Physicochemical properties of phenyltrifluoroborate-based room temperature ionic liquids

Article abstract of DOI:10.1016/j.molliq.2017.09.067

Novel room temperature ionic liquids (RTILs) were synthesized using phenyltrifluoroborate ([PhBF₃]^?), which is the simplest aryltrifluoroborate structure. The [PhBF₃]^?-based RTILs showed desirable transport properties, i.e., viscosity and ionic conductivity, and the anion has a rigid and bulky aromatic ring. The properties of 1-butyl-3-methylimidazolium phenyltrifluoroborate ([C₄mim][PhBF₃]) are comparable to those of 1-butyl-3-methylimidazolium tetrafluoroborate ([C₄mim][BF₄]). The molecular volume of the RTILs, which was estimated using the appropriate quantum chemical calculations, highly correlated with the transport properties. The cation volume is an important factor that can control the physicochemical properties in this RTIL system. In addition, the basic skeleton structure of the cation determined the electrochemical window.

Full text of DOI:10.1016/j.molliq.2017.09.067

Journal of Molecular Liquids 246 (2017) 236–243
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Physicochemical properties of phenyltrifluoroborate-based room
temperature ionic liquids
Kazuki Iwasaki a, Kazuki Yoshii a, Tetsuya Tsuda a, , Susumu Kuwabata
⁎
a,b,⁎⁎
a
Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
Department of Energy and Environment, Research Institute of Electrochemical Energy, National Institute of Advanced Industrial Science and Technology, 1-8-31 Midorigaoka, Ikeda, Osaka 563-
b
8577, Japan
a r t i c l e i n f o
a b s t r a c t
Novel room temperature ionic liquids (RTILs) were synthesized using phenyltrifluoroborate ([PhBF3]−), which is
the simplest aryltrifluoroborate structure. The [PhBF3]−-based RTILs showed desirable transport properties, i.e.,
viscosity and ionic conductivity, and the anion has a rigid and bulky aromatic ring. The properties of 1-butyl-3-
methylimidazolium phenyltrifluoroborate ([C4mim][PhBF3]) are comparable to those of 1-butyl-3-
methylimidazolium tetrafluoroborate ([C4mim][BF4]). The molecular volume of the RTILs, which was estimated
using the appropriate quantum chemical calculations, highly correlated with the transport properties. The cation
volume is an important factor that can control the physicochemical properties in this RTIL system. In addition, the
basic skeleton structure of the cation determined the electrochemical window.
Article history:
Received 17 August 2017
Received in revised form 15 September 2017
Accepted 17 September 2017
Available online 20 September 2017
Keywords:
Room temperature ionic liquid
Phenyltrifluoroborate
Fluoroanion
© 2017 Elsevier B.V. All rights reserved.
Aromatic structure
Physicochemical properties
1. Introduction
phosphonium. Their common side chains include straight alkyl,
aminoalkyl, hydroxyalkyl, alkoxyalkyl, and other groups, and AlCl4−
,
Room temperature ionic liquids (RTILs), liquid salts that contain
only a cation and an anion at room temperature, are a subset of molten
salts that possess unique features, such as negligible vapor pressures, in-
combustibility, wide electrochemical windows (EWs), and relatively
high ionic conductivities [1,2]. The features of RTILs, except for the han-
dling temperatures, are very similar to those of molten salts, which are
very useful anhydrous solvents for high-temperature electrochemical
technologies, including the Hall-Héroult process and molten carbonate
fuel cell systems. These appealing features are why many different ap-
plications have been proposed for RTILs, e.g., next generation energy de-
vices [3–8], recyclable and nonvolatile organic synthesis processes [9,
10], high-performance lubricants [11,12], and functional liquid mate-
rials for vacuum technologies [13–17]. The number of fundamental sci-
ence articles related to RTILs has increased in recent years because
scientists can readily design and synthesize the ionic species in RTILs
but not in conventional solvents. In the past several decades, numerous
cations and anions have been used to prepare various functional RTILs.
