Physicochemical Properties for the Binary Systems of Ionic Liquids [C

Article pubs.acs.org/jced

Physicochemical Properties for the Binary Systems of Ionic Liquids [Cnmim]Cl + N,N‑Dimethylformamide Xiao-Jing Yan, Shu-Ni Li,* Quan-Guo Zhai, Yu-Cheng Jiang, and Man-Cheng Hu* Key Laboratory of Macromolecular Science of Shaanxi Province, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an, Shaanxi 710119, P R China S Supporting Information *

ABSTRACT: Density ρ, refractive index nD, and viscosity η for ionic liquids [Cnmim]Cl (Cnmim = 1-alkyl-3-methylimidazolium; n = 2, 4, 6, 8 for ethyl, butyl, hetyl, and octyl) + N,N-dimethylformamide (DMF) binary systems have been investigated at atmospheric pressure from 288.15 K to 318.15 K. The molar excess Gibbs energy ΔG*E, volumetric properties including excess molar volume VE, and apparent molar volume Vφ,i have been calculated from the experimental data. Furthermore, the refractive index and viscosity deviations (ΔnD, Δη) from the ideal behavior have been obtained and well fitted to the Redlich−Kister equation. Our results show that DMF can effectively adjust the physicochemical properties of 1-alkyl-3-methylimidazolium ionic liquids.

1. INTRODUCTION Ionic liquids (ILs) have been attracting increased interest for use in both the industrial and academic fields. Their incombustibility, low vapor pressure, and good solvation potential are the basis for them often being classified as “green” solvents.1−3 Compared to conventional molecular organic solvents, a remarkable advantage of ILs is the designability to adjust the physicochemical properties.4,5 For various ILs, precise measurements of density, refractive index, viscosity, ultrasonic sound velocity, solubility, surface tension, compressibility behavior, and other thermodynamic properties including phase equilibria have been reported to date.6−10 However, the viscosity of pure ionic liquids is usually 1−3 orders of magnitude higher than that of conventional molecular solvents. Obviously, it is a major drawback for their application owing to the slow rate of mass transport within solution. A further “tuning” strategy is the utilization of mixing the ionic liquids with molecular solvents, such as water, alcohol, acetonitrile, and so on, which allows changing and controlling the properties of the systems.11−14 For example, Brennecke et al. studied the phase behavior and solution thermodynamics of imidazolium-based ILs and water mixtures by experiment and calculation methods,15−18 and the results show that the presence of water can significantly affect the physical properties of ILs. Moreover, such binary mixtures have also been effectively used for lipid extraction from biomass,19,20 electrochemical applications13,21−23 and absorption of CO2.24 Among the varieties of ILs, the imidazolium-based ILs (1-alkyl3-methylimidazolium) is the most commonly investigated group.25 The anions in the structure of ILs are mainly tetrafluoroborate BF4−, hexafluorophosphate PF6−, bis(trifluoromethylsulfonyl) imide [TF2N]−, trifluoromethanesulfonate [CF3SO3]−, and so on. © 2014 American Chemical Society

However, the data for 1-alkyl-3-methylimidazolium halide ([Cnmim]X) have been rarely reported up to now.26,27 This may be because the presence of hydrogen bonding in the structures of 1-alkyl-3-methylimidazolium halide salts tends to have much higher melting points and viscosities than ILs with other anions. Therefore, systematic and reliable physicochemical properties are urgently needed for the utilization of such ionic liquids in chemical and industrial processes. On the other hand, N,N-dimethylformamide (DMF) is a common molecular solvent widely used in industrial and laboratorial processes. It can be used as an aprotic and protophilic solvent due to its high boiling point (T = 526.15 K, 100 kpa), large dipole moment (μ = 3.24 D), and high dielectric constant (ε = 36.71)28 at 298.15 K. Thus, it can effectively dissolve organic or inorganic substances. Moreover, DMF is often used as a cosolvent with water, alcohol, or Table 1. Material Description in This Work chemical name 1-ethyl-3-methylimidazolium chloride 1-butyl-3-methylimidazolium chloride 1-hetyl-3-methylimidazolium chloride 1-octyl-3-methylimidazolium chloride N,N-dimethylformamide

source

initial mass fraction

final water mass fraction

Shanghai

> 0.99

0.3 %

Shanghai

> 0.99

0.3 %

Shanghai

> 0.99

0.1 %

Shanghai

> 0.99

0.1 %

Shanghai

> 0.995

0.05 %

Received: October 18, 2013 Accepted: April 10, 2014 Published: April 28, 2014 1411

dx.doi.org/10.1021/je4009238 | J. Chem. Eng. Data 2014, 59, 1411−1422

Journal of Chemical & Engineering Data

Article

other common polar and nonpolar solvents due to its versatile solvent character.29,30 Venkatesu et al. have characterized the

Figure 2. Density (a) and refractive index (b) as a function of ionic liquid mass fraction x1 for the binary systems with different ionic liquids: ■, [C2mim]Cl + DMF; ▲, [C4mim]Cl + DMF; ●, [C6mim]Cl + DMF; ⬟, [C8mim]Cl + DMF at 318.15 K. Symbols refer to the experimental data, while the lines present the results calculated from eq 1.

Table 2. Comparison of Density ρ, Refractive index nD, and Viscosity η with Literature Data for Pure Components at T = 298.15 K ρ/(g·cm−3)

nD

T/K

expt

lit.

expt

lit.

