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Chemistry Letters Vol.33, No.12 (2004)
Effect of Methyl Groups onto Imidazolium Cation Ring on Liquid Crystallinity
and Ionic Conductivity of Amphiphilic Ionic Liquids
Tomohiro Mukai, Masafumi Yoshio,y Takashi Kato,y and Hiroyuki Ohnoꢀ
Department of Biotechnology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588
yDepartment of Chemistry and Biotechnology, School of Engineering, The University of Tokyo,
Hongo, Bunkyo-ku, Tokyo 113-8656
(Received September 27, 2004; CL-041136)
Phase transition behavior of imidazolium dodecylsulfonate
over night. The mixture was cooled to 0 ꢁC, and then methane-
sulfonic acid methyl ester was added and the mixture was further
stirred for 2 h. The mixture was refluxed again for 1 day. Then
precipitated CH3SO3Na was removed by filtration. The solution
was concentrated in vacuo. The residue was purified by distilla-
tion under reduced pressure. 1,4,5-Trimethylimidazole was pre-
pared according to the reported method.8 1-Methyl-, and 1,2-
dimethylimidazole (Aldrich) were purified by distillation under
reduced pressure before use. N,N0-Dimethylimidazolium iodides
were obtained by the reaction of N-methylimidazoles and iodo-
methane in acetonitrile for 1 day at room temperature. The prod-
uct was purified by recrystallization from ethylacetate/acetoni-
trile. Silver dodecylsulfonate was obtained by ion-exchange re-
action between sodium dodecylsulfonate and silver nitrate in wa-
ter. The white precipitate was corrected by the filtration and
purified by the recrystallization from methanol. N,N0-Dimethyli-
midazolium dodecylsulfonates were obtained by ion-exchange
reaction of N,N0-dimethylimidazolium iodides and silver dode-
cylsulfonate in dichloromethane at room temperature. The pre-
cipitated silver iodide was removed by filtration, solvent was re-
moved in vacuo. The obtained white powder was purified by re-
crystallization from acetonitrile three times. The product was
collected by filtration and dried in vacuo at 45 ꢁC for 3 days.
Phase transition behavior was characterized by DSC meas-
urement and polarized optical microscopic observation. Dynam-
ic ionic conductivity was carried out by complex impedance
method with comb-shaped gold electrode cell system.
First, we studied the liquid crystallinity of the obtained
AILs. Table 1 shows the summary of phase transition behavior
of AILs (a, b, c, and d). 1,3-Dimethylimidazolium salt (a)
showed enantiotropic smectic A phase (SA). Imidazolium salts
having methyl groups on the 1, 3, and 4-position (c) showed
monotropic SA phase. Furthermore, 1,3,4,5-tetramethylimidazo-
lium salt (d) showed no liquid crystalline phase. These results
strongly suggested that the increase in the number of methyl
groups onto imidazolium ring gradually suppressed the liquid
crystallinity. Seddon et al.9 reported that the temperature range
of mesophase for N-alkylpyridinium hexafluorophosphate was
narrowed by further methyl substitution.9 There is no further
study on the effect of methyl substitution on the properties of
AILs.
was considerably affected by the introduction of methyl groups
onto the imidazolium cation ring. Methyl group on the 2-position
eliminated the liquid crystallinity. That on the 4-position was ef-
fective to suppress the crystallization.
Ionic liquids were prepared by coupling asymmetric organic
cations and charge-delocalized anions.1 Since these ionic liquids
(ILs) show such excellent properties as high ionic conductivity,
non-volatility, non-flammability, etc., ILs have been studied vig-
orously in wide variety of research fields.2 Taking the possibility
of functional design into account, it is important to analyze the
effect of structure on their properties. For example, a proton
on the 2-position of imidazolium ring has recently been revealed
to have an important role to interact with anion through hydro-
gen bond.3 It is also known that introduction of a methyl group
on the 2-position of imidazolium ring elevated the melting point
of the corresponding ILs.3
On the other hand, amphiphilic ionic liquids (AILs) such as
salts containing long hydrocarbon chain or perfluorocarbon
chain on the imidazolium cation show thermotropic liquid crys-
talline phase in the bulk.4 These AILs provide phase-separated
layers of polar ionic liquid domain and nonpolar alkyl chains.
We have already reported that these AILs were effective to pro-
vide 1-5 and 2-dimentional6 ion conductive path by using self-as-
semble characteristics of these AILs. Such assembled AILs show
dramatic change in the ionic conductivity through the phase tran-
sition. Since this transition can be used as switching devices, it is
necessary to study the effect of their structures on the transition
behavior. There are a lot of studies4,7 on the phase transition and
assembled structure of alkylpyridinium salts by changing alkyl
chain length of cations. We focused on the methyl group substi-
tution on the imidazolium cation ring expecting fine tuning of
thermal behavior. In the present study, we prepare a series of
methyl substituted imidazolium dodecylsulfonates, and analyze
their phase transition behavior and ionic conductivity.
1,4-Dimethylimidazole was synthesized under Ar atmos-
phere as follows. 4-Methylimidazole was dissolved in THF
and the resulting solution was added to a NaH/THF turbid mix-
ture at 0 ꢁC. After stirring the mixture for 2 h, it was refluxed
On the other hand, the importance of proton at the 2-position
of imidazolium ring has already been reported through the ex-
1
periments analyzed with X-ray diffraction study and H NMR.3
The salt b where proton at the 2-position was substituted into
methyl group showed no liquid crystallinity. This result clearly
shows that the proton at the 2-position is quite important for
the liquid crystallinity of AILs.
Figure 1. Structure of various methyl substituted imidazolium
dodecylsulfonate.
Copyright Ó 2004 The Chemical Society of Japan
Mukai, Tomohiro