The basic skeletons for typical cations include imidazolium, pyridinium,
pyrrolidinium, piperidinium, quaternary ammonium, sulfonium, and
BF−4 , PF−6 , CF3SO−3 , N(SO2CF3)2−, N(CN)2−, and (FH)nF− (1 ≤ n ≤ 3) have
been widely used as the anion component [18,19]. The physicochemical
properties of RTILs strongly depend on the cation and anion species
combination [20–26]. For example, 1-butyl-1-methylpyrrolidinium
bis(trifluoromethylsulfonyl)amide ([C4mpyr][N(SO2CF3)2]) has a vis-
cosity of 76 mPa·s, but when the cation component is changed to 1-
ethyl-3-methylimidazolium ([C2mim]+), the viscosity significantly de-
creases to 35 mPa·s [27,28]. Changing the anion component also results
in changes in the properties of the RTIL. Unfortunately, the design and
synthesis of novel anion species is difficult compared to that of the cat-
ions that have been synthesized using a simple nucleophilic substitution
reaction [29] because the anion synthetic process is not well-established
and laborious.
Very recently, we successfully synthesized nearly 40 types of
aryltrifluoroborate anions ([ArBF3]−) and reported the unique physico-
chemical properties of alkali metal salts with [ArBF3]− [30]. Potassium
and cesium salts with [ArBF3]− can be easily prepared on a large scale
using commercially available reagents [31]. [ArBF3]− is an analog of tet-
rafluoroborate ([BF4]−), which is a typical RTIL anion first reported by
Wilkes and Zaworotko [32], and in [ArBF3]−, one fluorine in [BF4]− is re-
placed with a designable aromatic ring. Because of the ring, organic salts
with [ArBF3]− are likely to have favorable physicochemical properties
because of the decreased interionic interaction energy between the an-
ions and cations due to the lower surface charge density on the anions
as a result of adding an electron-withdrawing phenyl group [27,33–
⁎
Corresponding author.
⁎⁎ Correspondence to: Department of Applied Chemistry, Graduate School of
Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan.
E-mail addresses: ttsuda@chem.eng.osaka-u.ac.jp (T. Tsuda),
kuwabata@chem.eng.osaka-u.ac.jp (S. Kuwabata).
https://doi.org/10.1016/j.molliq.2017.09.067
0167-7322/© 2017 Elsevier B.V. All rights reserved.

K. Iwasaki et al. / Journal of Molecular Liquids 246 (2017) 236–243
237
Fig. 1. Chemical structures of [PhBF3]−-based RTILs with different organic cations and their abbreviations.
40]. In this study, we attempted to produce undiscovered RTILs by com-
bining different onium cations, e.g., imidazolium, pyridinium,
pyrrolidinium, piperidinium, and quaternary ammonium, with the
phenyltrifluoroborate ([PhBF3]−) anion, which has the simplest struc-
ture among the [ArBF3]− anions. The physicochemical properties of
the resulting [PhBF3]−-based salts were examined. The factors control-
ling their properties were explored via systematic data gathering.
by NMR spectroscopy, mass spectrometry, and elemental analysis. 1-
Butyl-3-methylimidazolium tetrafluoroborate ([C4mim][BF4]) was pur-
chased from Kanto Chemical Co. and used for comparison. It was thor-
oughly vacuum dried for 24 h before use to remove any residual water.
Thermogravimetric (TG) analyses were performed using a Bruker
TG-DTA2000SA instrument. Samples were placed on an open aluminum
pan and heated from room temperature to 773 K at a rate of 5 K·min−1
under flowing dry nitrogen gas. The thermal degradation temperature
was determined from the 5 wt% loss point of the TG curve. Differential
scanning calorimetry (DSC) was conducted using a Bruker DSC3100SA
instrument. The samples were sealed in an aluminum pan with an alu-
minum top. The sealed pan was heated and cooled at a rate of
5 K·min−1. The glass-transition temperature and melting point were
obtained from the DSC curve of the second heating process. These
values were estimated using the tangential intersection method near
the temperature at which a phase transformation occurred. These two
instruments were controlled with a Bruker MTC1000SA workstation
utilizing the Bruker WS003 software. All the specimens for these mea-
surements were prepared in an argon gas-filled glove box (Vacuum At-
mospheres Co., Omni-Lab, O2 and H2O b 1 ppm).