DMF

298.15

0.944 26

1.428 58

1.4288034

[C6mim]Cl [C8mim]Cl

298.15 298.15

1.042 20 1.015 97

0.9445033 0.9442135 1.0396726 1.0088226

1.509 53 1.487 09

1.5172026 1.5098726

Component

thermodynamic properties of the binary mixers of DMF with a series of ammonium ILs containing the acetate, dihydrogen phosphate, and hydrogen sulfate anions.31,32 Moreover, the molecular interaction was discussed based on the measured properties. Thus, four cheap and familiar imidazolium chloride ionic liquids, 1-ethyl-3-methylimidazolium chloride ([C2mim]Cl), 1-butyl-3methylimidazolium chloride ([C4mim]Cl), 1-hetyl-3-methylimidazolium chloride ([C6mim]Cl), and 1-octyl-3-methylimidazolium chloride ([C8mim]Cl) are selected to form binary systems

Figure 1. Density (a), refractive index (b), and viscosity (c) as a function of ionic liquid mole fraction x1 for the binary system [C6mim]Cl + DMF at different temperatures (■, 288.15 K; ▲, 298.15 K; ●, 308.15 K; ◆, 318.15 K). Symbols refer to the experimental data, while the lines present the results calculated from eq 1 1412



Article

Table 3. Density ρ, Refractive Index nD, Viscosity η for [Cnmim]Cl + DMF at (288.15, 298.15, 308.15, and 318.15) K with Pressure P = 0.1 MPaa ρ/(g·cm−3) x1

288.15 K

298.15 K

308.15 K

η/(mPa·s)

nD 318.15 K

288.15 K

0.0000 0.0102 0.0203 0.0310 0.0417 0.0524 0.0635 0.0752 0.0867 0.0988 0.1107 0.1232 0.1361 0.1489 0.1626 0.1759

0.953 79 0.958 86 0.963 47 0.968 07 0.972 24 0.976 38 0.980 82 0.984 98 0.989 33 0.993 32 0.997 33 1.001 39 1.005 60 1.009 60 1.013 74 1.017 70

0.944 26 0.949 52 0.954 18 0.958 86 0.963 18 0.967 27 0.971 86 0.976 05 0.980 59 0.984 64 0.988 67 0.992 88 0.997 07 1.001 24 1.005 40 1.009 49

0.934 69 0.940 08 0.944 84 0.949 62 0.954 02 0.958 21 0.962 87 0.967 15 0.971 78 0.975 93 0.980 02 0.984 31 0.988 59 0.992 84 0.997 09 1.001 24

0.925 16 0.930 60 0.935 40 0.940 37 0.944 85 0.949 12 0.953 87 0.958 24 0.962 96 0.967 20 0.971 44 0.975 73 0.980 13 0.984 44 0.988 83 0.993 01

1.433 04 1.435 65 1.438 36 1.440 91 1.443 54 1.445 88 1.448 22 1.450 67 1.453 08 1.455 42 1.457 84 1.460 21 1.462 53 1.464 77 1.467 21 1.469 42

0.0000 0.0086 0.0172 0.0260 0.0351 0.0443 0.0541 0.0638 0.0738 0.0841 0.0947 0.1056 0.1167 0.1282 0.1399 0.1520

0.953 79 0.957 70 0.961 03 0.964 09 0.967 36 0.970 51 0.973 65 0.976 87 0.979 65 0.982 76 0.985 77 0.988 72 0.991 56 0.994 51 0.997 33 1.000 15

0.944 26 0.948 35 0.951 98 0.954 91 0.958 49 0.961 51 0.964 75 0.968 06 0.970 86 0.974 08 0.977 24 0.980 16 0.983 01 0.986 10 0.988 96 0.991 87

0.934 69 0.938 90 0.942 61 0.945 63 0.949 34 0.952 42 0.955 77 0.959 15 0.962 05 0.965 29 0.968 56 0.971 53 0.974 49 0.977 64 0.980 61 0.983 60

0.925 16 0.929 42 0.933 20 0.936 41 0.940 14 0.943 38 0.946 75 0.950 28 0.953 24 0.956 58 0.959 89 0.962 99 0.966 03 0.969 28 0.972 26 0.975 40

1.433 04 1.435 62 1.437 57 1.440 10 1.442 23 1.444 42 1.446 60 1.448 61 1.450 83 1.452 82 1.454 92 1.457 66 1.459 09 1.461 11 1.463 15 1.465 16

0.0000 0.0312 0.0646 0.1020 0.1456 0.1960 0.2502 0.3144 0.3893 0.4796 0.5910 0.7249 1.0000

0.953 79 0.964 44 0.973 48 0.982 00 0.990 33 0.998 49 1.005 77 1.013 08 1.020 04 1.026 84 1.033 55 1.039 53 1.047 99

0.944 26 0.955 32 0.964 62 0.973 43 0.982 06 0.990 57 0.998 08 1.005 74 1.012 97 1.019 96 1.026 93 1.033 24 1.042 20

0.934 69 0.946 13 0.955 75 0.964 86 0.973 81 0.982 59 0.990 42 0.998 35 1.005 87 1.013 14 1.020 36 1.026 94 1.036 43

0.925 16 0.936 91 0.946 92 0.956 28 0.965 63 0.974 62 0.982 82 0.990 96 0.998 80 1.006 38 1.013 87 1.020 70 1.030 62

1.433 04 1.440 88 1.448 18 1.454 88 1.461 75 1.468 36 1.474 73 1.480 80 1.486 93 1.492 78 1.498 86 1.504 29 1.512 50

0.0000 0.0306 0.0647 0.1021 0.1457 0.1939 0.2496 0.3143 0.3900 0.4809 0.5908

0.953 79 0.962 61 0.970 04 0.976 96 0.983 52 0.989 54 0.995 13 1.000 40 1.005 26 1.009 79 1.013 76

0.944 26 0.953 41 0.961 21 0.968 34 0.975 24 0.981 46 0.987 32 0.992 89 0.997 98 1.002 82 1.006 97

0.934 69 0.944 19 0.952 27 0.959 68 0.966 87 0.973 37 0.979 50 0.985 36 0.990 69 0.995 80 1.000 19

