Densities, Viscosities, and Conductivities of the Imidazolium Ionic

Article pubs.acs.org/jced

Densities, Viscosities, and Conductivities of the Imidazolium Ionic Liquids [Emim][Ac], [Emim][FAP], [Bmim][BETI], [Bmim][FSI], [Hmim][TFSI], and [Omim][TFSI] Andreas Nazet,† Sophia Sokolov,† Thomas Sonnleitner,† Takashi Makino,‡ Mitsuhiro Kanakubo,*,‡ and Richard Buchner*,† †

Institut für Physikalische und Theoretische Chemie, Universität Regensburg, D-93040 Regensburg, Germany National Institute of Advanced Industrial Science and Technology (AIST), 4-2-1 Nigatake, Miyagino-ku, Sendai 983-8551, Japan

‡

S Supporting Information *

ABSTRACT: Data for the transport properties electrical conductivity, κ, and dynamic viscosity, η, of the imidazolium ionic liquids [Emim][FAP], [Emim][Ac], [Bmim][BETI], [Bmim][FSI], [Hmim][TFSI], and [Omim][TFSI] (κ only) is presented. Electrical conductivity has been studied in the wide temperature range of (273.15 to 468.15) K, whereas η was determined in the range of (273.15 to 408.15) K. The data could be well fitted by the empirical Vogel−Fulcher−Tammann equation. Additionally, the densities of these ionic liquids, showing a linear dependence on temperature, were collected from (273.15 to 363.15) K.

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INTRODUCTION Ionic liquids (ILs) are molten salts with anarbitrarily definedmelting point < 373 K.1 These compounds exhibit a variety of highly desirable properties, including but not limited to a wide liquid range, very low vapor pressure, and high thermal stability. Accordingly, these compounds have aroused considerable interest during the past decade.2−4 In particular, ILs are highly promising, either in neat form or in mixtures with proton conductors, as electrolytes in high-temperature fuel cells or other electrochemical devices.5−8 Especially for such applications but also in many other areas knowledge on the transport properties, namely, dynamic viscosity, η, and electrical (ionic) conductivity, κ, over a temperature range as wide as possible is essential. Density, ρ, is also a frequently needed fundamental property. Numerous data for ρ, η, and κ have been published for ILs. However, a perusal of the literature reveals that most of them coverat bestonly a very limited temperature range. Also, data from different sources vary often considerably. In this contribution we report data collected in our laboratories on the dynamic viscosity, electrical conductivity, and density of the neat 1-alkyl-3-methylimidazolium ionic liquids listed in Table 1, covering the temperature range of (273.15 to 408.15) K for η, (273.15 to 468.15) K for κ, and (273.15 to 363.14) K for ρ. Especially for electrical conductivity, the present results considerably extend the existing database to high temperatures and complement previous investigations in our laboratories.9−12 The selection of ILs (Table 1) was mainly guided by ongoing investigations © XXXX American Chemical Society

into their molecular-level dynamics, but [Hmim][TFSI] was chosen because this IL was used for a IUPAC round-robin (i.e., an interlaboratory test with measurements performed independently in several laboratories using batches of the same, specially prepared, sample)13,14 with the aim of establishing standards for the determination of the physical properties of ionic liquids. Thus, the results of refs 13 and 14 are an excellent base to crosscheck and validate the present data. Data for ρ and η of [Omim][TFSI] will be discussed separately by Harris and Kanakubo.12 For the present contribution we complemented ambient-pressure conductivity data for this IL from ref 12 to cover the temperature range of (273.15 to 468.15) K and discuss the combined data sets. To permit interpolation and for easier comparison with the literature, the present values were fitted with appropriate equations, namely, a linear equation for ρ and the Vogel− Fulcher−Tamann (VFT) equation for η and κ (see below). To assess the quality of the present results and their agreement with published data, the relative deviation δY = (Y − Yfit)/Yfit

(1)

of the experimental value, Y(T) [= ρ, η, κ], of this investigation or the literature from the present fit, Yfit(T), is discussed. Received: March 26, 2015 Accepted: July 3, 2015

A

DOI: 10.1021/acs.jced.5b00285 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Table 1. ILs Studied, Their Abbreviations, CAS Numbers, Suppliers, Overall Purities (w/w) (Halide Mass Fraction in Brackets), and Water Mass Fractions, w(H2O), after Drying IL

abbreviation

1-ethyl-3-methylimidazolium acetate 1-ethyl-3-methylimidazolium tris(pentafluoroethyl) trifluorophosphate 1-butyl-3-methylimidazolium bis(perfluoroethylsulfonyl) imide 1-butyl-3-methylimidazolium bis(fluorosulfonyl)imide 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

[Emim][Ac] [Emim] [FAP] [Bmim] [BETI] [Bmim][FSI] [Hmim] [TFSI] [Omim] [TFSI]

CAS no.

supplier

w(H2O)a

purity (w/w) b

143314-17-4 377739-43-0

IoLiTec Merck

254731-29-8 1235234-58-8 382150-50-7

prepared analogous to [Bmim] [TFSI]15 Kanto Chemical Co. IoLiTec

178631-04-4

see ref 12 for preparation

> 0.95 (< 0.01) > 0.999b ( 0.99d (not detect.)e > 0.99b ≥ 0.99b > 0.99d (not detect.)e

b,c

1.25·10−3 24·10−6 170·10−6 3·10−6 14·10−6 460·10−6

a Karl Fischer titration with 10% relative standard uncertainty for w(H2O) < 200·10−6 and ∼5 % otherwise; bManufacturer’s specification. cIon chromatography. dNMR 1H and 13C resonances. eBelow detection limit (∼10−5 mol·L−1) of AgNO3 testing.

