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Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Density, Electrical Conductivity, Dynamic Viscosity, Excess Properties, and Molecular Interactions of Ionic Liquid 1‑Cyanopropyl-3-methylimidazolium Tetrafluoroborate and Binary System with Acetonitrile Qingguo Zhang,*,†,‡ Dongye Liu,‡ Qing Li,‡ Xinyuan Zhang,† and Ying Wei‡ †

College of New Energy, Bohai University, Jinzhou 121013, Liaoning Province, China College of Chemistry and Chemical Engineering, Bohai University, Jinzhou 121013, Liaoning Province, China

‡

S Supporting Information *

ABSTRACT: In this work, a novel binary mixture system containing the ionic liquid (IL) 1-cyanopropyl-3-methylimidazolium tetrafluoroborate ([PCNmim][BF4]) and acetonitrile (AN) was prepared. The density, dynamic viscosity, and electrical conductivity of the pure IL and the binary system at different ratios were determined at atmospheric pressure from 288.15 to 323.15 K. We have estimated a series of important thermodynamic parameters of the pure IL by empirical equations and correlated the temperature dependences of the electrical conductivity and dynamic viscosity of the binary system using both the Vogel−Fulcher−Tamman and Arrhenius equations. We also focused on the excess molar volume, VE, of the binary mixture system, and the results of the VE were correlated to the Redlich−Kister polynomial equation. Then, the interstice model and excess volume were employed to investigate the structural properties and packing structure, respectively, of the pure [PCNmim][BF4] and IL + AN mixtures. For a deep understanding of the intermolecular interactions of the binary system, the structure, energy, and COSMO charge distribution of the cation and anion of the IL and the AN molecule were calculated by density functional theory calculations to evaluate the intermolecular interactions of the IL and AN by means of the variations in the infrared (IR) spectrum.

1. INTRODUCTION Ionic liquids (ILs) are liquid salts composed of large-volume organic cations with either small-volume organic or inorganic anions, which have attracted great attention in recent years for their unique physicochemical properties.1−11 Due to their high viscosity and weak transport capabilities,12 the preparation of mixed systems involving ILs, solvents, or other liquids has become an easy way to widen the applications of ILs.13,14 ILs containing CN groups are expected to exhibit unusual properties and relatively high stabilities, useful for multipurpose applications.15 They have been applied as ligands or suitable reaction media in catalytic reactions,16 as electrolytes in lithium batteries,17 or as solvents for the extraction and dissolution of cellulose.18 They also exhibit better tribological properties than common ILs.15 However, most nitrile-functionalized ILs are solid at room temperature or below, which reduces their potential applications. Therefore, the development of new nitrile-functionalized ILs that are liquid at low temperatures is highly desired. Additionally, it should be noted that detailed characterization of the physicochemical properties of these nitrile-functionalized ILs and IL mixtures have not yet been investigated. Zhang et al.15 synthesized 1-cyanopropyl-3methylimidazolium tetrafluoroborate ([PCNmim][BF4]) and studied its physicochemical properties. Liu et al.17 synthesized an © XXXX American Chemical Society

IL functionalized with a cyano group, 1-cyanopropyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([PCNmim][NTf2]), and determined its physicochemical properties at different temperatures. Gonfa et al.19 prepared the IL 1-butyl-3methylimidazolium thiocyanate ([C4C1im][SCN]) and calculated the excess molar volume (VE) of its binary system with methanol by fitting the data to the Redlich−Kister (R−K) polynomial equation. Furthermore, the ILs 1-(3-cyanoprophyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C1C3CNIm][NTf2]), 1-(3-cyanopropyl)-3-methylimidazolium dicyanamide ([C1C3CNIm][DCA]), and several other nitrile-functionalized ILs were used by Moura et al. for ethane and ethylene absorption.20 As a common organic solvent, acetonitrile (AN) and its mixtures with ILs have been applied in diverse areas such as electrolytes for batteries and reaction media in organic syntheses.21,22 Herein, the IL [PCNmim][BF4] and AN were selected to prepare binary systems over the whole mole fraction range. First, the density, electrical conductivity, dynamic viscosity, and surface tension of pure [PCNmim][BF4] were measured at different Received: October 13, 2017 Accepted: March 30, 2018

