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Xu, Hang; Zhang, Dandan

doi:https://doi.org/10.1155/2017/7428097

Journal of Chemistry

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Abstract Introduction Experimental Results and Discussion Conclusions Conflicts of Interest Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2017 | Article ID 7428097 | https://doi.org/10.1155/2017/7428097

Viscosities and Conductivities of [BMIM]Zn(Ac)_xCl_y(; ) Ionic Liquids at Different Temperatures

Hang Xu¹and Dandan Zhang¹

Academic Editor: Elena Gomez

Received19 Mar 2017

Revised21 Apr 2017

Accepted21 May 2017

Published07 Jun 2017

Abstract

Viscosity and conductivity data of ionic liquids were detected at temperature ranging from 323.15 to 353.15 K with an interval of 5 K. The conductivities of different ionic liquids at the same temperature followed the trend [BMIM][ZnAcCl₂] > [BMIM][ZnAc₂Cl] > [BMIM][ZnCl₃] > [BMIM][ZnAc₃]. The viscosities of different ionic liquid abided by the order [BMIM][ZnCl₃] > [BMIM][ZnAcCl₂] > [BMIM][ZnAc₂Cl] > [BMIM][ZnAc₃]. Acetate ion could reduce the viscosity of ionic liquids. The relationship between viscosity/conductivity and temperature obeyed the Arrhenius equation and Vogel-Fulcher-Tammann (VFT) equation very well with above 0.99 correlation coefficients.

1. Introduction

Ionic liquid (IL) is a kind of liquid which is composed of ions under room temperature. Physical properties of ionic liquids are being odorless, being noncombustible, strong electrical conductivity, and very low vapor pressure which can be used as solvent in high vacuum system and reduce the problems of environmental pollution due to volatility [1]. Compared with traditional organic solvents and electrolytes, ionic liquid has a great ability to dissolve a large number of inorganic and organic substances and, sometimes, exhibits a double functional role of solvent and catalyst [2]. Ionic liquid can be used in the fields of chemical reaction solvent and catalytic reaction carrier. For example, Mehnert et al. [3] used [BMIM][PF₆] as chemical solvent for homogeneous hydroformylation catalysis. Bagheri et al. [4] prepared heteropolytungstate-ionic liquid as carrier which was supported on the surface of silica coated magnetite nanoparticles. Wang et al. [5] prepared ZnCl₂-[BMIM]Cl which was used in cocatalyzed coupling reaction of CO₂ with epoxides. Sun and Zhao [6] used [BMIM]Cl/[FeCl₃] ionic liquid as catalyst for alkylation of benzene with 1-octadecene. Liu et al. [7] studied using ionic liquid [BMIM][Ac] as a catalyst for methanolysis of polycarbonate.

In most cases, the physical properties of ionic liquid, especially conductivity and viscosity, have a great influence for chemical reaction process. In this study, four ionic liquids of [BMIM][ZnCl₃], [BMIM][ZnAcCl₂], [BMIM][ZnAc₂Cl], and [BMIM][ZnAc₃] were prepared and those viscosities and conductivities were measured at different temperatures. Arrhenius equation and Vogel-Fulcher-Tammann (VFT) were used to describe the relationship between the conductivity/viscosity and temperature.

2. Experimental

2.1. Materials

1-Butyl-3-methylimidazolium chloride ([BMIM]Cl) and 1-butyl-3-methylimidazolium acetic acid ([BMIM]Ac) ionic liquids were purchased from Linzhou Keneng Material Technology Co., Ltd. Purity grades of [BMIM]Cl and [BMIM]Ac were above 99% and they could be used after drying treatment. Zinc chloride and Zinc acetate were analytically pure and used after drying process.

2.2. Preparation of Ionic Liquids

The ionic liquids had been prepared based on [8]. [BMIM][ZnCl₃] ionic liquid was prepared by mixing 0.3 mol [BMIM]Cl and 0.3 mol ZnCl₂ at 90°C for 24 h. [BMIM][ZnAcCl₂] ionic liquid was prepared by mixing 0.3 mol [BMIM]Ac and 0.3 mol ZnCl₂ at 90°C for 24 h. [BMIM][ZnAc₂Cl] ionic liquid was prepared by mixing 0.3 mol [BMIM]Cl and 0.3 mol ZnAc₂ at 90°C for 24 h. [BMIM][ZnAc₃] ionic liquid was prepared by mixing 0.3 mol [BMIM]Ac and 0.3 mol ZnAc₂ at 90°C for 24 h. All the samples should be dried in vacuum drying oven at 90°C for 24 h.

