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Physical Characterization of an Aromatic Extraction Solvent Formed by [bpy][BF4] and [4bmpy][Tf2N] Mixed Ionic Liquids Marcos Larriba, Silvia García, Pablo Navarro, Julián García,* and Francisco Rodríguez Department of Chemical Engineering, Complutense University of Madrid, E-28040 Madrid, Spain ABSTRACT: The binary mixture of N-butylpyridinium tetrafluoroborate ([bpy][BF4]) and 4-methyl-N-butylpyridinium bis(trifluoromethylsulfonyl)imide ([4bmpy][Tf2N]) ionic liquids (ILs) has recently been proposed as an environmentally friendly solvent in the liquid−liquid extraction of toluene from heptane. In addition to the extractive properties, a solvent must possess adequate physical properties to be used in an industrial process. In this paper, we have measured refractive indices, densities, and dynamic viscosities of {[bpy][BF4] + [4bmpy][Tf2N]} IL mixtures over the whole range of compositions at temperatures from 293.15 K to 353.15 K. A comparative analysis between physical properties of binary IL mixtures and those of sulfolane has been made. The accuracy of the group contribution method proposed by Ye and Shreeve in predicting experimental densities of a binary pyridinium-based IL mixture composed of four different ions has been evaluated. The Bingham and the Grunberg−Nissan mixing rules have also been used to estimate the viscosities of the binary IL mixtures.

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behavior of IL mixtures.10−17 To study the behavior of this pyridinium-based IL mixture, we have measured refractive indices, and we have calculated the excess properties quantifying the deviation from the ideal behavior. Likewise, we have compared the experimental densities of this binary IL mixture with those estimated by the Ye and Shreeve method18 and extended by Gardas and Coutinho.19 Also, we have studied the accuracy of the Grunberg−Nissan20 and the Bingham21 viscosity mixing rules.

INTRODUCTION A large number of studies performed during past decade have demonstrated that ionic liquids (ILs) can be considered as potential solvents to replace organic compounds, such as sulfolane, in the liquid−liquid extraction of aromatic hydrocarbons, requiring less energy consumption and fewer steps in the aromatic separation unit.1−3 However, only a very small number of pure ILs has shown simultaneously an aromatic/ aliphatic selectivity and an aromatic distribution coefficient higher than the sulfolane values.1 Fletcher et al. suggested that the properties of a binary IL mixture could be better than those of the pure ILs,4 and Plechkova and Seddon recommended the addition of a second IL to fine-tune the properties of the IL-based solvents.5 Because of this, we suggested the use of binary IL mixtures as aromatic extraction solvents, and we determined that the extractive properties of a binary IL mixture are intermediate than those of pure ILs.6,7 In our most recent work,7 we have studied an IL-based solvent composed of an IL with high aromatic/aliphatic selectivity ([bpy][BF4]),8 mixed with other IL with high aromatic distribution coefficient ([4bmpy][Tf2N]).9 We have concluded that a {[bpy][BF4] + [4bmpy][Tf2N]} IL mixture with a [bpy][BF4] mole fraction of 0.7 shows a higher selectivity and a higher toluene distribution coefficient than those of sulfolane.7 To check that the {[bpy][BF4] + [4bmpy][Tf2N]} IL mixture also possesses adequate physical properties to replace sulfolane, we have measured densities and dynamic viscosities of this binary IL mixture over the whole range of compositions and at temperatures from 293.15 K to 353.15 K. We have performed a comparative analysis between the physical properties shown in this work and those of sulfolane. The number of works involving physical properties of binary IL mixtures is still insufficient to allow a full understanding of the © XXXX American Chemical Society