Table 2. Densities, ρ, of the Investigated ILs at Pressure p = 0.1 MPa.a ρ/kg·m−3 T/K 273.15 278.15 283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15 358.15 363.15

[Emim][Ac]

[Emim][FAP]

111(1.95)

1732.(93)

110(5.67)

1720.(83)

109(9.44)

1708.(84)

109(3.24)

1696.(96)

108(7.13)

1685.(16)

108(1.09)

1673.(43)

107(5.11)

1661.(79)

106(9.17)

1650.(23)

106(3.30)

1638.(73)

[Bmim][BETI]

[Bmim][FSI]

153(9.60) 153(4.00) 152(8.50) 152(3.88) 151(8.55) 151(3.25) 150(7.96)

137(9.20) 137(4.73) 137(0.30) 136(5.90) 136(1.53) 135(7.17) 135(2.85)

149(7.47)

134(4.27)

148(7.06)

133(5.79)

147(6.73)

132(7.38)

146(6.47)

131(9.05)

145(6.27)

131(0.80)

144(6.14)

130(2.61)

[Hmim][TFSI] 139(1.02) 138(6.34) 138(1.65) 137(6.98) 137(2.35) 136(7.74) 136(3.15) 135(8.58) 135(4.02) 134(9.49) 134(4.97) 134(0.47) 133(5.98) 133(1.51) 132(7.05) 132(2.61) 131(8.18) 131(3.77)

Standard uncertainty u(p) = 10 kPa; for the particular samples investigated the standard uncertainty of ρ is 0.01 kg·m−3, but due to the limited purity of the ILs ur(ρ) = 0.0001 for [Emim][FAP], 0.001 for [Bmim][BETI], [Bmim][FSI] and [Hmim][TFSI], and 0.005 for [Emim][Ac]. Accordingly, uncertain digits of the present data are bracketed. a

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EXPERIMENTAL SECTION Materials and Sample Handling. Except for [Emim][FAP], which was taken from a freshly opened ampule of the provider, the samples of this study were dried for 1 week at 313 K with a high-vacuum line (pressure p < 10−9 bar). The residual water mass fraction, w(H2O) (Table 1), was determined by coulometric Karl Fischer titration (Mitsubishi Moisturemeter MCI CA-02). No further purification steps were performed prior to use. All ILs were clear liquids and, except for the yellowish [Emim][Ac], practically colorless. After drying the ILs were stored in a N2-filled glovebox until use. To prevent uptake of atmospheric moisture, the protective N2 atmosphere was maintained during all steps of the measurement protocols for density, viscosity, and electrical conductivity by using syringe techniques. Only a moderate increase (Δw(H2O) ≤ 70·10−6) of the water content of randomly selected samples was found after the experiments. All measurements were done at ambient pressure (∼0.1 MPa). Density. For all investigated ILs the reported densities, ρ (Table 2), were determined in the temperature range of

(273.15 to 363.15) K with vibrating-tube densimeters (Anton Paar DMA 5000) in Regensburg ([Emim][Ac], [Emim][FAP], [Hmim][TFSI]), and Sendai ([Bmim][BETI], [Bmim][FSI]). The temperature stability of these instruments was ≤ 0.005 K. Deionized water and air were used for their calibration by the internal calibration routine. The repeatability of the measurements was ± 5·10−3 kg·m−3, but considering the possible sources of error in calibration and measurement, the estimated standard uncertainty of ρ for the particular IL samples investigated in this study is 0.01 kg·m−3, and we give data in Table 2 to this decimal to allow reproduction of our density fits; see Results and Discussion. However, as discussed in ref 16, due to the limited purity of the samples (Table 1) the relative standard uncertainty of the present IL data is only ur(ρ) = 0.0001 for [Emim][FAP], 0.001 for [Bmim][BETI], [Bmim][FSI] and [Hmim][TFSI], and 0.005 for [Emim][Ac]. Accordingly, uncertain digits are bracketed in Table 2. Viscosity. The dynamic viscosities, η, of [Emim][Ac], [Emim][FAP] and [Hmim][TFSI] were measured with an automated rolling-ball viscometer (Anton Paar AMVn) in the B



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Table 3. Dynamic Viscosities, η, of the Investigated ILs at p = 0.1 MPaa η/mPa·s T/K 273.15 278.15 283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15 358.15 363.15 368.15 378.15 388.15 398.15 408.15 a

[Emim][Ac]b 701.2

[Emim][FAP]b 168.1

288.0

96.54

141.1

60.53

78.27 47.69 31.26 21.77 16.01 12.19 9.558 7.710 6.362 5.356 4.589

[Bmim][BETI]c

[Bmim][FSI]c

579.1 398.8 282.2 205.3 153.3 115.9 89.72

98.81 76.48 60.41 48.60 39.71 32.93 27.64

56.57

20.22

37.86

15.35

26.63

12.01

40.82

[Hmim][TFSI]c 210.6 116.7 70.51 45.51

28.98

31.03

21.41

22.14

16.29

16.40 19.48

9.651

14.74

7.918

11.48

6.626

12.78

12.59

10.23

9.957

8.367 6.967 5.892 5.045

8.066 6.673 5.623 4.819 4.190

Standard uncertainty u(p) = 10 kPa. bExpanded (k = 2) relative uncertainty, Ur(η) = 0.015. cUr(η) = 0.02.

The other κ values collected in Table 4 were determined in Regensburg. For [Emim][FAP] and [Hmim][TFSI] at (278.15, 288.15, and 298.15) K, a Huber Unistat 505 connected to an appropriately sized external oil bath was used to maintain the temperature of the conductivity cells within ± 0.005 K. Otherwise, the previously described17 homemade precision thermostat operating in the temperature range of (298.15 to 468.15) K with ≤ 0.003 K temperature stability was used. For both thermostats the actual measurement temperature was recorded with a calibrated Pt100 sensor connected to an ASL F250 precision thermometer (overall standard uncertainty 0.025 K), whereas the resistance, R(ν), of the conductivity cells was obtained as a function of ν (500 ≤ ν /Hz ≤ 10000) with a LCR Bridge (Hameg HM 8118) and a switching device to multiplex up to six homemade capillary cells. The raw data for R(ν) were corrected for lead resistance and, to eliminate electrode polarization effects, subsequently extrapolated to infinite frequency, R∞, using the empirical function R(ν) = R∞ + A/νa, where A is specific to the cell and 0.5 < a < 1. A typical example for such an extrapolation is shown in Figure S1 of the Supporting Information. For each of the two thermostats a set of four capillary cells18 was calibrated with aqueous solutions of (0.01, 0.1, and 1) mol·kg−1 KCl using the reference data reported by Pratt et al.19 The so-determined cell constants, C, ranging from 6.54 to 91.40 cm−1 (Ur(C) = 0.008), were used to calculate κ = C/R∞. No corrections for possible changes of C with temperature were applied as earlier studies found these to be negligible.10,18,20 Keeping in mind all possible sources of error an expanded (k = 2) relative uncertainty of Ur(κ) = 0.015 is estimated for the conductivity measurements performed in Regensburg. For [Bmim][BETI], [Bmim][FSI], and [Omim][TFSI], where data from Sendai