2. Experimental section
Potassium phenyltrifluoroborate (K[PhBF3]) was synthesized using
phenylboronic acid (PhB(OH)2) (Wako Pure Chemical Industries, Ltd.),
potassium fluoride (KF) (Wako Pure Chemical Industries, Ltd.), and L-
tartaric acid (Wako Pure Chemical Industries, Ltd.) in the following pro-
cedure [31]. An aqueous solution of KF (200 mmol, 20 mL) was added to
a solution of PhB(OH)2 (50 mmol) in acetonitrile (200 mL), and the
mixture was stirred for 15 min at ambient temperature. L-Tartaric acid
(2.05 equiv.) dissolved in tetrahydrofuran (THF) (100 mL) was slowly
added to the mixture, and then, a white by-product immediately precip-
itated. The reaction mixture was vigorously stirred for 1 h at ambient
temperature and filtered to remove the precipitate. The resultant filtrate
was concentrated in vacuo, and the crude potassium salt was obtained
as a solid. The crude product was purified by recrystallization to obtain
pure K[PhBF3]. The obtained salt was dried at 353 K under vacuum for
1 h. The final product was confirmed by nuclear magnetic resonance
(NMR) spectroscopy, mass spectrometry, and elemental analysis.
Seven kinds of onium salts, 1-ethyl-3-methylimidazolium chloride
([C2mim]Cl) (Tokyo Chemical Industry Co., Ltd.), 1-butyl-3-
methylimidazolium chloride ([C4mim]Cl) (Kanto Chemical Co., Inc.),
1-butylpyridinium chloride ([C4py]Cl) (Tokyo Chemical Industry Co.,
Ltd.), 1-butyl-1-methylpyrrolidinium chloride ([C4mpyr]Cl) (Sigma-Al-
drich, Inc.), 1-butyl-1-methylpiperidinium chloride ([C4mpip]Cl)
(Tokyo Chemical Industry Co., Ltd.), trimethylpropylammonium bro-
mide ([N1,1,1,3]Br) (Tokyo Chemical Industry Co., Ltd.), and
tributylmethylammonium chloride ([N4,4,4,1]Cl) (Sigma-Aldrich, Inc.)
Density measurements were conducted using a Kyoto Electronics
Manufacturing DA-640 resonant frequency oscillation density/specific
gravity meter in the range of 298–353 K. The viscosity was measured
were used as the cationic species for the preparation of the [PhBF3]−
-
based organic salts. K[PhBF3] was prepared via the previously men-
tioned protocol and was used as the anion source. The synthesis of the
[PhBF3]−-based organic salts was performed using the metathesis pro-
tocol explained below. K[PhBF3] (40 mmol) was added to a solution of
an onium halide (40 mmol) in acetonitrile (60 mL), and the mixture
was stirred for 1 h at ambient temperature. After the reaction, the mix-
ture was filtered to remove the precipitated by-product, KCl or KBr, and
the filtrate was condensed under vacuum. The crude product was ex-
tracted by CH2Cl2 and rinsed with ultrapure water several times to re-
move the unreacted halides and by-product. The organic layer was
concentrated in vacuo. The resultant onium phenyltrifluoroborate was
dried at 373 K under vacuum for 12 h. The final product was confirmed
Fig. 2. Results of the TG analysis for (•••) [C4mim][BF4], ( ) [C4mim][PhBF3], (
[C2mim][PhBF3], [C4py][PhBF3], [C4mpyr][PhBF3],
)
)
(
)
(
)
(
[C4mpip][PhBF3], ( ) [N1,1,1,3][PhBF3], and ( ) [N4,4,4,1][PhBF3]. The measurements
were conducted at a rate of 5 K·min−1
.

238
K. Iwasaki et al. / Journal of Molecular Liquids 246 (2017) 236–243
Table 1
Physicochemical properties of [PhBF3]−-based RTILs and salts.
RTILs and salt
FWa
Tgb/K
Tmc/K
Tdd/K
de/g·cm−3
ηf/mPa·s
σg/mS·cm−1
Λh/S·cm2·mol−1
Vcationi/nm3
Vanionj/nm3
[C4mim][BF4]
226
284
255
281
287
301
274
345
190
209
210
224
208
228
―
―
―
―
―
―
―
350
―
611
492
477
490
497
514
495
496
1.201
1.145
1.192
1.151
1.114
1.108
―
101
115
60
131
241
1150
―
4.21
2.42
7.09
2.20
1.44
0.328
―
0.795
0.602
1.54
0.539
0.371
0.0895
―
0.207
0.207
0.160
0.203
0.211
0.231
0.160
0.319
0.072
0.164
0.164
0.164
0.164
0.164
0.164
0.164
[C4mim][PhBF3]
[C2mim][PhBF3]
[C4py][PhBF3]
[C4mpyr][PhBF3]
[C4mpip][PhBF3]
[N1,1,1,3][PhBF3]
[N4,4,4,1][PhBF3]
227
1.028
1870
0.112
0.0377
The ionic volume was calculated with the Gaussian 09 program using a B3LYP/6-31G+(d) level calculation.
a
Formula weight.
