Density and Viscosity of Binary Mixtures of 1-Ethyl-3

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX-XXX

pubs.acs.org/jced

Density and Viscosity of Binary Mixtures of 1‑Ethyl-3methylimidazolium Heptachlorodialuminate and Tetrachloroaluminate Ionic Liquids Yong Zheng,*,† Yongjun Zheng,† Qian Wang,‡ Zhen Wang,† and Dayong Tian† †

College of Chemistry and Environmental Engineering, Anyang Institute of Technology, Anyang, 455000 Henan, P. R. China State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China

‡

S Supporting Information *

ABSTRACT: The densities and viscosities of binary mixtures of 1-ethyl-3methylimidazolium heptachlorodialuminate and tetrachloroaluminate ionic liquids with different molar compositions were measured by high-precision vibrating tube densimeter and automated microviscometer from 293.15 to 353.15 K. On this basis, the excess molar volumes and excess viscosities of binary mixtures were also calculated for the first time. All the experimental data were well fitted by the empirical equations. According to the results of computational calculations, enhanced molecular symmetry and molar concentration of heptachlorodialuminate anion usually lead to higher density. The hydrogen bonding among ions is confirmed as one of the most important structural parameters in determining the viscosity of ionic liquids. It shows that Lewis acidic 1-ethyl-3-methylimidazolium chloroaluminate can be treated as the classic binary mixtures of heptachlorodialuminate and tetrachloroaluminate ionic liquids. Looser packing and/or weaker interionic interaction probably occurs after the formation of binary mixtures. The results and conclusions of this work will help to promote future research and application of chloroaluminate ionic liquids.

1. INTRODUCTION Ionic liquids (ILs), the so-called room-temperature molten salts, have attracted worldwide attention over the past few

tunable acidity, chloroaluminate ILs usually serve as efficient electrolytes and catalysts in many chemical reactions.11−13 Among these ILs, one of the most commonly used systems is Lewis acidic 1-ethyl-3-methylimidazolium chloroaluminate ([Emim]Cl/AlCl3) (see Figure 1).14−17 Although significant progress has been achieved in relevant research studies, the density, viscosity, excess molar volume, excess viscosity, and structure of Lewis acidic [Emim]Cl/AlCl3 are still less understood, which is probably attributed to the chemical instability of chloroaluminate ILs in moisture and air.18 From the perspective of industrial application, there is much work to be done in this field.

Figure 1. Chemical structure of the [Emim]+ cation.

decades. Owing to their novel structure and excellent physicochemical properties, ILs can be used as green reaction media in numerous research fields, including electrochemistry, extraction, catalysis, synthesis, biomass processing, gas capture, and so on.1−4 The history of first-generation ILs began in 1951, when Hurley et al.5 reported that a mixture of organic halide salt and aluminum chloride became liquid at low temperature. Since then, extensive studies have been carried out to explore the chemical composition of these systems.6−8 It was found that the resulting liquids were chloroaluminate-based molten salts. As the mole fraction of aluminum chloride increases from 0.5 to 0.66, the predominant form of chloroaluminate anion changes from neutral tetrachloroaluminate ([AlCl4]−) to Lewis acidic heptachlorodialuminate ([Al2Cl7]−) (see eq 1).9,10 With their © XXXX American Chemical Society

Cl− + AlCl3 → [AlCl4]− + AlCl3 → [Al 2Cl 7]−

(1)

Under the above background, this work aimed to make perform further and systematic research on the typical properties of [Emim]Cl/AlCl3. Therefore, a series of Lewis acidic [Emim]Cl/AlCl3 ILs with different molar compositions were prepared by first mixing precise molar quantities of [Emim][AlCl4] and [Emim][Al2Cl7]. Then, the densities and viscosities of these ILs mixtures were measured from 293.15 to Received: July 31, 2017 Accepted: October 20, 2017

A

DOI: 10.1021/acs.jced.7b00702 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Information of Chemical Samples Used in This Work chemical name

CAS no.

initial mass fraction purity

source

1-ethyl-3-methylimidazolium chloride anhydrous aluminum chloride

65039-09-0

Tokyo Chemical Industry

>0.980

7446-70-0

>0.990

1-ethyl-3-methylimidazolium tetrachloroaluminate 1-ethyl-3-methylimidazolium heptachlorodialuminate

80432-05-9

Sinopharm Chemical Reagent Co., Ltd. synthesized

a

synthesized

final mass fraction purity

analysis method

recrystallization; vacuum drying sublimation

>0.995

NMRa; EAb; KFc PTd; KF

electrolysis vacuum drying electrolysis vacuum drying

>0.995e

purification method

>0.995

>0.995e

NMR; EA; KF NMR; EA; KF

Nuclear magnetic resonance. bElemental analysis. cKarl Fisher titration. dPotentiometric titration. eWater content < 50 mg/kg.