A

DOI: 10.1021/acs.jced.7b00898 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

glovebox. The infrared (IR) spectrum of a binary mixture (xIL = 0.4999) was recorded on a Nicolet iS50 FT-IR spectrometer. 2.3. Measurements. Density. A Mettler Toledo DM45 Delta Range Density Meter was used to measure the density at different concentration ratios and temperatures ranging from 288.15 to 323.15 K with a step size of 5 K under atmospheric pressure (Mettler Toledo’s vibrating U-tube technology). The density meter was calibrated with water and ultrapure dry air before performing the measurements, and its reproducibility is 10−5 g· cm−3. The uncertainty was 0.001 g·cm−3 for the density and 0.01 K for the temperature. Electrical Conductivity. The electrical conductivity was measured with a DDSJ-308A conductivity meter operating with a DJS-10C conductivity electrode under dry argon atmosphere. The conductivity electrode was calibrated with the standard aqueous KCl solution before use. The temperature was controlled from 288.15 to 323.15 K within 0.01 K by means of a thermostatic water bath. The uncertainty in the electrical conductivity measurements was estimated to be 0.05 mS·cm−1. Dynamic Viscosity. A Lovis 2000 M (Anton Paar) viscometer employing the rolling-ball principle was used to measure the viscosity of the pure IL and IL-solvent binary system. The measurement temperature ranged from 288.15 to 323.15 at a 5 K interval under atmospheric pressure. The dynamic viscosity measurements were made with different angles using three calibrated capillaries with d = 1.59 mm, d = 1.8 mm, and d = 3 mm, and calibration was performed using viscosity standard oils provided by the suppliers. The temperature measurement accuracy was 0.01 K. The uncertainty in the dynamic viscosity measurements was 0.05 mPa·s. Surface Tension. A tensiometer (BZY-101 type produced by Shanghai Fangrui instrument Co. Ltd.) was employed to measure the surface tension of the pure IL by the platinum plate method in a glovebox. The sample was thermostatically set at each temperature with an accuracy of 0.01 K. First, the surface tension of degassed water (boiling and cooling) was measured in the temperature range of 288.15 to 323.15 K. The uncertainty in the surface tension measurements was 0.1 mJ·m−2. All the experimental data for pure [PCNmim][BF4] and the binary system are listed in Table 2. 2.4. Calculations. In this paper, the 3D structures and energy of the investigated IL complexes were calculated by Turbomole32 and the COSMO−RS methodology33 based on DFT calculations. The B3LYP functional and the TZVP basis set from the Turbomole library were used.34 All interaction energies were obtained under the supermolecular ansatz. The structural characteristics and H-bonding interactions were optimized and evaluated. The calculation conditions were those of the ideal gas phase.

temperatures, from 288.15 to 323.15 K. In addition, a series of significant thermodynamic parameters, such as the standard molar entropy, lattice energy, thermal expansion coefficient, and interstice parameters of the IL were estimated according to Glasser’s theory, the interstice model theory, and empirical methods.23−27 The temperature dependence of the electrical conductivity and dynamic viscosity of the IL and its binary mixture ([PCNmim][BF4] + AN) was described by the Vogel− Fulcher−Tamman (VFT) method28,29 and the Arrhenius equation,30,31 respectively. The excess molar volume (VE) of the mixture ([PCNmim][BF4] + AN) was also calculated to discuss the interactions in the binary system at different molar ratios and temperatures ranging from 288.15 to 323.15 K. Infrared (IR) spectroscopy and density functional theory (DFT) calculations combined with the COSMO−RS methodology were also used to evaluate the intermolecular interactions between the IL and organic solvent molecules. In addition, the optimized structures of the cation/anion and solvent molecules, the charge distribution, energy, and hydrogen bonding were calculated to identify the interactions and structural changes in the binary system with the help of IR spectrum.

2. EXPERIMENTAL SECTION 2.1. Materials. The reagent 1-cyanopropyl-3-methylimidazolium tetrafluoroborate ([PCNmim][BF4]) was purchased from Shanghai Chengjie Chemical Co., Ltd., and the AN was purchased from Sinopharm (China) (Figure 1). The IL was dried

Figure 1. Structures of [PCNmim][BF4] IL and AN.

under vacuum for 24 h at 333.15 K before use. The AN was distilled and dried before use. The purity and sources of the materials are summarized in Table 1. 2.2. Preparation and Characterization of [PCNmim][BF4] and the [PCNmim][BF4] + AN Binary System. The IL was characterized by IR spectroscopy, 1H nuclear magnetic resonance (NMR) spectroscopy, and elemental analysis (EA) (see Supporting Information). Binary system samples were prepared by weight on a Shanghai JKFA2004N analytical balance with a precision of 0.0001 g. The experimental uncertainty in the mole fractions was less than 0.0001. A Karl Fischer moisture titrator (ET08, Mettler EasyPlus) was used to measure the water content at atmospheric pressure. The water content of the IL was 0.020 wt % before the properties were determined and approximately 0.035 wt % after the experiments. The water content measurement uncertainty was 0.001 wt %. All determinations were performed under dry argon atmosphere in a

3. RESULTS AND DISCUSSION 3.1. Estimation of the Volumetric and Surface Properties of [PCNmim][BF4]. The definition of the thermal expansion coefficient (α) of an IL is shown in eq 1:

Table 1. Specifications of Chemical Samples chemical name

a

CAS registry numbers

AN

75-05-8

[PCNmim][BF4]