temperature range of (278.15 to 408.15) K. The standard temperature uncertainty of this instrument was 0.05 K, its stability 0.01 K. Before starting the actual measurements, the employed capillary tubes with diameters of (1.6, 1.8, and 3.0) mm were calibrated with standard-viscosity oils (Cannon S3, N14 and N44) as a function of temperature and capillary inclination. The nominal relative standard uncertainty of the instrument was 0.5 %. However, from the observed dependence of η on capillary inclination, we estimate an expanded (k = 2) relative uncertainty of Ur(η) = 0.015 for the present data. The ILs [Bmim][BETI] and [Bmim][FSI] were investigated in the temperature range of (273.15 to 363.15) K using the Stabinger-type densitimeter/viscometer (Anton Paar SVM 3000) at Sendai. This instrument has a standard temperature uncertainty of 0.01 K and a stability of 0.005 K. Its expanded (k = 2) relative uncertainty in η is 0.02. All obtained η values are summarized in Table 3. Electrical Conductivity. For the ILs [Bmim][BETI], [Bmim][FSI], and [Omim][TFSI] measurements of electrical conductivity, κ, were performed in Sendai in the temperature range of (273.15 to 353.15) K with an impedance analyzer (Bio Logic SP-150 potentiostat/galvanostat with built-in EIS option) and homemade cells calibrated at 298.15 K with 0.01 mol L−1 aqueous KCl (cell constants C = (0.320 and 0.360) cm−1; a correction for thermal expansion was applied at other temperatures). The cell impedance was determined in the frequency range of 0.1 ≤ ν /Hz ≤ 1·106 and the solution resistance, R∞, and thus κ = C/R∞, determined from the ν → ∞ intercept of a Nyquist plot (ℑ(Z*) vs ℜ(Z*)) of the complex impedance, Z*(ν), with the ℜ(Z*) axis. The standard uncertainty in temperature was 0.01 K, the expanded (k = 2) relative uncertainty of κ was estimated to be Ur(κ) = 0.02. C



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Table 4. Electrical Conductivities, κ, of the Investigated ILs at p = 0.1 MPa.a κ/S·m−1 T/K 273.15 278.15 283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15 358.15 368.15 378.15 388.15 398.15 408.15 418.15 428.15 438.15 448.15 458.15 468.15 a

[Emim][Ac]

[Emim][FAP]

[Bmim][BETI] b

4.467 2.759 0.2776

1.854

0.4709

1.322

0.7329

0.9825

1.068

0.7586

1.477

0.6031

1.956

0.4930

2.503 3.114 3.802 4.531 5.310 6.126 6.917

0.4117 0.3501 0.3023 0.2645 0.2339 0.2089 0.1884 0.1710 0.1563 0.1436 0.1327 0.1233

0.03243 0.04599b 0.06341b 0.08528b 0.1122b 0.1450b 0.1829b 0.2274 0.2781b 0.3364 0.4000b 0.4747 0.5489b 0.6382 0.7244b 0.8307 0.9282b 1.050 1.293 1.564 1.856 2.163 2.488 2.827 3.186 3.561 3.937 4.335 4.719

[Bmim][FSI]

[Hmim][TFSI]

b

0.2915 0.3704b 0.4622b 0.5610b 0.6815b 0.8164b 0.9534b 1.117 1.273b 1.465 1.636b 1.861 2.038b 2.293 2.475b 2.767 2.944b 3.280 3.826 4.398 4.990 5.601 6.229 6.872 7.531 8.201 8.892 9.618 10.28

0.07762 0.1359 0.2178 0.3275 0.4660 0.6337 0.8303 1.054 1.304 1.579 1.875 2.192 2.526 2.878 3.243 3.622 4.012 4.414 4.822 5.237

[Omim][TFSI] 0.03083b 0.04320b 0.05895b 0.07844b 0.1023b 0.1311b 0.1639b 0.2039 0.2463b 0.2979 0.3512b 0.4151 0.4785b 0.5563 0.6271b 0.7201 0.8009b 0.9064 1.114 1.340 1.588 1.850 2.126 2.419 2.722 3.036 3.359 3.691 4.029

Standard uncertainty u(p) = 10 kPa. bDetermined in Sendai with Ur(κ) = 0.02; other data, with Ur(κ) = 0.015, are from Regensburg.

Figure 1. (a) Density, ρ(T), of [Hmim][TFSI] from this study (■) and associated fit with eq 2 (solid line); (b) corresponding relative deviation, δ, from the fit. Also included are the data (a) and its deviation from the present fit (b) of Kato et al.,21 Jacquemin et al.,22 Tariq et al.,23 Fröba et al.,24 Kolbeck et al.,25 Tariq et al.,26 Seoane et al.,27 Gangamallaiah et al.,28 Iguchi et al.,29 and Tenney et al.30 The dash−dotted lines in (a) and (b) are the IUPAC recommendation14 for this IL and its relative deviation from the present fit, respectively. In (b) the bold dashed line indicates δ = 0, and the thin dashed lines represent arbitrary margins of 100 δ = ± 0.5.