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EXPERIMENTAL SECTION [bpy][BF4] and [4bmpy][Tf2N] ILs were purchased from Iolitec GmbH with a mass fraction purity higher than 0.99. The purities of the ILs were measured by the supplier using NMR analysis. Halide and water mass fractions were estimated lower than 0.0001, being determined by the manufacturer using ion chromatography and Karl Fisher titration, respectively. Chemicals were used as received without further purification. To avoid water hydration, they were stored in their original tightly closed bottles in a desiccator. Their handling was carried out under a dry nitrogen atmosphere inside a glovebox. Binary IL mixtures in the whole range of compositions were prepared in airtight vials using a Mettler Toledo AB104 balance with a precision of ± 1·10−4 g. Physical properties of the mixtures were determined at the end of the mixing process with a Labnet Vortex Mixer. The estimated uncertainty in the mole fraction of the binary mixtures was lower than 0.0008. Densities, refractive indices, and dynamic viscosities of the pure [bpy][BF4] and [4bmpy][Tf2N] ILs and of the binary IL mixtures were measured Received: September 6, 2012 Accepted: May 7, 2013

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dx.doi.org/10.1021/je300972a | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Table 1. Experimental Refractive Indices (nD), Densities (ρ), and Dynamic Viscosities (η) of [bpy][BF4] (1) + [4bmpy][Tf2N] (2) IL Mixtures on Mole Fraction (x) as a Function of Temperature and at P = 0.1 MPaa T/K x1

303.15

313.15

0.0000 0.0575 0.1136 0.2311 0.3515 0.4566 0.5692 0.6730 0.7836 0.8890 0.9535 1.0000

1.4480 1.4479 1.4477 1.4474 1.4472 1.4471 1.4469 1.4468 1.4468 1.4469 1.4470 1.4470

1.4449 1.4448 1.4446 1.4444 1.4443 1.4441 1.4441 1.4440 1.4441 1.4442 1.4443 1.4444

1.4419 1.4418 1.4416 1.4414 1.4414 1.4413 1.4413 1.4413 1.4413 1.4416 1.4417 1.4418

0.0000 0.0575 0.1136 0.2311 0.3515 0.4566 0.5692 0.6730 0.7836 0.8890 0.9535 1.0000

1.4166 1.4093 1.4019 1.3851 1.3660 1.3478 1.3262 1.3040 1.2775 1.2491 1.2302 1.2171

1.4073 1.4001 1.3928 1.3762 1.3574 1.3394 1.3181 1.2961 1.2700 1.2419 1.2232 1.2103

1.3981 1.3910 1.3838 1.3674 1.3488 1.3311 1.3100 1.2883 1.2625 1.2347 1.2162 1.2035

0.0000 0.0575 0.1136 0.2311 0.3515 0.4566 0.5692 0.6730 0.7836 0.8890 0.9535 1.0000 a

293.15

70.6 75.3 77.8 88.7 101.1 115.1 135.2 153.4 171.9 195.0 216.1 227.1

45.2 47.3 49.3 55.5 63.3 72.7 81.7 90.6 98.9 109.6 118.8 123.6

323.15 nD 1.4388 1.4388 1.4386 1.4385 1.4385 1.4384 1.4385 1.4385 1.4386 1.4389 1.4390 1.4392 ρ/g·cm−3 1.3888 1.3819 1.3747 1.3586 1.3403 1.3228 1.3020 1.2805 1.2550 1.2276 1.2093 1.1967 η/mPa·s 21.8 22.6 23.5 26.4 29.6 32.4 35.4 38.2 41.0 44.1 46.5 48.1

30.6 31.9 33.0 37.0 42.3 47.2 52.1 56.6 61.3 66.9 72.5 74.8

333.15

343.15

353.15

1.4358 1.4357 1.4356 1.4356 1.4356 1.4356 1.4357 1.4358 1.4359 1.4362 1.4364 1.4366

1.4328 1.4328 1.4327 1.4327 1.4327 1.4328 1.4330 1.4331 1.4333 1.4336 1.4338 1.4341

1.4298 1.4298 1.4297 1.4298 1.4299 1.4300 1.4302 1.4304 1.4306 1.4310 1.4313 1.4315

1.3797 1.3728 1.3658 1.3498 1.3318 1.3145 1.2940 1.2728 1.2476 1.2205 1.2024 1.1899

1.3705 1.3638 1.3569 1.3411 1.3233 1.3063 1.2860 1.2651 1.2402 1.2134 1.1956 1.1832