683224-99-9

source Sinopharm (China) Shanghai Chengjie (China)

initial mole fraction purity

purification method

final mole fraction purity

analysis method

water content before experiment

water content after experiment

>0.990

distillation

>0.997

GCa, KFb

0.010 wt %

0.015 wt %

>0.990

vacuum desiccation

>0.990

1

0.020 wt %

0.035 wt %

H NMR, FTIR, KFb

Gas−liquid chromatography. bKarl Fischer titration. B



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Table 2. Experimental Values of Density ρ, Electrical Conductivity σ, Dynamic Viscosity η, and Surface Tension γ for the [PCNmim][BF4] + AN Binary System at Temperature T = 288.15−323.15 K and at Pressure p = 0.1 MPaa T, K xIL 1.0000 lit. 0.8885 0.8024 0.7001 0.5996 0.4999 0.3996 0.3013 0.2003 0.1004 0.0000 lit. 1.0000 0.8885 0.8024 0.7001 0.5996 0.4999 0.3996 0.3013 0.2003 0.1004

288.15

293.15

1.31607

1.31233

1.30235 1.28962 1.27185 1.24827 1.21822 1.17983 1.13124 1.05856 0.95103 0.78733

1.29872 1.28611 1.26821 1.24416 1.21412 1.17556 1.12673 1.05428 0.94691 0.78196 0.782045

0.58 1.26 1.57 2.80 3.65 5.58 10.5 16.8 26.2 37.1

0.85 1.74 2.08 3.52 4.67 6.85 12.4 19.3 28.9 39.8

1.0000 lit. 0.8885 0.8024 0.7001 0.5996 0.4999 0.3996 0.3013 0.2003 0.1004

590.6

417.9

372.8 287.0 150.9 105.8 72.81 30.38 13.17 5.545 1.783

266.5 207.7 113.9 80.64 56.55 24.74 11.15 4.901 1.645

1.0000 lit.

49.1

48.7

298.15 1.30861 1.31915 1.29492 1.28271 1.26441 1.24019 1.21015 1.17147 1.12241 1.04989 0.94292 0.77661 0.776665 1.23 2.36 2.86 4.70 5.88 8.34 14.6 22.1 32.0 42.8 302.6 35215 195.6 154.3 87.6 62.96 45.13 20.18 9.572 4.361 1.520 48.4 48.315

303.15

308.15

313.15

318.15

323.15

ρ, g·cm−3 1.30481

1.30105

1.29732

1.29359

1.28986

1.28773 1.27569 1.25704 1.23229 1.20189 1.16302 1.11343 1.04136 0.93473 0.76573 0.765785

1.28414 1.27233 1.25343 1.22843 1.19789 1.15893 1.10904 1.03703 0.93078 0.76025 0.760295

1.28051 1.26886 1.24969 1.22439 1.19383 1.15462 1.10471 1.03273 0.92661 0.75472

1.27687 1.26538 1.24599 1.22051 1.18985 1.15056 1.10031 1.02851 0.92240 0.74916

1.29138 1.27928 1.26073 1.23629 1.20598 1.16721 1.11784 1.04559 0.93882 0.77118 0.771245 σ, mS·cm−1 1.71 3.08 3.72 5.85 7.19 9.97 17.0 25.0 35.1 45.6 η, mPa·s 223.3 147.0 117.1 68.7 50.09 36.59 16.91 8.303 3.908 1.412 γ, mJ·m−2 48.1

2.31 3.92 4.75 7.19 8.70 11.9 19.7 28.0 38.3 48.4 169.9 112.9 90.99 54.90 40.62 30.15 14.47 7.266 3.529 1.315 47.7

3.05 4.91 5.99 8.70 10.4 14.0 22.7 31.0 41.6 51.3 128.7

3.93 6.03 7.34 10.37 12.2 16.3 25.9 34.0 44.9 54.1 100.6

4.98 7.30 8.72 12.2 14.2 18.7 29.5 37.0 48.2 56.9 79.82

88.12 71.79 44.63 33.47 25.23 12.49 6.436 3.204 1.228

70.08 57.48 36.82 28.00 21.38 10.85 5.741 2.936 1.153

56.76 47.05 30.77 23.76 17.96 9.380 5.152 2.695 1.084

47.4

47.1

46.8

Standard uncertainties u are u(xIL) = 0.0001, u(T) = 0.01 K, u(p) = 10 kPa, and u(γ) = 0.1 mJ·m−2. The relative standard uncertainties are ur(ρ) = 0.001, ur(σ) = 0.05, and ur(η) = 0.05. a

α=−

1 ⎛ ∂ρ ⎞ ⎜ ⎟ ρ ⎝ ∂T ⎠ P

Vm(298.15 K) = M ·(N ·ρ)−1 (1)