0.98). Prior to use, all of the chemicals were purified. Detailed information on the chemicals is listed in Table 1. 2.2. Synthesis of ILs. All the following synthesis and subsequent purification processes were performed in an argonfilled glovebox (Universal, MIKROUNA, China) in which the mass contents of water and oxygen were both less than 1 mg/ kg. The mass measurements were conducted by an electronic balance (MS204S, Mettler Toledo, Switzerland) with accuracy of ±0.0001 g. [Emim][AlCl 4]. Dry [Emim]Cl was first mixed with anhydrous aluminum chloride at the precise molar ratio of 1:1 around room temperature. Then, the resulting liquid was filtered through a quartz glass frit and purified by potentiostatic electrolysis method to remove the organic and/or inorganic impurities. The electrolysis process was conducted in an electrolytic cell with two aluminum electrodes through an electrochemical workstation (CHI660D, Chenhua, China). The final product was obtained as a colorless liquid and dried at −0.1 MPa and 323 K in a vacuum oven (DZF-6020, Jinghong, China) before use. [Emim][Al2Cl7]. Dry [Emim]Cl was first mixed with anhydrous aluminum chloride at the precise molar ratio of 1:2 around ambient temperature. After that, the resulting liquid was filtered through a quartz glass frit and purified by potentiostatic electrolysis method to remove the organic and/ or inorganic impurities. The electrolysis process was conducted in an electrolytic cell with two aluminum electrodes through the electrochemical workstation. The final product was obtained as a colorless liquid and dried at −0.1 MPa and 323 K in the vacuum oven prior to use. Lewis Acidic [Emim]Cl/AlCl3 (Binary Mixtures of [Emim][Al2Cl7] and [Emim][AlCl4]). The binary mixtures used were prepared by mixing precise molar quantities of [Emim][Al2Cl7] and [Emim][AlCl4] at room temperature. The corresponding standard uncertainty u(x) of the mole fraction is 0.0001. 2.3. Characterization of ILs. The molecular structure and chemical composition of [Emim]Cl, [Emim][Al2Cl7], and [Emim][AlCl4] ILs were characterized by NMR spectra, elemental analysis, and water content analysis. NMR spectra were obtained on an NMR spectrometer (av-400 MHz, Bruker, Switzerland). In the measurement of NMR for [Emim][Al2Cl7] and [Emim][AlCl4], acetone-d6 was used as an external standard to suppress the interference from other solvents. The elemental analyzer (Vario El cube, Elementar, Germany) was used to determine the chemical composition of ILs. [Emim]Cl. 1H NMR (DMSO-d6, 400 MHz): δ × 106 = 9.492 (s, 1H), 7.858 (d, 1H), 7.757 (d, 1H), 4.180 (m, 2H), 3.836 (s, 3H), and 1.370 (t, 3H). EA: C (w = 0.4895), N (w = 0.1903), H (w = 0.0747), and Cl (w = 0.2412).

353.15 K with high-precision vibrating tube densimeter and automated microviscometer. On this basis, the resulting excess molar volumes and excess viscosities of binary mixtures have been calculated by empirical equations. For illustrating the relationship between physicochemical properties and molecular structure of ILs, computational calculations were also conducted by the Gaussian09 program. Finally, all of the experimental results were evaluated and analyzed in detail according to the empirical equations and theory of the liquid binary mixtures.

2. EXPERIMENTAL SECTION 2.1. Chemicals. 1-Ethyl-3-methylimidazolium chloride ([Emim]Cl) was purchased from Tokyo Chemical Industry with a mass fraction purity higher than 0.99. Anhydrous aluminum chloride was obtained from Sinopharm Chemical Reagent Co., Ltd. at analytic grade (mass fraction purity > Table 2. Comparison of the Experimental Densities ρ and Viscosities η of [Emim][Al2Cl7] and [Emim][AlCl4] at Temperature T with Literature Valuesa ρ/g cm−3 T/K

this work

293.15 303.15 313.15 323.15 333.15 343.15 353.15

1.395287 1.385833 1.376465 1.367190 1.357991 1.348876 1.339819

293.15

1.299317

303.15

1.291130

313.15

1.283046

323.15

1.275029

333.15

1.267095

343.15 353.15

1.259221 1.251412

η/mPa s lit.

[Emim][Al2Cl7] 1.393419 1.384219 1.375019 1.365919 1.356719 1.347519 1.338419 [Emim][AlCl4] 1.298119 1.297920 1.290119 1.290820 1.282119 1.283320 1.274019 1.275920 1.266019 1.268920 1.258019 1.249919

this work

lit.