D



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determined for the IUPAC Technical Report were fitted by a straight line, eq 2, with parameters aρ = 1640.94 kg·m−3 and bρ = 0.9012 kg·m−3·K−1 valid for 258 < (T/K) < 373.14 These fit parameters agree well with the present intercept (1643.3 kg· m−3, Table 5) and slope (0.908 kg·m−3·K−1), so that within the temperature range of the present investigation, (278 to 363) K, the relative deviation between both fits is below 0.03 %. However, from Figure 1 it is obvious that the deviation is systematic and almost certainly caused by the upward curvature of the present ρ(T) values. Except for the data of Tariq et al.,26 the recently published ρ(T) values for [Hmim][TFSI] are smaller than those of the present investigation (Figure 1). Best agreement was found with the results of Tenney et al.30 and Iguchi et al.29 who used samples with similar water contents (w(H2O) = 8·10−6 and 20· 10−6). However, there is no clear correlation between average deviation and w(H2O) as the ρ values of Seoane et al.27 (w(H2O) ≈ 70·10−6) are closer to the present data than those of Tariq et al.,26 (w(H2O) < 20·10−6). In particular, contamination by water cannot explain the large deviation (0.52 ≤ 100δ ≤ 0.81) of the ρ(T) values of Fröba et al.24 and Kolbeck et al.25 Interestingly, the plots of most of the available data sets and deviations are parallel to the present ρ(T) or δ(T). They also exhibit a convex curvature in the temperature range of 280 ≲ T/K ≲ 370, followed by a concave shape at higher T for the only data set extending to 473 K (Figure 1).26 It seems unlikely that this oscillating behavior of ρ(T) is an inherent property of [Hmim][TFSI] as to some extent such a curvature also shows up for [Emim][Ac] (Figure S2). Since all ρ(T) values exhibiting such behavior were determined with vibrating-tube densimeters, this oscillation probably reflects deficiencies of the measurement technique for highly viscous samples31 which were not fully compensated by the automatic correction routine implemented in newer instruments, like the Anton Paar DMA

and Regensburg were combined, the expanded (k = 2) relative uncertainty is 0.035.

■

RESULTS AND DISCUSSION Density. The present density values, collected between (273.15 and 363.15) K, are summarized in Table 2 and displayed in Figure 1 and Figures S2−S5 of the Supporting Information, together with available literature data. Following the IUPAC Technical Report13,14 on the thermodynamic and thermophysical properties of the reference IL [Hmim][TFSI], a straight line, ρ = aρ − bρ ·T

(2)

was used to fit ρ(T) for all studied ILs. The obtained parameters aρ and bρ are summarized in Table 5 together with the corresponding fit standard error, σfit. Table 5. Parameters aρ and bρ of the Straight-Line Fit, eq 2, to the Density Data of the Investigated ILs and Associated Fit Standard Error, σfit IL

aρ/kg·m−3

bρ/kg·m−3·K−1

σfit

[Emim][Ac] [Emim][FAP] [Bmim][Beti] [Bmim][FSI] [Hmim][TFSI]

1280.8 2059.9 1822.7 1611.2 1643.3

0.608 1.177 1.038 0.851 0.908

0.2 0.3 0.2 0.3 0.2

[Hmim][TFSI]. Specially prepared samples of this IL (mole fraction purity > 0.995; w(H2O) = 0.00001) were used for a IUPAC round robin aiming at the establishment of a reference for the determination of the thermodynamic and thermophysical properties of ILs.13,14 Accordingly, a comparison of the present data with literature should start with this wellinvestigated compound. The density data of [Hmim][TFSI]

Figure 2. (a) Viscosity, η, of [Emim][Ac] from this study (■) and corresponding fit with eq 3 (solid line, for parameters see Table 6); (b) corresponding relative deviation, δ, from the fit. In (b) the thin dashed lines represent arbitrary margins of 100δ = ± 3. Also included are the data (a) and their deviation from the present fit (b) of Fendt et al.,56 Freire et al.,32 Hou et al.,39 Pereiro et al.,36 Quijada-Maldonado et al.,55 Araujo et al.,34 and Castro et al.54 E



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Table 6. Parameters ln(Aη/η°), Bη, and T0,η of the VFT Fit eq 3 to the Dynamic Viscosity, η, of the Investigated ILs and Fit Standard Error, σfit; η° = 1 mPa·s

5000 used in this investigation. For this reason we abstained from extending the polynomial of eq 2 to achieve a better fit of the experimental data. [Emim][Ac]. As for [Hmim][TFSI] the present ρ(T) values deviate systematically from the fit curve, eq 2 (Figure S2). The data of Fröba et al.24 and Freire et al.32 exhibit a similar curvature but otherwise are in excellent agreement with the present results. Most of the other literature data run parallel to the present fit curve, see Figure S2, with either smaller33−35 or larger36,37 ρ(T) values and 100|δ| < 0.25. Larger deviations were found for the data of Shiflett et al.38 and especially of Hou et al.39 The latter exhibit a strange temperature dependence (Figure S2). It must be noted that probably all experimental data reported for this IL are affected by the rather high water and (where stated) halide contents of the investigated samples. Ma et al.37 tried to correct for w(H2O) by adding defined amounts of water and extrapolating ρ to w(H2O) = 0, so their data for 298 ≤ T/K ≤ 338 are probably the most reliable. [Emim][FAP]. Several authors40−44 studied the density of this IL as a function of temperature. Their values and the deviations from our fit are shown in Figure S3. Generally, this data is in good agreement with the present ρ(T) (−0.13 ≤ 100δ ≤ 0.17), but that of Neves et al.44 deviates significantly (−0.97 ≤ 100δ ≤ −0.35) with a clear trend. Note that, in contrast to the previously discussed ILs, the present experimental values scatter randomly around the fit curve, suggesting that here viscosity was not significantly affecting the density measurements. [Bmim][BETI]. Also for this IL, where only a few studies of ρ(T) were found in the literature, 45−47 the present experimental densities scattered randomly around the fit curve, eq 2 (Figure S4), and are in excellent agreement (0.07 ≤ 100δ ≤ 0.02) with the data reported by Tokuda et al.45 The densities of Liu et al.47 between (293 and 353) K are in fair agreement (−0.06 ≤ 100δ ≤ −0.23) but show a systematic trend. The value reported by Rollins et al.46 for [Bmim][BETI] containing 0.28 mol·L−1 D2O exemplifies the strong effect of water contamination on IL density. [Bmim][FSI]. To the best of our knowledge, no data were previously reported for the density of this IL. Similar to some of the previously discussed compounds the present ρ(T) deviate systematically from a straight line (Figure S5). This may result from insufficient viscosity correction of the raw data by the built-in routine of the Anton Paar DMA 5000 densimeter, but no further discussion is possible. Viscosity. The present values for dynamic viscosity, η, collected between (273.15 and 363.15 or 408.15) K, are summarized in Table 3 and displayed in Figures 2 and S6−S9, together with literature data where available. For glass forming liquids not too far above their glass transition temperature, Tg, the temperature dependence of their transport properties, Y (= η, κ, ...), generally follows the Vogel− Fulcher−Tamman (VFT) equation, ln(Y /Y °) = ln(AY /Y °) + BY /(T − T0, Y )