0.925 16 0.934 98 0.943 27 0.951 06 0.958 48 0.965 30 0.971 71 0.977 79 0.983 45 0.988 76 0.993 39

1.433 04 1.439 20 1.445 35 1.450 40 1.455 98 1.460 74 1.465 31 1.469 67 1.474 07 1.477 91 1.481 69

298.15 K

308.15 K

[C2mim]Cl+DMF 1.428 58 1.423 81 1.431 06 1.426 54 1.433 46 1.428 88 1.436 40 1.431 89 1.438 72 1.434 16 1.441 44 1.436 91 1.443 61 1.438 96 1.446 26 1.441 90 1.448 55 1.443 72 1.451 10 1.446 78 1.453 54 1.449 07 1.455 97 1.451 67 1.458 27 1.454 06 1.460 65 1.456 21 1.463 02 1.458 88 1.465 36 1.461 02 [C4mim]Cl+DMF 1.428 58 1.423 81 1.430 88 1.426 11 1.433 13 1.428 36 1.435 37 1.430 60 1.437 61 1.432 85 1.439 80 1.435 05 1.442 06 1.437 31 1.444 22 1.439 48 1.446 38 1.441 63 1.448 52 1.443 78 1.450 63 1.445 90 1.452 71 1.447 99 1.454 75 1.450 03 1.456 76 1.452 05 1.458 71 1.454 01 1.460 64 1.455 95 [C6mim]Cl+DMF 1.428 58 1.423 81 1.436 34 1.431 87 1.443 59 1.439 09 1.450 59 1.446 32 1.457 51 1.453 30 1.464 33 1.460 38 1.470 86 1.466 95 1.477 02 1.473 35 1.482 13 1.478 56 1.489 33 1.485 95 1.495 67 1.492 37 1.501 08 1.497 97 1.509 53 1.506 67 [C8mim]Cl+DMF 1.428 58 1.423 81 1.434 59 1.430 07 1.441 13 1.436 10 1.446 13 1.441 89 1.452 03 1.447 66 1.456 41 1.452 41 1.461 73 1.457 31 1.465 88 1.462 12 1.470 66 1.466 72 1.474 29 1.470 89 1.478 35 1.475 04

1413

318.15 K

288.15 K

298.15 K

308.15 K

318.15 K

1.419 09 1.422 18 1.424 50 1.427 50 1.429 77 1.432 52 1.434 77 1.437 65 1.439 64 1.442 53 1.445 00 1.447 46 1.449 86 1.451 92 1.454 77 1.456 79

0.9360 1.0228 1.1556 1.1876 1.3271 1.3986 1.5353 1.7652 1.8671 2.1232 2.2499 2.4292 2.7658 3.0017 3.2931 3.6453

0.8250 0.8935 1.0075 1.0304 1.1466 1.1980 1.3123 1.5005 1.5768 1.7844 1.8789 2.0143 2.2779 2.4531 2.6721 2.9224

0.7360 0.7902 0.8895 0.9058 1.0043 1.0452 1.1392 1.2966 1.3549 1.5273 1.6245 1.7053 1.9158 2.0514 2.2508 2.4056

0.6650 0.7059 0.7940 0.8045 0.8901 0.9230 1.0023 1.1359 1.1807 1.3268 1.4040 1.4679 1.6398 1.7478 1.9050 2.0203

1.419 09 1.421 47 1.423 79 1.426 11 1.428 42 1.430 69 1.433 03 1.435 26 1.437 48 1.439 70 1.441 88 1.444 02 1.446 12 1.448 20 1.450 21 1.452 19

0.9360 1.0639 1.1303 1.2335 1.3581 1.4408 1.5708 1.6780 1.8647 2.0040 2.2403 2.3965 2.5887 2.9498 3.1963 3.6846

0.8250 0.9285 0.9836 1.0690 1.1734 1.2337 1.3432 1.4436 1.5899 1.6785 1.7998 1.9785 2.1259 2.3786 2.5823 2.9066

0.7360 0.8223 0.8668 0.9394 1.0217 1.0972 1.1663 1.2386 1.3615 1.4318 1.5245 1.6649 1.7824 1.9629 2.1348 2.3508

0.6650 0.7351 0.7720 0.8352 0.8995 0.9641 1.0257 1.0784 1.1755 1.2407 1.3112 1.4236 1.5180 1.6515 1.7965 1.9442

1.419 09 1.427 65 1.434 68 1.442 19 1.449 09 1.456 52 1.462 93 1.469 74 1.474 97 1.482 64 1.489 16 1.494 98 1.503 90

0.936 1.271 1.715 2.392 4.166 6.280 10.768 17.976 32.588 65.305 134.943 232.475

0.825 1.097 1.451 2.064 2.874 4.815 7.927 12.517 21.487 40.253 76.976 178.301

0.736 0.961 1.249 1.733 2.348 3.817 6.056 9.115 15.077 26.458 47.616 75.172 2044.156

0.665 0.852 1.091 1.489 1.957 3.113 4.770 6.880 10.945 18.366 31.332 47.692 884.198

1.419 09 1.425 62 1.431 43 1.438 00 1.443 42 1.448 49 1.453 47 1.458 60 1.463 06 1.467 65 1.471 79

