Thermodynamic properties of PEG-based functionalized imidazolium toluenesulfonate ionic liquids

Thermodynamic properties of PEG-based functionalized imidazolium toluenesulfonate ionic liquids

Journal of Molecular Liquids 204 (2015) 39–43 Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier...

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Journal of Molecular Liquids 204 (2015) 39–43

Contents lists available at ScienceDirect

Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Thermodynamic properties of PEG-based functionalized imidazolium toluenesulfonate ionic liquids Jianying Wang, Ying Chen, Lizhe Zhang, Chao Liu, Yongqi Hu ⁎ College of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, China

a r t i c l e

i n f o

Article history: Received 12 November 2014 Received in revised form 6 January 2015 Accepted 12 January 2015 Available online 14 January 2015 Keywords: Ionic liquid Thermophysical properties Density Surface tension

a b s t r a c t A series of PEG-based functionalized imidazolium toluenesulfonate ionic liquids (PBILs), which exhibit extremely high SO2 solubility, were prepared. The density and surface tension of PBILs ([M-PEG350Mim][Tosyl], [MPEG500Mim][Tosyl] and [M-PEG700Mim][Tosyl]) have been measured in the temperature range from 293.15 K to 333.15 K. The coefficients of thermal expansion were calculated from the experimental density results using an empirical correlation. Molecular volume and standard entropies of these ILs were calculated from the experimental density values. The surface properties of ILs were investigated. The critical temperatures and enthalpies of vaporization were also discussed. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Air pollution has been drawing much attention in the world. As one of the main air pollutants, SO2 is emitted from burning of fossil fuels, and it is harmful to the environment and the human health [1–3]. Hence, it is important to separate and recycle SO2 from the industrial waste gas. As one promising SO2 selective separation media, room-temperature ionic liquids (RTILs) have been attracting extensive interest in recent years because of their unique properties such as chemical stability, negligible vapor pressure and high thermal stability [4,5]. Recently, many attempts have been made to explore functional ILs for SO2 absorption through addition of functional groups on the cation or anion of ILs [6–9], particularly, ether-based functionalized ILs. Hong [3] had synthesized a series of ether-functionalized imidazolium methanesulfonates which manifest quite high SO2 solubility; [E3mim][Tetz] was synthesized by Cui et al. [7], and it was found that the IL exhibits an extremely high SO2 capacity and excellent reversibility via a combination of chemical and physical absorption. We have developed a series of PEG-based functionalized imidazolium toluenesulfonates (PBILs) with a long ether chain which provides multiple active sites for SO2 absorption on cation, and the ILs were able to absorb SO2 with extremely high capacity and efficient regeneration performance. The thermodynamic properties of PBILs, such as density and surface tension, are very important for their industrial application in flue gas desulfurization (FGD). It is also helpful for the development of ⁎ Corresponding author. E-mail address: [email protected] (Y. Hu).

http://dx.doi.org/10.1016/j.molliq.2015.01.017 0167-7322/© 2015 Elsevier B.V. All rights reserved.

correlations and predictive method for these properties [10–14]. So it is necessary to study their physical properties. The aim of this work is to measure the basic physical properties of series of PBILs over the temperature range from 293.15 K to 333.15 K under atmospheric pressure. The series of PBILs, namely 1-heptaethylene glycol monomethyl ether-3-methylimidazolium toluenesulfonate ([M-PEG350Mim][Tosyl]), 1-undecaethylene glycol monomethyl ether-3-methylimidazolium toluenesulfonate ([M-PEG500Mim][Tosyl]) and 1-pentadecaethylene glycol monomethyl ether-3-methylimidazolium toluenesulfonate ([MPEG700Mim][Tosyl]), have been synthesized and characterized, and the density and surface tension were measured. The volumetric and surface properties of PBILs are also discussed systematically.

