Electrical Conductivities, Viscosities, and Densities of N-Acetoxyethyl

Jan 11, 2013 - The temperature dependences of the density, viscosity, and electrical conductivity were investigated in four ammonium ionic liquids wit...
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Electrical Conductivities, Viscosities, and Densities of N‑Acetoxyethyl‑N,N‑dimethyl‑N‑ethylammonium and N,N‑Dimethyl‑N‑ethyl‑N‑methoxyethoxyethylammonium Bis(trifluoromethanesulfonyl)amide and Their Nonfunctionalized Analogues Takashi Makino,† Mitsuhiro Kanakubo,*,† Tatsuya Umecky,†,‡ and Akira Suzuki† †

National Institute of Advanced Industrial Science and Technology (AIST), 4-2-1 Nigatake, Miyagino-ku, Sendai 983-8551, Japan Saga University, 1 Honjo-machi, Saga, 840-8502, Japan

‡

S Supporting Information *

ABSTRACT: The temperature dependences of the density, viscosity, and electrical conductivity were investigated in four ammonium ionic liquids with the bis(trifluoromethanesulfonyl)amide anion ([Tf2N]−). The cations for the ionic liquids were N-acetoxyethyl-N,N-dimethyl-N-ethylammonium ([N112,2OCO1]+), N,N-dimethyl-N-ethyl-N-methoxyethoxyethylammonium ([N112,2O2O1]+), N,N-dimethyl-N-ethyl-N-pentylammonium ([N1125]+), and N,N-dimethyl-N-ethyl-N-heptylammonium ([N1127]+). Measurements were performed over the temperature range T = (273.15 to 363.15) K at atmospheric pressure. The densities were fitted to quadratic functions of temperature. The ionic liquids with the functionalized cations ([N112,2OCO1]+ and [N112,2O2O1]+) have the higher densities than the corresponding counterparts with the nonfunctionalized cations ([N1125]+ and [N1127]+). The viscosities and electrical conductivities were analyzed using the Litovitz and Vogel−Fulcher−Tamman equations. The ester functionalized ionic liquid, [N112,2OCO1][Tf2N], has poorer transport properties (that is, higher viscosity and lower electrical conductivity) than the nonfunctionalized [N1125][Tf2N], whereas the ether functionalized ionic liquid, [N112,2O2O1][Tf2N], has superior transport properties to its analogue, [N1127][Tf2N]. Empirical Walden plots (double logarithmic graph of molar conductivity vs reciprocal viscosity) give straight lines with slopes less than unity.

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INTRODUCTION Room-temperature ionic liquids (ILs) are salts, the melting points of which are at or below ambient temperatures. They generally consist of a large asymmetric cation and an organic or inorganic anion. ILs generally have some favorable characteristics such as negligible vapor pressure, nonflamability, high thermal and chemical stability, a wide electrochemical window, high solubility of certain specific gases, and so on. Because of these features, ILs have attracted much attention, for example, as electrolytes in batteries, solvents for chemical reactions, and media for separation processes. An understanding of the nature of ILs is of importance for related technologies; therefore, a large number of investigations have been performed on the physical and chemical properties of ILs. Physicochemical properties of ILs can be changed by the introduction of functional groups (e.g., ether, ester, carboxyl, hydroxyl, cyano, and amine groups) into cations and anions.1−10 Of special interest, ether functionalized ILs have been synthesized and characterized in the past decade,1,5,11−18 because the ether group is effective in improving the transport properties and in reducing the melting temperature of ILs without any significant degradation of electrochemical stability. However, even in the case of such well-studied ILs, only limited data are available for © 2013 American Chemical Society

the temperature dependencies of the volumetric and transport properties. Particularly, transport properties depend strongly on temperature, and therefore such information is required for developing technologies and designing processes. Our research group has accurately measured the densities, viscosities, and electrical conductivities of a number of ILs as a function of temperature and pressure.19−26 Here, the densities, viscosities, and electrical conductivities were measured over the temperature range T = (273.15 to 363.15) K at atmospheric pressure for quaternary ammonium ILs both with and without functional groups coupled with bis(trifluoromethanesulfonyl)amide anion ([Tf2N]−). The functionalized ILs are N-acetoxyethyl-N,N-dimethyl-N-ethylammonium bis(trifluoromethanesulfonyl)amide ([N112,2OCO1][Tf2N]) and N,N-dimethyl-N-ethyl-N-methoxyethoxyethylammonium bis(trifluoromethanesulfonyl)amide ([N112,2O2O1][Tf2N]). The former IL has an ester group, and the latter has two ether groups. Nonfunctionalized ILs are their analogues, that is, N,N-dimethyl-N-ethyl-N-pentylammonium Received: September 13, 2012 Accepted: December 30, 2012 Published: January 11, 2013 370