Glass-transition temperature.
Melting point.
Thermal degradation temperature at 5 wt% loss.
Density at 298 K.
Viscosity at 298 K.
Ionic conductivity at 303 K.
Equivalent conductivity at 303 K.
b
c
d
e
f
g
h
i
Volume of the cation.
Volume of the anion.
j
using a Kyoto Electronics Manufacturing EMS-1000 electromagnetically
spinning viscometer in the range of 298–353 K. Ionic conductivity mea-
surements were performed using a Horiba DS-51 digital conductivity
meter in the range of 303–353 K with a glass conductivity cell after
the cell was calibrated with a 0.1 M KCl aqueous solution. The electro-
chemical measurements were conducted with an IVIUM Technologies
CompactStat portable electrochemical analyzer. All electrochemical ex-
periments were performed in a three-electrode cell. The working elec-
trode was a glassy carbon disk (1.6 mm in diameter), which was
polished with an alumina suspension (0.06 μm in diameter) prior to
use. A platinum wire (0.5 mm in diameter) was used as the counter
electrode, and the reference electrode was constructed by placing a
1.0 mm diameter Ag wire into a 6 mm diameter Vycor® glass tube filled
with a [C4mim][N(SO2CF3)2] RTIL containing 0.05 M Ag[N(SO2CF3)2]. All
the measurements, except the viscosity, were performed in an argon-
filled glove box. The viscosity measurements were performed using
the special airtight cells for the electromagnetically spinning viscome-
ter. The Gaussian 09 program [41] was used for the molecular orbital
calculations. We used the basis sets implemented in the Gaussian pro-
gram. All the ion structures used in this study were optimized at the
HF/6-311G(d,p) level. The ionic volumes of the cations and anions
were calculated at the B3LYP/6-31G+(d) level [20]. We determined
the ionic volume using the average value from 10 calculations.
3. Results and discussion
3.1. General characteristics
The chemical structures of the organic salts prepared in this study
are summarized in Fig. 1 with their abbreviations. We successfully syn-
thesized seven types of [PhBF3]−-based organic salts. All the salts,
Fig. 4. Temperature dependence of the density for (●) [C4mim][BF4], ( ) [C4mim][PhBF3],
Fig. 3. DSC curves of (a) [C4mim][BF4], (b) [C4mim][PhBF3], (c) [C2mim][PhBF3], (d)
(
(
) [C2mim][PhBF3], ( ) [C4py][PhBF3], ( ) [C4mpyr][PhBF3], ( ) [C4mpip][PhBF3], and
) [N4,4,4,1][PhBF3]. The solid lines depicted on each plot are the approximate lines
[C4py][PhBF3], (e) [C4mpyr][PhBF3], (f) [C4mpip][PhBF3], (g) [N1,1,1,3][PhBF3], and (h)
[N4,4,4,1][PhBF3]. The measurements were conducted at a rate of 5 K·min−1
.
calculated using the least-square method.

K. Iwasaki et al. / Journal of Molecular Liquids 246 (2017) 236–243
239
Table 2
Unfortunately, certain information about such phase transitions could
only be obtained using a specially manufactured DSC at this time [45].
Fitted parameters for the density of the RTILs with various cations.
RTILs
a/g·cm−3
b × 104/g·cm−3·K−1
|R|
3.3. Physicochemical properties
[C4mim][BF4]
1.413
1.339
1.392
1.340
1.297
1.288
1.207
−7.113
−6.510
−6.711
−6.362
−6.136
−6.048
−6.020
N0.9999
N0.9999
N0.9999
N0.9999
N0.9999
N0.9999
N0.9999
[C4mim][PhBF3]
[C2mim][PhBF3]
[C4py][PhBF3]
[C4mpyr][PhBF3]
[C4mpip][PhBF3]
[N4,4,4,1][PhBF3]
The density data for the [PhBF3]−-based RTILs at 298 K are given in
Table 1 along with other physicochemical properties. The temperature
dependence of the density (d) of the [PhBF3]−-based RTILs is shown
in Fig. 4. The solid lines depicted in the figure are straight lines that
were calculated using the least-squares method. In general, density
can be represented as a function of absolute temperature by the follow-
ing equation:
except [N1,1,1,3][PhBF3], were liquid salts at room temperature. K[PhBF3]
has a melting point of 568 K [30], but the cation exchange of K+ for the
organic cations caused a sudden drop in the melting point due to the
larger cation size and the asymmetric structure of the organic cations.