2. Experimental 2.1. Materials and apparatus Polyethylene glycol monomethyl ether (industrial, CAS no. 900474-4) was purchased from Han Tai Chemical Co., Ltd. (Nantong, China). Tosyl chloride (99%, CAS no. 98-59-9, J&K), 1-methylimidazole (99%, CAS no. 616-47-7) was purchased from Shanghai Cheng Jie Chemical Co. LTD. Properties of PEG-based functionalized ILs were characterized: 1H NMR of the ILs was measured on a Bruker AVANCF 500 MHz spectrometer, using CDCl3 as a solvent with TMS as the internal standard. Thermal stability of these ILs was measured by SDT Q600 Simultaneous DSC-TGA Instrument (V20.9 Build 20, TA Co. LTD.). Such ionic liquids

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J. Wang et al. / Journal of Molecular Liquids 204 (2015) 39–43

exhibit high thermal stability (Fig. 1S), the decomposition temperatures center on about 300–400 °C. 2.2. Synthesis of PEG-based functionalized ILs All the PEG-based functionalized imidazolium toluenesulfonate ILs, namely ([M-PEG350Mim][Tosyl], [M-PEG500Mim][Tosyl], and [MPEG700Mim][Tosyl]) were synthesized via a two-step reaction (Reaction process is shown in Scheme 1.). For example, the synthesis of [MPEG350Mim][Tosyl], CH3-O-PEG350-OH (M-PEG350-OH) and tosyl chloride reacted to generate an intermediate (M-PEG350-OTs) [15], then, the intermediate and equimolar 1-methylimidazole were added into a dried 100 ml flask and heated to 70 °C. The reaction was allowed to proceed for 24 h [16]. At last, a pale yellow viscous liquid was obtained. [MPEG500Mim][Tosyl] was pasted below 25 °C, and [M-PEG700Mim][Tosyl] was pasted below 32 °C. 2.2.1. [M-PEG350Mim][Tosyl] 1 H NMR (500 MHz, CDCl3) δ ppm: 2.31(s, 3H), 3.34(m, 3H), 3.51(m, 2H), 3.59(m, 25H), 3.79(t, 2H), 3.94(d, 3H), 4.45(t, 2H), 7.12(d, 2H), 7.34(m,1H), 7.59(m,1H), 7.75(d, 2H), 9.73(t, 1H). 2.2.2. [M-PEG500Mim][Tosyl] 1 H NMR (500 MHz, CDCl3) δ ppm: 2.34(s,3H), 3.37(m,3H), 3.54(t,2H), 3.62(m, 39H), 3.85(t, 2H), 3.99(s, 3H), 4.52(t, 2H), 7.14(d, 2H), 7.29(s,1H), 7.64(s,1H), 7.80(d, 2H), 9.91(s,1H). 2.2.3. [M-PEG700Mim][Tosyl] 1 H NMR (500 MHz, CDCl3) δ ppm: 2.34(s,3H), 3.37(s, 3H), 3.54(t, 2H), 3.63(m, 51H), 3.86(t, 2H), 3.40(s, 3H), 4.53(t, 2H), 7.14(d, 2H), 7.26(s,1H), 7.64(s,1H), 7.80(d, 2H), 9.99(s,1H). 2.3. Physicochemical property measurements The PBILs are hygroscopic. In order to reduce the content of water, all the IL samples were dehydrated under a vacuum at 80 °C for at least 48 h. In the process of dehydration, they were weighed every 15 min until a constant weight prior to the measurements. The water contents of the three PBILs were lower than 0.1% using a Karl Fischer analysis. To avoid surface contamination and the absorption of water by the ILs, all the measurements were performed under dry atmosphere. Circulating water from a thermostatically regulated bath was around the measuring cell to maintain the temperature with a temperature stability of ±0.02 K [5].