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Table 1. Chemical Structures, Melting Temperatures Tm, Glass Transition Temperatures Tg, and Decomposition Temperatures Td of the Present Ammonium ILsa

u(Tm) = 1 K and u(Td) = 1 K, where the uncertainties are given at a 95 % confidence level. b[Tf2N]− = [(CF3SO2)2N]−. cThe decomposition temperatures are defined as 10 % mass loss.

a

bis(trifluoromethanesulfonyl)amide ([N1125][Tf2N]) and N,Ndimethyl-N-ethyl-N-heptylammonium bis(trifluoromethanesulfonyl)amide ([N1127][Tf2N]). The chemical structures of the present ILs are summarized in Table 1.

The thermal gravimetry measurements were performed with Seiko Instruments Inc., TG/DTA220 from (293 to 873) K at a constant heating rate 10 K·min−1 under nitrogen atomosphere to determine the decomposition temperatures (Td), as defined as 10 % mass loss. Tm and Td are summarized in Table 1. Tg was not detected over the present temperature range. The expanded uncertainties of Tm and Td are ± 1 K. Apparatus and Procedure. The instruments and the experimental equipments were the same as previously described elsewhere.25,26 The densities were measured using a vibrating tube densimeter (Anton Paar, DMA 5000M). The built-in viscosity correction for this instrument was employed as previously confirmed by the references with known viscosities.19−21 The instrumental constants were calibrated using dry air and distilled water, which was purified by a Millipore Simpli Lab Purification Pack. The viscosities were determined with a rotating-cylinder viscometer (Anton Paar, Stabinger SVM 3000). The reliability and validity of the viscosities were confirmed by measuring the reference samples supplied by Cannon Instrument Company as described in the previous studies.25,26 The impedance measurement was performed with an impedance analyzer (Bio Logic, SP-150). The solution resistance (Rsol) was obtained from the Nyquist plot by fitting the measured impedances to the feasible electric circuit. A syringe-type cell with a pair of platinum electrodes was employed. The cell constant was 35.4 m−1 at 298.15 K, and the correction for thermal expansion was made for different temperatures, as stated in the previous reports.22,23 The sample temperature was kept within ± 0.01 K at most in all of the measurements. The instrumental accuracy for the densities is less than ± 0.05 kg·m−3, whereas the expanded uncertainty for the densities is ± 0.1 % because of the sample impurities. The expanded uncertainties for the viscosities and electrical conductivities are less than ± 2 %.