16.11 12.41 9.790 7.943 6.571 5.526 4.721

16.0619 12.3419 9.75219 7.90219 6.53819 5.50819 4.71319

20.62

20.5319

15.69

15.5919

12.31

12.2519

9.952

9.89619

8.214

8.18619

6.927 5.945

6.90819 5.92819

Standard uncertainties u are u(T) = 0.01, and 0.01 K for ρ and η, respectively, the relative standard uncertainties for the densities Ur(ρ) = 0.0005, and the relative expanded uncertainties for the viscosities Ur(η) = 0.005 (0.95 confidence level ). a

B



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Table 3. Experimental Values of Density ρ for [Emim][Al2Cl7] (1) + [Emim][AlCl4] (2) Binary Mixtures from 293.15 to 353.15 K as a Function of Mole Fraction x1 at 0.1 MPaa ρ/g cm−3 x1

293.15 K

303.15 K

313.15 K

323.15 K

333.15 K

343.15 K

353.15 K

0.0000 0.1016 0.2046 0.3035 0.4003 0.5041 0.5962 0.6955 0.7963 0.8901 1.0000

1.299317 1.311437 1.323232 1.334146 1.344261 1.354516 1.363068 1.371773 1.380103 1.387334 1.395287

1.291130 1.303063 1.314695 1.325469 1.335448 1.345572 1.354021 1.362614 1.370832 1.377972 1.385833

1.283046 1.294785 1.306242 1.316875 1.326738 1.336733 1.345066 1.353545 1.361663 1.368703 1.376465

1.275029 1.286574 1.297853 1.308347 1.318086 1.327975 1.336207 1.344572 1.352559 1.359515 1.367190

1.267095 1.278441 1.289546 1.299898 1.309503 1.319285 1.327419 1.335662 1.343536 1.350405 1.357991

1.259221 1.270362 1.281285 1.291503 1.300986 1.310657 1.318694 1.326817 1.334583 1.341369 1.348876

1.251412 1.262347 1.273072 1.283154 1.292539 1.302085 1.310018 1.318032 1.325698 1.332392 1.339819

a Standard uncertainties u are u(T) = 0.01 K, u(p) = 0.1 KPa, and u(x) = 0.0001, and the relative standard uncertainties for the densities Ur(ρ) = 0.0005 (0.95 confidence level).

[Emim][AlCl4]. 1H NMR (acetone-d6, 400 MHz): δ × 106 = 8.325 (s, 1H), 7.437 (d, 1H), 7.384 (d, 1H), 4.267 (m, 2H), 3.955 (s, 3H), and 1.522 (t, 3H). 27Al NMR (acetone-d6, 400 MHz): δ × 106 = 102.154. EA: C (w = 0.2570), N (w = 0.1002), H (w = 0.0393), and Cl (w = 0.5068). [Emim][Al2Cl7]. 1H NMR (acetone-d6, 400 MHz): δ × 106 = 8.077 (s, 1H), 7.158 (d, 1H), 7.118 (d, 1H), 4.032 (m, 2H), 3.726 (s, 3H), and 1.351 (t, 3H). 27Al NMR (acetone-d6, 400 MHz): δ × 106 = 103.386. EA: C (w = 0.1739), N (w = 0.0673), H (w = 0.0264), and Cl (w = 0.5989). The structure of ILs was confirmed according to the peaks and chemical shifts obtained in NMR spectra. The results of elemental analysis were consistent with the chemical composition of ILs. The water content of chloroaluminate ILs was determined by Karl Fisher titration (751 GPD Titrino, Metrohm, Switzerland). On the basis of the above measurements, the mass fraction purity of ILs was higher than 0.995. 2.4. Density Measurements. A high-precision vibrating tube densimeter (DMA 5000, Anton Paar, Austria) was applied in the density measurement of ILs. The relative standard uncertainty Ur(ρ) was 0.0005 (0.95 confidence level). Before the experiment, the densimeter was calibrated with dry air and ultrapure water. The following measurement was conducted from 293.15 to 353.15 K at 0.1 MPa in the argon-filled glovebox. The precision of the experimental temperature was as high as ±0.001 K, which was controlled by integrated Pt 100 platinum thermometers. Experimental pressure was regulated with a programmable logic controller in the glovebox, and the standard uncertainty U(p) is 0.1 KPa. 2.5. Viscosity Measurements. The viscosities of ILs were measured by an automated falling ball microviscometer (AMVn, Anton Paar, Austria) at 0.1 MPa in an argon-filled glovebox. The relative expanded uncertainty Ur(η) was 0.005 (0.95 confidence level). Calibration was conducted with ultrapure water and viscosity standard oils (No. H117, Anton Paar, Austria). The experimental temperature ranged from 293.15 to 353.15 K with precision of ±0.01 K, which was controlled by a built-in precise Peltier thermostat. Experimental pressure was regulated by programmable logic controller in the glovebox, and the standard uncertainty U(p) is 0.1 KPa. 2.6. Computational Method. All the computational calculations for ILs were conducted by the Gaussian09 program based on density functional theory (DFT). The 6-31+G(d,p) basis set is known as a split-valence double-ζ basis set, which