The resulting organic salts were stable in air without any undesirable re-
actions with oxygen and moisture even though they were moderately
hygroscopic. All the salts had a lower water miscibility than the salts
with [BF4]−, and the hydrophobic phenyl group in [PhBF3]− causes
this characteristic.
d ¼ a þ bT
ð1Þ
where a is the density at 0 K (g·cm−3), b is a volume expansion coeffi-
cient (g·cm−3·K−1), and T is the absolute temperature (K). The results
fitted by the least-squares method are summarized in Table 2. The cor-
relation coefficient (|R|) indicates the precision of the fitting. The densi-
ties of the [PhBF3]−-based liquid salts were 1.028–1.192 g·cm−3 at
298 K (Table 1), and these values were closely related to the cationic
volume. The ascending order of the cation volume based on the
B3LYP/6-31G+(d) level calculations using the Gaussian 09 program
[41] is [C2mim]+ (0.160 nm3) b [C4py]+ (0.203 nm3) b [C4mim]+
(0.207 nm3) b [C4mpyr]+ (0.211 nm3) b [C4mpip]+ (0.231 nm3) b
3.2. Thermal behavior
The TG analysis results for the [PhBF3]−-based salts are shown in Fig.
2 along with the results for [C4mim][BF4] as a comparison. The thermal
degradation temperatures determined at a 5 wt% loss are given in Table
1. All the salts exhibited clear weight losses at temperatures in the range
from 477 to 514 K, and their thermal stabilities were inferior to that of
[C4mim][BF4]. This result is attributed to the [PhBF3]− anion containing
a B–C bond because the bond dissociation energy of the B–C bond
[N4,4,4,1]
+ (0.319 nm3), and this order coincides with the descending
order of the density. Several research groups have concluded that the
molecular volumes of RTILs are related to some of their physicochemical
parameters, e.g., density, viscosity, melting point, and dielectric con-
stant [46–48]. As we will discuss later, a similar relationship was also
observed in our RTILs.
(92 kcal·mol−1) is smaller than that of the B–F bond (181 kcal·mol−1
)
The [PhBF3]−-based RTILs showed viscosities (η) ranging from 60 to
1870 mPa·s at 298 K and ionic conductivities (σ) ranging from 0.112 to
7.09 mS·cm−1 at 303 K (Table 1). The RTILs with aromatic cations, i.e.,
[C4mim]+, [C2mim]+, and [C4py]+, had favorable transport properties.
Most RTILs with a π-conjugated cation show a similar tendency [49,
50]. The compact volume of aromatic cations, not the liquid structure
of RTILs, contributes to the transport properties, as shown in Fig. 5,
and this indicates that the molecular volume is related to the viscosity
and conductivity. The following equations proposed by Krossing et al.
were used to fit the data [48].
in [BF4]− [42]. The TG curves also indicated that the thermal degrada-
tion reaction proceeds via multiple steps, whereas [C4mim][BF4]
showed a simple one-step curve. As expected, the weight losses ob-
served for the first one or two steps were very close to the wt% of the
[PhBF3]− in the original RTILs, e.g., 51 wt% in [C4mim][PhBF3] and
56 wt% in [C2mim][PhBF3]. The DSC curves for the same RTILs are
given in Fig. 3. Almost all the salts showed a glass-transition tempera-
ture (Tg) at 208–228 K, but they did not show significant crystallization
or melting behaviors, except for [N1,1,1,3][PhBF3], which showed a sharp
exothermic peak in the cooling process at ca. 313 K and a noticeable en-
dothermic peak in the heating process at 353 K without any glass-tran-
sition behavior (Fig. 3g). In the case of [C4mpy][PhBF3] and
[N4,4,4,1][PhBF3], small crystallization and melting behaviors were ob-
served (Fig. 3e and h). These behaviors are induced by the very slow dy-
namics derived from the unique properties of RTILs [43,44].