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EXPERIMENTAL SECTION Materials. [N112,2OCO1][Tf2N] was synthesized as follows: a 133.6 g (0.80 mol) aliquot of 2-bromoethyl acetate (Wako Pure Chemical Industries, Ltd.) was slowly added with stirring to a solution of a 70.2 g (0.96 mol) N,N-dimethylethylamine (Tokyo Chemical Industry Co., Ltd.) and 401.0 g of acetonitrile at 75 °C for 24 h. The mixture was dried under reduced pressure, and about 184.0 g of solid product was obtained. The solid was dissolved in 184.0 g of distilled water and washed with an about 364.0 g aliquot of toluene twice, then, 365.1 g of N-acetoxyethylN,N-dimethyl-N-ethylammonium bromide ([N112,2OCO1][Br]) aqueous solution was obtained. To exchange [Br]− with [Tf2N]−, the aqueous solution was mixed and stirred for 9 h at room temperature with 314.7 g (0.75 mol) of 73.4 wt % lithium bis(trifluoromethanesulfonyl)amide (Li[Tf2N]) aqueous solution and 551.0 g of dichloromethane. After that, the organic phase was separated and washed with about 185.0 g of distilled water twice. Dichloromethane was evaporated from the organic mixture, and then 328.7 g of [N112,2OCO1][Tf2N] was obtained. The halogen contents of aqueous solutions in contact with the samples were less than the detection limit of AgNO3 testing. 1H spectra were recorded on a JEOL AL400 NMR spectrometer with DMSO-d6 as the standard at ∼293 K: δ/ppm = 1.21−1.28 (t, 3H), 2.06 (s, 3H), 3.04 (s, 6H), 3.37−3.44 (q, 2H), 3.58− 3.62 (m, 2H), 4.38−4.44 (m, 2H). The purity was identified as ≥ 99 mol %. The synthetic methods of the other ILs were similar to that of [N112,2OCO1][Tf2N] and are described in the Supporting Information. Any excess water in the IL was further removed by evacuation at 343 K for approximately 30 h just prior to measurements. Dried ILs were transferred to a closed electrical cell or other instruments by the use of an airtight syringe under dry nitrogen or argon. The water contents of [N112,2OCO1][Tf2N], [N112,2O2O1][Tf2N], [N1125][Tf2N], and [N1127][Tf2N] were (46, 45, 37, and 49)·10−6 mass fractions, respectively, as determined by Karl Fischer coulometric titration. The melting points (Tm) and glass transition temperatures (Tg) were measured by using a differential scanning calorimeter (Seiko Instruments Inc., DSC220) with increasing temperature from (223 to 293) K at a constant heating rate of 5 K·min−1.

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RESULTS AND DISCUSSION The second columns in Tables 2, 3, 4, and 5 list the densities (ρ/kg·m−3) at atmospheric pressure of [N112,2OCO1][Tf2N], [N112,2O2O1][Tf2N], [N1125][Tf2N], and [N1127][Tf2N], respectively. The densities were fitted to the following polynomial in the temperature range investigated: ρ = a + bT + cT 2

(1)

The coefficients and their standard errors are summarized in Table 6. The residuals (experimental data − calculated values) 371

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Table 2. Densities ρ, Viscosities η, Electrical Conductivities κ, and Molar Conductivities Λ of [N112,2OCO1][Tf2N] from T = (273.15 to 363.15) K at Atmospheric Pressurea

Table 4. Densities ρ, Viscosities η, Electrical Conductivities κ, and Molar Conductivities Λ of [N1125][Tf2N] from T = (273.15 to 363.15) K at Atmospheric Pressurea

T/K

ρ/kg·m−3

η/mPa·s

κ/S·m−1

Λ/μS·m2·mol−1

T/K

ρ/kg·m−3

η/mPa·s

κ/S·m−1

Λ/μS·m2·mol−1

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

1481.9b 1477.1b 1472.2b 1467.3b

1539 953.7 617.3 415.3 289.1 207.5 153.0 89.40 56.56 38.15 27.10 20.06 15.37

0.01589 0.02507 0.03784 0.05491 0.07714 0.1052 0.1395 0.2287 0.3481 0.4980 0.6800 0.8913

4.721 7.476 11.32 16.48 23.22 31.75 42.25 69.71 106.7 153.7 211.2 278.5

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

1363.19 1358.79 1354.43 1350.08 1345.71 1341.37 1337.04 1328.44 1319.93 1311.48 1303.10 1294.77 1286.49

688.8 464.2 323.1 231.2 169.9 127.6 97.88 60.94 40.43 28.25 20.60 15.56 12.12

0.02856 0.04151 0.05842 0.07997 0.1067 0.1392 0.1779 0.2754 0.4011 0.5560 0.7407 0.9545

8.891 12.96 18.31 25.14 33.65 44.06 56.47 87.98 129.0 179.9 241.3 312.9

1463.24 1458.65 1454.07 1444.94 1435.92 1426.99 1418.13 1409.35 1400.64

a

a

u(T) = 0.01 K, ur(ρ) = 0.001, ur(η) < 0.02, ur(κ) < 0.02, and ur(Λ) < 0.02. Uncertainties except for temperature are 95 % confidence limits. b Densities were measured by a vibrating tube densimeter incorporated in Anton Paar, Stabinger SVM 3000, because Anton Paar, DMA 5000 M exhibited “out of time error”.

u(T) = 0.01 K, ur(ρ) = 0.001, ur(η) < 0.02, ur(κ) < 0.02, and ur(Λ) < 0.02. Uncertainties except for temperature are 95 % confidence limits.