0.936 1.617 2.007 2.763 4.132 5.958 8.425 13.282 20.458 34.231 60.632

0.825 1.348 1.629 2.262 3.234 4.582 6.215 9.498 14.027 22.181 37.327

0.736 1.172 1.448 1.909 2.615 3.636 4.753 7.070 10.118 15.154 24.360

0.665 1.013 1.289 1.661 2.182 2.970 3.753 5.427 7.536 10.765 16.683



Article

Table 3. continued ρ/(g·cm−3) x1

288.15 K

0.7259 1.0000 a

1.017 15 1.022 06

298.15 K 1.010 60 1.015 97

308.15 K 1.004 44 1.009 80

η/(mPa·s)

nD 318.15 K 0.997 95 1.003 62

288.15 K 1.485 34 1.490 08

298.15 K

308.15 K

318.15 K

288.15 K

298.15 K

308.15 K

318.15 K

[C8mim]Cl+DMF 1.482 12 1.478 97 1.487 09 1.484 13

1.475 89 1.481 22

122.327

69.310

43.044 155.026

28.247 65.946

Uncertainties: u(x) = 1·10−4, u(ρ) = 0.00005 g·cm−3, u(nD) = 0.00004, u(T) = 0.03 K, u(η) = 0.02 mPa·s

Table 4. Values of Parameters of Equation 1 for Systems [Cnmim]Cl + DMF at (288.15, 298.15, 308.15, and 318.15) Ka A0 ρ

nD

η

ρ

nD

η

ρ

nD

η

ρ

nD

η

a

A1

0.9544 0.9449 0.9353 0.9258 1.4331 1.4285 1.4240 1.4194 0.9865 0.8630 0.7663 0.6858

0.4467 0.4556 0.4651 0.4747 0.2560 0.2570 0.2534 0.2631 5.7763 4.7536 3.8529 3.3033

0.9543 0.9449 0.9354 0.9258 1.4332 1.4286 1.4238 1.4191 0.9789 0.8539 0.7586 0.6812

0.3904 0.3992 0.4097 0.4217 0.2693 0.2720 0.2723 0.2811 10.4531 9.0622 7.6340 6.3067

0.9561 0.9467 0.9372 0.9277 1.4345 1.4300 1.4253 1.4207 −0.0857 0.4703 0.6033 0.6154

0.2712 0.2809 0.2908 0.3006 0.2152 0.2161 0.2205 0.2238 58.7238 24.7614 12.4792 7.3648

0.9559 0.9464 0.9369 0.9274 1.4343 1.4297 1.4250 1.4203 0.7606 0.7541 0.7345 0.6749

0.2222 0.2310 0.2390 0.2478 0.1712 0.1746 0.1772 0.1819 25.2631 16.1480 11.3501 9.2182

A2 [C2mim]Cl + DMF −0.5300 −0.5399 −0.5518 −0.5621 −0.3012 −0.2876 −0.2326 −0.3000 50.9049 43.6271 39.9755 34.4257 [C4mim]Cl + DMF −0.6126 −0.6260 −0.6413 −0.6613 −0.4206 −0.4318 −0.4309 −0.4463 −12.7385 −17.1154 −13.6617 −8.4000 [C6mim]Cl + DMF −0.3224 −0.3338 −0.3453 −0.3562 −0.2425 −0.2399 −0.2442 −0.2460 −350.5809 −113.2688 −29.2963 1.3488 [C8mim]Cl + DMF −0.2901 −0.2994 −0.3059 −0.3155 −0.2100 −0.2127 −0.2124 −0.2173 −56.8449 −14.6424 0.9908 3.0476

A3

σ

T/K

0.1618 0.1590 0.1555 0.1522 0.0815 0.0846 −0.0632 0.0801 9.3996 −24.9434 −49.9382 −56.4339

0.0002 0.0003 0.0003 0.0003 0.0001 0.0001 0.0002 0.0002 0.0316 0.0266 0.0232 0.0203

288.15 298.15 308.15 318.15 288.15 298.15 308.15 318.15 288.15 298.15 308.15 318.15

0.1466 0.1432 0.1392 0.1339 0.1924 0.1955 0.1995 0.1919 389.0312 299.3724 211.2734 142.3194

0.0002 0.0002 0.0002 0.0002 0.0002 0.0000 0.0000 0.0000 0.0305 0.0180 0.0120 0.0087

288.15 298.15 308.15 318.15 288.15 298.15 308.15 318.15 288.15 298.15 308.15 318.15

0.1436 0.1489 0.1543 0.1590 0.1057 0.1036 0.1054 0.1058 1076.9631 490.7492 241.3983 125.4495

0.0011 0.0012 0.0012 0.0012 0.0007 0.0008 0.0008 0.0009 0.7654 0.3170 0.1555 0.1090

288.15 298.15 308.15 318.15 288.15 298.15 308.15 318.15 288.15 298.15 308.15 318.15

0.1344 0.1384 0.1403 0.1443 0.0949 0.0958 0.0945 0.0966 313.3873 155.4232 80.0897 45.8471

0.0010 0.0010 0.0011 0.0011 0.0007 0.0007 0.0007 0.0007 0.2402 0.1455 0.0999 0.0811

288.15 298.15 308.15 318.15 288.15 298.15 308.15 318.15 288.15 298.15 308.15 318.15

δ = [∑(Ycal − Yexp)2/N ]0.5, where N is the number of experimental points 1414



Article

Figure 3. Density (a, b) and viscosity (c, d) comparison of the ILs + molecule solvent binary systems in the literatures and this work at 298.15 K.

analyzed. An average value of three independent measurements was obtained for each solution. The uncertainty of mole fraction (xi) in the measurement was about 0.01 %. The density, refractive index, and viscosity of the ILs + DMF binary solutions were determined using a DMA 4500 densimeter (Anton Paar, Austria), RXA 170 refractometer (Anton Paar, Austria), and AMVn viscometer (Anton Paar, Austria). The apparatus calibrations were performed periodically under atmospheric pressure using double distilled water prior to initiation of each series of measurements. The accuracy of the density, refractive index, and viscosity are ± 1·10−5 g·cm−3, ± 4·10−5, and ± 1·10−4 mPa·s, respectively. The measurements were done in three duplicate runs and the average values were considered. The density, refractive index, and viscosity values of the pure solvents are given in Table 2 and compared with the literature values.

with DMF. Herein, the density, refractive index, and viscosity of the mixtures [Cnmim]Cl (Cnmim = 1-alkyl-3-methylimidazolium; n = 2, 4, 6, 8) + DMF were reported at (288.15, 298.15, 308.15, and 318.15) K and atmospheric pressure, which are well fitted to empirical equations. The molar excess Gibbs, excess molar volume, apparent molar volume, and the refractive index and viscosity deviations from the ideal behavior have been also been calculated and discussed.