1.3615 1.3549 1.3480 1.3325 1.3149 1.2981 1.2781 1.2575 1.2328 1.2063 1.1888 1.1765

16.1 16.8 17.5 19.5 21.6 23.3 25.2 27.0 28.9 30.8 32.2 33.0

12.3 12.8 13.3 14.6 16.0 17.2 18.5 19.7 20.9 22.1 23.0 23.8

9.8 10.1 10.5 11.4 12.4 13.3 14.1 14.9 15.8 16.6 17.2 17.7

Standard uncertainties u are u(nD) = 0.0003, u(ρ) = 0.0008 g·cm−3, u(η) = 1.2 %.

± 0.01 K. The uncertainty of the dynamic viscosities was found to be less than ± 1.2 %. In previous works, we have checked the reliability of the measurement methods comparing experimental physical properties with published data for several pure ILs.15,22,23 To assess the reliability of the physical properties gathered in this work, we have made a comparison between our results and literature data for the [bpy][BF4] and [4bmpy][Tf2N] pure ILs. The average absolute difference between our refractive indices of the pure [bpy][BF4] and those from literature was 0.01 %.24 The average deviation between experimental densities and literature data for the [bpy][BF4] was less than 0.17 %,25−27 and for the [4bmpy][Tf2N] pure IL was less than 0.02 %.28 Lastly, the average absolute deviations between experimental and published viscosities of the [bpy][BF4] and [4bmpy][Tf2N] pure ILs were less than 5.19 %27 and 1.59 %,28 respectively. It is important to highlight that the [bpy][BF4] employed by Mokhtarani et al. was

from 293.15 K to 353.15 K. A detailed description of equipment and of the procedure can be found elsewhere.15 Refractive indices were measured employing a Rudolph Research Analytical J357 refractometer with a temperature precision of ± 0.1 K. The uncertainty of the experimental refractive indices was estimated to be less than ± 3·10−4. Densities were determined using an Anton Paar DMA-5000 oscillating U-tube density meter. The temperature of the measurement was determined by two integrated Pt100 probes with a precision of ± 0.01 K. The effect of the viscosity of the sample on density determination was automatically corrected by a viscosity correction factor. The estimated uncertainties of the densities were less than ± 8·10−4 g·cm−3. Dynamic viscosities of the pure or mixed [bpy][BF4] and [4bmpy][Tf2N] ILs were measured using an Anton Paar Automated Micro Viscometer (AMVn) based on the falling ball principle. The temperature was controlled with a precision of B



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Figure 1. Refractive indices of [bpy][BF4] (1) + [4bmpy][Tf2N] (2) IL mixtures as a function of [bpy][BF4] mole fraction and temperature: ◇, 293.15 K; ×, 303.15 K; △, 313.15 K; □, 323.15 K; ∗, 333.15 K; ○, 343.15 K; +, 353.15 K. Dashed lines represent refractive indices of ideal binary mixtures.

Figure 2. Deviations from ideality of the refractive indices of [bpy][BF4] (1) + [4bmpy][Tf2N] (2) IL mixtures as a function of temperature and composition: ◇, 293.15 K; ×, 303.15 K; △, 313.15 K; □, 323.15 K; ∗, 333.15 K; ○, 343.15 K; +, 353.15 K. Dashed lines are the fitting curves using Redlich−Kister equations.

temperature are graphically presented in Figure 1. As expected, refractive indices of the pure ILs and of the binary IL mixtures decreased by increasing the temperature. In Figure 1, dashed lines are refractive indices of the binary IL mixtures predicted by an ideal mixing rule:

synthesized in their laboratory with higher water mass fraction (6.25·10−4) than the IL used here (1·10−4),27 whereas the [4bmpy][Tf2N] used by Oliveira et al. was also supplied by Iolitec GmbH with the same purity as the IL used in this work.28 Also, the largest deviations were found for the viscosity data as a result of the important effect of water content and impurities on the dynamic viscosity of ILs.29

