Article pubs.acs.org/jced
Density, Viscosity, and Electrical Conductivity of Protic Amidium Bis(trifluoromethanesulfonyl)amide Ionic Liquids Masaki Watanabe,† Daisuke Kodama,*,† Takashi Makino,‡ and Mitsuhiro Kanakubo‡ †
Department of Chemical Biology and Applied Chemistry, College of Engineering, Nihon University, 1 Nakagawara, Tokusada, Tamura-machi, Koriyama, Fukushima 963-8642 Japan ‡ Research Institute for Chemical Process Technology, National Institute of Advanced Industrial Science and Technology (AIST), 4-2-1 Nigatake, Miyagino-ku, Sendai, Miyagi 983-8551 Japan S Supporting Information *
ABSTRACT: We investigated the densities, viscosities, and electrical conductivities of the protic amidium-based ionic liquids with bis(trifluoromethanesulfonyl)amide ([TFSA]−). The cations were N,N-dimethylformamidium, [DMFH]+, N,N-dimethylacetamidium, [DMAH]+, and N,N-dimethylpropionamidium, [DMPH]+. The physical properties were measured over the temperature range from 273.15 to 363.15 K at atmospheric pressure. The densities were correlated with the linear or quadratic equations, and the transport properties were reproduced well with the Vogel−Fulcher− Tammann equation. The densities and the viscosities increased in the following order: [DMAH][TFSA] > [DMPH][TFSA] > [DMFH][TFSA]. The opposite trend was observed for the electrical conductivities. The empirical Walden plots gave the straight lines in all the present ionic liquids. It was found that the data points for [DMFH][TFSA] appreciably fall below [DMPH][TFSA] and [DMAH][TFSA] on the Walden plot.
1. INTRODUCTION Ionic liquids (ILs) are salts that melt at temperatures lower than 100 °C.1 They generally consist of a large asymmetric cation and organic or inorganic anion. ILs have negligibly small vapor pressures, nonflamability, high thermal and chemical stability, wide electrochemical windows, and high solubilities of acidic gases such as CO2, NOx and SOx.2−4 On the basis of these characteristics, ILs have attracted attention as electrolytes for secondary batteries, media for the separation processes, and solvents of chemical reactions, and so on. In addition, the physicochemical properties of ILs are optimized for individual applications by the chemical modifications as well as the combinations of cation and anion. A number of studies have been performed to understand the nature of ILs. One of the unfavorable features for the industrial applications is the higher production cost. Protic ionic liquids (PILs) are a class of ILs and have similar characteristics to typical aprotic ILs. Moreover, PILs are readily synthesized by the neutralization of a Brønsted acid and base. The physicochemical properties of the PILs have been investigated; for example, the volumetric and transport properties have been reported for the ammonium and phosphonium based PILs.5−7 It is pointed out that the degree of the proton transfer from an acid to a base strongly affects the physical properties, in particular, transport properties. We have also reported that the protic N,Ndimethylformamidium bis(trifluoromethanesulfonyl)amide ([DMFH][TFSA]) absorbs a larger amount of CO2 (in © XXXX American Chemical Society
volume concentration scale) than the typical aprotic 1-butyl3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([BMIM][TFSA]), which shows the highest class of CO2 solubility.8 In the present study, the physical properties of the amidiumbased PILs were investigated over the wide temperature range. There are very limited literature data,9 therefore, there is little information concerning such kinds of physical properties of the amidium ILs. However, the volumetric and transport properties are fundamental engineering data, which are essential to design some chemical processes. We report the densities, viscosities, and electrical conductivities of a series of amidium bis(trifluoromethanesulfonyl)amide PILs at atmospheric pressure over the temperature range from 273.15 to 363.15 K. The amidium cations were [DMFH]+, N,N-dimethylacetamidium ([DMAH]+), and N,N-dimethylpropionamidium ([DMPH]+). The effects of cation species on the physical properties were discussed based on the length of the alkyl chain. Furthermore, densities and viscosities of the precursor amides, namely N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), and N,N-dimethylpropionamide (DMP), were reported under the same conditions, in order to compare the effects of the addition of H[TFSA] on the properties. Special Issue: Proceedings of PPEPPD 2016 Received: July 2, 2016 Accepted: November 14, 2016
A
DOI: 10.1021/acs.jced.6b00575 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
Table 1. Chemicals Used in the Present Study chemical
purity (mol %)
source
analysis method
N,N-dimethylformamidium bis(trifluoromethanesulfonyl)amide ([DMFH][TFSA]) N,N-dimethylacetamidium bis(trifluoromethanesulfonyl)amide ([DMAH][TFSA]) N,N-dimethylpropionamidium bis(trifluoromethanesulfonyl)amide ([DMPH] [TFSA]) N,N-dimethylformamide (DMF) N,N-dimethylacetamide (DMA) N,N-dimethylpropionamide (DMP) lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]) bis(trifluoromethanesulfonyl)amine (H[TFSA]) methanol dichloromethane nitric acid
synthesized synthesized synthesized
>98 >99 >99
NMR NMR NMR
Tokyo Chemical Industry Co., Ltd. Tokyo Chemical Industry Co., Ltd. Tokyo Chemical Industry Co., Ltd. Kanto Chemical Co., Inc. Morita Chemical Industries Co., Ltd. Wako Pure Chemical Industries, Ltd. Wako Pure Chemical Industries, Ltd. Wako Pure Chemical Industries, Ltd.
