Self-assembled N-alkylimidazolium perfluorooctanesulfonates

Article abstract of DOI:10.1246/cl.2005.442

Thermostability of a smectic A phase composed of N-alkyl-imidazolium heptadecafluorooctanesulfonate was improved by the formation of nonpolar layer composed of mixed perfluoroalkyl and aliphatic chains. Copyright

Full text of DOI:10.1246/cl.2005.442

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Chemistry Letters Vol.34, No.3 (2005)
Self-assembled N-Alkylimidazolium Perfluorooctanesulfonates
y
y
ꢀ
Tomohiro Mukai, Masafumi Yoshio, Takashi Kato, and Hiroyuki Ohno
Department of Biotechnology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588
y
Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo,
Hongo, Bunkyo-ku, Tokyo 113-8656
(Received November 17, 2004; CL-041386)
Thermostability of a smectic A phase composed of N-alkyl-
urement and polarized optical microscopic observation. X-ray
diffraction measurement was carried out at the temperature 10
imidazolium heptadecafluorooctanesulfonate was improved by
the formation of nonpolar layer composed of mixed perfluor-
oalkyl and aliphatic chains.
ꢂ
C lower than clearing point. Dynamic ionic conductivity in par-
allel direction of ionic layer was carried out by complex impe-
6
dance method over frequency range from 10 to 5 ꢃ 10 Hz, us-
6
ing comb-shaped gold electrode cell system. Cell constants of
0
N,N -Dialkylimidazolium salts composed of charge-delocal-
1
these cells were calibrated by 0.01 M KCl solution before using.
All of a-n showed an enantiotropic smectic A (SA) phase.
The SA phase for a-1 was seen in wider temperature range than
ized anions are well known as room temperature ionic liquids.
2
,3
They have been investigated in various research fields because
of their attractive properties such as high ionic conductivity,
non-volatility, and non-flammability. In order to obtain excellent
ionic liquids showing low viscosity and low melting point, fluo-
0
that for N-methyl-N -alkylimidazolium trifluoromethanesulfo-
8
nate. Figure 2 shows a phase diagram as a function of aliphatic
9
chain length of a-n on heating. When carbon number was fewer
ꢁ
ꢁ
ꢁ
ꢁ
rinated anion such as TFSI , PF6 , CF3SO3 , and BF4 are of-
than 3, melting points and clearing points fell with increasing the
aliphatic chain length. Short aliphatic chains on imidazolium
cation decreased molecular symmetry and disturbed the molecu-
lar packing. On the other hand, when n ꢄ 3, clearing points was
raised with increasing the aliphatic chain length. Because per-
fluoroalkane is immiscible with hydrocarbons, the increase of
thermostability of the SA phase composed of a-n is attributed
to enhancement of molecular orientation which is based on the
formation of aliphatic chain layer phase-separated each from
ionic and perfluoroalkyl chain layer.
2
ten employed. Generally, ionic liquids composed of trifluoro-
methanesulfonate anions shows lower melting point and viscos-
ity than those composed of methanesulfonate anions. Such flu-
4
orinated anions show weaker Coulombic interaction with cations
because of electron-withdrawing property to delocalize the
anionic charge.
It is known that a number of amphiphilic imidazolium salts
form thermotropic liquid–crystalline phase with nano-order
phase separation of nonpolar aliphatic or perfluoroalkyl chain
5
and polar ionic groups. In our pervious study, we have investi-
gated specific ion conductive behavior of monodomain orientat-
The smectic layer spacing of a-2 was determined from dif-
fraction peak in the smallest-angle. Layer spacing of a-2 was
ꢀ
23.2 A. This spacing is approximately twice the length of the
6
ed liquid–crystalline imidazolium salts. In order to attain both
ꢀ
fully extended anion (12 A). In the case of a-n having short ali-
high ionic conductivity in these aggregated ionic domain and
stabilized liquid–crystalline phases, it should be effective to in-
troduce perfluoroalkyl chain toward anion. Furthermore, there
has been no study on self-assembled structure of amphiphilic
imidazolium salts having long perfluoroalkyl and aliphatic chain
so far. Here we report the effect of aliphatic chain length of neu-
tralized imidazolium salts prepared from heptadecafluorooctane-
sulfonic acid and N-alkylimidazole (Figure 1; a-n.) on the phase
transition behavior, self-assembled structure, and ionic conduc-
tivity.
phatic chain, their layer spacing suggested that perfluoroalkyl
chains form bilayer structure.
Effect of aliphatic chain length on smectic layer spacing was
Salts a-n were prepared by neutralization of N-alkylimida-
7
zole with heptadecafluorooctanesulfonic acid in ethanol at room
temperature. The mixture was stirred for 12 h, then the solvent
was removed in vacuo. The obtained white powder was purified
by recrystallization from chloroform or chloroform/diethyl
ether depending alkyl chain length. The product was collected
ꢂ
by filtration and dried in vacuo at 45 C for 4 days. The structure
1
of the samples was confirmed by H NMR and elemental analy-