The densities of all the PBILs [M-PEG 350Mim][Tosyl], and [MPEG500Mim][Tosyl], and [M-PEG 700Mim][Tosyl] ionic liquids were measured using a densimeter (MYX-1). The repeatability of the density measurement is ± 0.0002 g·cm 3. The density measuring cell was thermostated with a temperature stability of ± 0.02 K. The apparatus calibration was performed periodically, and water and air were used for calibration. The measurements were done in five replicate runs and the average value was considered for further study [17]. Measurements of surface tension of the PBILs were performed with a platinum ring with a DCAT21 (Dataphysics, Germany) digital tensiometer. The ring and vessel were thoroughly cleaned by immersion in a concentrated solution of nitric acid for several hours before experiments. Then it was rinsed with distilled water, carefully flamed in a Bunsen burner, washed again with distilled water and dried. The uncertainty of the measurements is ±0.15 mN·m−1 [17]. 3. Results and discussion Physicochemical properties of the synthesized ILs were characterized and shown in Tables 1 and 3. The value of density and surface tension for [M-PEG500Mim][Tosyl] at 293.15 K and for [MPEG700Mim][Tosyl] at 303.15 K were not given because it is pasted at room temperature. 3.1. Density of PEG-based functionalized ILs As one of the basic physicochemical properties of ILs, density is very important for a better understanding of the interactions in this kind of compound [18,19]. Our research group [17] and Fredlake et al. [20] reported that the density of ILs has a good linear relationship with temperature, which is similar to the results in this work. The experimental density values were measured over the temperature range from (293.15 to 333.15) K at atmospheric pressure, and the results are presented in Table 1. For a specific IL, The values of density are found to decrease with increasing temperature. The structures have an effect on densities of ILs. Generally, the PBILs that have the same anions will have a lower density if the substituent on the cation has a longer chain structure [21,22]. From Table 1 it can be seen that the densities decrease as the lengths of ether chain on the cations increase, which is in the order of [M-PEG350Mim][Tosyl] N [M-PEG500Mim][Tosyl] N [MPEG700Mim][Tosyl] at all measured temperatures. This trend is consistent with our previous results for ether-functionalized imidazolium ionic liquids [23]. The density values of PEG-based functionalized

Scheme 1. Synthetic route of PEG-based functionalized ILs.

J. Wang et al. / Journal of Molecular Liquids 204 (2015) 39–43

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Table 1 Density of PEG-based functionalized ILs in the temperature range from 293.15 K to 333.15 K under atmospheric pressure. Ionic liquids

[M-PEG350Mim] [Tosyl] [M-PEG500Mim] [Tosyl] [M-PEG700Mim] [Tosyl]

ρ/(g·cm−3) 293.15 K

303.15 K

308.15 K

313.15 K

323.15 K

333.15 K

1.1821 – –

1.1714 1.1568 –

1.1663 1.1520 1.1472

1.1606 1.1454 1.1422

1.1517 1.1359 1.1318

1.1418 1.1274 1.1224

imidazolium ILs in the current work are lower than that of the traditional imidazolium-based ILs ([bmim][BF4], [bmim][NTf2], [bmim][PF6], [emim][NTf2], [emim][EtSO4] and [N4111][NTf2]) [24], which is in the range of 1.20–1.53 g·cm− 3 at 293 K. The density value of [MPEG500Mim][Tosyl] is comparable with that of the [emim][L-lactate] [25] and [C2mim](CH3CH2O)2PO2 [17] at the measured temperature range. Density data for PBIL studied here were used to deduce the coefficient of thermal expansion. Fig. 1 shows the experimental values of lnρ against T. The values were fitted by the method of the leastsquare. Empirical equation was derived for the three ILs as follows: h  i −3 −4 ¼ 0:4198−8:6278  10 T=K ln ρ= g  cm h  i −3 −4 ln ρ= g  cm ¼ 0:4080−8:6649  10 T=K h  i −3 −4 ln ρ= g  cm ¼ 0:4079−8:7834  10 T=K: The correlation coefficient is 0.9995, 0.9979, and 0.9999 for [MPEG350Mim][Tosyl], [M-PEG500Mim][Tosyl], and [M-PEG700Mim][Tosyl], respectively. The thermal expansion coefficient of IL studied here is defined through the following equation: α¼

    1 ∂V ∂ ln ρ ¼− V ∂T p ∂T p

ð1Þ

where α is the coefficient of thermal expansion, V is the volume of the IL and ρ is the density of the IL. The value of α obtained from the fitting line is 8.63 × 10 − 4 K − 1, 8.66 × 10 − 4 K − 1 , and 8.78 × 10− 4 K − 1 for [M-PEG350 Mim][Tosyl], [M-PEG500 Mim] [Tosyl], and [M-PEG700Mim][Tosyl], respectively. The α value is comparable to the thermal expansion coefficients of [emim][L-lactate] (8.00 × 10−4 K−1) [25]. The molecular volume of ILs, Vm, is calculated from the experimental density value using the following equation at 298.15 K. V m ¼ M=ðN  ρÞ