>99.5 >99.0 >98.0 >99.7 >98.0 >99.8 >99.5 60
gas chromatography gas chromatography gas chromatography silver nitrate titration not available gas chromatography gas chromatography not available
2. EXPERIMENTAL SECTION 2.1. Materials. The chemicals used in the present work were listed in Table 1. The amidium PILs were prepared in our laboratory. DMF, DMA, and DMP were purchased from Tokyo Chemical Industry Co., Ltd. Lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]) and bis(trifluoromethanesulfonyl)amine (H[TFSA]) were obtained from Kanto Chemical Co., Inc. and Morita Chemical Industries Co., Ltd. Methanol, dichloromethane, and nitric acid were supplied by Wako Pure Chemical Industries, Ltd. All chemicals were used without further purification. 2.2. Synthesis of PILs. [DMFH][TFSA] was synthesized according to the procedure reported in our previous study.8 The synthetic methods for [DMAH][TFSA] and [DMPH][TFSA] were the same as described in the literature.10 An equimolar amount of DMA or DMP was slowly added to a methanol solution of H[TFSA] with stirring in an ice bath. The mixture was stirred at room temperature for 12 h. The solutions were evacuated at 333 K for 1 h to remove residual methanol by using a rotary evaporator, and then, the yellowish amidium PILs were obtained. The present PILs were further dried under vacuum for 96 h at 343 K. The products were identified with 1 H and 13C NMR (JEOL, JNM-ECX400), and the assignments were given in Supporting Information. The estimated purities were higher than 98% for [DMFH][TFSA], 99% for [DMAH][TFSA], and 99% for [DMPH][TFSA]. The water content (w/w) in the PILs was measured with a Karl Fischer titration (KEM, MKC-520) as 20 ppm for [DMFH][TFSA], 53 ppm for [DMAH][TFSA], and 27 ppm for [DMPH][TFSA]. 2.3. Apparatus and Procedure. The experimental apparatus for the density, viscosity, and electrical conductivity
Table 2. Potential Energies E of the N−H and O−H Type Amidiums N−H type amidium
O−H type amidium
MJ mol−1 −653.97 −757.35 −860.68
[DMFH]+ [DMAH]+ [DMPH]+
−654.05 −757.40 −860.73
Table 3. Density ρ, Viscosity η, Electrical Conductivity κ and Molar Conductivity Λ of [DMFH][TFSA] at Atmospheric Pressure (p = 0.1 MPa)a T K 273.15 278.15 283.15 288.15 293.15 298.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15
ρ kg·m
η −3
1517.50 1512.05 1506.63 1501.25 1495.89 1490.56 1485.25 1474.71 1464.25 1453.87 1443.57 1433.33 1423.15
κ
Λ −1
mPa·s
S·m
107.2 79.51 60.61 47.45 37.93 30.71 25.32 17.99 13.34 10.27 8.13 6.60 5.47
0.224 0.293 0.373 0.465 0.570 0.687 0.815 1.108 1.441 1.808 2.210 2.645
μS·m ·mol−1 2
52.3 68.6 87.7 109.8 134.9 163.3 194.4 266.2 348.7 440.5 542.3 653.8
a Standard uncertainties u are u(T) = 0.01 K and u(p) = 1 kPa. Relative expanded uncertainties Ur are Ur(ρ) = 0.004, Ur(η) = 0.02, Ur(κ) = 0.02, and Ur(Λ) = 0.02 (95% level of confidence).