2

nD =

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∑ nD,ixi i=1

RESULTS AND DISCUSSION Refractive indices (nD), densities (ρ), and dynamic viscosities (η) of the [bpy][BF4] and [4bmpy][Tf2N] pure ILs, and of the {[bpy][BF4] + [4bmpy][Tf2N]} IL mixtures over the whole range of compositions were determined at temperatures from 293.15 K to 353.15 K and atmospheric pressure. The experimental physical properties along with the standard uncertainties are shown in Table 1. Refractive Index. Refractive indices of the {[bpy][BF4] + [4bmpy][Tf2N]} IL mixtures as a function of composition and

(1)

where nD is the refractive index of the binary IL mixture, nD,i is the refractive index of the pure ILs at the same temperature, and xi denotes the IL mole fractions in the IL mixture. As observed, the experimental refractive indices of the {[bpy][BF4] + [4bmpy][Tf2N]} IL mixtures are slightly lower than that predicted for the ideal mixture. A similar behavior was observed in our previous work in {[bpy][BF4] + [bpy][Tf2N]} IL mixtures.15 To quantify the deviation of the refractive index from the ideal behavior, deviations from ideality (ΔnD) have been calculated as the difference between the experimental data of the {[bpy][BF4] C



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Figure 3. Densities of [bpy][BF4] (1) + [4bmpy][Tf2N] (2) IL mixtures at different temperatures and compositions: ◇, 293.15 K; ×, 303.15 K; △, 313.15 K; □, 323.15 K; ∗, 333.15 K; ○, 343.15 K; +, 353.15 K. Dashed lines represent densities estimated from the group contribution model proposed by Ye and Shreeve, and extended by Gardas and Coutinho, and the dotted line denotes the density of sulfolane at 313.15 K (from ref 32).

Figure 4. Excess molar volumes for [bpy][BF4] (1) + [4bmpy][Tf2N] (2) IL mixtures as a function of temperature: ◇, 293.15 K; ×, 303.15 K; △, 313.15 K; □, 323.15 K; ∗, 333.15 K; ○, 343.15 K; +, 353.15 K. Dashed lines are the fitting curves using Redlich−Kister polynomial equations.

where Q is the deviation from ideality of the physical property, Ai are fitting parameters of the Redlich−Kister model, and n is the polynomial expansion order. The order of the polynomial was selected to minimize the standard deviations of the fitting. The parameters obtained for the Redlich−Kister equation in the fit of the experimental deviations from ideality of the refractive indices are listed in Table 3, whereas the ΔnD calculated from the Redlich−Kister expression are graphically shown as dashed lines in Figure 2 together with experimental deviations. Density. Experimental densities of {[bpy][BF4] + [4bmpy][Tf2N]} IL at temperatures between 293.15 K and 353.15 K are plotted in Figure 3. The density of the binary IL mixture decreases as the [bpy][BF4] mole fraction increases at constant temperature, because the density of this pure IL is considerably lower than that of [4bmpy][Tf2N]. Due to the wide number of possible combinations between the commercially available ILs to form binary mixtures, the implementation of predictive models of physical properties of

+ [4bmpy][Tf2N]} mixture and the refractive indices predicted using eq 1. ΔnD values are represented in Figure 2 as a function of temperature and composition. As can be observed, the highest absolute values of ΔnD have been obtained in mixtures with [bpy][BF4] mole fraction between 0.5 and 0.7 and at 353.15 K. Therefore, an increase in temperature causes a slight negative deviation from the ideal behavior of the IL mixture. In the {[bpy][BF4] + [bpy][Tf2N]} IL mixture, the effect of the [bpy][BF4] mole fraction and of the temperature was similar, but the maximum negative deviations was almost three times higher than the maximum ΔnD obtained in this work.15 A fourth order Redlich−Kister polynomial expansion30 was used to fit the deviations from the ideal behavior of the three physical properties measured in this paper: 4

Q = xixj ∑ Ai (xi − xj)n n=0

(2) D



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Table 2. Refractive Index Deviations (ΔnD), Excess Molar Volumes (VE), and Dynamic Viscosity Deviations (Δη) of [bpy][BF4] (1) + [4bmpy][Tf2N] (2) IL Mixtures T/K x1