Table 5. Densities ρ, Viscosities η, Electrical Conductivities κ, and Molar Conductivities Λ of [N1127][Tf2N] from T = (273.15 to 363.15) K at Atmospheric Pressurea

Table 3. Densities ρ, Viscosities η, Electrical Conductivities κ, and Molar Conductivities Λ of [N112,2O2O1][Tf2N] from T = (273.15 to 363.15) K at Atmospheric Pressurea T/K

ρ/kg·m−3

η/mPa·s

κ/S·m−1

Λ/μS·m2·mol−1

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

1414.1b 1409.65 1404.97 1400.31 1395.68 1391.07 1386.49 1377.40 1368.39 1359.45 1350.57 1341.75 1332.98

243.5 173.6 127.5 96.35 74.27 58.57 47.19 32.03 22.89 17.02 13.14 10.39 8.433

0.06811 0.09358 0.1249 0.1622 0.2061 0.2571 0.3152 0.4513 0.6164 0.8099 1.025 1.269

21.98 30.30 40.58 52.88 67.38 84.36 103.8 149.6 205.6 271.9 346.5 431.7

T/K

ρ/kg·m−3

η/mPa·s

κ/S·m−1

Λ/μS·m2·mol−1

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

1311.95 1307.78 1303.56 1299.38 1295.20 1291.00 1286.82 1278.51 1270.28 1262.09 1253.97 1245.90 1237.87

944.3 634.1 437.8 310.5 225.7 167.7 127.6 77.77 50.63 34.68 24.84 18.49 14.18

0.01570 0.02302 0.03280 0.04538 0.06135 0.08104 0.1049 0.1668 0.2489 0.3529 0.4796 0.6308

5.415 7.965 11.39 15.80 21.43 28.40 36.89 59.02 88.66 126.5 173.1 229.1

a

u(T) = 0.01 K, ur(ρ) = 0.001, ur(η) < 0.02, ur(κ) < 0.02, and ur(Λ) < 0.02. Uncertainties except for temperature are 95 % confidence limits.

a

u(T) = 0.01 K, ur(ρ) = 0.001, ur(η) < 0.02, ur(κ) < 0.02, and ur(Λ) < 0.02. Uncertainties except for temperature are 95 % confidence limits. b Densities were measured by a vibrating tube densimeter incorporated in Anton Paar, Stabinger SVM 3000, because Anton Paar, DMA 5000 M exhibited “out of time error”.

for the densities in % are shown in Figure 1. The standard errors of the fits are also listed in Table 6. Figure S1 shows the densities of ILs as a function of temperature. Both the ester and ether functionalized ILs have larger densities than the corresponding nonfunctionalized counterparts. The IL with a shorter alkyl chain, [N1125][Tf2N], is denser than [N1127][Tf2N]. Similar behaviors in volumetric properties have been reported in some literature.5,11 The molar volumes (Vm/cm3·mol−1) and expansion coefficients (β/K−1; β ≡ (1/V)(∂V/∂T)p) of the present ILs at 298.15 K are summarized in Table 7. The nonfunctionalized ILs ([N1125][Tf2N] and [N1127][Tf2N]) have almost the same expansion coefficients, although the molar volume of [N1125][Tf2N] is smaller than that of [N1127][Tf2N]. The ester functionalized [N112,2OCO1][Tf2N] has a smaller molar volume and expansion coefficient than [N1125][Tf2N]. Similary, the molar volume of the ether functionalized [N112,2O2O1][Tf2N] is

Figure 1. Differences between the experimental density ρexp and the calculated density ρcal as a function of temperature T. ■, [N112,2OCO1][Tf2N]; ●, [N112,2O2O1][Tf2N]; □, [N1125][Tf2N]; ○, [N1127][Tf2N]. Data points of [N112,2OCO1][Tf2N], [N112,2O2O1][Tf2N], and [N1125][Tf2N] are covered with those of [N1127][Tf2N]. 372

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Table 6. Coefficients of the Best Fits for the Densities ρ A″/kg·m−3 B″/kg·m−3·K−1 C″·104/kg·m−3·K−2 std. error of fit/%