takes p- and d-type polarization, as well as diffuse function for hydrogen, carbon, nitrogen, and other elements into consideration. This basis set is very important for improving the accuracy of computations when investigating the interaction energy and hydrogen bonding in ionic liquids. The optimized structure and energy obtained by this method shows good precision and reproducibility in calculating these structural parameters of ion pairs. The results of computations are well consistent with the experimental data. Consequently, the structure of [Emim][AlCl4] and [Emim][Al2Cl7] ion pairs was studied and optimized at the Becke−three−parameter− Lee−Yang−Parr (B3LYP) level with 6-31+G(d,p) basis set. Under such conditions, each optimized structure has been obtained as a true minimum by frequency calculations. The effect of basis set superposition errors (BSSE) on interaction energy between ions has been considered through counterpoise method. Table 2 shows the comparison of densities and viscosities of [Emim][Al2Cl7] and [Emim][AlCl4] measured in this work with literature values at the same temperatures. The relative error of densities and viscosities of [Emim][Al2Cl7] and [Emim][AlCl4] between this work and literature is less than 0.15 and 0.65%, respectively.19,20 It can be concluded that all the experimental values are well consistent with those reported in the literature.

3. RESULTS AND DISCUSSION 3.1. Density. The densities ρ of [Emim][Al2Cl7] and [Emim][AlCl4] binary mixtures obtained from 293.15 to 353.15 K are listed in Table 3. As shown in Figure 2, the values of density fall in the range from 1.251412 to 1.395287 g cm−3 and are well fitted by the classic equation ρ = A + BT + CT 2

(2)

in which A (g cm−3), B (g cm−3 K−1), and C (g cm−3 K−2) are empirical parameters.21,22 The values of best-fit parameters can be found in Table 4. All of the densities of IL mixtures obviously decrease as the experimental temperature increases, which indicates that the increasing temperature enhances the volume of ions, resulting in smaller mass per unit volume. Meanwhile, the chemical composition of chloroaluminate ILs has a significant effect on the density values. At the same temperature, the density of ILs C



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Table 5. Interaction Energy ΔE, Dipole Moment μ, and Hydrogen Bonds of [Emim][AlCl4], [Emim][Al2Cl7], [Bmim][AlCl4], and [Bmim][Al2Cl7] Ion Pairs Calculated by DFT Method ΔE

μ

C2−H··· Cla

C6−H··· Cla

C7−H··· Cla

IL

kJ mol−1

Db

10−10 m

10−10 m

10−10 m

[Emim][AlCl4] [Emim] [Al2Cl7] [Bmim][AlCl4] [Bmim] [Al2Cl7]

293.83 273.60

15.14 15.29

2.619 2.663

2.728 2.765

2.812 2.862

290.987 270.617

15.327 15.567

2.5887 2.6167

2.7117 2.7477

2.6977 2.7257

a The distance of hydrogen bond between H and Cl atoms. bD = 3.33564 × 10−30 C m.

Figure 2. Density ρ of [Emim][Al2Cl7] (1) + [Emim][AlCl4] (2) binary mixtures as a function of temperature T at different mole fractions x1 of [Emim][Al2Cl7] = (▶) 0.0000, (△) 0.1016, (■) 0.2046, (○) 0.3035, (▼) 0.4003, (□) 0.5041, (◀) 0.5962, (▷) 0.6955, (⧫) 0.7963, (◁) 0.8901, and (▲) 1.0000. The solid curves are correlated with eq 2, and the symbols represent experimental values.