293.15

303.15

313.15

0.0000 0.0575 0.1136 0.2311 0.3515 0.4566 0.5692 0.6730 0.7836 0.8890 0.9535 1.0000

0.0000 −0.0001 −0.0002 −0.0003 −0.0004 −0.0005 −0.0005 −0.0005 −0.0004 −0.0002 −0.0001 0.0000

0.0000 −0.0001 −0.0002 −0.0004 −0.0004 −0.0005 −0.0006 −0.0005 −0.0004 −0.0002 −0.0001 0.0000

0.0000 −0.0001 −0.0003 −0.0004 −0.0005 −0.0005 −0.0006 −0.0005 −0.0005 −0.0003 −0.0002 0.0000

0.0000 0.0575 0.1136 0.2311 0.3515 0.4566 0.5692 0.6730 0.7836 0.8890 0.9535 1.0000

0.00 0.05 0.09 0.19 0.28 0.32 0.35 0.37 0.38 0.36 0.29 0.00

0.00 0.04 0.08 0.18 0.27 0.31 0.34 0.36 0.37 0.35 0.28 0.00

0.00 0.04 0.08 0.18 0.27 0.31 0.33 0.36 0.37 0.35 0.28 0.00

0.0000 0.0575 0.1136 0.2311 0.3515 0.4566 0.5692 0.6730 0.7836 0.8890 0.9535 1.0000

0.0 −4.4 −10.6 −18.0 −24.5 −26.9 −24.5 −22.6 −21.4 −14.8 −3.8 0.0

0.0 −2.4 −4.8 −7.8 −9.5 −8.3 −8.2 −7.4 −7.8 −5.3 −1.2 0.0

323.15 ΔnD 0.0000 −0.0001 −0.0003 −0.0004 −0.0005 −0.0006 −0.0006 −0.0006 −0.0005 −0.0003 −0.0002 0.0000 VE/cm3·mol−1 0.00 0.03 0.07 0.17 0.26 0.30 0.33 0.36 0.36 0.34 0.28 0.00 Δη/mPa·s 0.0 −0.7 −1.2 −1.4 −1.5 −1.4 −1.4 −1.3 −1.4 −1.1 −0.3 0.0

0.0 −1.3 −2.6 −3.8 −3.9 −3.6 −3.6 −3.8 −3.9 −3.0 −0.2 0.0

2

∑i = 1 Wx i i 2

N (∑i = 1 V0,ixi)(a + bT + cP)

343.15

353.15

0.0000 −0.0001 −0.0003 −0.0004 −0.0005 −0.0006 −0.0006 −0.0006 −0.0005 −0.0003 −0.0002 0.0000

0.0000 −0.0001 −0.0003 −0.0004 −0.0005 −0.0006 −0.0006 −0.0006 −0.0005 −0.0003 −0.0002 0.0000

0.0000 −0.0001 −0.0003 −0.0004 −0.0005 −0.0006 −0.0006 −0.0006 −0.0005 −0.0004 −0.0002 0.0000