IL

ln(Aη /η°)

Bη/K

T0,η/K

102·σfit

[Emim][Ac] [Emim][FAP] [Bmim][Beti] [Bmim][FSI] [Hmim][TFSI]

−1.657 −1.835 −2.393 −1.565 −1.966

673.7 827.2 968.5 709.8 819.4

196.1 159.1 162.6 157.9 166.3

0.3 0.7 0.4 0.1 0.7

also determined. The then obtained η values in the temperature range of 258 < (T/K) < 433 were described by the polynomial ln(η /η°) = −13.941 + 6721.88/(T /K) − 2.24584 ·106 /(T /K)2 + 3.70841· 108/(T /K)3

(4)

where η° = 1 Pa·s. For 298 ≤ (T/K) ≤ 388 the present experimental values and the corresponding VFT fit, eq 3, are in very good agreement with η(T) calculated with eq 4 (|δ | ≲ 0.02), but at lower and higher temperatures the present values are systematically too large, reaching δ = −0.076 at 408.15 K (Figure S6). However, note that in the temperature range of the present data |δ| is always smaller than the combined uncertainties of both investigations. Figure S6 also shows η values published since the IUPAC report.14 Nearly all of them are in excellent agreement with the present data exhibiting relative deviations < 3 %.27−29,49−51 An exception are the results of Ahosseini et al.52 and Rupp et al.53 reaching δ = −0.12 due to their markedly different temperature dependence, with smaller dη/dT for the first and larger slope for the second. [Emim][Ac]. The present η(T), scattering by less than 0.4 % around the fit with eq 3 (Figure 2), are in good agreement with the data of Freire et al.32 (278.15 ≤ (T/K) ≤ 363.15; 0.018 ≤ δ ≤ 0.029) and especially Castro et al.54 (278.15 ≤ (T/K) ≤ 338.15; −0.018 ≤ δ ≤ −0.0017. The viscosities published by Pereiro et al.36 (298.15 ≤ (T/K) ≤ 333.15) and Araujo et al.34 (283.15 ≤ (T/K) ≤ 343.15) essentially coincide, as might be expected for data determined by the same group with the same instrument for a sample from the same provider but are systematically smaller than the present values with δ increasing from (−4 to −12) % with decreasing T (Figure 2). A similar, albeit less pronounced, trend was found for the data of QuijadaMaldonado et al.55 Despite the obvious differences, all of the above studies, performed with [Emim][Ac] samples from the same provider (IoLiTec) and with similar, albeit rather high, water and halide ion contents (w(H2O) ≈ (1.2 to 1.5)·10−3 and w(X−) > 0.01) agree reasonably well. This is not the case for the η(T) values reported by Fendt et al.56 for a sample of > 95 % purity from Sigma-Aldrich, studied with a rotational viscometer of 2 % stated relative standard uncertainty (Figure 2). Most likely, this is due to excessive water content of w(H2O) ≈0.012. Almost certainly, water contamination cannot be the reason for the huge deviation of all of the above [Emim][Ac] data from the viscosities reported by Hou et al.39 The latter exhibit a completely different and rather unphysical temperature dependence. [Emim][FAP]. For this IL only limited information is available so far. The present experimental data oscillate with a relative amplitude of ±1% around the fit curve (Figure S7). They are in excellent agreement with the η(T) values published 14

(3)

where AY, BY, and the so-called VFT temperature, T0,Y, are fit parameters.48 This is also the case for the present data for viscosity (Y° = η° = 1 mPa·s) and electrical conductivity (Y° = κ° = 1 S·m−1; see below). For η(T) the obtained values for ln(Aη/η°), Bη and T0,η are summarized in Table 6 together with the corresponding fit standard error, σfit. [Hmim][TFSI]. Within the IUPAC round robin13,14 on the thermophysical properties of [Hmim][TFSI], the viscosity was F



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Figure 3. (a) Electrical conductivity, κ(T), of [Emim][Ac] from this study (■) and corresponding fit with eq 3 (solid line, for parameters see Table 7); (b) corresponding relative deviation, δ, from the fit. In (b) the thin dashed lines represent arbitrary margins of 100δ = ± 3. Also included are the data (a) and their deviation from the present fit (b) of Hou et al.39 (□), Pereiro et al.36 (○), and Araujo et al.34 (△).

by Neves et al.,44 exhibiting a similar oscillation. Good agreement, albeit with a small systematic shift, was also observed with the results of Dutt57 (0 < δ < 0.029) and Alimantariotis et al.41 (−0.029 ≤ δ ≤ −0.014). The fit published by Seki et al.58 for η(T) in the temperature range of (283.15 to 353.15) K exhibits relative deviations of only −0.034 ≤ δ ≤ 0.01 from the present fit but the dependence of η on temperature is somewhat smaller. [Bmim][BETI]. For this IL the present experimental values scatter within ± 0.7 % around the fit with eq 3 (Figure S8). Over most of the temperature range investigated by Tokuda et al.,45 their results appear to be systematically shifted to smaller values compared to the present data, but at T ≈ 298 K the slope of their η(T) suddenly changes so at 283 K their value is larger (Figure S8). Nevertheless, with δ ranging from −4.2 % to 3.6 %, the agreement with the results of this investigation is very good. The data reported by Shirota et al.59 and Fox et al.,60 both determined at a single temperature only, are already ∼6.5 % smaller, whereas on average those of Liu et al.,47 covering the exceptionally wide temperature range of (298 to 573) K, are shifted by −17 % (Figure S8).61 Note that all of the [Bmim][BETI] samples were prepared by the investigating groups who generally gave w(H2O) but no further information on sample purity. [Bmim][FSI]. The present experimental η values, determined with an Anton Paar SVM 3000, scatter within −0.0018 ≤ δ ≤ 0.0015 around the fit with eq 3 (Figure S9). These relative deviations are well within the expanded relative uncertainty of the instrument, Ur(η) = 0.02. To the best of our knowledge no viscosity data is reported yet for this IL so that the present η(T) cannot be crosschecked. Electrical Conductivity. The present values for electrical conductivity, κ(T), generally collected between (273.15 and 468.15) K, are summarized in Table 4 and displayed in Figures 3−5 and S10−S12, together with literature data where available. As for viscosity, also κ(T) is well fitted by the