measurements were the same as described in our previous studies.11−14 The densities of [DMFH][TFSA] and [DMPH][TFSA] were determined by using a vibrating tube densimeter (Anton Paar, DMA 5000M). The instrumental constants were calibrated with dry air and Milli-Q water (Millipore Direct-Q3 UV). The density of [DMAH][TFSA] and the viscosities of the present PILs were measured by using a rotating-cylinder viscometer (Anton Paar, Stabinger SVM 3000) that was equipped with a built-in densimeter. We have confirmed the validity of this viscometer using the reference samples supplied by Cannon Instrument Company. The samples were transferred to a gastight syringe under dry nitrogen atmosphere (the dew point was less than 243 K) and injected into the instruments without contacting moisture. The densities and viscosities of DMF, DMA, and DMP were measured by the
Figure 1. Chemical structures of the present cations and anion: (a) N−H type amidium; (b) O−H type amidium; (c) [TFSA]−; (d) [DMFH]+; (e) [DMAH]+; (f) [DMPH]+. B
DOI: 10.1021/acs.jced.6b00575 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
Table 4. Density ρ, Viscosity η, Electrical Conductivity κ, and Molar Conductivity Λ of [DMAH][TFSA] at Atmospheric Pressure (p = 0.1 MPa)a ρ
T K 313.15 323.15 333.15 343.15 353.15 363.15
kg·m
η −3
1549.8 1539.4 1529.1 1518.6 1508.1 1497.8
κ
Λ −1
mPa·s
S·m
67.83 44.71 31.31 22.85 17.35 13.51
0.403 0.574 0.782 1.024 1.295
μS·m ·mol−1 2
95.7 137.3 188.3 248.3 316.1
a
[DMAH][TFSA] was solid at the lower temperatures than 313.15 K, so we could not measure the physical properties. Standard uncertainties u are u(T) = 0.01 K and u(p) = 1 kPa. Relative expanded uncertainties Ur are Ur(ρ) = 0.001, Ur(η) = 0.02, Ur(κ) = 0.02, and Ur(Λ) = 0.02 (95% level of confidence).
Figure 2. Temperature dependence of the density ρ for protic amidium ionic liquids. Circle (filled), [DMFH][TFSA]; triangle (filled), [DMAH][TFSA]; square (filled), [DMPH][TFSA]; circle (open), dimethylformamide; triangle (open), dimethylacetamide; square (open), dimethylpropionamide; cross, dimethylformamide (ref 23); plus, dimethylacetamide (ref 23). The solid lines are regressed by eq 1 or 2.
Table 5. Density ρ, viscosity η, Electrical Conductivity κ, and Molar Conductivity Λ of [DMPH][TFSA] at Atmospheric Pressure (p = 0.1 MPa)a T K 273.15 278.15 283.15 288.15 293.15 298.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15
ρ kg·m
η −3
1546.80 1541.52 1536.20 1530.91 1525.62 1520.35 1515.12 1504.73 1494.43 1484.21 1474.06 1463.98 1453.97
κ
Λ −1
mPa·s
S·m
583.6 401.3 284.2 207.2 154.9 117.7 91.33 57.92 39.33 27.84 20.58 15.67 12.28
0.053 0.075 0.103 0.136 0.177 0.225 0.280 0.415 0.585 0.787 1.014 1.277
μS·m ·mol−1 2
13.2 18.6 25.5 34.0 44.3 56.5 70.7 105.4 149.7 202.6 263.0 333.4
a Standard uncertainties u are u(T) = 0.01 K and u(p) = 1 kPa. Relative expanded uncertainties Ur are Ur(ρ) = 0.001, Ur(η) = 0.02, Ur(κ) = 0.02, and Ur(Λ) = 0.02 (95% level of confidence).
Figure 3. Temperature dependence of viscosity η for protic amidium ionic liquids. Circle (filled), [DMFH][TFSA]; triangle (filled), [DMAH][TFSA]; square (filled), [DMPH][TFSA]; circle (open), dimethylformamide; triangle (open), dimethylacetamide; square (open), dimethylpropionamide; cross, dimethylformamide (ref 24); plus, dimethylacetamide (ref 25). The solid lines are regressed by eq 3.
same procedure as the PILs. These amides were degassed under vacuum for 20 min at 313 K just before the measurements. The electrical conductivity was measured using an impedance analyzer (Bio Logic, SP-150). A syringe-type cell with a pair of platinum electrodes was employed. The solution resistance (Rsol) was obtained from the Nyquist plot by fitting the measured impedances to the best-fit form of an electric circuit. The sample temperature was kept within ±0.01 K in all the measurements. The expanded uncertainties for the densities, viscosities, and electrical conductivities are 0.05 kg m−3 (0.5 kg m−3 for [DMAH][TFSA]), < 2%, and