η ¼ a1eb
ð2Þ
ð3Þ
1Vm
2Vm
σ ¼ a2e−b
where a1 (mPa·s) and a2 (mS·cm−1) are the empirical pre-exponential
factors, b1 (nm−3) and b2 (nm−3) are the empirical constants, and Vm
(nm3) is the molecular volume of each organic cation and the
[PhBF3]− anion in the RTILs. Their volumes were estimated using quan-
tum chemical calculations, and the data are summarized in Table 1. The
fitting parameters for Eqs. (2) and (3) are a1 = 4.00 × 10−2, b1 = 22.9,
a2 = 3.48 × 104, and b2 = 26.8. Interestingly, even though
[C4mpip][PhBF3] has some deviations, the plots for all the RTILs agree
well with the equations, suggesting that the cation volume is an impor-
tant factor in the [PhBF3]−-based RTIL system. The obtained slopes, b1
and b2, were larger than those for the other anions, e.g., [BF4]−
,
[N(CN)2]−, and [N(SO2CF3)2]− [48]. The values for b1 are related to
the strength of the intermolecular interactions in the RTILs, and the
same applies to b2. In the [PhBF3]−-based RTILs, a stronger interaction
was observed compared to the RTILs with other anions, which was con-
trary to our expectation that the interionic interaction energy would de-
crease due to the electron-withdrawing phenyl group on the anion. The
unexpected behavior could be due to the extra interaction induced by
the dynamic contact resistance between the organic cation and the
bulky phenyl group on [PhBF3]−.
Fig. 5. Correlation between the ( ) ionic pair volume and viscosity at 298 K and the (
ionic pair volume and ionic conductivity at 303 K. The original data are given in Fig. 4 and
Table 1.
)

240
K. Iwasaki et al. / Journal of Molecular Liquids 246 (2017) 236–243
Fig. 6. Arrhenius plots of the (a) viscosities and (b) ionic conductivities for (●) [C4mim][BF4], ( ) [C4mim][PhBF3], ( ) [C2mim][PhBF3], ( ) [C4py][PhBF3], ( ) [C4mpyr][PhBF3], (
)
[C4mpip][PhBF3], and ( ) [N4,4,4,1][PhBF3].
Arrhenius plots of the viscosity and ionic conductivity for the RTILs
with [PhBF3]− are shown in Fig. 6a and b, respectively. Each plot
shows gentle curves, and the curves are common for glass-forming
RTILs. In general, the activation energies of the transport properties, vis-
cosity and ionic conductivity can be discussed using Arrhenius plots.
However, using the plots to discuss the temperature dependence of
glass-forming liquids is difficult because of the complex system generat-
ed by the super-Arrhenius behavior [51]. In this case, the Vogel–
Tammann–Fulcher (VTF) equation, as expressed below, can be used to
fit the transport properties. Eqs. (4) and (5) show the VTF equation for
the viscosity and the equivalent ionic conductivity (Λ/S·cm2·mol−1),
respectively [52,53].
the following equations, which involve the partial differentiation of Eqs.
(4) and (5), respectively, with respect to temperature [52,53].
ꢀ
ꢁ
RkηT2
∂ ln η
∂T
RT
2
Ea;η ¼ −RT2
¼
−
ð7Þ
ð8Þ
2
ðT−T0Þ
ꢀ
ꢁ
∂ ln Λ
∂T
RkΛ T2
RT
2
Ea;Λ ¼ RT2
¼
−
2
ðT−T0Þ
The resulting plots of Ea,η and Ea,Λ versus the absolute temperature
are shown in Fig. 7. The ionic transport activation energies of the RTILs
strongly depended on the cation volume and temperature. The RTILs
with larger cations had higher activation energies, and this suggests
that the cation structure has a large effect on the activation energy,
which directly influences the transport properties. The activation ener-
gies decreased as the temperature increased, and almost all the Ea,η
values were higher than the Ea,Λ values at each temperature [20,22,
54]. Given that the differential capacitance measured via an AC imped-
ance technique is different from the capacitance estimated using the
static method because of the ultra-slow response to the electric double
layer formation [55], the observed differences between the Ea,η and Ea,Λ
values may be due to the AC impedance method used to perform the
ionic conductivity measurement.