2. EXPERIMENTAL SECTION 2.1. Materials. The purity levels and sources of the substances used in this study are reported in Table 1. DMF was dried using molecular sieves, and the water content was determined to be 0.05 % using a Karl Fischer titrator. For [C2mim]Cl, [C4mim]Cl, [C6mim]Cl, and [C8mim]Cl, water mass fractions were determined to be 0.3 % after the materials were heated at 333 K under vacuum for 48 h. The density and refractive index for the pure components determined in this work are compared with those in the literature (Table 2). 2.2. Apparatus and Procedure. Since [C2mim]Cl and [C4mim]Cl are solid at room temperature, their solubility in DMF should be determined first. [C6mim]Cl and [C8mim]Cl are completely miscible with DMF. The samples were prepared by dissolving excess ILs into DMF via weighing on an analytical balance (Mettler Toledo, AL 204, Switzerland) with an accuracy of ± 1·10−4 g. The prepared samples were sealed in resistantcorrosion sample tubes, stirred for 48 h, and allowed to settle for a further 24 h at the experimental temperatures controlled by a water bath to ensure that the equilibrium was established. Afterward, the solution of the samples was withdrawn and

3. RESULTS AND DISCUSSION 3.1. Density, Refractive Index, Viscosity. The experimental results for the density, refractive index, and viscosity for the [Cnmim]Cl (n = 2, 4, 6, 8) + DMF systems at (288.15, 298.15, 308.15, and 318.15) K are listed in Table 3. More recently, the viscosity and density of [C4mim]Cl + DMF system at 298.15 K have been reported by Yang et al.,36,37 the values of which are in good agreement with our results. A simple empirical equation can correlate the original data for density, refractive index, and viscosity for all the systems. Y = A 0 + A1w1 + A 2w12 + A3w13 1415

(1)



Article

Figure 5. Experimental values of apparent molar volume ((a) [C6mim]Cl (b) DMF) as a function of ionic liquid mole fraction x1 for the binary systems [C6mim]Cl + DMF at ■, 288.15 K; ▲, 298.15 K; ●, 308.15 K; ◆, 318.15 K. Symbols refer to the experimental data.

Figure 4. (a) Experimental values of excess molar volume as a function of ionic liquid mole fraction x1 for the binary systems [C6mim]Cl + DMF at ■, 288.15 K; ▲, 298.15 K; ●, 308.15 K; ◆, 318.15 K. Symbols refer to the experimental data, while the lines present the results calculated from eq 3. (b) Excess molar volume compared with the literature values at 298.15 K.

Cl + DMF systems in this work is dramatically smaller than that of [Cnmim]Cl + water26 (Figure 3a). That is, DMF can decrease the density of ILs more effectively. The comparison of the viscosity for [C8mim]Cl with water, methanol, ethanol, 1-proponal in the literature27 and DMF in our study (Figure 3c) also suggests that DMF is a good solvent for decreasing the viscosity of ILs. Moreover, the DMF + other Ils38 and the experiment results in this study are plotted in Figure 3b and 3d. The results indicate that adding the high polar solvent DMF to ILs can smoothly reduce the density and viscosity of ILs, even for the imidazolium chloride ILs with the highest density and viscosity. 3.2. Volumetric Behavior of the Binary Mixtures. The intermolecular forces between the different molecules of the mixture are less or more stronger than they are in the pure liquids. The information can be reflected by the value of the excess molar volume VE. Furthermore, the apparent molar volume Vφ,i of ionic liquids in DMF was also calculated. Variation of VE with mole fraction of ILs are plotted in Figure 4a, which show that negative excess molar volumes appear for the [C6mim]Cl/[C8mim]Cl + DMF binary mixtures. Moreover, the results of VE for systems [C6mim]Cl/[C8mim]Cl + H2O26 and TMAH/TEAH/TPAH + DMF32 from the literature are plotted

where Ai (i = 0−3) are adjustable parameters, x1 is the mole fraction of ionic liquids in the mixed solutions, and Y represents the ρ, nD, and η. Values of fitting parameters as well as the standard deviations are given in Table 4. Figure 1 displays that the density, refractive index, and viscosity values are in a direct proportion relationship with mole fraction of IL (xIL) and inversely with the increase of temperature for all the binary systems. That is, adding DMF to ionic liquids will obviously decrease the density, refractive index, and viscosity, and when the temperature is higher, the values of all the properties are smaller. It can be seen from Table 3, that the viscosities of [C6mim]Cl/[C8mim]Cl + DMF systems decreased dramatically with an increase of DMF. Moreover, the shorter the carbon chains of the ILs are, the larger the density, refractive index, and viscosity are. In other words, the values are [C2mim] Cl > [C4mim]Cl > [C6mim]Cl > [C8mim]Cl as shown in Figure 2. This may be affected by the fact that ionic liquids with longer carbon chain cannot be close packed in the microscopic structure. For comparison, the results of other ILs + molecular solvent systems are depicted (Figure 3). The density of [Cnmim] 1416



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Table 5. Excess Molar Volume VmE, Apparent Molar Volume Vφ,i, Viscosity Deviation Δη, the Excess Gibbs Free Energies ΔG*E and Refractive Indice Deviation ΔnD for [C6mim]Cl/[C8mim]Cl + DMF Systems from 288.15 K to 318.15 K with Pressure P = 0.1 MPaa V mE

