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T. Umecky et al. / Journal of Molecular Liquids 209 (2015) 557–562
and a 5.0 mmϕ outer tube (Shigemi, PS-001-7) containing sample
solution for the chemical shift measurements. A 0.75 wt.% 3-
(trimethylsilyl)propionic-2,2,3,3-d4 acid (TSP) in D2O solution
(Sigma-Aldrich), 3.00 mol dm−3 lithium chloride (N99.99%,
Sigma-Aldrich) in D2O (99.96 at.%D, Sigma-Aldrich), nitromethane-d3
(99 at.%D, Sigma-Aldrich), and hexafluorobenzene (N99.5%, Sigma-
Aldrich) were used as external references for 1H and 13C, 7Li, 15N, and
19F nuclei, respectively.
Self-diffusion coefficient measurements were conducted on pure ILs
and LiTFSA-IL solutions using the JEOL ECA-600 spectrometer. To sup-
press convection flow as much as possible, an NMR microtube (Shigemi,
DMS-005J) was used in the diffusion experiments. The sample solution
in the NMR tube was less than 5 mm in height. Self-diffusion coefficients
of Dcation, Danion, and DLi were determined from 1H NMR peak of the ter-
minal methyl group of the cations, 19F and 7Li peaks of TFSA− and Li+,
respectively. A pulsed field gradient spin-echo sequence was used in
all the self-diffusion coefficient measurements. The diffusion time was
fixed at 100 ms. The gradient magnitude (g) or the gradient pulse
width (δ) was varied during the measurements. The gradient magni-
tudes were calibrated using the self-diffusion coefficient of water at
313.2 K [28]. The self-diffusion coefficients determined from both
methods with varying g and δ were in good agreement with each
other within the deviation of 3%. The self-diffusion coefficients deter-
mined by both methods were averaged to obtain the final values.
7Li longitudinal relaxation times (T1,Li) were measured using the
above two NMR spectrometers. The sample solutions were sealed into
NMR microtubes (Shigemi, SP-001). A standard inversion recovery
sequence was used in the T1,Li measurements.
Fig. 1. Molecular structures of C8dabco+ and C8mim+ with the position-numbers in the
parentheses according to the IUPAC numeration.
[C8dabco]Br was precipitated in the solution. Ethyl acetate phase was
removed by filtration, and ethyl acetate adsorbed on the salt was re-
moved by heating at ~343 K for 6 h under vacuum. The salt was recrys-
tallized twice from isopropyl alcohol (N99.7%, Wako). The refined salt of
[C8dabco]Br was filtrated from the solvent, and then the solvent was
evaporated at ~343 K for 6 h under reduced pressure. On the contrary,
[C8mim]Br was obtained from the reaction of 1-methylimidazole
(N99.0%, Tokyo Kasei) with 1-bromooctane in acetonitrile (N99.8%,
Wako) by the conventional method [26]. The salt of [C8mim]Br was re-
crystallized from toluene (N99.8%, Wako). The refined salt of [C8mim]Br
was heated at ~343 K for 6 h under reduced pressure to remove the
excess solvent. Aqueous solutions of KTFSA (N99.8%, Kanto Kagaku)
and HTFSA (N99.0%, Kanto Kagaku) were added to aqueous solutions
of [C8dabco]Br and [C8mim]Br, respectively, to replace Br− with TFSA−
. Note that in the synthesis of [C8dabco][TFSA] highly Lewis-acidic
ions, such as H+ and Li+, cannot be used as the counter cation of
TFSA− because such ions strongly interact with Lewis-basic tertiary
amine of C8dabco+. After stirring the biphasic mixture of each IL system
for 24 h, the upper aqueous phase was decanted, and then the lower
nonaqueous phase was washed more than five times with pure water
(Millipore, Elix UV 3 water purification systems). Water in the IL
phase was removed under vacuum at ~323 K for 8 h or more. Finally,
each colorless viscous liquid of [C8dabco][TFSA] and [C8mim][TFSA]
thus obtained was identified by 1H and 13C NMR spectroscopic mea-
surements and elemental analyses for the H, C, and N atoms; Anal.
Calc. (%) for C16H29N3O4F6S2 (C8dabcoTFSA): C, 38.01, H, 5.78, N, 8.31,
Found: C, 37.96, H, 5.65, N, 8.58 and Anal. Calc. (%) for C14H23N3O4F6S2
(C8mimTFSA): C, 35.36, H, 4.88, N, 8.84, Found: C, 35.44, H, 4.82, N,
9.03. These results showed no major organic impurity in both ILs. More-
over, residual Br− in the ILs could not be detected by a fluorescence X-
ray analysis (Shimadzu, Rayny EDX-800HS). The water contents of
[C8dabco][TFSA] and [C8mim][TFSA] were determined by a Karl-Fisher
titration to be 73 and 70 ppm, respectively. LiTFSA (N99.95%, Sigma-
Aldrich) was dissolved into each IL at xLi = 0.1 under nitrogen
atmosphere.
3. Results and discussion
3.1. Density (ρ), viscosity (η), and electrical conductivity (κ)
The ρ, η, and κ values at 313.2 K are summarized in Table 1. The ρ
value of pure [C8mim][TFSA] agrees with the literature data within
1% [29–31]. The other physicochemical properties of the C8dabco+-
and C8mim+-IL systems at the temperature investigated are unfortu-
nately unavailable in the literature. For both IL systems, the ρ and η
values increase by the dissolution of Li+ into the ILs, while the κ values
decrease. These features are comparable with the previous results of
pyrrolidinium- and ammonium-ILs [2,8,10,14]. The molar conductivi-
ties (Λ) of the IL solutions at xLi = 0 and 0.1 were estimated from the
values of ρ and κ by
κ ꢀ Mmix
Λ ¼
;
ð1Þ
ρ
Table 1
Densities (ρ), viscosities (η), electrical conductivities (κ), molar conductivities (Λ), Walden
products (ηΛ), self-diffusion coefficients (Dcation, Danion, DLi), Nernst–Einstein deviation pa-
rameters (Δ), and hydrodynamic radii of cation (rcation), anion (ranion), and Li+ (rLi) in
C8dabco+- and C8mim+-IL solutions with LiTFSA at xLi = 0 and 0.1 and 313.2 K.
IL
[C8dabco][TFSA]
[C8mim][TFSA]
xLi
0
0.100
0
0.100
ρ/g cm−3
1.32
276
0.0375
1.44
3.98
0.34
0.37
–
1.34
543
1.31
47.7
0.246
8.95
4.27
2.4
2.1
–
0.20
0.23
–
1.33
60.3
0.199
6.85
4.13
1.9
η/mPa s
κ/S m−1
0.0207
0.748
4.06
0.19
0.19
0.022
0.22
0.22
1.9
Λ/10−5 S m2 mol−1
ηΛ/10−6 Pa s S m2 mol−1
Dcation/10−11 m2 s−1
Danion/10−11 m2 s−1
DLi/10−11 m2 s−1
rcation/nm
Density (ρ), viscosity (η), and electrical conductivity (κ) of pure ILs
and LiTFSA-IL solutions were determined at 313.2 K using the same
equipments as those in the previous report [27].
1.5
0.17
0.20
0.26
2.2
0.24
0.22
–
NMR spectra of 1H, 7Li, 13C, 15N, and 19F nuclei were recorded on the
Agilent 400 MHz NMR system. A double tube was constructed from a
3.0 mmϕ inner tube (Shigemi, PN-001) including the reference solution
ranion/nm
rLi/nm
Δ
0.44
0.43
0.44
0.40