probably due to the enhanced molar concentration of [Al2Cl7]− anion with larger volume and heavier molecular weight.7,23 The difference between the molecular structure of [Emim][Al2Cl7] and [Emim][AlCl4] can be seen in Figure 3. On the other hand, the density of [Emim]Cl/AlCl3 is higher than that of another well-known chloroaluminate IL, 1-butyl-3methylimidazolium chloroaluminate ([Bmim]Cl/AlCl3), at the same temperature and molar composition.7,19 Consequently, it can be found that [Emim]Cl/AlCl3 has lower molar volume under similar conditions. For example, the molar volumes of [Emim]Cl/AlCl3 and [Bmim]Cl/AlCl3 calculated from the density at 293.15 K and x1 = 0.4 are 248.0 and 280.7 cm3 mol−1, respectively.7 It is apparent that the structure of imidazolium cation, especially N-alkyl side chains, is vital to the density of chloroaluminate ILs. On the basis of the results of computational calculations, the dipole moment μ of ion pairs follows the order [Emim][AlCl4] < [Bmim][AlCl4] and [Emim][Al2Cl7] < [Bmim][Al2Cl7] (see Table 5). Consequently, it is proper to state that the decreased length of N-alkyl side chain enhances the geometric symmetry of chloroaluminate ILs, resulting in higher packing density of ions. It is probably the main reason why [Emim]Cl/AlCl3 ILs have higher densities than [Bmim]Cl/AlCl3 ILs under the same conditions. 3.2. Viscosity. Table 6 lists the viscosities η of [Emim][Al2Cl7] and [Emim][AlCl4] binary mixtures measured in the temperature range from 293.15 to 353.15 K. The temperature dependency of viscosity at different mole fractions is presented

Table 4. Fitted Values of the Empirical Parameters A−C and Standard Deviation σ for the Densities of Binary Mixtures of [Emim][Al2Cl7] (1) + [Emim][AlCl4] (2) as a Function of Mole Fraction x1 Based on Eq 2 103B

A x1 0.0000 0.1016 0.2046 0.3035 0.4003 0.5041 0.5962 0.6955 0.7963 0.8901 1.0000

g cm

−3

1.57157 1.58751 1.60203 1.61762 1.63306 1.64816 1.66091 1.67264 1.68495 1.69570 1.70816

g cm

−3

K

107C −1

−1.03713 −1.04458 −1.04682 −1.06448 −1.08754 −1.10821 −1.12589 −1.13510 −1.15063 −1.16521 −1.18601

g cm−3 K−2

105σ

3.69762 3.50714 3.26667 3.32500 3.49167 3.63214 3.74643 3.70952 3.77619 3.86429 4.05001

0.9805 0.9679 1.1555 1.0418 0.7643 1.4161 1.5626 1.2804 0.6976 0.8452 0.8025

increases with increasing mole fraction of [Emim][Al2Cl7]. According to the literature and our previous work, this trend is

Figure 3. Optimized structure of [Emim][AlCl4] and [Emim][Al2Cl7] ion pairs calculated at the B3LYP/6-31+G(d,p) level. Hydrogen bonds are indicated by dashed lines. D



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Table 6. Experimental Viscosities η of [Emim][Al2Cl7] (1) + [Emim][AlCl4] (2) Binary Mixtures Measured from 293.15 to 353.15 K as a Function of Mole Fraction x1 at 0.1 MPaa η/mPa s x1

293.15 K

303.15 K

313.15 K

323.15 K

333.15 K

343.15 K

353.15 K

0.0000 0.1016 0.2046 0.3035 0.4003 0.5041 0.5962 0.6955 0.7963 0.8901 1.0000

20.62 19.96 19.38 18.85 18.40 17.95 17.57 17.19 16.82 16.47 16.11

15.69 15.23 14.81 14.44 14.10 13.77 13.49 13.21 12.93 12.68 12.41

12.31 11.98 11.67 11.38 11.12 10.86 10.64 10.42 10.21 10.01 9.790

9.952 9.697 9.461 9.239 9.033 8.826 8.650 8.468 8.286 8.127 7.943

8.214 8.016 7.821 7.643 7.474 7.304 7.157 7.008 6.859 6.726 6.571

6.927 6.762 6.598 6.447 6.306 6.161 6.037 5.906 5.777 5.660 5.526

5.945 5.806 5.664 5.533 5.408 5.282 5.173 5.059 4.946 4.842 4.721

a Standard uncertainties u are u(T) = 0.01 K, u(p) = 0.1 KPa, and u(x) = 0.0001, and the relative expanded uncertainties for the viscosities Ur(η) = 0.005 (0.95 confidence level).