Vogel−Fulcher−Tamann equation, eq 3. The obtained values for ln(Aκ/κ°), Bκ and T0,κ are summarized in Table 7, together with the corresponding fit standard error, σfit. Table 7. Parameters ln(Aκ/κ°), Bκ, and T0,κ of the VFT Fit eq 3 to the Electrical Conductivity, κ, of the Investigated ILs and Fit Standard Error, σfit; κ° = 1 S m−1 IL

ln(Aκ/κ°)

Bκ/K

T0,κ/K

102·σfit

[Emim][Ac] [Emim][FAP] [Bmim][Beti] [Bmim][FSI] [Hmim][TFSI] [Omim][TFSI]

4.725 4.156 4.109 4.387 4.015 3.967

−623.1 −620.8 −749.4 −634.1 −699.0 −762.7

194.4 168.2 173.9 160.2 171.9 170.9

0.3 0.4 0.7 0.7 0.2 0.6

[Hmim][TFSI]. The κ(T) values for this IL, covering the temperature range of 273.15 ≤ (T/K) ≤ 468.15 were all determined in Regensburg. They are well described by eq 3 with |δ | ≤ 0.0052 although a systematic oscillation around the fit curve is obvious (Figure S10). The data of the IUPAC round robin,13,14 determined between (278 and 323) K, were described by the polynomial κ /(S ·m−1) = 0.2903 + 0.01969·(T /K) − 1.666 ·10−4 ·(T /K)2 + 3.345·10−7 ·(T /K)3

(5)

Conductivities calculated with this equation are in excellent agreement with the present experimental data (and their fit curve) for 298 ≤(T/K) ≤358 but deviate systematically at lower T. As expected from the limited temperature range of the IUPAC data, extrapolation of eq 5 beyond ∼360 K is problematic (Figure S10). Since the publication of the IUPAC report14 only three other authors measured the electrical conductivity of [Hmim][TFSI]. Calado et al.62 used a setup developed in their laboratory, G



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Figure 4. (a) Electrical conductivity, κ(T), of [Bmim][BETI] from this study (■) and corresponding fit with eq 3 (solid line, for parameters see Table 7); (b) corresponding relative deviation, δ, from the fit. In (b) the thin dashed lines represent arbitrary margins of 100 δ = ± 3. Also included are the data (a) and their deviation from the present fit (b) of Tokuda et al.45 (□).

rare. The conductivity values published by Pereiro et al.36 for 298.15 ≤ (T/K) ≤ 323.15 and by Araujo et al.34 for 283.15 ≤ (T/K) ≤ 323.15, with claimed relative uncertainties of 1 %, are in fair agreement with the present data, albeit generally systematically higher (δ ≤ 0.13). In view of sample specifications34,36 comparable to ours (Table 1) such deviations seem surprising. As for density (Figure S2) and viscosity (Figure 2), the conductivity values published by Hou et al.39 for this IL exhibit a completely different temperature dependence. Only around room temperature their κ(T) is similar to the other available data (Figure 3), possibly indicating problems with temperature setting in that study. [Emim][FAP]. With |δ| ≤ 0.0072 the present conductivity data collected between (278.15 and 468.15) K for this IL in Regensburg showed somewhat larger deviations from their fit with eq 3 than [Emim][Ac] (see above). Only two publications with comparable data could be found. Ignatev et al.66 reported a single κ value, determined at 293.15 K with a commercial conductometer of 0.5% relative standard uncertainty, which is 0.9 % below the present fit (Figure S12). Using an unspecified sample, Seki et al.58 investigated the temperature range of 243.15 ≤ (T/K) ≤ 353.15 with an impedance analyzer and a commercial conductivity cell, claiming a relative uncertainty of 0.35 % for κ. Only a single numerical value at 303.15 K was published, together with the parameters of a VFT fit to their data. It appears that the κ(T) values of ref 58 scatter considerably as the published conductivity value at 303.15 K and the corresponding fit curve deviate considerably from each other and from the present data; see Figure S12. Note that a similar discrepancy also exists for η, where the experimental point (43 mPa·s) of ref 58. was not included in Figure S7. [Bmim][BETI]. [Bmim][BETI] was partly measured in Sendai (273.15 ≤ (T/K) ≤ 353.15) and partly in Regensburg (308.15 ≤ (T/K) ≤ 468.15) using the same sample. The combined data was reasonably well fitted by eq 3 although δ shows oscillatory behavior (Figure 4b), suggesting that a VFT