(2)

where M is the molar mass and N is the Avogadro number. The values of Vm listed in Table 3 are those of selected imidazoliumbased ILs with similar carbon chains. A comparison of said values revealed different contributions of the cations and anions to the molecular volume.17,35,38 The standard entropy (S0) and lattice energy (UPOT) were estimated using the relationships established by Glasser:23

where ρ is the density of the pure IL. The subscript P is the pressure. The thermal expansion coefficient was thus estimated at 298.15 K. The value, listed in Table 3, is similar to that of [PCNmim][NTf2] bearing the same cation and greater than that of [C4mim][BF4] bearing the same anion.17,35 The value of the thermal expansion coefficient implies that the volume change of the IL with the increasing temperature is close to that of glycerol36 and could thus be considered independent of the temperature.37 The molecular volume Vm of an IL was calculated from the experimental density by the following eq 2:

S 0 , J·K−1·mol−1 = 1246.5(Vm , nm 3) + 29.5

(3)

UPOT , kJ·mol−1 = 1981.2(ρ /M )1/3 + 103.8

(4)

The standard entropy reported for [PCNmim][BF4] is listed in Table 3. This standard entropy is lower than those of other ILs C



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Table 3. Estimated Parameters of Thermal Expansion Coefficient α, Molecular Volume Vm, Standard Entropy S0, Lattice Energy UPOT, Surface Excess Entropy Sa, Surface Excess Energy Ea, and Enthalpy of Vaporization ΔlgHm0 of Pure [PCNmim][BF4] IL at T = 298.15 K IL

α, 10−4 K−1

Vm, nm3

Vm+, nm3

Vm−, nm3

S0, J·K−1·mol−1

UPOT, kJ·mol−1

Sa, mJ·m−2

Ea, mJ·m−2

ΔlgHm0, kJ·mol−1

[PCNmim][BF4] [PCNmim][NTf2] [C4mim][BF4]

5.73 6.0917 3.6535

0.3009 0.472117 0.312435

0.194 ± 0.01538 0.194 ± 0.01538 0.196 ± 0.02138

0.073 ± 0.00938 0.232 ± 0.01538 0.073 ± 0.00938

404.6 617.917 418.935

454 40517 45035

0.0655 − 0.074335

67.9 − 66.835

149.1 − 141.335

Table 4. Interstice Parameters of [PCNmim][BF4] IL at 298.15 K IL

v, 10−24 cm3

∑v, cm3 (formula unit)−1

α (cal), 10−4 K−1

α (exp), 10−4 K−1

∑v/V (%)

[PCNmim][BF4] [C4mim][BF4]

16.8 19.035

20.22 22.8435

5.62 5.4535

5.73 3.6535

11.2 10.835

V = 2Nv + Vi

in Table 3, which may be the result of the good matching between the cation and the anion, ultimately affording a relatively low standard entropy.23 The lattice energy of [PCNmim][BF4] is very low compared to that of fused salts; for example, fused CsI39 has the lowest lattice energy among all alkali halides (613 kJ·mol−1). The low liquid-state temperature of the IL may result from its low lattice energy. One of the fundamental reasons for the formation of ILs at room temperature is a low lattice energy. The measured surface tension (γ) values of [PCNmim][BF4] from 288.15 to 323.15 K are listed in Table 2; the experimental data is in good agreement with the literature at 298.15 K15 as calculated with eq 5: γ , mJ·m−2 = a − bT

If the expansion of the IL is entirely caused by the expansion of the interstices with the increasing temperature, the coefficient α can be calculated based on the interstice model: α=

The surface excess entropy (Sa) and surface excess energy (Ea) were also estimated from the surface tension using eqs 6 (the slope of eq 5) and 7 at 298.15 K. The adjusted parameters a and b were obtained by linear fitting, with a linear correlation coefficient of >0.999. (6)

Ea , mJ·m−2 = γ − T (∂γ /∂γ )p

(7)

The estimated surface excess entropy and surface excess energy values for [PCNmim][BF4] are listed in Table 3. The enthalpy of vaporization Δ lgH m0 (298.15 K) of [PCNmim][BF4] was estimated from the empirical equation by Kabo et al.:40 Δgl Hm0 , kJ·mol−1 = A(γV 2/3N1/3) + B

(8)

where A and B are empirical parameters (A = 0.01121, B = 2.4 kJ·mol−1), V is molar volume, and γ is the surface tension of the pure IL at 298.15 K. The enthalpy of vaporization for pure [PCNmim][BF4] was calculated with eq 8, and the result is presented in Table 3. [PCNmim][BF4] has an enthalpy of vaporization similar to those of other common ILs.4,24 3.2. Interstice Model for the IL. To date, several theories have been put forward to describe the structural and transport properties of pure ILs.41−44 Herein, a simple interstice model25−27 was employed for the selected IL. The interstice volume (ν) was estimated from classical statistical mechanics, as shown in eq 9: v = 0.6791(k bT /γ )3/2

1 ⎛⎜ ∂V ⎞⎟ 3Nv = ⎝ ⎠ V ∂T p VT

(11)