ð2Þ

where M is the molar mass of [M-PEGMim][Tosyl], and N is the Avogadro's constant. The calculated value of Vm for [M-PEGMim] [Tosyl] are shown in Table 2. At the same temperature, all the molar mass and molecular volume of the PBILs synthesized in this work increase with the increase of ether chain length on the imidazolium cation. The standard entropy can be expressed in terms of molecular volume by the following equation:     o −1 −1 3 ¼ 1246:5 V m =nm þ 29:5: S ð298Þ= J  mol  K

ð3Þ

Eq. (3) was proposed by Glasser [26], and the standard entropy values of the PBILs are given in Table 2. As can be seen the standard entropy values have shown an increasing trend as the length of the ether chain on the imidazolium cation increases. The standard entropy values of the PBILs are in the range of 1060–1705 J·mol−1·K− 1, which are much higher than some imidazolium-based ILs, e.g., standard entropy value of [Mim]Ac and [emim][L-lactate] is 303.2 J·mol−1·K−1 [27] and 392.6 J·mol−1·K−1 [25], respectively. Lattice energy reflects the interaction of the cation and anion, and density is also critically needed for the estimation of lattice energy (UPOT) of an ionic species which then can be utilized to calculate the heat of formation [28]. According to Glasser's theory [26], UPOT, is calculated using the following equation:   −1 1=3 U POT = kJ  mol ¼ 1981:2ðρ=MÞ þ 103:8:

ð4Þ

The lattice energy of PEG-based functionalized ILs calculated by Eq. (4) is also shown in Table 2. The lattice energy values of the PBILs have a decreasing trend with the increase of length of the ether chain. The lattice energy values are 353.7, 334.4, and 316.4 kJ·mol−1, respectively. This value is less than those of series of imidazolium-based ILs [26], which are in the range of 432–470 kJ·mol−1. This indicates that the interaction of cation and anion of PBIL is weaker than that of imidazolium-based ILs, which have strong hydrogen bonding interaction between cation and anion [29]. Moreover, the low lattice energy may explain the low liquid-state temperature of room temperature ionic liquids [30]. 3.2. The properties of surface for PEG-based functionalized ILs The surface tension of the ILs is higher than that of general organic solvents, but lower than that of water, thus, using ILs can accelerate the phase separation process [31–33]. Experimental surface tension results were determined over the temperature range from (293.15 to 333.15) K at atmospheric pressure and are presented in Table 3. The cations have an effect on the surface tension of ILs. The surface tension

Table 2 Molecular volume (Vm), standard entropy (So) and lattice energy (UPOT) of PEG-based functionalized ILs at 298.15 K and atmospheric pressure.

Fig. 1. Plot of experimental values of lnρ against T for PEG-based functionalized ILs.

IL

Vm/nm3

S°(298)/(J·mol−1·K−1)

UPOT/(kJ·mol−1)

[M-PEG350Mim] [Tosyl] [M-PEG500Mim] [Tosyl] [M-PEG700Mim] [Tosyl]

0.8275 1.0527 1.3435

1060.9 1341.6 1704.2

353.7 334.4 316.4

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Table 3 Surface tensions of PEG-based functionalized ILs in the temperature range from 293.15 K to 333.15 K under atmospheric pressure. Ionic liquids

[M-PEG350Mim] [Tosyl] [M-PEG500Mim] [Tosyl] [M-PEG700Mim] [Tosyl]

γ/(mN·m−1) 293.15 K

303.15 K

308.15 K

313.15 K

323.15 K

333.15 K

38.95 – –

38.07 40.42 –

37.62 39.97 40.58

37.06 39.46 40.10

36.25 38.49 39.17

35.21 37.57 38.10

increases with an increase of ether chain length of the cation at the same temperature. The surface tension of the synthesized PBILs in this work are comparable with that of the [bmim][L-lactate] [34], which is in the range of 40.56–43.14 mN·m−1. The linear variation of surface tensions with temperature for pure PBILs is shown in Fig. 2. An empirical equation of surface tension was obtained.   −1 ¼ a−bT=K γ= mN  m