Vφ,1 −1

cm ·mol 3

x1

ΔG*E

Vφ,2 −1

cm ·mol 3

−1

cm ·mol 3

ΔnD

Δη

kJ·mol−1

[C6mim]Cl + DMF 288.15 0.0000 0.0312 0.0646 0.1020 0.1456 0.1960 0.2502 0.3144 0.3893 0.4796 0.5910 0.7249 1.0000 298.15 0.0000 0.0312 0.0646 0.1020 0.1456 0.1960 0.2502 0.3144 0.3893 0.4796 0.5910 0.7249 1.0000 308.15 0.0000 0.0312 0.0646 0.1020 0.1456 0.1960 0.2502 0.3144 0.3893 0.4796 0.5910 0.7249 1.0000 318.15 0.0000 0.0312 0.0646 0.1020 0.1456 0.1960 0.2502 0.3144 0.3893 0.4796 0.5910 0.7249 1.0000

K 0.000 −0.297 −0.494 −0.651 −0.776 −0.880 −0.938 −0.980 −0.977 −0.927 −0.822 −0.598 0.000

183.942 185.781 187.056 188.107 188.949 189.688 190.321 190.927 191.504 192.047 192.612 193.437

0.000 −0.316 −0.519 −0.681 −0.811 −0.923 −0.979 −1.028 −1.022 −0.956 −0.839 −0.611 0.000

184.400 186.479 187.834 188.940 189.800 190.601 191.243 191.886 192.518 193.091 193.668 194.512

0.000 −0.333 −0.546 −0.716 −0.853 −0.965 −1.026 −1.074 −1.067 −0.995 −0.865 −0.623 0.000

184.929 187.135 188.578 189.738 190.669 191.494 192.179 192.854 193.519 194.131 194.736 195.594

0.000 −0.346 −0.576 −0.749 −0.902 −1.009 −1.082 −1.122 −1.119 −1.047 −0.907 −0.649 0.000

185.633 187.774 189.355 190.506 191.547 192.373 193.128 193.824 194.515 195.163 195.802 196.697

76.642 76.336 76.113 75.917 75.733 75.548 75.390 75.213 75.042 74.861 74.633 74.468

0 0.0054 0.0100 0.0137 0.0171 0.0198 0.0218 0.0228 0.0230 0.0216 0.0189 0.0137 0

77.415 77.089 76.861 76.656 76.466 76.267 76.110 75.916 75.741 75.578 75.363 75.193

0 0.0052 0.0098 0.0138 0.0172 0.0199 0.0220 0.0230 0.0220 0.0219 0.0192 0.0138 0

78.208 77.864 77.624 77.411 77.210 77.007 76.839 76.642 76.461 76.295 76.093 75.945

0 0.0055 0.0099 0.0141 0.0174 0.0203 0.0224 0.0235 0.0240 0.0224 0.0196 0.0141 0.0000

0 −63.594 −131.406 −207.472 −295.942 −397.651 −505.974 −634.041 −781.085 −954.226 −1160.83 −1406.82 0

0 0.0816 0.1122 0.2262 0.1546 0.4113 0.5207 0.2878 0.0707 −0.3214 −1.1001 −2.6995 0.0000

79.013 78.657 78.398 78.179 77.958 77.758 77.570 77.377 77.182 77.002 76.796 76.655

0.0000 0.0059 0.0101 0.0145 0.0177 0.0208 0.0226 0.0240 0.0244 0.0229 0.0200 0.0144 0.0000

0 −27.406 −56.613 −89.314 −127.364 −170.690 −216.968 −271.554 −333.645 −406.008 −491.520 −593.436 0

0.0000 0.0974 0.1528 0.2986 0.2291 0.5349 0.6604 0.4323 0.2507 −0.0968 −0.8249 −2.3113 0.0000

K

K

K

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Table 5. continued x1

V mE

Vφ,1

Vφ,2

cm3·mol−1

cm3·mol−1

cm3·mol−1

ΔG*E ΔnD

Δη

kJ·mol−1

[C8mim]Cl + DMF 288.15 0.0000 0.0306 0.0647 0.1021 0.1457 0.1939 0.2496 0.3143 0.3900 0.4809 0.5908 0.7259 1.0000 298.15 0.0000 0.0306 0.0647 0.1021 0.1457 0.1939 0.2496 0.3143 0.3900 0.4809 0.5908 0.7259 1.0000 308.15 0.0000 0.0306 0.0647 0.1021 0.1457 0.1939 0.2496 0.3143 0.3900 0.4809 0.5908 0.7259 1.0000 318.15 0.0000 0.0306 0.0647 0.1021 0.1457 0.1939 0.2496 0.3143 0.3900 0.4809 0.5908 0.7259 1.0000

K 0.000 −0.254 −0.417 −0.568 −0.690 −0.793 −0.864 −0.912 −0.922 −0.887 −0.763 −0.517 0.000

217.493 219.349 220.242 221.063 221.708 222.337 222.898 223.436 223.955 224.507 225.087 225.799

0.000 −0.264 −0.440 −0.587 −0.719 −0.817 −0.888 −0.940 −0.945 −0.914 −0.773 −0.517 0.000

218.516 220.360 221.400 222.220 222.941 223.593 224.161 224.730 225.252 225.843 226.441 227.152

0.000 −0.277 −0.457 −0.610 −0.745 −0.847 −0.920 −0.976 −0.977 −0.946 −0.799 −0.603 0.000

219.488 221.477 222.571 223.426 224.173 224.853 225.436 226.035 226.573 227.189 227.710 228.540

0.000 −0.288 −0.467 −0.634 −0.769 −0.879 −0.956 −1.007 −1.017 −0.976 −0.822 −0.631 0.000

220.534 222.731 223.737 224.672 225.417 226.118 226.745 227.340 227.918 228.557 229.078 229.948