in Figure 4. All the experimental data have been fitted according to the well-known Vogel−Fulcher−Tamman (VFT) equation η = η0exp(B /(T − T0))

where η0 (mPa s), B (K), and T0 (K) are adjustable parameters.24,25 The best fitted parameters are given in Table 7. As illustrated in Figure 4, the viscosities of IL mixtures range from 4.721 to 20.62 mPa s and decrease remarkably when the temperature increases. It can be inferred that the increase in temperature weakens ILs’ resistance to internal motion and interionic friction, which mainly results from the hydrogen bonding and electrostatic force among ions.26 At the same temperature, the viscosities of [Emim]Cl/AlCl3 ILs decrease as the mole fraction of [Emim][Al2Cl7] increases. According to Table 5 and Figure 3, [Emim][Al2Cl7] has higher ion-pair interaction energy than that of [Emim][AlCl4], and the distances of hydrogen bonds in [Emim][Al2Cl7] are longer than those in [Emim][AlCl4]. These experimental and computational results reflect the presence of stronger cation− anion interaction and hydrogen bonding in [Emim][AlCl4]. Consequently, the increased molar concentration of [Emim][Al2Cl7] reduces the interionic force and friction, which leads to lower viscosity of [Emim][AlCl4] and [Emim][Al2Cl7] binary mixtures. Compared with [Bmim]Cl/AlCl3 reported in our previous work,7 [Emim]Cl/AlCl3 ILs have lower viscosities at the same temperature and molar composition. Considering the relationship between physiochemical properties and molecular structure of ILs, it can be found that the strength of hydrogen bonding is one of the most important intrinsic factors in determining the viscosity of chloroaluminate ILs.27,28 As shown in Table 5, the distances of hydrogen bonds in these ion pairs follow the order [Emim][AlCl4] > [Bmim][AlCl4] and [Emim][Al2Cl7] > [Bmim][Al2Cl7]. It reflects the presence of weaker hydrogen bonding in [Emim][AlCl4] and [Emim][Al2Cl7]. Therefore, the lower viscosities of [Emim][AlCl4] and [Emim][Al2Cl7] binary mixtures are mainly ascribed to the weaker interionic hydrogen bonding. 3.3. Excess Molar Volume and Excess Viscosity. As described in the literature, Lewis acidic [Emim]Cl/AlCl3 ILs can be regarded as the binary mixtures of [Emim][AlCl4] and [Emim][Al2Cl7].9,29 Thus, it is necessary to investigate the corresponding excess molar volume VE and excess viscosity Δη of [Emim]Cl/AlCl3 ILs.

Figure 4. Viscosity η of [Emim][Al2Cl7] (1) + [Emim][AlCl4] (2) binary mixtures as a function of temperature T at different mole fractions x1 of [Emim][Al2Cl7] = (▲) 0.0000, (○) 0.1016, (■) 0.2046, (▷) 0.3035, (⧫) 0.4003, (□) 0.5041, (★) 0.5962, (◊) 0.6955, (▶) 0.7963, (☆) 0.8901, and (▼) 1.0000. The solid curves are correlated with eq 3, and the symbols represent experimental values.

Table 7. Fitted Values of the Empirical Parameters η0, B, T0, and Standard Deviation σ for the Viscosities of [Emim][Al2Cl7] (1) + [Emim][AlCl4] (2) Binary Mixtures as a Function of Mole Fraction x1 Based on the VFT Equation η0

B

x1

mPa s

K

K

σ

0.0000 0.1016 0.2046 0.3035 0.4003 0.5041 0.5962 0.6955 0.7963 0.8901 1.0000

0.289 0.268 0.252 0.245 0.238 0.227 0.215 0.200 0.182 0.162 0.155

621.4 643.1 657.6 662.1 664.9 674.7 687.6 706.5 731.6 768.2 776.1

147.5 144.0 141.7 140.7 140.2 138.8 137.0 134.6 131.5 126.9 126.1

0.0099 0.0082 0.0075 0.0094 0.0086 0.0101 0.0110 0.0109 0.0070 0.0104 0.0111

(3)

T0

E



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Table 8. Excess Molar Volumes VE for [Emim][Al2Cl7] (1) + [Emim][AlCl4] (2) Binary Mixtures Calculated from 293.15 to 353.15 K as a Function of Mole Fraction x1 at 0.1 MPaa VE/cm3 mol−1 x1

293.15 K

303.15 K

313.15 K

323.15 K

333.15 K

343.15 K

353.15 K

0.1016 0.2046 0.3035 0.4003 0.5041 0.5962 0.6955 0.7963 0.8901

0.1352 0.2027 0.2022 0.1807 0.1402 0.1051 0.0659 0.0259 0.0091

0.1407 0.2103 0.2095 0.1888 0.1477 0.1106 0.0707 0.0307 0.0126

0.1472 0.2207 0.2196 0.1957 0.1531 0.1163 0.0756 0.0329 0.0149

0.1544 0.2330 0.2318 0.2067 0.1588 0.1196 0.0787 0.0402 0.0195

0.1624 0.2454 0.2446 0.2201 0.1667 0.1245 0.0849 0.0468 0.0238

0.1723 0.2618 0.2605 0.2347 0.1772 0.1322 0.0940 0.0557 0.0300

0.1824 0.2815 0.2799 0.2480 0.1889 0.1424 0.1032 0.0629 0.0362

a

Standard uncertainties u are u(T) = 0.01 K, u(p) = 0.1 KPa, and u(x) = 0.0001, and the expanded uncertainties for excess molar volumes U(VE) = 0.003 cm3 mol−1 (0.95 confidence level).