The correlation parameters of the IL at 298.15 K were estimated according to the interstice model and are presented in Table 4. Typically, ∑v/V is around 10−15% for volume changes from the ion solid to the melt, and the results for the studied [PCNmim][BF4] are very close to those with empirical regularity. Comparison of the thermal expansion coefficients α (exp) and α (cal) obtained from the experimental and calculated data for a series of ILs revealed that most deviations are around 10−15%,4,45 and the value for [PCNmim][BF4] implies that the interstice model is appropriate to describe this IL. 3.3. Excess Volumetric Properties of the [PCNmim][BF4] + AN Binary System. The densities of pure [PCNmim][BF4] and [PCNmim][BF4] + AN mixtures were measured at temperatures from 288.15 to 323.15 K, and the results are listed in Table 2. The density of pure AN in this work shows nearly the same as the literature from 293.15 to 313.15 K.5 A comparison between the density of [PCNmim][BF4] IL (298.15 K) in this work and that reported15 (298.15 K) in the literature shows that the density is not in agreement. The value measured in this paper is lower than that of the literature. This difference may be the result of other impurities such as residual salt from the synthesis.46 As shown in Figure 2a, the density values decrease with the increasing temperature and also decrease with the increasing AN content. This behavior is consistent with that of other binary IL systems with AN.5,10 From the measured density values, the excess molar volume (VE) of the mixture was calculated using the following equation, as presented in Table 5:1,3,10

(5)

Sa , mJ·m−2 = −b = −(∂γ /∂T )p

(10)

⎛ ⎞ ⎛ ⎞ 1 1 1 1 V E = x1M1⎜⎜ − ⎟⎟ + x 2M 2⎜⎜ − ⎟⎟ ρ1 ⎠ ρ2 ⎠ ⎝ ρ1,2 ⎝ ρ1,2

(12)

where ρ1, ρ2, and ρ1,2 are the density values of the IL, AN, and mixture, respectively; M1 and M2 denote the molar masses of the IL and AN, respectively; and x1 and x2 are the mole fractions of the IL and AN, respectively. The experimental values of VE (Table 5, Figure 2b) were then fitted to a Redlich−Kister (R−K) polynomial equation with four parameters:47

(9)

where kb is Boltzmann’s constant. The ν value is listed in Table 4. The molar volume of the IL (V) comprises the inherent volume (Vi) and the total volume of all interstices (∑v = 2Nv), that is,

V E = x1x 2∑ Ak (x1 − x 2)k k

D

(13) DOI: 10.1021/acs.jced.7b00898 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


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Figure 2. (a) Density ρ for the [PCNmim][BF4] + AN binary system as a function of temperature T at different mole fractions xIL of the IL: ■, 1.0000; ●, 0.8885; ▲, 0.8024; ▼, 0.7001; ◆, 0.5996; ◀, 0.4999; ▶,0.3996; ●, 0.3013; ★, 0.2003; ■, 0.1004; ●, 0.0000. Solid lines represent the linear correlation (r = 1). (b) Excess molar volumes VE for the [PCNmim][BF4] + AN mixtures at different temperatures T: ■, 288.15 K; ●, 293.15 K; ▲, 298.15 K; ▼, 303.15 K; ◆, 308.15 K; ◀, 313.15 K; ▶, 318.15 K, ●, 323.15 K. Solid lines represent the Redlich−Kister correlation.

Table 5. Excess Molar Volumes VE for [PCNmim][BF4] + AN Binary System at Temperature T = 288.15−323.15 Ka T, K xIL

288.15

293.15

298.15

303.15

0.000 −0.658 −1.187 −1.683 −1.920 −2.104 −2.224 −2.325 −2.063 −1.335 0.000

V , cm ·mol 0.000 −0.712 −1.269 −1.753 −1.982 −2.156 −2.287 −2.380 −2.152 −1.441 0.000 E

1.0000 0.8885 0.8024 0.7001 0.5996 0.4999 0.3996 0.3013 0.2003 0.1004 0.0000 a

0.000 −0.613 −1.049 −1.573 −1.844 −1.991 −2.104 −2.209 −1.908 −1.133 0.000

0.000 −0.647 −1.112 −1.638 −1.875 −2.042 −2.157 −2.260 −1.989 −1.229 0.000

3

308.15

313.15

318.15

323.15

0.000 −0.749 −1.329 −1.819 −2.032 −2.216 −2.356 −2.448 −2.248 −1.552 0.000

0.000 −0.790 −1.415 −1.893 −2.097 −2.285 −2.435 −2.520 −2.340 −1.676 0.000

0.000 −0.827 −1.490 −1.955 −2.145 −2.351 −2.498 −2.601 −2.439 −1.791 0.000

0.000 −0.864 −1.565 −2.023 −2.213 −2.429 −2.588 −2.680 −2.549 −1.909 0.000

−1

Standard uncertainty u is u(VE) = 0.01 cm3·mol−1.