ð5Þ

where γ is the surface tension, a and b are fitting coefficients, and T is the absolute temperature. The corresponding fitting equation of [MPEG350Mim][Tosyl], [M-PEG500Mim][Tosyl], and [M-PEG700Mim] [Tosyl] was as follows:   −1 ¼ 66:2742−0:09310T=K γ= mN  m   −1 ¼ 69:4223−0:09566T=K γ= mN  m   −1 ¼ 70:9749−0:09858T=K: γ= mN  m The correlation coefficient is 0.9992, 0.9999, and 0.9994 for [MPEG350Mim][Tosyl], [M-PEG500Mim][Tosyl], and [M-PEG700Mim][Tosyl], respectively. The surface entropy and surface enthalpy can be derived from the experimental surface tension data. The surface entropy is obtained from the slopes Ss = b = −(∂γ/∂T)P of the temperature plots of the surface tension. The surface enthalpies Es = a = γ − T(∂γ/∂T)P are constant in the temperature range of (293.15 to 333.15) K. The surface entropy and the surface enthalpy value for the PBILs are shown in Table 4. It is shown that the surface entropy value of all the PBILs are lower than 0.1 mJ·K·m− 2, which is lower than that of [emim][L-lactate] (0.1065 mJ·K·m− 2) [25]. The increase of surface entropy of ionic liquids is in line with the growth of the cationic ether. And there is a same trend for the surface enthalpy value of the PBILs. The surface enthalpy value of [M-PEG350Mim][Tosyl], [M-PEG500Mim][Tosyl], and [M-PEG700Mim][Tosyl] are 66.27, 69.42, and 70.97 mJ·m− 2,

respectively. These values are also smaller than those of [emim] [ L-lactate] (83.46 mJ·m − 2 ) [25], which also illuminates the interaction of cation and anion of imidazolium-based ILs is stronger than that of [M-PEGMim][Tosyl] since the surface enthalpy is dependent on interaction energy (lattice energy) between ions [35]. 3.3. Estimation of critical temperatures and vaporization enthalpies Critical temperatures (Tc) of ionic liquids is one of the most relevant thermophysical properties since it can be used in many corresponding state correlations for equilibrium and transport properties of fluids [36]. However, it is hard to acquire reliable data of Tc due to the intrinsic nature of ILs. Guggenheim [32] empirical equation was usually used to predict the critical temperature value by many research groups [36,37]. The empirical equation was described as follows:   T 11=9 γ ¼ K 1− Tc

ð6Þ

where γ is the surface tension, Tc is the critical temperature, and K is an empirical constant. The equation reflects the fact that γ becomes null at the critical point [36,37]. The estimated critical temperatures of PBILs are shown in Table 4. Rebelo et al. [38] proposed an expression of estimating the normal boiling point temperature, Tb, in terms of critical temperature, Tc, which is Tb = 0.6Tc. The normal boiling point temperature value estimated from Guggenheim and expression is 480 K, 490 K, and 485 K for [M-PEG350Mim][Tosyl], [M-PEG 500 Mim][Tosyl], and [MPEG700Mim][Tosyl], respectively. Low vapor pressure is one of the most important properties of ILs, and can be studied by the enthalpy of vaporization. The measurable vapor pressures in a temperature interval are large enough for a proper evaluation of thermodynamic parameters such as enthalpy of vaporization. Zaitsau et al. [39] proposed an empirical equation of estimating the enthalpy of vaporization,   g ο 2=3 1=3 þB Δl H m ¼ A γV M N

ð7Þ

where N is Avogadro's constant, VM is molar volume, A and B are empirical parameters and their values are 0.01121 and 2.4 kJ·mol−1, respectively. The molar enthalpies of vaporization for ILs in this work estimated from Eq. (7) are found to be 231.6, 288.1, and 344.2 kJ·mol−1 (see Table 4). The enthalpies of vaporization calculated by Zaitsau et al. for [Cnmim][NTf2](n = 2,4,6,8) were in the range of 135.3–150.0 kJ·mol− 1 [39]. This suggests that higher enthalpies of vaporization for PBILs may be due to their large molecular volume.