together with a commercial four electrode cell, for the determination of κ(T) between (288.27 and 333.01) K. Their results for an IoLiTec sample with the same specifications as ours are in excellent agreement with the present values and fit (|δ | ≤ 0.01). Similar agreement (−0.008 ≤ δ ≤ 0.013) was found for the data determined by Santos et al.49 with a commercial cell and an impedance analyzer in the temperature range of 298.15 ≤ (T/K) ≤ 333.15 for a sample provided by NIST. Interestingly, for both literature data the relative deviations from the present fit curve exhibit the same oscillation as our κ(T). This possibly suggests that the present VFT description with the parameters of Table 7 is not fully appropriate. Also included in Figure S10 is the fit published by Rupp et al.53 which predicts conductivities that are considerably smaller (−0.077 ≤ δ ≤ −0.053) than all other available data. [Omim][TFSI]. This data was partly determined in Sendai and partly in Regensburg. No systematic deviation between both sets for κ(T) was found, which scatter around the VFT fit within −0.008 ≤ δ ≤ 0.014 (Figure S11). The conductivities reported by Tokuda et al.45 for 268 ≤ (T/K) ≤ 373 exhibit similar relative deviations (−0.009 ≤ δ ≤ 0.013). Also in good agreement (δ = 0.01) is the value reported by Yamaguchi et al.63 for 298.15 K. Although systematically smaller (−0.062 ≤ δ ≤ −0.029), the fit of Rupp et al.53 is still in fair agreement with the present results. However, the fit curve published by Martinelli et al.64 for the temperature range of (270 to 400) K differs considerably, as does in particular the value given by Fitchett et al.65 (Figure S11). Keeping in mind the similar rather high water contents of the samples of Yamaguchi et al.63 (w(H2O) = 3.0·10−4) and Fitchett et al.65 (5.48·10−4) water contamination does not seem to be the reason for this excessive deviation. [Emim][Ac]. Conductivity measurements of this IL in Regensburg covered the temperature range of (298 to 418) K. The obtained κ(T) scatter within ± 0.3 % around the VFT fit, eq 3 (Figure 3). Literature data for comparison are rather H



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Figure 5. (a) Electrical conductivity, κ(T), of [Bmim][FSI] from this study (■) and corresponding fit with eq 3 (solid line, for parameters see Table 7); (b) corresponding relative deviation, δ, from the fit. In (b) the thin dashed lines represent arbitrary margins of 100δ = ± 3.

fit may not be fully appropriate. The only conductivity data published so far for this IL seem to support this view. With random deviations of |δ | ≤ 0.035 from the present fit curve the results of Tokuda et al.45 at 273.15 ≤ (T/K) ≤373.15 agree rather well. However, at 263.15 K their κ value is 10 % larger than predicted by our fit curve (Figure 4). Nevertheless, we abstained from choosing a different fit function because of the small but systematic offset between the Sendai and Regensburg data in the overlapping temperature range (303.15 ≤ (T/K) ≤ 353.15). The combined expanded relative uncertainty of Sendai and Regensburg data (3.5 %) is too large to justify a more elaborate fitting equation. [Bmim][FSI]. With relative deviations in the range −0.021 ≤ δ ≤ 0.010 the present combined data from Sendai and Regensburg are well-described by eq 3 (Figure 5), but a clear offset can be seen for the overlapping temperature range (303.15 ≤ (T/K) ≤ 353.15). From the inspection of the corresponding Walden plot (Figure 6), where a kink can be seen, we suspect problems with the Sendai data for T > 310 K although one has to stress that even at 353.15 K both data sets differ less than their combined uncertainties. In contrast to [Bmim][BETI], the experimental values do not oscillate around the fit curve. To the best of our knowledge no reference data is available for this IL.

Figure 6. Walden plot, Λ = f(η −1), for red ▲, [Emim][FAP]; red ■, [Emim][Ac]; blue ▼ [Bmim][BETI]; [Bmim][FSI]:blue ●, Sendai data; blue ○, κ values from Regensburg data with interpolated ρ and η; ⧫, [Hmim][TFSI]; and △, [Omim][TFSI]. The solid line indicates “ideal” behavior defined by the data for 1 M KCl(aq).69

As expected, the transport behavior, here η(T) and κ(T), of the investigated ILs is well-described by eq 3. Characteristic quantities for the low-T behavior are the VFT temperature, T0,Y (Y = η, κ), and the fragility parameter, |BY|/T0,Y, of the sample. The former is generally found to be (20 to 30) K below the glass transition temperature, Tg, determined with scanning calorimetry, whereas the second is a measure for the deviation of Y(T) from Arrhenius behavior, where |BY|/T0,Y → ∞.48 Except for [Emim][Ac], where T0,η ≈ T0,κ ≈ Tg and [Emim][FAP] and [Bmim][FSI] where to our knowledge no glass-transition temperatures have been published, the present values for T0,η (Table 6) and T0,κ (Table 7) are in the range expected for them relative to Tg (Table 8). However, it must be noted that T0,η is generally ∼10 K lower than T0,κ. Similarly, the Bη and Bκ parameters and accordingly the fragility parameters |

■

CONCLUSIONS The present results for ρ(T), η(T), and κ(T) of [Hmim][TFSI] are in very good agreement with values determined for this IL in the IUPAC round-robin,13,14 lending thus credit to the present data for [Emim][FAP], [Emim][Ac], [Bmim][BETI], [Bmim][FSI], and [Omim][TFSI]. For each investigated IL the density values obtained in the temperature range of (273.15 to 363.14) K are well-described by a linear fit, eq 2, with the parameters summarized in Table 5. Small but apparently systematic oscillatory deviations probably reflect deficiencies of the vibrating-tube technique for highly viscous samples.31 I



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Table 8. Fragility Parameters |Bκ|/T0,κ and |Bη|/T0,η of the Investigated ILs, Together with Their Glass-Transition Temperatures, Tg, from the Literature IL [Emim][Ac] [Emim][FAP] [Bmim][Beti] [Bmim][FSI] [Hmim][TFSI] [Omim][TFSI] a