The Walden plot method and pulsed-field-gradient spin-echo
(PGSE) NMR [56] spectroscopy are often used to estimate the dissocia-
tion degree of RTILs, i.e., the ionicity. Here, to investigate the ionicity of
the [PhBF3]−-based RTILs at each temperature, we evaluated the degree
of the ionic dissociation using the Walden plot method that requires
data on the equivalent ionic conductivity and the reciprocal of the vis-
cosity [20,22,57]. In Fig. 8, the Walden plots of the [PhBF3]−-based
RTILs are shown, and the plots of [C4mim][BF4] are included for compar-
ison. The diagonal line in the figure is an ideal line for a 1 M KCl aqueous
kη
T−T0
1
2
lnη ¼
þ
ln T− ln Aη
ð4Þ
ð5Þ
kΛ
T−T0
1
− lnΛ ¼
þ
ln T− ln AΛ
2
where kη (K) is a constant related to the Arrhenius activation energy for
the viscous behavior, kΛ (K) is a constant related to the conduction be-
havior, T0 (K) is an ideal glass-transition temperature, Aη is a scaling fac-
tor for the viscosity, and AΛ is a scaling factor for the equivalent ionic
conductivity. The experimental results for the specific ionic conductivity
can be converted to the equivalent conductivity using the following
equation.
Λ ¼ σM=d
ð6Þ
where M is the formula weight (g·mol−1) and d is the density
(g·cm−3). The fitted parameters obtained from Eqs. (4) and (5) are
summarized in Table 3. The temperature-dependent activation energies
for the viscosity (Ea,η) and the equivalent conductivity (Ea,Λ) are given by
Table 3
Fitted parameters for the VTF equations for the viscosity and equivalent conductivity of the [PhBF3]−-based RTILs.
RTILs
Derived from the viscosity
Derived from the equivalent conductivity
T0 /K
kη /K
ln Aη
|R|
T0 /K
kΛ /K
ln AΛ
|R|
[C4mim][BF4]
171
168
166
176
176
195
182
8.71 × 102
9.61 × 102
8.45 × 102
9.03 × 102
9.89 × 102
1.00 × 103
1.31 × 103
5.08
5.48
5.15
5.37
5.46
5.53
6.59
0.9999
0.9999
0.9999
0.9999
0.9999
0.9999
0.9999
148
152
132
148
162
180
178
2.70 × 103
1.04 × 103
1.41 × 103
1.19 × 103
1.09 × 103
2.78 × 103
4.14 × 103
13.2
9.21
10.6
9.90
9.62
14.5
17.2
0.9999
0.9999
0.9999
0.9999
0.9999
0.9999
0.9999
[C4mim][PhBF3]
[C2mim][PhBF3]
[C4py][PhBF3]
[C4mpyr][PhBF3]
[C4mpip][PhBF3]
[N4,4,4,1][PhBF3]

K. Iwasaki et al. / Journal of Molecular Liquids 246 (2017) 236–243
241
Fig. 7. Temperature-dependent activation energies for the (a) viscosity and (b) equivalent conductivity for (●) [C4mim][BF4], ( ) [C4mim][PhBF3], ( ) [C2mim][PhBF3], (
)
[C4py][PhBF3], ( ) [C4mpyr][PhBF3], ( ) [C4mpip][PhBF3], and ( ) [N4,4,4,1][PhBF3].
solution that is regarded as an ideal dissociation state. If there are par-
tially associated ion pairs in RTILs, the plots deviate from the ideal
line, and the degree of ionic dissociation can be visually estimated
based on the deviation. Interestingly, the plots of most of the
[PhBF3]−-based RTILs and [C4mim][BF4] were close to the ideal line.
The small gaps between the ideal line and the plots imply that the
RTILs have favorable dissociation degrees. In terms of the ionicity, the
introduction of the phenyl group to BF3 did not have a large influence.
(vs. Ag(I)/Ag), but the potentials were substantially lower than those
with [BF4]− (ca. 1.65 V). This was probably due to the oxidizable elec-
tron-rich phenyl group in [PhBF3]−. In contrast, the cathodic stabilities
differed based on the cation structure. The quaternary ammonium cat-
ions, [C4mpyr]+, [C4mpip]+, and [N4,4,4,1 +, had a higher stability than
]
that of the other aromatic cations. [C4py]+ had the lowest cathodic sta-
bility, and the stability difference from [C2mim]+ was ca. 1.1 V. This is an
acceptable potential gap considering the reported values observed in
[C2mim][AlCl4] and [C4py][AlCl4] [59].
3.4. Electrochemical analyses
The electrochemical stability, which is generally called the EW [58],
of the RTILs with [PhBF3]− was examined via linear sweep voltammetry.