Figure 2b shows that, at all investigated temperatures, the VE values of the [PCNmim][BF4] + AN binary system are negative over the whole concentration range. It is distinctly observed that a VE minimum appears at xIL ≈ 0.3. The temperature influence on VE is illustrated in Figure 2b. The variation in VE becomes more negative with the increasing temperature, implying that more efficient packing and/or attractive interactions occur upon mixing the IL with AN.3,10,50 This negative trend is similar to that of many binary systems (IL + organic solvent),10,48,50 which suggests (1) the interstices in the IL provide space for additional solvent molecules and (2) the number of solvent molecules and the temperature are both key factors for the micropacking and interactions in the binary system. 3.4. Transport Properties of [PCNmim][BF4] and the [PCNmim][BF4] + AN Binary System. The transport properties of ILs, such as the dynamic viscosity and electrical conductivity, are important characteristics in chemical and engineering processes. The electrical conductivities of the binary system were measured from 288.15 to 323.15 K. The relation between the electrical conductivity and temperature of the [PCNmim][BF4] + AN binary system (molar ratio of IL from 1 to 0.1) was fitted by the Vogel−Fulcher−Tamman (VFT) equation as follows:4

where x1 and x2 are the mole fractions of the IL [PCNmim][BF4] and AN, respectively. Ak is an adjustable parameter from 288.15 to 323.15 K determined using the least-squares method. In the present investigation, the k values were taken from 0 to 3, which is a common range for most IL systems.6,48,49 The standard deviation (SD) was calculated from eq 14:48 ⎡ n ⎤0.5 E E 2 SD = ⎢∑ (Vexp , i − Vcal , i) /(n − m)⎥ ⎢⎣ i = 1 ⎦⎥

(14)

where n is the number of experimental data and m is the number of parameters. The R−K polynomial parameters and corresponding standard deviations, SD, are presented in Table 6; the Table 6. Coefficients Ak of the Redlich−Kister Polynomial Equation of [PCNmim][BF4] + AN Binary System and Standard Deviations, SD T, K

A0

A1

A2

A3

SD

288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15

−8.211 −8.370 −8.579 −8.775 −8.988 −9.237 −9.452 −9.733

3.514 3.392 3.316 3.118 3.051 2.880 2.853 2.816

−2.694 −3.505 −4.137 −5.088 −5.904 −6.798 −7.706 −8.500

1.392 2.163 3.016 3.833 4.683 5.668 6.431 7.265

0.078 0.070 0.059 0.050 0.045 0.041 0.044 0.047

D = D0 exp[−E D/(T − T0)]

(15)

where D is the electrical conductivity and D0, ED, and T0 are adjustable parameters. R2 is correlation coefficient (see Figure 3a). The fitted parameters and correlation coefficient are listed in Table 7. To the best of our knowledge there is an obvious lack of systematic available experimental data in the literature for the

standard deviations of the binary system are within the acceptable range.48 E



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Figure 3. (a) Electrical conductivity σ and (b) dynamic viscosity η for the [PCNmim][BF4] + AN binary system as a function of temperature T at different mole fractions xIL of the IL. Solid lines represent the Vogel−Fulcher−Tammann fitting. (c) ln σ vs T−1 and (d) ln η vs T−1 for the [PCNmim][BF4] + AN binary system at different mole fractions of the IL. Solid lines represent the Arrhenius equation: ■, 1.0000; ●, 0.8885; ▲, 0.8024; ▼, 0.7001; ◆, 0.5996; ◀, 0.4999; ▶,0.3996; ●, 0.3013; ★, 0.2003; ■, 0.1004.

to the behavior of most binary IL + solvent systems.50 In addition, the electrical conductivity of the binary system increases with the temperature. The relation of the dynamic viscosity and temperature of the system across the whole concentration range was also fitted by the VFT equation as follows:29

Table 7. Adjustable Parameters of the Vogel−Fulcher− Tamman Equation and Correlation Coefficient, R2 σ, mS·cm−1 xIL

D0, mS·cm−1

1.0000 0.8885 0.8024 0.7001 0.5996 0.4999 0.3996 0.3013 0.2003 0.1004

2062 801.1 563.0 724.1 819.8 2048 12147 318.4 609.2 326.5

E D, K

T0, K

R2

799.7 604.6 491.4 534.5 566.0 798.3 1438 278.8 455.3 310.2 η, mPa·s

190.4 194.5 205.1 192.3 183.6 153.1 84.31 193.5 143.6 145.6

0.9999 0.9999 0.9996 0.9997 0.9999 0.9999 0.9999 0.9999 0.9999 0.9999

xIL

η0, mPa·s

B, K

T0, K

R2

1.0000 0.8885 0.8024 0.7001 0.5996 0.4999 0.3996 0.3013 0.2003 0.1004

0.0283 0.1083 0.1238 0.1445 0.2625 0.2167 0.2714 0.2060 0.1543 0.0672

−1380 −946.9 −889.5 −818.1 −632.4 −650.5 −503.0 −499.4 −496.4 −639.9

149.4 171.9 173.4 170.5 182.7 176.3 181.6 168.0 149.6 92.97

0.9999 0.9999 0.9999 0.9999 0.9999 0.9999 0.9997 0.9999 0.9999 0.9999

η = η0 exp[−B /(T − T0)]

