Wide polarized potential windows at the interface between water and an ionic liquid, tetraheptylammonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate

Article abstract of DOI:10.1246/cl.2007.1166

Wide polarized potential windows against interfacial ion transfer are available at the interface between a hydrophobic ionic liquid (IL), tetraheptylammonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate ([THpA ⁺][TFPB^-]) and an aqueous electrolyte solution (W) of MgSO₄. The width of the potential window, 1.1V at 60°C, enables electrochemical studies of charge transfer at the IL/W interface for a variety of ions and redox couples. Copyright

Full text of DOI:10.1246/cl.2007.1166

1166
Chemistry Letters Vol.36, No.9 (2007)
Wide Polarized Potential Windows at the Interface between Water and an Ionic Liquid,
Tetraheptylammonium Tetrakis[3,5-bis(trifluoromethyl)phenyl]borate
Ryoichi Ishimatsu, Naoya Nishi, and Takashi Kakiuchiꢀ
Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510
(Received June 25, 2007; CL-070680; E-mail: kakiuchi@scl.kyoto-u.ac.jp)
Wide polarized potential windows against interfacial ion
electrode was employed. The current due to the flow of cations
from W to IL was taken to be positive. Cyclic voltammograms
were recorded at the micro IL/W interface formed at the tip of
a glass pipette.4 The phase IV (W) was filled in a micropipette4
whose inner diameter at the tip was 1 ꢃ 0:3 mm. The phase II
(Wref) was in a glass tube, whose inner diameter was 0.5 mm.
For ion-transfer studies, tetrabutylammonium chloride (TBACl),
NaClO4, and NaSCN were dissolved in W, and the concentra-
tions were 1 mmol dmꢁ3. All voltammetric measurements were
made at 60 ꢂC.
transfer are available at the interface between a hydrophobic ion-
ic liquid (IL), tetraheptylammonium tetrakis[3,5-bis(trifluoro-
methyl)phenyl]borate ([THpAþ][TFPBꢁ]) and an aqueous
electrolyte solution (W) of MgSO4. The width of the potential
window, 1.1 V at 60 ꢂC, enables electrochemical studies of
charge transfer at the IL/W interface for a variety of ions and
redox couples.
When an ionic liquid (IL) is sufficiently hydrophobic, the in-
terface between the IL and an aqueous electrolyte solution (W)
can be electrochemically polarizable to allow us to use conven-
tional electrochemical methods, such as voltammetry for study-
ing equilibrium and nonequilibrium properties of charge transfer
and other interfacial properties in IL–W two-phase systems.1–5
The wider the potential window, the larger the degree of freedom
we have in choosing the subject of electrochemical studies at the
IL/W interface. Using a very hydrophobic anion, tetrakis[3,5-
bis(trifluoromethyl)phenyl]borate (TFPBꢁ) as the IL constitut-
ing anion, the wide window of about 0.8 V at 56 ꢂC was achiev-
ed.4 The transfer of moderately hydrophilic cations and hydro-
phobic anions such as choline4 and bis(trifluoromethylsulfo-
nyl)imide4 (C1C1Nꢁ), and the facilitated transfer of alkali metal
ions across the IL/W interface were successfully studied at this
interface.5 To detect the transfer of various cations and anions
across the IL/W interface, an even wider polarized potential
window is preferable. In this paper, we report a wide polarized
potential window of 1.1 V at 60 ꢂC at the micro interface be-
tween tetraheptylammonium tetrakis[3,5-bis(trifluoromethyl)-
phenyl]borate ([THpAþ][TFPBꢁ]) and an aqueous MgSO4 solu-
tion. The window is 0.3 V wider than that at the N-octadecyl-
isoquinolinium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate
([C18Iqþ][TFPBꢁ])/W interface.4
N-tetradecylisoquinolinium chloride (C14IqCl) was synthe-
sized by mixing isoquinoline (Aldrich) and 1-chlorotetradecane
(Aldrich) at 140–150 ꢂC for 20 h. Sodium tetrakis[3,5-bis(tri-
fluoromethyl)phenyl]borate (NaTFPB) was synthesized using a
procedure similar to that previously reported.6 [THpAþ]-
[TFPBꢁ] and N-tetradecylisoquinolinium tetrakis[3,5-bis(tri-
fluoromethyl)phenyl]borate ([C14Iqþ][TFPBꢁ]) were prepared
following a procedure as described elswhere.4 The melting point
of [THpAþ][TFPBꢁ] was 58–59 ꢂC. [C14Iqþ][TFPBꢁ] was
liquid at room temperature. The electrochemical cell in the pres-
ent study is represented as Cell(1), where X = THpA or C14Iq.