The obtained voltammograms are shown in Fig. 9. Each scan was initiat-
ed from the open circuit potential, and all the cathodic limiting poten-
tials (Ecathodic), the anodic limiting potentials (Eanodic), and the EWs
(EW = Eanodic − Ecathodic) were estimated from Fig. 9. The electrochem-
ical data are summarized in Table 4. The ascending order of the RTIL
4. Conclusions
We successfully synthesized six novel RTILs and one organic salt
with [PhBF3]−. Their thermal stabilities are moderate and do not ex-
ceed that of the [BF4]−-based RTILs due to the decomposition of
[PhBF3]−, which has a thermally unstable B–C bond. However, the
other physicochemical properties of the [PhBF3]−-based RTILs were
basically favorable even though [PhBF3]− is a relatively rigid and
large anion compared to the typical anion components in RTILs. In-
troducing the phenyl group to the BF3 structure was not problematic.
In RTIL systems, the cationic species as well as the anionic ones has a
major effect on the physicochemical properties. Several design
criteria for RTIL systems were successfully obtained. The fundamen-
tal findings reported in this article will be useful for creating current-
ly unknown functional RTILs.
EWs is [C4py][PhBF3] (2.42 V)
b [C2mim][PhBF3] (3.71 V) b
[C4mim][PhBF3] (3.78 V) b [N4,4,4,1][PhBF3] (4.40 V) b [C4mpip][PhBF3]
(4.55 V) b [C4mpyr][PhBF3] (4.67 V). The RTILs with a quaternary am-
monium cation, which is known to be more electrochemically stable
than aromatic cations, showed wider EWs. Typically, the cathodic and
anodic limiting reactions in RTILs are the decompositions of the cation
and anion, respectively [27,36,38]. The anodic limiting potentials of
the RTILs with [PhBF3]− had relatively similar values of 0.584–0.804 V
Fig. 8. Walden plots for (●) [C4mim][BF4], ( ) [C4mim][PhBF3], ( ) [C2mim][PhBF3], (
)
Fig. 9. Linear sweep voltammograms recorded at a glassy carbon electrode in (- - -)
[C4py][PhBF3], ( ) [C4mpyr][PhBF3], ( ) [C4mpip][PhBF3], and ( ) [N4,4,4,1][PhBF3]. The
solid line in the figure is an ideal line constructed from the data for a 1 M KCl aqueous
solution.
[C4mim][BF4], ( ) [C4mim][PhBF3], ( ) [C2mim][PhBF3], ( ) [C4py][PhBF3], (
[C4mpyr][PhBF3], ( ) [C4mpip][PhBF3], and ( ) [N4,4,4,1][PhBF3] at 298 K. The scan
rate was 10 mV·s−1
)
.

242
K. Iwasaki et al. / Journal of Molecular Liquids 246 (2017) 236–243
[18] T. Torimoto, T. Tsuda, K. Okazaki, S. Kuwabata, New frontiers in materials science
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Cathodic and anodic limits and electrochemical windows for the [PhBF3]−-based RTILs.
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RTILs
Ecathodica/V vs. Ag(I)/Ag
Eanodicb/V vs. Ag(I)/Ag
EWc/V
[C4mim][BF4]
−2.99
−3.04
−2.97
−1.84
−3.86
−3.76
−3.66
1.65
4.64
3.78
3.71
2.42
4.67
4.55
4.40
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[C4mim][PhBF3]
[C2mim][PhBF3]
[C4py][PhBF3]
[C4mpyr][PhBF3]
[C4mpip][PhBF3]
[N4,4,4,1][PhBF3]
0.745
0.739
0.584
0.804
0.787
0.735
a
Cathodic limiting potential at the cut-off current density of −0.3 mA·cm−2
.
b
c
Anodic limiting potential at the cut-off current density of 0.3 mA·cm−2
.
Electrochemical window; EW = Eanodic − Ecathodic
.
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kylation, Angew. Chem. Int. Ed. 52 (2013) 13392–13396.
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This research was partially supported by JSPS KAKENHI Grant Num-
bers JP15H03591, JP15K13287, JP15H02202, and JP16K14539 and by
the Advanced Low Carbon Technology Research and Development Pro-
gram (ALCA) for Specially Promoted Research for Innovative Next Gen-
eration Batteries (SPRING), Japan Science and Technology Agency (JST).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.molliq.2017.09.067.
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