Table 4 Surface entropies (Ss), surface enthalpies (Es), critical temperatures (Tc) and enthalpy of vaporization (Δgl Hοm) of PEG-based functionalized ILs at 298.15 K and atmospheric pressure. IL

Fig. 2. Plot of experimental values of surface tension (γ) against T for PEG-based functionalized ILs.

Es/(mJ·m−2) Ss/(mJ·K·m−2) Tc/K Δgl Hοm/(kJ·mol−1)

[M-PEG350Mim] [Tosyl] 66.27 [M-PEG500Mim] [Tosyl] 69.42 [M-PEG700Mim] [Tosyl] 70.97

0.09310 0.09566 0.09858

800 816 808

231.6 288.1 344.2

J. Wang et al. / Journal of Molecular Liquids 204 (2015) 39–43

4. Conclusions Three PEG-based functionalized ILs were designed and synthesized. Their density and surface tension have been measured as a function of temperature under atmospheric pressure. The length of the ether chain on the imidazolium cation has an influence on both of density and surface tension values. The density values decrease with increasing length of the ether chain in the temperature range covered in the current study. The values of surface tension reveal a reverse trend. The thermal expansion coefficient of the PBILs was determined using the experimental density results. The molecular volume, standard entropy and lattice energy of the ILs were calculated from the experimental densities at T = 298.15 K. Surface properties of the PBILs including surface entropy and surface enthalpy have been obtained from the surface tension values. The critical temperature and normal boiling point temperature value were carried out by using the Guggenheim empirical equations. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.molliq.2015.01.017. Acknowledgments This work is supported by the National Natural Science Foundation of China (21206030), Natural Science Foundation of Hebei Province (B2012208084), and Hebei Research Center of Pharmaceutical and Chemical Engineering, Hebei University of Science and Technology, china. References [1] M.J. Jin, Y.C. Hou, W.Z. Wu, S.H. Ren, S.D. Tian, L. Xiao, Z.G. Lei, J. Phys. Chem. B 115 (2011) 6585–6591. [2] Y. Shang, H.P. Li, S.J. Zhang, H. Xu, Z.X. Wang, L. Zhang, J.M. Zhang, Chem. Eng. J. 175 (2011) 324–329. [3] S.Y. Hong, J. Im, J. Palgunadi, S.D. Lee, J.S. Lee, H.S. Kim, M. Cheong, K.D. Jung, Energy Environ. Sci. 4 (2011) 1802–1806. [4] C.M. Wang, G.K. Cui, X.Y. Luo, Y.J. Xu, H.R. Li, S. Dai, J. Am. Chem. Soc. 133 (2011) 11916–11919. [5] K. Paduszynski, U. Domanska, Ind. Eng. Chem. Res. 51 (2012) 591–604. [6] S. Lee, Chem. Commun. (2006) 1049–1063.