(16)

where η0, B, and T0 are the adjustable parameters and R2 is correlation coefficient. The fitted parameters and correlation coefficient are listed in Table 7. The dynamic viscosity of the binary system was measured from 288.15 to 323.15 K, as listed in Table 7 and graphically depicted in Figure 3b. The fitting correlation coefficients are all >0.9997, indicating that the VFT equation is also suitable to describe the temperature dependence of the dynamic viscosity. Viscosity of the pure [PCNmim][BF4] IL (Table 2) was measured in the temperature range from 288.15 to 323.15 K. Our viscosity data for IL (298.15 K) is lower than that of the literature15 data. Residual salt from the synthesis may be the reason for the deviation.46 The pure [PCNmim][BF4] viscosity decreases with the increasing temperature, from 590.6 mPa·s at 288.15 K down to 79.82 mPa·s at 323.15 K. Additionally, the viscosity of the mixture also decreases with the increasing temperature. The viscosity of the binary system was also examined upon AN addition from 288.15 to 323.15 K. As shown in Figure 3b, it was found that the viscosity of the binary system significantly decreased after AN addition to the pure [PCNmim][BF4], especially in the case of dilute solutions. The Coulomb interactions between the [BF4]− anions and [PCNmim]+ cations are weakened upon mixing with polar organic compounds, leading to decreased viscosities of the mixtures.48 Such behavior can be interpreted by the formation of hydrogen bonding between the IL and AN.51,52 Additionally, the temperature dependence of the dynamic viscosity and eletrical conductivity for the [PCNmim][BF4] + AN binary system over the temperature range from 288.15 to

electrical conductivity of pure [PCNmim][BF4], yet. The measured conductivity results of pure IL listed in Table 2 in this work are the first time reported. As shown in Table 7, the fitting correlation coefficients are all >0.9996, which indicates that the VFT equation suitably describes the effect of the temperature on the electrical conductivity of the mixed system. It can be seen that the electrical conductivity sharply increases when AN is added to pure [PCNmim][BF4]. This change in the viscous nature of the IL upon dilution with the solvent is similar F



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Table 8. Arrhenius Parameters of [PCNmim][BF4] + AN Binary System and Correlation Coefficient, R2 σ, mS·cm−1

η, mPa·s xIL

η∞(10 ), mPa·s

1.0000 0.8885 0.8024 0.7001 0.5996 0.4999 0.3996 0.3013 0.2003 0.1004

5.307 10.16 15.45 61.04 105.3 192.2 624.9 2250 6976 17768

6

−3

−1

Eη(10 ), kJ·mol −44.31 −41.63 −40.00 −35.18 −33.02 −30.69 −25.79 −20.74 −15.97 −11.03

R

2

0.9988 0.9978 0.9977 0.9978 0.9971 0.9981 0.9974 0.9981 0.9987 0.9997

323.15 K was fitted with the logarithmic form of the Arrhenius equation (eq 17 and eq 18):31 ln η = ln η∞ − E D/RT

−1

σ∞(10 ), mS·cm 261.1 14.16 15.01 2.594 1.034 0.4200 0.1484 0.0260 0.0076 0.0020

Eσ(10−3), kJ·mol−1

R2

47.60 38.77 38.42 32.85 29.99 26.86 22.88 17.55 13.58 9.486

0.9967 0.9959 0.9960 0.9961 0.9970 0.9989 0.9998 0.9965 0.9990 0.9989

AN weakens the C−H···anion H-bonding interactions in [PCNmim][BF4]. Furthermore, the blue shift of the C2−H and C4,5−H bands of ∼6 cm−1 and that of the H band of the alkyl chain of ∼5 cm−1 (Figure 4) at around 3122, 3165, and 2969 cm−1, respectively, indicate the existence of H-bonding interactions involving the H atoms of the imidazolium ring and alkyl chain with the solvent molecule. The band for the CN groups on the imidazolium ring and in AN also exhibits a blue shift of 3 cm−1 (Figure 4) at 2250 cm−1, implying that H-bonding occurs between the H and the CN moieties. The above H-bonding interactions were further confirmed by the following DFT calculations. DFT calculations were carried out to understand the intermolecular interactions between the IL and AN. First, the structures of the IL cation and anion were optimized and the stable conformers were identified. Then, ion pairs consisting of the cations and anions were optimized to obtain IL structures. The most stable IL structures (ion pairs) and conformations of the complexes consisting of one AN molecule and the IL are listed in Figure 5. However, there may be many more possible

(17)

where η is the dynamic viscosity, η∞ is an empirical constant, and ED is the activation energy for the viscous flow. ln σ = ln σ∞ − E D/RT