[N112,2OCO1][Tf2N]

[N112,2O2O1][Tf2N]

[N1125][Tf2N]

[N1127][Tf2N]

1766.15 (7.21) −1.14761 (0.04578) 3.88752 (0.72133) 0.012

1694.93 (2.15) −1.12190 (0.01367) 3.44842 (0.21542) 0.004

1628.23 (0.59) −1.05900 (0.00376) 3.24906 (0.05926) 0.001

1562.48 (1.01) −0.987179 (0.006401) 2.56841 (0.10086) 0.002

Table 7. Molar Volumes Vm and Expansion Coefficients β of the Ammonium Based Ionic Liquids at 298.15 Ka Vm/cm3·mol−1

β/10−4 K−1

301.908 316.412

6.27837 6.45058

[N112,2OCO1][Tf2N] [N1125][Tf2N] a

[N112,2O2O1][Tf2N] [N1127][Tf2N]

Vm/cm3·mol−1

β/10−4 K−1

328.110 350.485

6.58682 6.46029

ur(Vm) = 0.001 and ur(β) = 0.002, where uncertainties are given at the 95 % confidence level.

Table 8. Coefficients of the Best Fits for the Litovitz Equation for the Viscosities η, Electrical Conductivities κ, and Molar Conductivities Λ κ/S·m−1

η/mPa·s [N112,2OCO1][Tf2N] standard error of fit/% [N112,2O2O1][Tf2N] standard error of fit/% [N1125][Tf2N] standard error of fit/% [N1127][Tf2N]

ln A′ B′/R ln A′ B′/R ln A′ B′/R ln A′ B′/R

standard error of fit/%

−0.7420 (0.0650) 1.623 (0.018)·108 K3 5.76 −0.3681 (0.0293) 1.184 (0.008)·108 K3 2.62 −0.5424 (0.0230) 1.434 (0.008)·108 K3 2.41 −0.4742 (0.0168) 1.487 (0.005)·108 K3 1.51

3.422 (0.078) −1.327 (0.010)·108 6.21 2.777 (0.035) −1.103 (0.009)·108 2.73 3.007 (0.037) −1.327 (0.010)·108 2.94 2.744 (0.027) −1.399 (0.007)·108 2.11

Λ/μS·m2·mol−1 K3

K3

K3

K3

9.205 (0.075) −1.539 (0.020)·108 5.19 8.648 (0.032) −1.123 (0.009)·108 2.10 8.840 (0.034) −1.347 (0.009)·108 2.36 8.679 (0.024) −1.418 (0.006)·108 1.65

K3

K3

K3

K3

Table 9. Coefficients of the Best Fits for the VFT Equation for the Viscosities η, Electrical Conductivities κ, and Molar Conductivities Λ [N112,2OCO1][Tf2N]

standard error of fit/% [N112,2O2O1][Tf2N]

standard error of fit/% [N1125][Tf2N]

standard error of fit/% [N1127][Tf2N]

standard error of fit/% a

ln A B T0 Da ln A B T0 Da ln A B T0 Da ln A B T0 Da

η/mPa·s

κ/S·m−1

Λ/μS·m2·mol−1

−1.852 (0.040) 820.3 (9.9) K 184.0 (0.7) K 4.458 (0.071) 0.58 −1.574 (0.017) 702.4 (4.6) K 173.8 (0.4) K 4.041 (0.036) 0.21 −2.177 (0.034) 903.9 (9.6) K 169.5 (0.7) K 5.333 (0.079) 0.42 −2.466 (0.050) 1.017·103 (0.015·103) K 164.0 (1.0) K 6.207 (0.128) 0.54

4.159 (0.037) −702.2 (8.5) K 188.6 (0.7) K −3.723 (0.059) 0.48 3.756 (0.011) −620.0 (2.7) K 176.9 (0.3) K −3.505 (0.021) 0.11 4.285 (0.031) −772.2 (8.1) K 174.7 (0.7) K −4.420 (0.064) 0.34 4.391 (0.044) −895.0 (12.1) K 168.5 (0.9) K −5.312 (0.100) 0.41

10.02 (0.03) −727.1 (7.6) K 187.3 (0.6) K −3.882 (0.053) 0.42 9.733 (0.009) −654.3 (2.2) K 174.6 (0.2) K −3.747 (0.017) 0.10 10.22 (0.03) −806.3 (7.8) K 172.9 (0.6) K −4.692 (0.033) 0.27 10.44 (0.04) −932.9 (11.8) K 166.6 (0.9) K −5.590 (0.111) 0.37

Angell strength factor (D = B/T0).