0.00 0.02 0.07 0.17 0.25 0.29 0.32 0.35 0.36 0.34 0.28 0.00

0.00 0.02 0.06 0.16 0.25 0.28 0.32 0.34 0.35 0.35 0.28 0.00

0.00 0.02 0.06 0.15 0.24 0.28 0.31 0.34 0.35 0.35 0.27 0.00

0.0 −0.3 −0.6 −0.6 −0.5 −0.5 −0.5 −0.5 −0.5 −0.3 −0.1 0.0

0.0 −0.2 −0.3 −0.4 −0.3 −0.4 −0.3 −0.4 −0.4 −0.4 −0.2 0.0

0.0 −0.1 −0.1 −0.2 −0.1 −0.1 −0.2 −0.2 −0.2 −0.1 −0.1 0.0

[bpy][Tf2N]} IL mixture.15 In this work, we have studied the accuracy of this method to predict the density of a binary IL mixture composed of four different ions. The volumes of the two cations and two anions forming the binary IL mixture studied were taken from the works of Ye and Shreeve and of Gardas and Coutinho: [bpy] (230 Å3),19 [4bmpy] (258 Å3),19 [Tf2N] (248 Å3),19 and [BF4] (73 Å3).18 Densities predicted of {[bpy][BF4] + [4bmpy][Tf2N]} mixtures by this group contribution method are represented as dashed lines in Figure 3 along with the experimental densities. As can be seen, the accuracy of the predictions decreased as the content of [bpy][BF4] in the binary IL mixture increased. The mean percent deviation between experimental and predicted densities was 0.45 %. Thus, the Ye and Shreeve predictive method seems to be a useful tool for estimating densities of binary IL mixtures formed by four different ions without requiring experimental data.

binary IL mixtures is absolutely essential. The Ye and Shreeve group contribution method for the prediction of densities of pure ILs,18 was extended to binary IL mixtures by Gardas and Coutinho:19 ρ=

333.15

(3)

where ρ indicates the predicted density of the binary mixture in kg·m−3, Wi is the IL molecular weight in kg·mol−1, N is the Avogadro number, V0,i is the molecular volume of the pure IL calculated as the sum of the volumes of anion and cation in m3·molecule−1, T is the temperature in K, and P is the pressure in MPa. The parameters a, b, and c obtained by Gardas and Coutinho were 8.005·10−1, 6.652·10−4 K−1, and −5.919·10−4 MPa−1, respectively.19 In our previous paper, this predictive method was used to appropriately estimate the densities of the {[bpy][BF4] + E



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Table 3. Fitting Parameters of the Redlich−Kister Polynomial Equation (A0, A1, A2, A3, A4) and Standard Deviations (s) for the Excess Properties of [bpy][BF4] + [4bmpy][Tf2N] IL Mixtures at Different Temperatures T/K 293.15 303.15 313.15 323.15 333.15 343.15 353.15

A0 −1.95·10−3 −2.13·10−3 −2.20·10−3 −2.26·10−3 −2.23·10−3 −2.26·10−3 −2.34·10−3

A1 −6.36·10−4 −5.56·10−4 −3.99·10−4 −4.48·10−4 −5.29·10−4 −3.30·10−4 −4.19·10−4

293.15 303.15 313.15 323.15 333.15 343.15 353.15

1.38 1.33 1.31 1.29 1.26 1.23 1.20

0.27 0.27 0.26 0.27 0.26 0.26 0.27

293.15 303.15 313.15 323.15 333.15 343.15 353.15

−102.01 −32.58 −13.73 −5.46 −1.96 −1.32 −0.52

4.97 8.51 −0.84 0.47 −0.61 0.03 −0.41

A2

A3

ΔnD −9.51·10−4 5.15·10−4 −3 −1.05·10 6.49·10−4 −3 −1.28·10 9.59·10−5 −1.06·10−3 −1.26·10−4 −1.68·10−3 −8.45·10−5 −3 −1.47·10 −3.85·10−4 −3 −1.62·10 −6.14·10−4 VE/cm3·mol−1 −0.22 2.76 −0.11 2.72 −0.18 2.78 −0.12 2.77 −0.10 2.85 −0.08 2.90 0.01 2.93 Δη/mPa·s −57.24 −49.11 −59.58 −15.91 −41.70 1.99 −8.51 1.57 −4.16 4.09 −2.00 −0.75 −1.37 0.89

A4 1.54·10−3 1.19·10−3 1.06·10−3 4.80·10−4 8.58·10−4 4.99·10−4 2.98·10−4

s 1·10−5 2·10−5 2·10−5 2·10−5 1·10−5 2·10−5 2·10−5

3.17 2.86 2.96 2.79 2.73 2.71 2.52

0.02 0.02 0.02 0.02 0.02 0.02 0.02

58.06 57.79 38.53 −0.64 1.87 −2.19 −0.19

1.26 0.47 0.32 0.08 0.03 0.02 0.01

Figure 5. Dynamic viscosities of [bpy][BF4] (1) + [4bmpy][Tf2N] (2) IL mixtures as a function of [bpy][BF4] mole fraction and temperature: ◇, 293.15 K; ×, 303.15 K; △, 313.15 K; □, 323.15 K; ∗, 333.15 K; ○, 343.15 K; +, 353.15 K. Dashed lines represent the viscosities calculated with Grunberg−Nissan mixing law, and the dotted line denotes the viscosity of sulfolane at 313.15 K (from ref 32).