Table 9. Excess Viscosities Δη for [Emim][Al2Cl7] (1) + [Emim][AlCl4] (2) Binary Mixtures from 293.15 to 353.15 K as a Function of Mole Fraction x1 at 0.1 MPaa Δη/mPa s x1

293.15 K

303.15 K

313.15 K

323.15 K

333.15 K

343.15 K

353.15 K

0.1016 0.2046 0.3035 0.4003 0.5041 0.5962 0.6955 0.7963 0.8901

−0.2018 −0.3173 −0.4012 −0.4147 −0.3965 −0.3611 −0.2933 −0.2087 −0.1357

−0.1268 −0.2089 −0.2545 −0.2770 −0.2666 −0.2445 −0.1988 −0.1481 −0.0905

−0.0740 −0.1244 −0.1652 −0.1812 −0.1797 −0.1676 −0.1373 −0.0933 −0.0570

−0.0509 −0.0800 −0.1033 −0.1148 −0.1133 −0.1042 −0.0867 −0.0662 −0.0368

−0.0311 −0.0568 −0.0724 −0.0823 −0.0818 −0.0774 −0.0633 −0.0467 −0.0256

−0.0227 −0.0424 −0.0548 −0.0602 −0.0598 −0.0547 −0.0466 −0.0344 −0.0200

−0.0146 −0.0306 −0.0405 −0.0470 −0.0460 −0.0423 −0.0347 −0.0243 −0.0135

a

Standard uncertainties u are u(T) = 0.01 K, u(p) = 0.1 KPa, and u(x) = 0.0001, and the relative expanded uncertainties for the excess viscosities Ur(Δη) = 0.02 (0.95 confidence level).

Figure 6. Excess viscosity Δη vs mole fraction x1 of [Emim][Al2Cl7] for [Emim][Al2Cl7] (1) + [Emim][AlCl4] (2) binary mixtures at different temperatures T = (■) 293.15, (●) 303.15, (◀) 313.15, (▼) 323.15, (▶) 333.15, (⧫) 343.15, and (▲) 353.15 K. The solid curves are calculated with the Redlich−Kister equation, and the symbols represent experimental values.

Figure 5. Excess molar volume VE vs mole fraction x1 of [Emim][Al2Cl7] for [Emim][Al2Cl7] (1) + [Emim][AlCl4] (2) binary mixtures at different temperatures T = (■) 293.15, (●) 303.15, (▲) 313.15, (⧫) 323.15, (▶) 333.15, (◀) 343.15, and (▼) 353.15 K. The solid curves are calculated with the Redlich−Kister equation, and the symbols represent experimental values.

VE =

On the basis of the experimental densities and viscosities described above, the values of VE and Δη have been calculated by the expression

⎛x M x1M1 + x 2M 2 xM ⎞ − ⎜⎜ 1 1 + 2 2 ⎟⎟ ρ ρ2 ⎠ ⎝ ρ1

Δη = η − (x1η1 + x 2η2) F

(4) (5)



Article

Table 10. Coefficients of the Redlich−Kister Equation A0, A1, A2, A3, and Standard Deviation σ for Excess Molar Volumes VE and Excess Viscosities Δη of [Emim][Al2Cl7] (1) + [Emim][AlCl4] (2) Binary Mixtures from 293.15 to 353.15 K property

T/K

A0

A1

A2

A3

σ

VE/cm3 mol−1

293.15 303.15 313.15 323.15 333.15 343.15 353.15 293.15 303.15 313.15 323.15 333.15 343.15 353.15

0.5793 0.6060 0.6319 0.6597 0.6957 0.7419 0.7938 −1.5837 −1.0645 −0.7189 −0.4502 −0.3310 −0.2412 −0.1871

−0.8581 −0.8754 −0.9122 −0.9735 −1.0245 −1.0794 −1.1569 0.6457 0.3488 0.1708 0.0737 0.0556 0.0584 0.0487

0.3304 0.3630 0.3990 0.4638 0.5167 0.5874 0.6538 −0.2893 −0.1318 0.0597 −0.0178 0.0462 0.0204 0.0587

−0.0446 −0.0406 −0.0320 0.0402 0.0820 0.1273 0.2003 −0.2065 −0.0918 −0.0380 0.0567 −0.0082 −0.0486 −0.0537