In this cell, the interface between phase II and III is electrochem-
ically nonpolarized, and the III and IV is the polarized interface.
The potential of the right-hand-side terminal of the cell with re-
spect to the left is hereafter denoted as E. When W contains LiCl
or MgCl2 an Ag/AgCl electrode was employed as the reference
electrode (V). In the case of Li2SO4 or MgSO4, an Ag/Ag2SO4
(1)
,
A
cyclic voltammogram at the interface between
[C14Iqþ][TFPBꢁ] and an aqueous 0.01 mol dmꢁ3 LiCl solution
is shown in Figure 1 (curve a). The width of the polarized poten-
E / V
-0.3
0.1
0.5
0.9
1.3
a
e
0.1 nA
b
c
d
-1.1
-0.7
-0.3
E / V
0.1
0.5
Figure 1. Cyclic voltammograms at the microinterface
between [C14Iqþ][TFPBꢁ] and an aqueous 0.01 mol dmꢁ3 LiCl
solution (a), and between [THpAþ][TFPBꢁ] and an aqueous
solution of 0.01 mol dmꢁ3 LiCl (b), Li2SO4 (c), MgCl2 (d), or
MgSO4 (e) at 60 ꢂC. The inner diameter of the micropipette
tip: 1 mm. The voltage scan was made from the middle of the
polarized potential window to the positive at the scan rate of
0.02 V sꢁ1
.
Copyright ꢀ 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.9 (2007)
1167
tial window was 0.85 V, which is comparable to those at
[C18Iqþ][TFPBꢁ]/W and trioctylmethylammonium tetrakis-
[3,5-bis(trifluoromethyl)phenyl]borate ([TOMAþ][TFPBꢁ])/W
interfaces (0.8 V).4 In fact, C18Iqþ and TOMAþ are much more
hydrophobic than C14Iqþ.7 The similarity in the width of the
polarized potential windows suggests that the potential at the
negative ends is independent of the hydrophobicity of the anions
constituting the ILs and is determined by the transfer of Clꢁ
from W to IL. Figure 1b shows a cyclic voltammogram at the
interface between [THpAþ][TFPBꢁ] and an aqueous 0.01
mol dmꢁ3 LiCl solution. The width of the polarized potential
window was 0.95 V, which is 0.1 V wider than that at
[C14Iqþ][TFPBꢁ]/W interface. This difference of the polarized
potential window is similar to that between [THpAþ][C2C2Nꢁ]/
W and [C14Iqþ][C2C2Nꢁ]/W interfaces8 and indicates a strong-
er affinity of Clꢁ with C14Iqþ, C18Iqþ, or TOMAþ than that with
THpAþ in these IL bulks.4,8
0.05 nA
a
b
c
-1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3
E / V
Figure 2. Cyclic voltammograms for the ion transfer of (a)
ClOꢁ4 , (b) TBAþ, and (c) SCNꢁ from W to IL side at the inter-
face between [THpAþ][TFPBꢁ] and an aqueous 0.01 mol dmꢁ3
LiCl solution at 60 ꢂC. The concentrations of the anion in W
were 1 mmol dmꢁ3, and the scan rate was 0.02 V sꢁ1. The dashed
lines show the level of zero currents and limiting currents.