2.3. Measurements of Conductivity and Viscosity

The conductivities of [BMIM][ZnCl₃], [BMIM][ZnAcCl₂], [BMIM][ZnAc₂Cl], and [BMIM][ZnAc₃] were measured by a Wayne-Kerr 6430B Autobalance Bridge fitted with a Shanghai DJS-1 electrode and the temperature was controlled within ±0.1 K using a HAAKEV26 temperature thermostat (Thermo Electron). In order to keep the ILs dry, the experiment should be carried out in a dry nitrogen atmosphere in the temperature range 323.15 to 353.15 K with an interval of 5 K. The viscosities of [BMIM][ZnCl₃], [BMIM][ZnAcCl₂], [BMIM][ZnAc₂Cl], and [BMIM][ZnAc₃] were measured using an Ostwald viscometer. We used a thermostatic bath to control the temperature to get a stability of ±0.1 K, which consumed 30 min to attain thermal equilibrium in the viscosity. Moreover, in order to prevent absorbing water from atmosphere, we should put the ILs in a dry nitrogen atmosphere when measuring the viscosity.

The water in the ionic liquid has a great influence on the data of conductivity and viscosity. So, moisture contents of ionic liquids were measured using a 851 Karl-Fischer Moisture Titrator (Metrohm). The results showed that the moisture contents of [BMIM][ZnCl₃], [BMIM][ZnAcCl₂], [BMIM][ZnAc₂Cl], and [BMIM][ZnAc₃] were 69.2 ppm, 83.5 ppm, 98.4 ppm, and 101.2 ppm, respectively. All the ionic liquids exhibited an extremely low moisture content in measuring process.

3. Results and Discussion

3.1. Conductivities of ILs at Different Temperature

The conductivities of [BMIM][ZnAc₃], [BMIM][ZnAc₂Cl], [BMIM][ZnAcCl₂], and [BMIM][ZnCl₃] ionic liquids are shown in Figure 1. From Figure 1, it is clearly seen that conductivities of ionic liquids increase with the rise of temperature. The conductivities of [BMIM][ZnAc₃] are 0.37 ms·cm⁻¹ at 323.15 K and 2.05 ms·cm⁻¹ at 353.15 K, respectively. The conductivities of [BMIM][ZnAc₂Cl] are 2.32 ms·cm⁻¹ at 328.15 K and 4.88 ms·cm⁻¹ at 348.15 K, respectively. The conductivities of [BMIM][ZnAcCl₂] are 3.36 ms·cm⁻¹ at 333.15 K and 4.86 ms·cm⁻¹ at 343.15 K, respectively. The conductivities of [BMIM][ZnCl₃] are 0.74 ms·cm⁻¹ at 323.15 K and 1.86 ms·cm⁻¹ at 338.15 K, respectively. With the rise of temperature, conductivities of ionic liquids have a great increase because the solvation degree and ion radius of ionic liquid are reduced. The ion motion is accelerated by the increase of temperature, which can also cause the improvement of the conductivity. Furthermore, the increase of temperature leads to the decrease of the solution viscosity and ions are easier to move. From Figure 1, the conductivities of different ionic liquids at the same temperature followed the trend [BMIM][ZnAcCl₂] > [BMIM][ZnAc₂Cl] > [BMIM][ZnCl₃] > [BMIM][ZnAc₃]. For example, in 338.15 K, the conductivities of [BMIM][ZnAc₃], [BMIM][ZnAc₂Cl], [BMIM][ZnAcCl₂], and [BMIM][ZnCl₃] are 1.08 ms·cm⁻¹, 3.65 ms·cm⁻¹, 4.08 ms·cm⁻¹, and 1.86 ms·cm⁻¹, respectively. Compared with [ZnAc₃]^-, [ZnCl₃]^- has the features of smaller ionic radius and lower mass which are advantage for ionic transportation. For another aspect, organic ion exhibits a weak ionization which is also bad for conductivity. So, conductivity of [ZnAcCl₂]^- is better than [ZnAc₂Cl]^-. Contrary to [ZnCl₃]^-, the symmetry of [ZnAc₂Cl]^- is low which is good for the ionic movement. The higher the symmetry in anion of ionic liquids, the lower the conductivity. The more the chlorine ion and the less the acetic acid, the greater the conductivity.

The relationship between conductivities of ionic liquid and temperature followed Arrhenius equation [9], which is described as

Here, is the conductivity and is the empirical constant. notes for the activation energy. is the temperature and is 8.314 J·mol⁻¹·K⁻¹. According to the Arrhenius equation and experimental data, the relationship between conductivity and temperature is shown in Figure 2 and the calculated values of , , and (correlation coefficient) are listed in Table 1. From Table 1, correlation coefficients () of four ionic liquids are above 0.99. The conductivities of ionic liquids at different temperatures follow Arrhenius relationship very well. The [BMIM]Zn(Ac)₃ ionic liquid has the maximal and compared to the other three ionic liquids. The values and values follow the order of [BMIM]Zn(Ac)₃ > [BMIM]ZnCl₃ > [BMIM]Zn(Ac)₂Cl > [BMIM]Zn(Ac)Cl₂.

