOI: 10.1021/la101928k
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Lin et al.
Article
Scheme 2. Schematic Illustration of Organized Structures in Meso-
phases of SEP-P or SEP-Ba
a The PMs are represented as blue polyhedrons. The alkyl and TEO
chains and quaternary ammonium headgroup of C18NEO3 are repre-
sented as black and red lines and green spheres, respectively. d is the layer
spacing.
chains are fully interdigitated, the layer spacings are still smaller
than the calculated one. This situation should originate from the
contraction of the alkyl chains. In contrast to the situation in
which PMs comprise hydrophilic domains by themselves, the
additional TEO chains expand the lateral area of hydrophilic
domains. As a result, the alkyl chains have to adopt a highly
folded conformation to fill space effectively, which shortens the
layer spacings. The highly folded alkyl chains also appear in some
ionic liquid crystals.30 Another possible reason may be the
entrance of the quaternary ammonium groups of the surfactants
into the PM sublayer, considering the pseudospherical structures
of PMs. The reported crystalline structure of [SiMo12O40]-
[C16H33N(CH3)3]4 has shown that cetyltrimethylammonium ca-
tions can enter the PM sublayer.46 Thus, we can propose a model
for the packing structures of SEP-P and SEP-B inthe SmB phases,
as shown in Scheme 2. The hydrophilic sublayer containing TEO
chains and polyanions is sandwiched by two uniform layers
consisting of fully interdigitated alkyl chains. Alkyl chains with
a highly folded conformation are perpendicular to the hydrophilic
sublayer and possess bond orientational order. SEP-P, which
possesses a lower alkyl chain density than that in SEP-B, has
much more contracted alkyl chains than does SEP-B. Thus, the
layer spacing of SEP-P in the SmB phase is visibly smaller than
that of SEP-B.
Figure 6. XRD patterns of (a) SEP-P and (b) SEP-B. (Inset)
Corresponding wide-angle XRD patterns.
SmB phase. The diffraction patterns at both 30 and 200 °C are
similar in the wide-angle region. However, the phase at 30 °C
cannot be definitely assigned as an LC phase because typical
textures and fluidity cannot be observed because of the high
viscosity. This phase might not be in a crystalline state but may be
in a highly ordered aggregated state.21 Thus, the phase-transition
sequence of SEP-B can be concluded to beanIso-SmB-X-solid
process.
On the basis of the above results, we tried to assume the
packing structures of the SmB phases. Because of the incompat-
ibility between the hydrophobic alkyl chain and the hydrophilic
TEO chain in the cationic C18NEO3, the two tail chains are
located on opposite sides of the quaternary ammonium group
through microphase segregation. The TEO chains of C18NEO3
are close to the PMs in the SEPs because both TEO chains and
PMs are hydrophilic and the TEO chains are anchored onto PMs
by the quaternary ammonium groups through electrostatic inter-
actions. The 1H NMR result also indicates that the TEO chains
and PMs are adjacent, as shown in Figure 1. Thus, hydrophobic
domains consisting of alkyl chains and hydrophilic domains
consisting of PMs and TEO chains will be generated, with the
interface of the quaternary ammonium groups interacting with
PMs. In addition, the length of the TEO chain with a fully
extended conformation is about 1.2 nm, which is similar to the
diameter (LPM) of the Keggin-type polyanion (1.0 nm).31,45
Therefore, the contribution of TEO chains to the thickness of
hydrophilic domains is faint and the thickness can be considered
to be 1.0 nm, consistent with the size of PMs. The calculated
length of the alkyl chain (LA) is about 2.3 nm. The layer spacings
(d) of both SEP-P (2.6 nm) and SEP-B (3.0 nm) in the SmB phase
are smaller than the sum of LA and LPM. Thus, even if the alkyl
Although SEP-P and SEP-B do not contain mesogenic groups,
they possess amphiphilic structures that are similar to those of
most ionic liquid crystals including C18NEO3 Ts, except for the
3
PM anions that possess large volumes and numerous charges.
Thus, the microphase segregation of incompatible parts and the
aggregation of compatible parts are still the main driving forces
for well-defined structures, and the long alkyl chains become
necessary for microphase segregation and the fluidity of LC
phases. When the anions are changed from p-toluenesulfonate
in C18NEO3 Ts to PMs in SEPs, the electrostatic interaction
3
between the anions and C18NEO3 becomes stronger. As a result,
the clearing points of SEP-P and SEP-B are higher than that of
(45) Bu, W. F.; Wu, L. X.; Zhang, X.; Tang, A. C. J. Phys. Chem. B 2003, 107,
13425–13431.
(46) Nyman, M.; Ingersoll, D.; Singh, S.; Bonhomme, F.; Alam, T. M.; Brinker,
C. J.; Rodriguez, M. A. Chem. Mater. 2005, 17, 2885–2895.
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Lin et al.
C18NEO3 Ts. Meanwhile, the electrostatic interactions in SEPs
should be strong enough to stabilize the LC phases.