Increasing the concentration of LiCl in W decreased the
polarized potential window at both positive and negative ends
at the [THpAþ][TFPBꢁ]/W interface (data are not shown). This
fact shows that the positive and negative ends are limited by the
transfer of Liþ and Clꢁ from W to the IL, respectively. It is then
possible to extend the polarized potential window by using more
hydrophilic ions in W. Curves c, d, and e in Figure 1 show cyclic
voltammograms at the interface between [THpAþ][TFPBꢁ] and
aqueous 0.01 mol dmꢁ3 Li2SO4, MgCl2, and MgSO4 solutions,
respectively. The widths of the polarized potential windows
are 1.0 (curve c), 1.05 (curve d), and 1.1 (curve e) V. The posi-
tive currents more sharply increased when the aqueous solution
In conclusion, a wider polarized potential window of 0.95 V
at the IL/W interface has been achieved by suppressing the
facilitated transfer of Clꢁ. By using more hydrophilic ions for
supporting electrolyte in W, such as Mg2þ and SO42ꢁ, the widest
polarized potential window over 1 V has been observed. This
wide potential window enables us to observe the transfer of
ꢁ
the moderately hydrophilic SCNꢁ and ClO4 and hydrophobic
contains Mg2þ (Figure 1 curves d and e), whereas the negative
TBAþ across the IL/W interface and provides a voltammetric
means for studying the ion transfer for a range of anions and
cations at the same interface and also the electron transfer for
a variety of redox couples.
2ꢁ
currents more sharply decreased when SO4
exists in W
(Figure 1 curves c and e). It is likely, therefore, that the positive
and negative ends of the windows are limited by the transfer
of these ions in W from W to IL.
Figure 2 shows cyclic voltammograms for the transfer of
ꢁ
tetrabutylammonium (TBAþ, curve a), ClO4 (curve b), and
References
1
SCNꢁ (curve c) across the [THpAþ][TFPBꢁ]/W interface at
the scan rate of 0.02 V sꢁ1. Plateau currents were observed for
all curves in Figure 2. Such steady-state voltammograms have
also been found at [C18Iqþ][TFPBꢁ]/W4 and tetrahexylammo-
nium bis(trifluoromethylsulfonyl)imide ([THxAþ][C1C1Nꢁ])/
W9 interfaces and have been ascribed to the tapered shape of
the micropipette.4,9 We evaluated the half-wave potentials,
E1=2, at the scan rate of 0.02 V sꢁ1 for the transfer of TBAþ,
ClO4ꢁ, and SCNꢁ across the interface, which are ꢁ0:70,
ꢁ0:75, and ꢁ0:81 V, respectively. The values of E1=2 for SCNꢁ
T. Kakiuchi, N. Tsujioka, Electrochem. Commun. 2003, 5,
253.
2
3
H. Katano, H. Tatsumi, Anal. Sci. 2003, 19, 651.
R. Ishimatsu, F. Shigematsu, T. Hakuto, N. Nishi, T.
Kakiuchi, Langmuir 2007, 23, 925.
4
5
6
7
8
9
N. Nishi, S. Imakura, T. Kakiuchi, Anal. Chem. 2006, 78,
2726.
N. Nishi, H. Murakami, S. Imakura, T. Kakiuchi, Anal.
Chem. 2006, 78, 5805.
H. Nishida, N. Takada, M. Yoshimura, T. Sonoda, H.
Kobayashi, Bull. Chem. Soc. Jpn. 1984, 57, 2600.
S. Tanaka, Y. Kitazumi, A. Suzuki, Y. Matsuoka, N. Nishi,
T. Kakiuchi, in preparation.
R. Ishimatsu, N. Nishi, T. Kakiuchi, Langmuir 2007, 23,
7608.
ꢁ
and ClO4 differ by 0.06 V, which is comparable to the value of
0.1 V at the nitrobeꢁnzene/W interface at 25 ꢂC.10 The transfer
of TBAþ and ClO4 has been found at [THxAþ][C1C1Nꢁ]/W
interface9 and that of SCNꢁ at tetraoctylammonium 2,4,6-
trinitrophenolate/W interface at 25 ꢂC.2 The transfer of these
moderately hydrophobic cation and moderately hydrophilic
anions were successfully observed when the ILs composed of
the hydrophobic symmetric quaternary ammonium, such as
THpAþ.
N. Tsujioka, S. Imakura, N. Nishi, T. Kakiuchi, Anal. Sci.
2006, 22, 667.
10 B. Hundhammer, S. Wilke, J. Electroanal. Chem. 1989, 266,
133.
Published on the web (Advance View) August 18, 2007; doi:10.1246/cl.2007.1166