76.642 76.379 76.195 76.010 75.834 75.658 75.490 75.312 75.130 74.933 74.777 74.755

0 0.0044 0.0086 0.0115 0.0146 0.0166 0.0180 0.0187 0.0188 0.0174 0.0150 0.0109 0

77.415 77.143 76.945 76.761 76.574 76.402 76.231 76.044 75.866 75.654 75.525 75.530

0 0.0042 0.0088 0.011 58 0.0149 0.0165 0.0186 0.0189 0.0193 0.0176 0.0152 0.0111 0

78.208 77.922 77.719 77.529 77.335 77.157 76.981 76.785 76.606 76.385 76.256 76.007

0 0.0044 0.0084 0.0119 0.0151 0.0169 0.0184 0.0194 0.0194 0.0181 0.0156 0.0114 0.0000

0 −69.753 −147.727 −233.118 −332.390 −441.934 −568.576 −714.785 −885.418 −1088.910 −1331.740 −1623.120 0

0 0.6133 0.5078 0.4986 0.4577 0.3541 −0.0679 −0.3557 −0.9816 −1.8237 −2.9029 −4.3032 0.0000

79.013 78.716 78.514 78.307 78.114 77.923 77.740 77.545 77.346 77.133 77.005 76.711

0.0000 0.0046 0.0083 0.0126 0.0153 0.0174 0.0189 0.0200 0.0197 0.0187 0.0160 0.0117 0.0000

0 −30.873 −65.405 −103.223 −147.175 −195.569 −251.616 −316.011 −391.160 −480.690 −586.885 −713.247 0

0.0000 0.5759 0.6083 0.6056 0.5347 0.4608 0.0399 −0.2111 −0.7955 −1.6195 −2.6248 −3.9300 0.0000

K

K

K

Uncertainties: u(x) = 1·10−4, u(VE) = 0.013 cm3·mol−1, u(Vφ) = 0.020 cm3·mol−1, u(ρ) = 0.00005 g·cm−3, u(nD) = 0.00004, u(T) = 0.03 K, u(p) = 10 kPa, u(η) = 0.02 mpa·s. a

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Figure 7. Experimental values of the molar excess Gibbs free energies as a function of ionic liquid mole fraction x1 for the binary system [C6mim] Cl + DMF at ■, 308.15 K; ▲, 318.15 K. Symbols refer to the experimental data, while the lines present the results calculated from eq 3

leads to the accelerated thermal movement of molecules. Thus, the molecular vibration range is increased and there is a corresponding increase in the spare volume of ILs, whereas the decrease of the apparent molar volume of DMF Vφ2 with the mole fraction of IL may occur because DMF accommodates the IL particles which finally leads to the volume change. 3.3. Refractive Index and Viscosity Deviations. The interaction information on multicomponent mixtures can be provided by the ΔnD and Δη data. Furthermore, the deviations in refractive index and viscosity can be calculated.40 The values of VmE, Vφ,i, and ΔnD, Δη for [C6mim]Cl/[C8mim] Cl + DMF systems from 288.15 K to 318.15 K are given in Table 5. The variations of ΔnD with mole fraction of ionic liquids at 318.15 K are shown in Figure 6 graphically. It displays that ΔnD values are positive for [C6mim]Cl/[C8mim]Cl + DMF systems at the investigated temperatures over the entire composition range. The positive deviations of ΔnD values indicate the strong interaction between two components of the mixtures. The values ΔnD present the direct proportion relationship with temperature for the binary mixtures, declaring that the interactions existing in ILs and DMF increase with a rise in temperature. Also, the data of ΔnD are found reverse to the law curves of excess molar volumes for [C6mim]Cl/[C8mim]Cl + DMF systems. Figure 6 displays the Δη for the system of [C6mim]Cl + DMF. For viscosity deviations, the sign is negative for all the binary systems, and the minimum lies at a mole fraction of approximately 0.7 for DMF + [C6mim]Cl/[C8mim]Cl. The viscosity deviations decrease with temperature increases, which is similar to that of the [C6mim]Cl/ [C8mim]Cl + H2O systems.26 3.4. The Molar Excess Gibbs Free Energies of Activation of Viscous Flow. Equation 2 is the systematic form of the equation for representing deviations of the viscosity from ideal solutions, expressed as the excess Gibbs free energy of activation for viscous flow ΔG*E.41

Figure 6. (a) Experimental values of refractive indice deviations as a function of ionic liquid mole fraction x1 for the binary system: ■, [C6mim]Cl + DMF; ▲, [C8mim]Cl + DMF at 318.15 K. Symbols refer to the experimental data, while the lines present the results calculated from eq 3. (b) Experimental values of viscosity deviations as a function of ionic liquid mole fraction x1 for the binary system [C6mim]Cl + DMF at ■, 308.15 K; ▲, 318.15 K.

in Figure 4b. It can be observed that both [C6mim]Cl/[C8mim] Cl + H2O and [C6mim]Cl/[C8mim]Cl + DMF systems show negative excess molar volumes. And the DMF systems present more negative character. However, for TMAH/TEAH/TPAH + DMF binary systems, the VE values reveal an inversion in the sign from negative to positive deviation. The results indicate that once ionic liquids are mixed with DMF or water, space is available in the network structure generated by the anions and cations. Therefore, small organic molecules embed in the gap when ILs and organic molecules mix, and a more effective accumulation occurs. Meanwhile, the ion-induced dipole interaction between the cations of the ionic liquid and DMF also reduces the molar volume of the mixture.39 The data of Vφ,i with mole fraction of ILs are plotted in Figure 5. The apparent molar volume of ILs, Vφ1, increases with an increased content of ILs at the fixed temperature, while the apparent molar volume of DMF, Vφ2, show the opposite trend. When the temperature increases, VE is more negative and the values of Vφ1, and Vφ2 are increased. This phenomenon may occur because the rise of the temperature