The viscosities (η/mPa·s), electrical conductivities (κ/S·m−1), and molar conductivities (Λ/μS·m2·mol−1; Λ ≡ κ/c = κM/ρ, where M is the molar mass) for the four ILs are also summarized in Tables 2 to 5. Each property was fitted to the Litovitz equation:

smaller than that of the corresponding [N1127][Tf2N]. Unlike in the case of [N112,2OCO1][Tf2N] and [N1125][Tf2N], the volumetric change with temperature for [N112,2O2O1][Tf2N] is more sensitive than that for [N1127][Tf2N]. 373

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(2)

and the Vogel−Fulcher−Tammann (VFT) equation: η , κ , or Λ = A exp(B /(T − T0))

(3)

where R is the gas constant. The coefficients of eq 2 (A′ and B′) and eq 3 (A, B, and T0) are given in Tables 8 and 9. The residuals between the experimental data and the calculated values for the viscosities, electrical conductivities, and molar conductivities are shown in Figures 2, 3, and 4. The standard deviations from the

Figure 4. Differences between the experimental molar conductivity Λexp and the calculated molar conductivity Λcal as a function of temperature T. ■, [N112,2OCO1][Tf2N]; ●, [N112,2O2O1][Tf2N]; □, [N1125][Tf2N]; ○, [N1127][Tf2N]. Black keys are for Litvitoz, and red keys are for VFT.

363.15) K. The squares of the correlation coefficients, R2, for the VFT equations are very close to unity. The Angell strength parameters D (= B/T0) are also listed in Table 9, indicating that the functionalized [N112,2OCO1][Tf2N] and [N112,2O2O1][Tf2N] are more fragile than [N1125][Tf2N] and [N1127][Tf2N], respectively. The temperature dependencies for the viscosities, electrical conductivities, and molar conductivities are presented in Figures S4, S5, and S6. [N112,2OCO1][Tf2N] has poorer transport properties to [N1125][Tf2N], and the temperature dependencies in [N112,2OCO1][Tf2N] are more sensitive than those in the nonfunctionalized analogue. Similar effects of the ester group have been observed elsewhere.10 On the other hand, [N112,2O2O1][Tf2N] has superior transport properties (lower viscosities and higher electrical conductivities) to [N1125][Tf2N] and [N1127][Tf2N], though [N112,2O2O1]+ has a longer side chain than [N1125]+. The viscosities and electrical conductivities of [N112,2O2O1][Tf2N] are less sensitive to temperature than those of the nonfunctionalized analogue. These improvements are attributable to the better flexiblility and the smaller volume of the ether functionalized side chain, which is also mentioned in previous literature.12,13,17,25,26 Figure 5 shows Walden plots, that is, the logarithmic projections of molar conductivity versus fluidity (reciprocal viscosity). The relation between the molar conductivity and viscosity can be expressed by the following genral form:

Figure 2. Differences between the experimental viscosity ηexp and the calculated viscosity ηcal as a function of temperature T. ■, [N112,2OCO1][Tf2N]; ●, [N112,2O2O1][Tf2N]; □, [N1125][Tf2N]; ○, [N1127][Tf2N]. Black keys are for Litvitoz, and red keys are for VFT.

Λη α = C

Figure 3. Differences between the experimental electrical conductivity κexp and the calculated electrical conductivity κcal as a function of temperature T. ■, [N112,2OCO1][Tf2N]; ●, [N112,2O2O1][Tf2N]; □, [N1125][Tf2N]; ○, [N1127][Tf2N]. Black keys are for Litvitoz, and red keys are for VFT.