−6

(18)

where σ is the electrical conductivity, σ∞ is an empirical constant, and ED is the activation energy for electrical conductivity. The fitted Arrhenius parameters are listed in Table 8. The 1000/T dependence of ln σ and ln η is illustrated in Figure 3c,d, respectively, in the temperature range from 288.15 to 323.15 K under the whole concentration range. The values of the correlation coefficient, R2, are also shown. The R2 values are lower than those obtained from the VFT equation. Thus, comparing to the Arrhenius equation, the experimental values of the electrical conductivity and dynamic viscosity of this IL mixture system are more suitable for the VFT equation. 3.5. Intermolecular Interactions in the [PCNmim][BF4] + AN Binary System. The chemical structure and atom numbering for [PCNmim][BF4] are shown in Figure 1. The IR spectra of pure IL and an IL + AN mixture (xIL = 0.4997) are shown in Figure 4. The stretching vibrations of the main functional groups

Figure 4. IR spectra of pure [PCNmim][BF4] IL (red line, xIL = 1.0000) and binary system (black line, xIL = 0.4999). Solid lines represent IR curves.

Figure 5. Optimized geometries for [PCNmim][BF4] + AN binary system. (a) [PCNmim][BF4] IL, total energy is −44.37 kJ·mol−1; (b) AN molecule; (c) AN molecule interacting with [PCNmim][BF4] IL, total energy is −51.81 kJ·mol−1; (d) [PCNmim][BF4] + 2AN, total energy is −63.67 kJ·mol−1; (e) 2 [PCNmim][BF4] + AN, total energy is −107.28 kJ·mol−1. H-bonds are denoted by dashed lines, and the corresponding H···N, H···O, and H···F distances (Å) are labeled.

(C2−H, C4,5−H, and H) of the alkyl chain in pure [PCNmim][BF4] are shown in Figure 4. In the IR spectra, the observed blue shifts of the C2−H, C4,5−H, and H bands of the alkyl chain on the imidazolium rings upon dilution indicate that G



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Table 9. Charge Distribution by the CosmothermX Interface of [PCNmim][BF4], AN, and the Binary System [PCNmim][BF4] + AN

interaction structures for the [PCNmim]+ cation, [BF4]− anion, and ion pairs with AN. The selected structure is the most stable optimized geometry from the number of possible mutual orientations of the interacting complexes examined. Although the hydrogen atoms on C2−H and C4,5−H carry positive charges, C2−H is more acidic since it is located between two electronegative nitrogen atoms.53 This was also observed from the COSMO surface charge density obtained from quantum calculations. In the stable geometries of [PCNmim][BF4] (Figure 5), as that for pure [PCNmim][BF4], it was found that H-bonding exists between the C2−H moiety on the imidazolium ring and an F atom of the [BF4]− anion. On the side chain, it was found that H-bonding exists between the C6−H and C7−H groups on the imidazolium ring and an F atom of the [BF4]− anion. At an IL/AN molar ratio of 1:1, additional H-bonding arises between the C5−H moiety and the N atom of AN. When an IL pair interacts with two AN molecules, the [PCNmim]+ cation shows good H-bonding with the AN molecules. For two IL ion pairs and one AN molecule, H-bonding exists between one H atom of AN and the C5−H of the IL cations. From the charge distribution calculations (Table 9), it appears that the highly localized negative charge in the [BF4]− anion can establish relatively strong cation−anion Coulombic interactions in the IL ion pairs.

were estimated by empirical or semiempirical equations based on the experimental data. The excess molar volume (VE) was calculated and fitted to the R−K polynomial equation, and the corresponding standard deviations were calculated. The VE values were always negative and decreased with the increasing temperature. The electrical conductivity and dynamic viscosity were fitted by the VFT and Arrhenius equations. It was found that the VFT equation was more suitable to describe the binary system [PCNmim][BF4] + AN. Excess volume calculations, IR spectra, and DFT calculations were employed to describe the intermolecular interactions between the IL ions and the solvent molecules. The interactions between the cation/anion pairs and solvent were briefly discussed. Through DFT calculations, the optimized structures, energies, charge distribution, and existing H-bonding in the binary system were determined; upon addition of AN, the [PCNmim]+ cation exhibits good H-bonding with the AN molecules.

4. CONCLUSIONS In this paper, a binary system of [PCNmim][BF4] + AN was prepared, and the physicochemical properties of pure [PCNmim][BF4] and the binary system were studied. A series of thermodynamic parameters of the pure [PCNmim][BF4] IL

Corresponding Author

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00898. FTIR, 1HNMR spectra, and EA for ILs in this work (PDF)

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AUTHOR INFORMATION

*(Q.Z.) E-mail: [email protected]. Tel.: + 86 18641695401. Fax: + 86 416 3400310. ORCID

Qingguo Zhang: 0000-0002-0172-9579 H



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This work was financially supported by the National Nature Science Foundation of China (No. 21373002, No. 21503020). The Nature Science Foundation of Liaoning Province (No. 201602016). The Doctoral Fund of Liaoning Province of China (No. 201601347). Program for Liaoning Excellent Talents in University, China (LJQ2015099). Notes

The authors declare no competing financial interest.

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