According to [10], the conductivity of ionic liquids also obeyed Vogel-Fulcher-Tammann (VFT) equation which is shown in

Here, , , and are the empirical constants. According to the VFT equation, the relationship between conductivity and temperature is shown in Figure 3 and , , , and are calculated and listed in Table 1. From Table 1, it is clearly seen that all the values are above 0.99 which means VFT equation is better to describe the relationship between conductivity and temperature than Arrhenius equation. The biggest and values are [BMIM]ZnCl₃ ionic liquid with 214.0 mS·cm⁻¹ and 441.7 K, respectively, compared to the other three ionic liquids. The smallest is [BMIM]ZnCl₃ ionic liquid with 245 K and the biggest is [BMIM]Zn(Ac)₂Cl with 283 K compared to others.

3.2. Viscosity of ILs at Different Temperature

The viscosities of [BMIM][ZnAc₃], [BMIM][ZnAc₂Cl], [BMIM][ZnAcCl₂], and [BMIM][ZnCl₃] ionic liquids are shown in Figure 4. From Figure 4, it is clearly seen that viscosities of ionic liquids express a decrease in tendency with the rise of temperature. The reason is that high temperature is a disadvantage for the intermolecular association. In low temperature, high intermolecular association can improve the viscosity of ionic liquid. For example, the viscosities of [BMIM][ZnAc₃] are 33 mPa·s at 323.15 K and 6 mPa·s at 353.15 K, respectively. The viscosities of [BMIM][ZnAc₂Cl] are 33 mPa·s at 328.15 K and 18 mPa·s at 348.15 K. The viscosities of [BMIM][ZnAcCl₂] are 58 mPa·s at 323.15 K and 22 mPa·s at 343.15 K. The viscosities of [BMIM][ZnCl₃] are 241 mPa·s at 323.15 K and 99 mPa·s at 338.15 K. The viscosities of [BMIM][ZnCl₃] are significantly higher than other ionic liquids. The viscosity order of four ionic liquids followed the tendency [BMIM][ZnCl₃] > [BMIM][ZnAcCl₂] > [BMIM][ZnAc₂Cl] > [BMIM][ZnAc₃]. Acetate ion can reduce the viscosity of ionic liquids. On the contrary, chloride ion can increase the viscosity of the ionic liquids.

According to [11], the Arrhenius equation exhibited in (3) can be used to describe the relationship between viscosity and temperature.

Here, is the empirical constant and denotes the activation energy for viscous flow. Figure 5 shows the relationship between viscosity of the logarithm and reciprocal of absolute temperature. From Figure 5, and is linear correlation. The fitting result is shown in Table 2. From Table 2, the correlation coefficients are greater than 0.99 which indicated that Arrhenius formula is good enough to describe the relationship between viscosity and temperature of ionic liquids. The values of [BMIM]Zn(Ac)₃ and [BMIM]ZnCl₃ are 52.51 kJ·mol⁻¹ and 49.48 kJ·mol⁻¹, respectively. The values of [BMIM] Zn(Ac)₂Cl and [BMIM]Zn(Ac)Cl₂ are 34.73 kJ·mol⁻¹ and 38.66 kJ·mol⁻¹, respectively.

The viscosity values of four ionic liquids are also fitted by the VFT equation which is shown in

Here, , , are the empirical constants. According to the VFT equation, relationship between dynamic viscosity and temperature is shown in Figure 6. From Figure 6, viscosity of ionic liquids at different temperature followed VFT equation very well. The fitting data was shown in Table 2. From Table 2, the correlation coefficients which are calculated by VFT equation are superior to Arrhenius model.

4. Conclusions

Four ionic liquids of [BMIM][ZnCl₃], [BMIM][ZnAcCl₂], [BMIM][ZnAc₂Cl], and [BMIM][ZnAc₃] were prepared and those viscosity and conductivity data of ILs were detected at temperature ranging from 323.15 to 353.15 K with an interval of 5 K. The conductivity order at the same temperature followed the trend: [BMIM][ZnAcCl₂] > [BMIM][ZnAc₂Cl] > [BMIM][ZnCl₃] > [BMIM][ZnAc₃]. The higher the symmetry in anion of ionic liquid, the lower the conductivity. The more the chlorine ion and the less the acetic acid, the greater the conductivity. The viscosity order of four ionic liquids is as follows: [BMIM][ZnCl₃] > [BMIM][ZnAcCl₂] > [BMIM][ZnAc₂Cl] > [BMIM][ZnAc₃]. Acetate ion could reduce the viscosity of ionic liquids. On the contrary, chloride ion could increase the viscosity of the ionic liquids. The relationship between viscosity/conductivity and temperature obeyed the Arrhenius equation and Vogel-Fulcher-Tammann (VFT) equation very well with above 0.99 correlation coefficients. According to the values, VFT equation was superior to Arrhenius model.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This work is supported by the National Natural Science Foundation of China (no. 21006057), Nature Science Foundation of Henan Province (no. 162300410083), and Youth Teacher Support Program of Henan Provincial University (no. 2014GGJS-055).

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Copyright

Copyright © 2017 Hang Xu and Dandan Zhang. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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