(4)

where α is an adjustable parameter and C is a constant. The slope and intercept in Figure 5 correspond to α and log C in eq 4, respectively: 0.938 and −0.205 for [N112,2OCO1][Tf2N], 0.944 and −0.292 for [N112,2O2O1][Tf2N], 0.939 and −0.260 for [N1125][Tf2N], and 0.951 and −0.335 for [N1127][Tf2N]. The slopes α are slightly smaller than unity, as reported for other ILs,25−29 which suggests that the conductive flows of ions relatively decrease with increasing temperature from the expected values of fluidity. Both functionalized ILs have better electrical conductivities at certain fluidities than the corresponding nonfunctionalized ILs. The intercepts, log C, decrease in the order of [N112,2OCO1][Tf2N] > [N1125][Tf2N] > [N112,2O2O1][Tf2N] > [N1127][Tf2N], in a good agreement with the trend of the inverse of the molar volumes 1/Vm as listed in Table 7, which indicates that the molecular size is also an important factor in the transport properties in the present ILs. Actually,

27−30

Litovitz equations are less than ± 3 % except for [N112,2OCO1][Tf2N], in which the deviation is more than ± 5 %. As in Figures 2, 3, and 4, η of [N112,2OCO1][Tf2N] has the mimimum residual around 303 K, whereas κ and Λ indicate the maximum at the almost same temperature. Actually, the viscosities at temperatures lower than 303 K are well fitted to one Litovits equation, and those at T > 303 K are reproduced as well with another Litovits equation with the standard deviations 1.64 % and 0.96 %, respectively, as shown in Figure S2 (the same for κ in Figure S3). On the other hand, the residuals from the VFT equations are less than 1 % over the entire temperature range T = (273.15 to 374

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The authors (TM, MK, TU) would thank Dr. Kenneth R. Harris at University of New South Wales, Australian Defence Force Academy for his helpful discussion and Ms. Eriko Niitsuma for her assistance with the measurements in the present study.

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Figure 5. Logarithmic projection of the molar conductivity Λ vs the fluidity (the reciprocal viscosity) η−1. ■, [N112,2OCO1][Tf2N]; ●, [N112,2O2O1][Tf2N]; □, [N1125][Tf2N]; ○, [N1127][Tf2N]. The solid line represents the Walden line with α = 1 and log C = 0, and the dashed and dotted lines are eye guides.

the ether −CH2O− and ester −OCO− groups have smaller van der Waals volumes (13.9 and 15.4 cm3·mol−1) than two methylene −CH2CH2− groups (20.46 cm3·mol−1).31

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CONCLUSION The densities, viscosities, and electrical conductivities have been measured for [N112,2OCO1][Tf2N], [N112,2O2O1][Tf2N], [N1125][Tf2N], and [N1127][Tf2N] at atmospheric pressure and over the temperature range (273.15 to 363.15) K. The densities were well-reproduced with second-order polynominals. Both fuctionalized ILs have higher densities (smaller molar volumes) than the corresponding nonfunctionalized analogues. The VFT equations were successfully applied to the viscosities, electrical conductivities, and molar conductivities with the standard deviations less than 1 %, while the Litovitz equation showed larger deviations ∼ 3 % except for [N112,2OCO1][Tf2N]. The viscosities decrease in the order of [N112,2OCO1][Tf2N] > [N1127][Tf2N] > [N1125][Tf2N] > [N112,2O2O1][Tf2N] at certain temperatures. [N112,2O2O1][Tf2N] is the most conductive followed by [N1125][Tf2N] > [N112,2OCO1][Tf2N] > [N1127][Tf2N]. Unlike the ether group, the ester group is not effective in improving the transport properties. The empirical Walden plots (log Λ vs log η−1) gave straight lines with slopes in the range 0.93 to 0.96, and the intercepts increase in [N1127][Tf2N] < [N112,2O2O1][Tf2N] < [N1125][Tf2N] < [N112,2OCO1][Tf2N]. The ion self-diffusion measurements for the present ammonium ILs will be carried out in due course, which could provide further insight into the properties reported in this study.

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ASSOCIATED CONTENT

S Supporting Information *

Electrical conductivities, viscosities, and densities of Nacetoxyethyl-N,N-dimethyl-N-ethylammonium, and N,N-dimethylN-ethyl-N-methoxyethoxyethylammonium bis(trifluoromethanesulfonyl)amide and their nonfunctionalized analogues. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; fax: +81-22-232-7002. 375

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