selectivity and toluene distribution ratio than the sulfolane

The density of a solvent is a key property to evaluate its potential use at industrial scale, because the difference between the density of the feed and the extraction solvent determines the hydrodynamic behavior in the extractor.31 Therefore, the densities of the sulfolane at 313.15 K are graphically shown in Figure 3 as a dotted line to be used as a benchmark.32 We have made the comparison between the densities of sulfolane and of the IL mixture at 313.15 K, since we used this temperature in the study of the extraction of toluene from heptane with the {[bpy][BF4] + [4bmpy][Tf2N]} mixture as a solvent.7 A mixture with a [bpy][BF4] mole fraction around 0.7 shows a higher

values.7 As seen in Figure 3, the experimental density of the IL mixture with this optimal composition at 313.15 K is significantly higher than that of sulfolane. Hence, as a result of its higher density, this binary IL mixture could improve the fluid dynamic performance in the liquid−liquid extractor. We have analyzed the deviation from ideality of the density by calculating the molar excess volume of the mixtures with the following expression: F



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Figure 6. Deviations from ideality of the dynamic viscosities of [bpy][BF4] (1) + [4bmpy][Tf2N] (2) IL mixtures at different temperatures: ◇, 293.15 K; ×, 303.15 K; △, 313.15 K; □, 323.15 K; ∗, 333.15 K; ○, 343.15 K; +, 353.15 K. Dashed lines are the fitting curves using the Redlich−Kister polynomial expansions.

⎛1

2

VE =

∑ xiWi ⎜⎜

⎝ρ

i=1

−

1⎞ ⎟⎟ ρi ⎠

the accuracy of the predictions obtained from the Bingham mixing rule is not too high. Because of this, we have also studied the Grunberg−Nissan mixing rule, since this equation was used by Navia et al.12 and by Stoppa et al.13 to estimate the viscosities of binary mixtures of imidazolium ILs:20

(4)

where VE denotes the excess molar volume of the IL mixture in cm3·mol−1, Wi is the molecular weight of the IL in g·mol−1, ρ indicates the density of the binary mixture in g·cm−3, and ρi is the IL density at the same temperature. Excess molar volumes of the {[bpy][BF4] + [4bmpy][Tf2N]} IL mixture are plotted in Figure 4 and presented in Table 2. As can be observed, the excess volumes of the binary IL mixtures were positive for all compositions, reaching the highest values for a [bpy][BF4] mole fraction between 0.7 and 0.9. We observed the same behavior and similar values for the excess volumes of the {[bpy][BF4] + [bpy][Tf2N]} IL mixture.15 Thus, the replacement of the [bpy] cation by the [4bmpy] cation has hardly modified the behavior of the density. On the other hand, the low values of excess volumes also highlights the nearly ideal behavior of the density of {[bpy][BF4] + [4bmpy][Tf2N]} IL mixtures. As in the case of the deviation from the ideality of refractive index, excess molar volumes have been adjusted to a fourth-order Redlich−Kister polynomial expansion (eq 2). Fitting parameters are listed in Table 3, and the calculated excess volumes are plotted in Figure 4 as dashed lines along with the experimental excess molar volumes. Viscosity. Dynamic viscosities of {[bpy][BF4] + [4bmpy][Tf2N]} IL mixtures as a function of temperature and composition are plotted in Figure 5. The Bingham mixing rule21 estimated accurately the viscosities of the {[bpy][BF4] + [bpy][Tf2N]} IL mixture15 and was also previously employed by Stoppa et al.13 to predict dynamic viscosities of binary mixtures of imidazolium-based ILs: 1 = η

2

∑ i=1

xi ηi

2

log10(η) =

∑ xi log10(ηi) i=1

(6)