⎡ ⎛ ηV ⎞ ⎛ η V ⎞⎤ ⎟⎟ − x1 ln⎜⎜ 1 1 ⎟⎟⎥ ΔG*E = RT ⎢ln⎜⎜ ⎢⎣ ⎝ η2V2 ⎠ ⎝ η2V2 ⎠⎥⎦

(2)

Here η, V, η1, η2, V1, and V2 represent viscosity of solution, its molar volume, viscosity of component 1, viscosity of component 2, 1419



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Table 6. Values of Parameters of Equation 3 at 308.15 K and 318.15 Ka T/K VE

ΔnD

ΔG*E

VE

ΔnD

ΔG*E a

A0

A1

288.15 298.15 308.15 318.15 288.15 298.15 308.15 318.15 308.15 318.15

−3.6908 −3.8135 −3.9616 −4.1553 0.0906 0.0906 0.0887 0.0906 −1.7066 −0.8060

288.15 298.15 308.15 318.15 288.15 298.15 308.15 318.15 308.15 318.15

−3.4952 −3.5831 −3.6881 −3.8060 0.0730 0.0730 0.0710 0.0730 −7.7984 −6.9051

A2

A3

[C6mim]Cl + DMF 1.6183 −0.3968 1.8370 −0.4246 2.0335 −0.3193 2.1690 −0.2838 −0.0491 0.0098 −0.0491 0.0098 −0.0468 0.0151 −0.0491 0.0098 −12.2590 −16.4028 −11.2564 −14.3768 [C8mim]Cl + DMF 1.8547 0.5092 2.0247 0.7633 2.1719 0.1673 2.3249 0.0739 −0.0404 0.0223 −0.0404 0.0223 −0.0395 0.0223 −0.0404 0.0223 −20.6315 −19.1094 −19.5687 −17.6153

σ

A4 1.8962 1.7730 1.7826 1.7662 −0.0015 −0.0015 −0.0115 −0.0015 −24.0340 −22.2836

−2.5040 −2.7384 −3.0968 −3.4227 0.0503 0.0503 0.0287 0.0503 −18.9704 −18.4829

0.0088 0.0114 0.0117 0.0105 0.0009 0.0009 0.0001 0.0001 0.0604 0.0644

0.8048 0.6955 −1.4413 −1.9937 −0.0137 −0.0137 −0.0134 −0.0137 −10.9571 −9.5160

−3.0193 −3.6385 −5.4486 −5.9587 0.0172 0.0172 0.0156 0.0172 11.2812 12.7856

0.0076 0.0073 0.0079 0.0082 0.0008 0.0008 0.0001 0.0002 0.0852 0.0683

δ = [∑(Ycal − Yexp)2/N ]0.5, where N is the number of experimental points.

deviations from the ideal behavior have been calculated. Also, the Redlich−Kister polynomial relation has been used to fit all of the excess and refractive index deviation properties to obtain their coefficients and standard deviations. The obtained volumetric and excess Gibbs free energies of activation of viscous flow data are helpful to understand mixing effects and other existing interactions during the ions transporting process.

molar volume of component 1, and molar volume of component 2, respectively.42−44 The variations of ΔG*E with mole fraction of ionic liquids at T = 308.15 K and 318.15 K are shown in Table 5 and in Figure 7. The original data reveal that ΔG*E for the ([C6mim]Cl + DMF) binary system are positive at the lower [C6mim]Cl mole fractions, but turn negative around x1 = 0.25021 at T = 308.15 K and 318.15 K. Negative minimum of the curves are located around the 0.72 mole fraction of [C6mim]Cl. The positive values of ΔG*E provide the information that strong interactions appear between unlike molecules. This demonstrates the view that chemical interactions which may involve association due to hydrogen bonding, dipole−dipole interaction, and formation of complexes due to charge transfer may lead to strong interactions.45 3.5. The Application of Redlich−Kister Polynomial Equation. The values of VEm, ΔG*E, and ΔnD correlate well with the Redlich−Kister equation:

■

Figures of density, refractive index, and viscosity for the binary system [C2mim]Cl/[C4mim]Cl/[C8mim]Cl + DMF at different temperatures; figures of excess molar volume, apparent molar volume, and molar excess Gibbs energy for the binary systems [C8mim]Cl + DMF at different temperatures. This material is available free of charge via the Internet at http://pubs.acs.org.

■

i=0

AUTHOR INFORMATION

Corresponding Author

n

F(x) = x1x 2 ∑ Ai (2x1 − 1)i

ASSOCIATED CONTENT

S Supporting Information *

*Tel.: +86-29-81530765. Fax: +86-29-81530727. E-mail: [email protected] (S.-N.L.); [email protected] (M.-C.H.).

(3)

Funding

where F(x) represents VEm, ΔG*E, and ΔnD, and Ai are fitting parameters based on a least-squares method, estimated by multiple regression analysis. The summary of Ai values along with the standard deviation σ is given in Table 6.

This work was supported by the National Natural Science Foundation of China (21171111, 21271123 and 21301114), the Natural Science Foundation of Shaanxi Province (2013JQ2009), the New Century Excellent Talents in University (NCET-120897) and the Youth Star of Science and Technology of Shaanxi Province (2014).

4. CONCLUSION The experimental data for the density, refractive index, and viscosity of the binary systems of [Cnmim]Cl + DMF over the temperature range from 288.15 K to 318.15 K has been presented in this paper. The experimental results have been fitted to empirical equations, which makes them reliable for the calculation of some thermophysical properties, such as the molar excess Gibbs and volumetric properties including excess molar volume and apparent molar volume. The refractive index

Notes

The authors declare no competing financial interest.

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REFERENCES

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