SEP-P, SEP-Si, and SEP-B. Considering that the PMs in SEPs
have the same size, increasing the charge numbers will increase the
charge density on the PMs and then enhance the electrostatic
interaction between C18NEO3 and PMs. Thus, the shifting scale
of the N-methyl group signal can be regarded as an indicator of
the strength of electrostatic interaction in SEPs, and the sequence
of the electrostatic interaction strength is SEP-P<SEP-PMo<
SEP-Si<SEP-B. In addition, the XRD patterns of the casting
films of SEP-Si (Figure S16) show equidistant diffraction peaks in
the small-angle region and several sharp diffraction peaks in the
wide-angle region at 30 °C, implying that SEP-Si is prone to
forming highly ordered layered structures. The SEP with the 1:4
cluster-to-surfactant ratio has been reported as a special situation
in which single crystals can be obtained.46 Here, SEP-Si should
adopt a similar high packing efficiency of ions. However, com-
pared with SEP-Si, the other three SEPs should possess lower
packing efficiencies and have noncompact or crowded packing of
alkyl chains because of more or fewer organic cations in these
SEPs than in SEP-Si. Besides, all of the SEPs possess phase
transitions with relatively large enthalpies below 0 °C (Figure 5),
which are difficult to assign precisely. We thought that these
transitions may be attributed to the phase transitions of alkyl
chains.37 The variation of enthalpies at these phase transitions
among SEPs may also originate from the different packing
efficiencies of ions in SEPs. Moreover, SEP-Si shows different
thermal properties than other SEPs; that is, only SEP-Si has phase
transitions between 0 and 150 °C, which may be relative to its
unique, highly ordered packing structures.
By combining the above situations, we can give a plausible
explanation for the dramatic difference in the thermal properties
of SEPs when varying only the employed PMs. SEP-P possesses a
lower packing efficiency than SEP-Si, that is, noncompact pack-
ing. Meanwhile, the electrostatic interaction in SEP-P is weakest
among those of the present SEPs. These factors contribute to the
relatively low clearing point of SEP-P. SEP-PMo contains the
same quantity of cations as SEP-P, which should result in a
similar packing efficiency to that of SEP-P. However, SEP-PMo
has stronger electrostatic forces than does SEP-P, which should
originate from the variation of peripheral atoms in PMs from W
to Mo and the corresponding change in the charge distribution on
the surfaces of PMs. Thus, SEP-PMo should possess a higher
transition temperature than that of SEP-P so that it decomposes
before melting. SEP-Si has stronger electrostatic forces than the
two former SEPs and the highest packing efficiency among the
present SEPs, which results in a transition temperature to an
isotropic liquid state that is higher than the transition temperature
of SEP-P. Consideringthe thermal stability, SEP-Si unfortunately
decomposes when reaching the isotropic liquid state. Although
SEP-B has the strongest electrostatic interactions, it may possess
the lowest packing efficiency of ions among the four SEPs. The
effect of low packing efficiency on the clearing point should
surpass that of strong electrostatic interactions in SEP-B. Thus,
SEP-B also has a relatively low clearing point and its LC behavior
can be investigated clearly. Meanwhile, the clearing point and the
corresponding entropy of SEP-B are a bit smaller than that of
SEP-P. The low packing efficiency should also make SEP-B
unable to form regular morphologies by recrystallization from
the mixed organic solvent, unlike other SEPs.
3
The present results are also consistent with the situation for the
SEPs in which the surfactants with double tail chains are grafted
with mesogenic groups (azobenzene or biphenyl groups).23,24 In
those cases, the match between the diameter of the PM and the
cross-sectional diameter of the surfactant is regarded as an
important factor that can determine the mesophases of the SEPs.
When the occupied space of the individual surfactant on the
surface of PM is large enough (i.e., HPW with only three charges
was employed or HSiW polyanions were encapsulated by un-
protonated surfactants), SmB phases can be observed.23,24 How-
ever, when the surface of PM becomes crowded, SEPs have a
SmA or smectic C phase if the charge number of polyanions
increases24 or if the surfactants are protonated for the SEP
containing HSiW polyanions.23 In contrast, cationic C18NEO3
in SEP-P or SEP-B possesses only one alkyl chain. Therefore, the
alkyl chains on the surfaces of PMs are not excessively crowded.
Because of the synergy and matching between C18NEO3 and the
polyanions, an ordered smectic phase such as the SmB phase
should be favorable to SEP-P. Obeying the same principle, SEP-B
exhibits a SmB phase as well, whereas SEP, in which the KBW
anions are encapsulated by the surfactants with double-tail
chains, has SmA and smectic C phases.24 The previous results
have shown the situation in which the alkyl chains are too
crowded to form the SmB phase,23,24 whereas the present results
indicate that the SmB phase can also form for SEP-P and SEP-B
through the synergy and matching of organic and inorganic
components, although only three alkyl chains per PM exist in
SEP-P.