The average percent deviation between experimental and viscosities estimated by eq 6 was 1.55 %. Thus, the Grunberg− Nissan mixing law has correctly predicted the experimental data, and this mixing rule appears to be adequate for estimating viscosities of pyridinium-based IL mixtures from data of pure ILs. In Figure 5, predicted values are plotted as dashed lines together with experimental dynamic viscosities. A comparative analysis between the viscosity of the sulfolane at 313.15 K32 and the experimental viscosities of {[bpy][BF4] + [4bmpy][Tf2N]} IL mixtures has been also made in Figure 5. As observed, the viscosities of the binary mixtures are substantially higher than that of sulfolane over the whole range of composition and temperatures. This fact could increase the mixing and pumping costs of the aromatic extraction unit, and it could be a drawback to the application of this binary IL mixture as an alternative solvent to sulfolane. To study the behavior of the viscosity of the IL mixture, deviations of dynamic viscosity from the ideal mixing behavior (Δη) have been calculated with the following expression: 2

Δη = η −

∑ xiηi i=1

(7)

Viscosity deviations from ideality (Δη) are listed in Table 3 and are graphically shown in Figure 6 along with the fitting curves using the fourth-order Redlich−Kister polynomial expansion. Adjusted parameters of the Redlich−Kister equation for the viscosity deviations are presented in Table 3. The viscosity of the {[bpy][BF4] + [4bmpy][Tf2N]} mixture has shown negative deviations in the whole range of composition, and the highest values of Δη have been obtained at 293.15 K. We observed the

(5)

where ηi is the viscosity of the pure IL and η denotes the predicted dynamic viscosity of the binary IL mixture at the same temperature. The average percentage difference between experimental viscosities of the {[bpy][BF4] + [4bmpy][Tf2N]} IL mixture and values predicted by eq 5 was 5.02 %. Therefore, G



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same effect of temperature for the [bpy][BF4] + [bpy][Tf2N] mixture.15 As in the density case, the substitution of the [bpy] cation for the [4bmpy] cation causes almost no change in the behavior of the viscosity.

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CONCLUSIONS Refractive indices, densities, and dynamic viscosities of {[bpy][BF4] + [4bmpy][Tf2N]} IL mixtures over the whole range of mole fractions have been measured at temperatures between 293.15 K and 353.15 K and atmospheric pressure. To study the behavior of the IL mixture, deviations from the ideality of the physical properties have been calculated from experimental data. Slight negative deviations from ideal mixing have been observed in refractive indices, whereas low positive values of excess molar volumes have been obtained. By contrast, the dynamic viscosity of the IL mixture has shown large negative deviations from ideality, increasing the nonideal behavior with decreasing temperature. The group contribution method of Ye and Shreeve, and improved by Gardas and Coutinho, has successfully predicted the densities of the binary IL mixtures, achieving the highest accuracy in mixtures with high content of [4bmpy][Tf2N]. The accuracy of the Grunberg−Nissan has been higher than that of the Bingham mixing law in predicting dynamic viscosities of the IL mixture from viscosities of pure ILs. A binary IL mixture with [bpy][BF4] mole fraction around 0.7 could be used as an alternative solvent to sulfolane, since this IL mixture has shown better extractive properties and a higher density than those of sulfolane. Nevertheless, the considerably higher dynamic viscosity of the {[bpy][BF4] + [4bmpy][Tf2N]} IL mixture compared to sulfolane could be an important drawback for the use of this IL mixture as an aromatic extraction solvent in industrial scale.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +34 91 394 51 19; fax: +34 91 394 42 43. E-mail address: [email protected]. Funding

The authors are grateful to the Ministerio de Economiá y Competitividad of Spain (MINECO) and the Comunidad de Madrid for financial support of Projects CTQ2011-23533 and S2009/PPQ-1545, respectively. S.G. thanks MINECO for awarding her an FPI Grant (Reference BES-2009-014703) under the CTQ2008-01591 project. M.L. also thanks Ministerio de Educación, Cultura y Deporte of Spain for awarding him an FPU grant (Reference AP2010-0318). Notes

The authors declare no competing financial interest.

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REFERENCES

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