sis. Phase transition behavior was characterized by DSC meas-
Figure 2. Phase diagram of a-n as a function of alkyl chain
length of imidazolium cation. Iso = Isotropic liquid, SA =
smectic A phase, Cr = crystal.
Figure 1. N-alkylimidazolium heptadecafluorooctanesulfonate.
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.3 (2005)
443
investigated. It is expected that smectic layer spacing would in-
crease with increasing chain length, when phase-separated struc-
ture is composed of three components of ionic groups, perfluor-
oalkyl, and aliphatic chains (a-n (n ꢄ 4)). X-ray diffraction
peaks of a-2, a-4, a-6, and a-8 were observed at almost the same
angle. Layer spacing of a-4, a-6, and a-8 were 23.0, 23.0, and
increasing aliphatic chain length. Similar to a-2, discontinuous
points of ionic conductivity for a-4, a-6, a-8, and a-10 agreed
with their melting point and clearing point.
N-Alkylimidazolium heptadecafluorooctanesulfonate shows
the enantiotropic smectic A phase. Ionic conduction in ionic lay-
er is faster than was observed in an isotropic phase. From ther-
modynamic and X-ray diffraction study, it was clarified that high
thermostability of the smectic A phase was attained by the for-
mation of nonpolar layer composed of perfluoroalkyl and ali-
phatic chains.
ꢀ
24.5 A respectively. It was thus clarified that aliphatic chain
length did not largely affect the layer spacing of a-n when
n ꢅ 8. From this result and Figure 2, it is considered that en-
hancement of thermostability with increasing aliphatic chain
length of a-n (n ꢄ 3) is attributable to the formation of nonpolar
layer structure composed of perfluoroalkyl and aliphatic chains
in the same plane. It has already been reported that steric effect
also allows formation of mixed layer composed of fluoroalkyl
One of authors (T. M.) acknowledges the financial support
from the Japan Society for the Promotion of Science (Research
Fellowships for Young Scientists). This study was carried out
under the 21st Century COE program for ‘‘Future Nanomateri-
als’’ at Tokyo University of Agriculture and Technology. The
present study was also supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan (No. 14205136).
1
0
and aliphatic chains. In the present study, strong electrostatic
interaction should be the major force to form the mixed layer
in the case of a-n (n ꢄ 4).
1
In the H NMR spectra, peak due to long aliphatic chain was
observed around ꢀ ¼ 1:3 as multiplet. Intensity of this multiplet
peak increased with increasing the chain length. In the case of a-
n, this multiplet was observed when n ꢄ 4. This result indicates
that aliphatic chain on imidazolium ring longer than n-butyl acts
to prepare nonpolar moiety. These aliphatic chains would be
phase-separated from ionic layer and prefer nonpolar layer.
Temperature dependence of ionic conductivity for a-2 is
shown in Figure 3 ( ). Activation energy in crystalline phase
was high, while low activation energy was observed in the SA
phase. In particular, ionic conductivity decreased after phase
transition from a SA phase to an isotropic phase. The decrease
of ionic conductivity might be due to lower ionic density in an
isotropic phase than is in aggregated ionic layer formed in the
SA phase. This suggests that successive ionic layers act as ion
conductive pathway. Ionic conductivity of a-4, a-6, a-8, and a-
References and Notes
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N-Methylimidazole, N-butylimidazole (Aldrich) and N-ethyl-
imidazole (Tokyo Kasei Co., Ltd.) were purified by distillation
under reduced pressure over KOH before use. Other N-alkylimi-
dazoles (-n-propyl-, -pentyl-, -hexyl-, -heptyl-, -octyl-, -nonyl-,
and -decyl-) were synthesized under Ar atmosphere as follows.
Imidazole (Kanto Chem. Co.) was dissolved in THF and the re-
ꢂ
sulting solution was added to NaH/THF turbid mixture at 0 C.
The mixture was stirred for 2 h, and then refluxed overnight, the
ꢂ
mixture was cooled to 0 C. Then 1-bromoalkane was added and
stirred for 2 h. The mixture was refluxed for 1 day. Then precipi-
tated NaBr was removed by filtration. The solution was concen-
trated in vacuo. The residue was purified by distillation under re-
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8
9
A. E. Bradley, C. Hardacre, J. D. Holbrey, S. Johnston, S. E. J.
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The phase transition temperatures ( C) on the second heating
ꢂ
0
0
cycle for a-n are: a-1 Cr 2.4 Cr 123.1 SA 184.7 Iso; a-2 Cr
0
7
8.0 Cr 100.7 SA 130.6 Iso; a-3 Cr ꢁ2:6 Cr 87.6 SA 107.9
0
0
0
0
Iso; a-4 Cr 44.3 Cr 54.3 Cr 89.5 SA 111.3 Iso; a-5 Cr 71.1
Cr 95.4 SA 121.8 Iso; a-6 Cr 112.3 SA 131.7 Iso; a-7 Cr 95.7
0
Figure 3. Temperature dependence of ionic conductivity of a
series of a-n at heating process. : a-2, : a-4, : a-6,
a-8, : a-10.
SA 146.6 Iso; a-8 Cr 3.8 Cr 101.1 SA 146.9 Iso.
10 D. Lose, S. Diele, G. Pelzl, E. Dietzmann, and W. Weissflog,
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Published on the web (Advance View) February 23, 2005; DOI 10.1246/cl.2005.442