43

[7] G.K. Cui, C.M. Wang, J.J. Zheng, Y. Guo, X.Y. Luo, H.R. Li, Chem. Commun. 48 (2012) 2633–2635. [8] G.K. Cui, J.J. Zheng, X.Y. Luo, W.J. Lin, F. Ding, H.R. Li, C.M. Wang, Angew. Chem. Int. Ed. 52 (2013) 10620–10624. [9] W.Z. Wu, B.X. Han, H.X. Gao, Z.M. Liu, T. Jiang, J. Huang, Angew. Chem. Int. Ed. 43 (2004) 2415–2417. [10] J. Wang, F. Zhao, Y. Liu, X. Wang, Y. Hu, Fluid Phase Equilib. 305 (2011) 114–120. [11] J. Wang, X. Zhang, Y. Liu, Y. Hu, J. Chem. Eng. Data 56 (2011) 3734–3737. [12] P. Wasserscheid, T. Welton, Ionic Liquids in Synthesis, VCHWiley, Weinheim, Germany, 2002. [13] T. Welton, Chem. Rev. 99 (1999) 2071–2083. [14] J. Dupont, C.S. Consorti, J. Spencer, J. Braz. Chem. Soc. 11 (2000) 337–344. [15] P.D. McCrary, P.A. Beasley, G. Gurau, A. Narita, P.S. Barber, O.A. Cojocaru, R.D. Rogers, New J. Chem. 37 (2013) 2196–2202. [16] H.S. Schrekker, D.O. Silva, M.A. Gelesky, M.P. Stracke, C.M.L. Schrekker, R.S. Goncalves, J. Dupont, J. Braz. Chem. Soc. 19 (2008) 426–433. [17] J.Y. Wang, F.Y. Zhao, R.J. Liu, Y.Q. Hu, J. Chem. Thermodyn. 43 (2011) 47–50. [18] H.C. Jiang, J.Y. Wang, F.Y. Zhao, G.D. Qi, Y.Q. Hu, J. Chem. Thermodyn. 47 (2012) 203–208. [19] E. Quijada-Maldonado, S. van der Boogaart, J.H. Lijbers, G.W. Meindersma, A.B. de Haan, J. Chem. Thermodyn. 51 (2012) 51–58. [20] C.P. Fredlake, J.M. Crosthwaite, D.G. Hert, S.N.V.K. Aki, J.F. Brennecke, J. Chem. Eng. Data 49 (2004) 954–964. [21] S.V. Dzyuba, R.A. Bartseh, ChemPhysChem 3 (2002) 161–166. [22] E.V. Ann, W.M. Reichert, R.P. Swatloski, H.D. Willauer, J.G. Huddleston, R.D. Rogers, ACS Symp. Ser. 818 (2002) 289–308. [23] Y. Zhao, J.Y. Wang, H.C. Jiang, Y.Q. Hu, J. Mol. Liq. 196 (2014) 314–318. [24] J. Jacquemin, A. Husson, A.H. Padua, et al., Green Chem. 8 (2006) 172–180. [25] J.Y. Wang, H.C. Jiang, Y.M. Liu, Y.Q. Hu, J. Chem. Thermodyn. 43 (2011) 800–804. [26] L. Glasser, Thermochim. Acta 421 (2004) 87–93. [27] W. Qian, Y.J. Xu, H.Y. Zhu, C.H. Yu, J. Chem. Thermodyn. 49 (2012) 87–94. [28] C.F. Ye, J.M. Shreeve, J. Phys. Chem. A 111 (2007) 1456–1461. [29] Y.Q. Hu, H.C. Jiang, X.C. Zhang, Z.P. Liu, J. Ren, R.J. Liu, J. Mol. Struc. (Theochem.) 915 (2009) 132–140. [30] J. Tong, M. Hong, W. Guan, J.B. Li, J.Z. Yang, J. Chem. Thermodyn. 38 (2006) 1416–1421. [31] J.G. Huddleston, A.E. Visser, R.D. Rogers, Green Chem. 3 (2001) 156–164. [32] P.A.Z. Suarez, S. Einloft, J.E.L. Dullius, R.F. de-Souza, J. Dupont, J. Chem. Phys. 95 (1998) 1626–1639. [33] G. Law, P.R. Watson, Langmuir 17 (2001) 6138–6141. [34] H.C. Jiang, Y. Zhao, J.Y. Wang, F.Y. Zhao, R.J. Liu, Y.Q. Hu, J. Chem. Thermodyn. 64 (2013) 1–13. [35] J.Z. Yang, Q.G. Zhang, B. Wang, J. Tong, J. Phys. Chem. B 110 (2006) 22521–22524. [36] M.J. Earle, J.M.S.S. Esperanca, M.A. Gilea, J.N. Canongia Lopes, L.P.N. Rebelo, J.W. Magee, K.R. Seddon, J.A. Widegren, Nature 439 (2006) 831–834. [37] E.A. Guggenheim, J. Chem. Phys. 13 (1945) 253–258. [38] L. Rebelo, L. Canongia, J. Esperanca, E. Filipe, J. Phys. Chem. B 109 (2005) 6040–6043. [39] D.H. Zaitsau, G.J. Kabo, A.A. Strechan, Y.U. Paulechka, J. Phys. Chem. A 110 (2006) 7303–7306.