Unlike SEP-P and SEP-B, SEP-Si and SEP-PMo do not show
typical LC behavior because no isotropic liquid state with thermal
stability can be observed. Figure 5 shows the measured DSC
traces of SEP-Si and SEP-PMo. Combined with the POM ob-
servation, SEP-Si reveals two solid-solid transitions at-30.8 and
72.0 °C and SEP-PMo possesses only a solid-solid transition
at -13.7 °C on the second heating. Moreover, SEP-Si obviously
decomposes while it reaches the isotopic liquid state (at ca. 310 °C
as measured by POM), and SEP-PMo decomposes completely at
ca. 277 °C before melting. These decompositions are in agreement
with the corresponding TGA results.
Influence of PMs on the Thermal Properties of SEPs.
Comparing the four SEPs, they clearly show remarkable differ-
ences in thermal properties. Finally, we would like to determine
the reason for this situation and the role of TEO chains. The
prerequisite for obtaining SEPs that exhibit typical thermotropic
LC properties is that the SEPs must maintain thermal stability
when becoming isotropic liquids. Except for improving the
thermal stability of the SEPs, adjusting the transition tempera-
tures should be another effective means to access SEPs showing
typical LC behavior. Although the electrostatic interaction still
exists in the isotropic liquid state, ordered structures including the
ionic sublayers should be effectively destroyed in ionic com-
pounds.7 Moreover, enhancing the interaction forces among the
components or forming highly ordered structures can increase the
transition temperatures to isotropic liquids.47 The complex of
C18NEO3 with different PMs affords the resulting SEPs with dif-
ferent electrostatic interaction strength and different ion packing
efficiencies and then varies the transition temperatures of SEPs.
As shown in Figures 1 and S10, the signals of the N-methyl groups
shift to low field with the increase in the charge number of PMs in
It is worth noting that grafting TEO chains onto common
cationic surfactants offers greater opportunity than using only the
simple surfactants to adjust the packing efficiency of ions and the
thermal properties of SEPs. The additional TEO chains can enter
the hydrophilic domains containing PMs and could influence the
thermal properties of SEPs. In contrast to the hydrophilic
(47) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis; Wiley-VCH :
Weinheim, Germany, 2002; Chapter 3.
13208 DOI: 10.1021/la101928k
Langmuir 2010, 26(16), 13201–13209

Lin et al.
Article
domains that contain only polyanions, the TEO chains can make
the hydrophilic domains more sensitive to heat, which promotes
the collapse of the hydrophilic domains on heating. The TEO
chains can also increase the volume of the hydrophilic parts in
SEPs and adjust the packing efficiency of all components in SEPs,
especially the packing and conformation of alkyl chains. There-
fore, introducing TEO chains contributes to the decrease in the
transition temperatures to the isotropic liquid state and endowing
SEP-P and SEP-B with typical thermotropic LC behavior.
Although the C18NEO3 cation is not universal for affording the
SEPs with LC properties among all of the PMs, the present method
provides a good opportunity to access SEPs showing typical
thermotropic LC behavior without containing mesogenic groups.
a near-room-temperature ionic liquid. Adjusting the packing
efficiency of the ions through introducing the TEO chains into
SEPs is an efficient way to influence the thermal properties of
SEPs, especially the transition temperatures to the isotropic liquid
state, although SEP-Si and SEP-PMo do not show typical LC
behavior. The present results indicate that the combination of
proper cationic amphiphiles and PMs can impart typical thermo-
tropic LC behavior to the obtained SEPs, and mesogenic groups
or hydrogen bonding precursors are not necessary. We believe
that the present method could facilitate the preparation and
application of PM-based LC materials and might be feasible for
other hybrid LCs based on nanoobjects.
Acknowledgment. This work was financially supported by the
National Basic Research Program (2007CB808003), the National
Natural Science Foundation of China (20973082, 20703019,
20921003), and the Open Project of State Key Laboratory of
Polymer Physics and Chemistry, CAS. We thank the 111 project
(B06009) for the visit and helpful discussions with Prof. Ulrich
Kortz at Jacobs University.
Conclusions
Without the introduction of mesogenic groups or hydrogen
bonding precursors, we have obtained SEPs that exhibit typical
thermotropic LC properties through encapsulating HPW or
KBW polyanions by using C18NEO3. The microphase segrega-
tion of incompatible parts and the aggregation of compatible
parts, the long alkyl chains, and the electrostatic interactions with
Supporting Information Available: Characterization of
C18NEO3 Ts, C18NEO3 PF6, and SEPs and XRD patterns
of the casting films of SEP-Si. This material is available free
of charge via the Internet at http://pubs.acs.org.
enough strength are key conditions for endowing C18NEO3 Ts,
3
SEP-P, and SEP-B with typical LC behavior. C18NEO3 Ts
3
3
3
shows a SmA phase with low transition temperatures, and SEP-P
and SEP-B show typical SmB phases, whereas C18NEO3 PF6 is
3
Langmuir 2010, 26(16), 13201–13209
DOI: 10.1021/la101928k 13209