0.0030 0.0028 0.0034 0.0033 0.0034 0.0040 0.0050 0.0059 0.0023 0.0024 0.0017 0.0006 0.0003 0.0004

Δη/mPa s

in which ρ and η are the density and viscosity of binary mixtures, respectively, and x1 and x2, M1 and M2, ρ1 and ρ2, and η1 and η2 are the mole fractions, molar masses, densities, and viscosities of [Emim][Al2Cl7] and [Emim][AlCl4], respectively.30,31 All the experimental values of VE and Δη were fitted on the basis of the Redlich−Kister polynomial equation

hydrogen bonding, resulting in fewer negative values of excess viscosity.33 In brief, the excess molar volumes and excess viscosities of [Emim][Al2Cl7] and [Emim][AlCl4] binary mixtures were well fitted by the Redlich−Kister polynomial equation. The experimental data have been analyzed according to the theory of the liquid binary mixtures, which shows that the strength of hydrogen bonding and interaction among ions becomes weaker after the formation of the mixture.

k

Y = x1(1 − x1) ∑ Ai (2x1 − 1)i i=0

(6)

4. CONCLUSIONS In this study, Lewis acidic [Emim]Cl/AlCl3 ILs with different mole fractions were prepared by mixing precise molar quantities of [Emim][Al2Cl7] with [Emim][AlCl4]. The densities and viscosities of binary mixtures were measured from 293.15 to 353.15 K. On this basis, the excess molar volumes and excess viscosities of binary mixtures were calculated. All the experimental data were well fitted by the empirical equations. For the relationship between molecular structure and properties of ILs to be investigated, the optimized geometry and parameters of [Emim][Al2Cl7] and [Emim][AlCl4] ion pairs were obtained according to the computational method, which shows that the enhanced molar concentration of [Al2Cl7]− and higher molecular symmetry could increase the density of binary mixtures. Meanwhile, the interionic interaction, especially hydrogen bonding, has a significant influence on the viscosity of ILs. Looser packing and/or weaker interionic interaction probably occurs after [Emim][Al2Cl7] is mixed with [Emim][AlCl4]. It is hoped that this work may provide important experimental data and conclusions for future research and application of Lewis acidic [Emim]Cl/AlCl3 ILs.

where Y represents VE or Δη, Ai is an adjustable parameter, and x1 is the mole fraction of [Emim][Al2Cl7] in binary mixtures. All the data of VE and Δη are listed in Tables 8 and 9. The plots of VE and Δη vs mole fraction of [Emim][Al2Cl7] are shown in Figures 5 and 6, respectively.32 The standard deviation σ has been determined according to the equation33,34 2 ⎡ ⎤1/2 ⎢ ∑ (Ycal − Yexp) ⎥ σ (Y ) = ⎢ ⎥ n−p ⎢⎣ ⎥⎦

(7)

in which n and p are the number of experimental data and coefficients of eq 6, respectively. The values of parameters Ai and standard deviations σ are listed in Table 10. As shown in Figure 5, all the excess molar volumes for the binary mixtures of [Emim][Al2Cl7] and [Emim][AlCl4] are positive at the experimental temperatures and molar compositions. The values of excess molar volume increase with temperature increasing from 293.15 to 353.15 K, which reaches the maximum value around mole fraction x1 = 0.2. It reflects that the mixing process of [Emim][Al2Cl7] and [Emim][AlCl4], as well as increase in temperature, could both lead to less close-packing and/or weaker hydrogen bonding and interionic interaction in [Emim]Cl/AlCl3 ILs.32,33 By contrast, the excess viscosities of binary mixtures are negative under the studied experimental conditions, which also increase as temperature increases. According to Figure 6, the minimum value is obtained around mole fraction x1 = 0.4 for all of the ILs. It can be concluded that the strength of hydrogen bonding among ions becomes weaker after the formation of binary mixtures.32 The increase in temperature weakens the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00702. 1 H NMR spectrum of [Emim]Cl, 1H and 27Al NMR spectra of [Emim][AlCl4] and [Emim][Al2Cl7], molar volume, fractional deviations of experimental density, viscosity, and excess molar volume and excess viscosity from the values reported in the literature and calculated by empirical equations (PDF) G



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Article

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 372 2909705. Fax: +86 372 2592068. ORCID

Yong Zheng: 0000-0002-6224-4073 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported financially by the National Natural Science Foundation of China (21406002, 51404230) and the Science & Technology Development Program of Anyang. The authors gratefully acknowledge Prof. Suojiang Zhang and Dr. Kun Dong from Institute of Process Engineering, Chinese Academy of Sciences for their guidance and assistance in the computational calculations.

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