|Bκ|/T0,κ 3.205 3.691 4.308 3.958 4.067 4.463

|Bη|/T0,η 3.435 5.199 5.956 4.495 4.928 5.647a

(3) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis, 2nd ed.; Wiley: Chichester, 2007. (4) Pârvulescu, V. I.; Hardacre, C. Catalysis in Ionic Liquids. Chem. Rev. 2007, 107, 2615−2665. (5) Belieres, J.-P.; Angell, C. A. Protic Ionic Liquids: Preparation, Characterization, and Proton Free Energy Level Representation. J. Phys. Chem. B 2007, 111, 4926−4937. (6) Ye, H.; Huang, J.; Xu, J. J.; Kodiweera, N. K. A. C.; Jayakody, J. R. P.; Greenbaum, S. G. New Membranes Based on Ionic Liquids for PEM Fuel Cells at Elevated Temperatures. J. Power Sources 2008, 178, 651−660. (7) Ohno, H. Electrochemical Aspects of Ionic Liquids; Wiley: Chichester, 2011. (8) Fedorov, M. V.; Kornyshev, A. A. Ionic Liquids at Electrified Interfaces. Chem. Rev. 2014, 114, 2978−3036. (9) Stoppa, A.; Zech, O.; Kunz, W.; Buchner, R. The Conductivity of Imidazolium-Based Ionic Liquids from (−35 to 195)°C. A. Variation of Cation’s N-Alkyl Chain. J. Chem. Eng. Data 2010, 55, 1768−1773. (10) Zech, O.; Stoppa, A.; Buchner, R.; Kunz, W. The Conductivity of Imidazolium-Based Ionic Liquids from (248 to 468) K. B. Variation of the Anion. J. Chem. Eng. Data 2010, 55, 1774−1778. (11) Makino, T.; Kanakubo, M.; Umecky, T.; Suzuki, A.; Nishida, T.; Takano, J. Electrical Conductivities, Viscosities, and Densities of NMethoxymethyl- and N-Butyl- N-methylpyrrolidinium Ionic Liquids with the Bis(fluorosulfonyl)amide Anion. J. Chem. Eng. Data 2012, 57, 751−755. (12) Harris, K.; Kanakubo, M. Self-diffusion, Velocity Crosscorrelation Coefficients and Distinct Diffusion Coefficients of the Ionic Liquid [BMIM][Tf2N] at High Pressure. Phys. Chem. Chem. Phys., to be submitted. (13) Marsh, K. N.; Brennecke, J. F.; Chirico, R. D.; Frenkel, M.; Heintz, A.; Magee, J. W.; Peters, C. J.; Rebelo, L. P. N.; Seddon, K. R. Thermodynamic and Thermophysical Properties of the Reference Ionic Liquid: 1-Hexyl-3-methylimidazolium Bis[(trifluoromethyl)sulfonyl]amide (Including Mixtures). Part 1. Experimental Methods and Results (IUPAC Technical Report). Pure Appl. Chem. 2009, 81, 781−790. (14) Chirico, R. D.; Diky, V.; Magee, J. W.; Frenkel, M.; Marsh, K. N. Thermodynamic and Thermophysical Properties of the Reference Ionic Lquid: 1-Hexyl-3-methylimidazolium Bis[(trifluoromethyl)sulfonyl]amide (Including Mixtures). Part 2. Critical Evaluation and Recommended Property Values (IUPAC Technical Report). Pure Appl. Chem. 2009, 81, 791−828. (15) Harris, K.; Kanakubo, M.; Woolf, A. Temperature and Pressure Dependence of the Viscosity of the Ionic Liquids 1-Hexyl-3methylimidazolium Hexafluorophosphate and 1-Butyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide. J. Chem. Eng. Data 2007, 52, 1080−1085. (16) Chirico, R. D.; et al. Improvement of Quality in Publication of Experimental Thermophysical Property Data: Challenges, Assessment Tools, Global Implementation, and Online Support. J. Chem. Eng. Data 2013, 58, 2699−2716. (17) Stoppa, A.; Hunger, J.; Buchner, R. Conductivities of Binary Mixtures of Ionic Liquids with Polar Solvents. J. Chem. Eng. Data 2009, 54, 472−479. (18) Barthel, J.; Wachter, R.; Gores, H.-J. Temperature Dependence of Conductance of Electrolytes in Nonaqueous Solutions. Mod. Aspects Electrochem. 1979, 13, 1−79. (19) Pratt, K. W.; Koch, W. F.; Wu, Y. C.; Berezansky, P. A. Molalitybased Primary Standards of Electrolytic Conductivity. Pure Appl. Chem. 2001, 73, 1783−1793. (20) Barthel, J.; Feuerlein, F.; Neueder, R.; Wachter, R. Calibration of Conductance Cells at Various Temperatures. J. Solution Chem. 1980, 9, 209−219. (21) Kato, R.; Gmehling, J. Systems with Ionic Liquids: Measurement of VLE and γ∞ Data and Prediction of their Thermodynamic Behavior Using Original UNIFAC, mod. UNIFAC(Do) and COSMORS(Ol). J. Chem. Thermodyn. 2005, 37, 603−619.

Tg/K 195;

67

197.7

68

19069 189.8;50 189;70 192;71 18672 188.4;50 186.7;73 193;71 189;74 184.3;75 187;76 183;77 18578

Obtained with the data of ref 12.

Bη|/T0,η and |Bκ|/T0,κ (Table 8) differ considerably with |Bκ|/T0,κ < |Bη|/T0,η. Probably, this arises from a too-small temperature range of the present experiments, in particular for η(T), and the lack of experimental values at T < 273 K for both η and κ. A useful means for discussing possible relations between electrical conductivity and dynamic viscosity of electrolytes is the Walden plot, Λ = f(η−1) (Figure 6), with Λ(T) = κ(T)·M/ ρ(T) as the equivalent conductivity at a given T; M is the molar mass of the IL.69 From the present samples [Emim][FAP] essentially lies on the “ideal” line defined by the data for 1 M KCl(aq), reflecting the weak interactions of the large tris(pentafluoroethyl)trifluorophosphate anion. All other investigated ILs are somewhat below the KCl line but still can be classified as “good” ionic liquids.79

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ASSOCIATED CONTENT

■

AUTHOR INFORMATION

S Supporting Information *

A figure illustrating the determination of R∞ and graphs comparing the data for ρ, η and κ of this investigation with appropriate fits. Also included in this comparison are literature values where available. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.5b00285. Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Funding

The authors gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft for T.S. and R.B. within the framework of Priority Program SPP 1191, “Ionic Liquids”. M.K. acknowledges support by the JSPS Institutional Program for Young Researcher Overseas Visits, and R.B. thanks JSPS for a BRIDGE Fellowship. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS It is a pleasure to thank Ms. Eriko Niitsuma for her careful assistance with the measurements at AIST Sendai. REFERENCES

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Densities, Viscosities, and Conductivities of the Imidazolium Ionic

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