Ionic liquid 의 합성과 물성 측정

신배근, 김기섭, Demberel Nymba, 이흔

한국과학기술원 화학공학과

Synthesis and Properties Measurement of the Ionic Liquids

Bae Kun Shin, Ki Sub Kim,  Demberel Nymba, Heun Lee

Department of Chemical Engineering, KAIST

Introduction

Ionic liquids have received growing attention in the aspect of energy and environment problems.

Room temperature ionic liquids typically consist of a heterocyclic cation based on substituted imidazole or pyridine and an organic anion such as[eqn: [AlCl_4 ]^- ,~[BF_4 ]^- ,~or [PF_6 ]^-] etc. Ionic liquids of this type have many benefits; They have liquid property at room temperature, low viscosity, little vapor pressure, high thermal and chemical stability and are non-corrosive.[1] Because of these properties, ionic liquids are studied in various areas such as the replacement solvents of VOC and the electrolytes of batteries and fuel cells. Also, ionic liquids are applied to environmentally-friendly alternatives to organic solvents for liquid/liquid extractions and catalysis.

In spite of the important characters of the ionic liquids, physical and  thermodynamic properties are hardly presented on the literature. In this study, three ionic liquids, [eqn: BMImBr,~BMImI,~BMImBF_4], were synthesized and their basic properties such as density, viscosity and refractive index were measured. These properties are essential to apply ionic liquids to various area.

Experimental

1.Materials

Reactants for the synthesis of the ionic liquids were supplied from Aldrich-Sigma chemical Co and was used without further purification. The  solution was prepared using triple-distilled water.

2. Apparatus and procedure

The literature method for synthesis of ionic liquids is the reaction of N-alkylimidazole, RIm, ith alkyl salts, R'X in appropriate organic solvents

[eqn: RIm+R'X→RR'Im^+ X^-]

where [eqn: X^-] is the anion of interest. The problem of this reaction is that in some cases the liquid and solid salts in the reaction product are miscible and difficult to be completely separated because they are involatile. The choice of cation M is important in such a case to make a pure ionic liquid [2]

The densities, viscosity, and refractive indices of the [eqn: BMImBr,~BMImI,~BMImBF_4] were measured. Densities of ionic liquids were measured using pycnometers. A water bath was used to maintain the temperature of the sample solution within a pycnometer. The temperature can be controlled within ±0.1K. The temperature was measured by mercury-filled glass thermometer. The mass of the ionic liquids was measured by balance with a precision of 0.0001g

Viscosities of ionic liquids were measured by appropriate Ubbelohde type viscometers (Witeg) of various capillary sizes. The temperature of solution  was maintained by the water bath in which controlled to within ±0.1K. Electronic stopwatch was used to measure the efflux times of liquid solutions with an accuracy of 0.01s. The viscosities were calculated by following equation.

[eqn: □=tK□]

where K is kinematic viscosity and t is efflux time and □ is density. K and □ is the given data and t must be measured. The efflux time of each data set was measured at least three times and averaged.

Refractive indices were measured by a precision Abbe refractometer 3T (American Optical Co.) An external bath circulator was used to control the experimental temperature within ±0.1 K. Every measurement was performed three times

Results and Discussion

1. Density

For the check of reliability of our experiment, the density of pure Aceton was measured at 297.15K.  Density of pure Aceton was good agreement with literature data. The density values of Ionic liquids([eqn: BMImBr,~BMImI,~BMImBF_4])were measured over the temperature ranging from 297.15 to 323.15K.  Each data set was repeated three times and averaged. In case of [eqn: BMImBF_4], The deviation between the experimental and the calculated values was 0.21%

Figure 1 shows densities of pure ionic liquids. As the temperature increased, the densities of pure ionic liquids decreased linearly. In Figure 1, [eqn: BMImI] has the highest value of three ionic liquids and has the lowest value.

2. Viscosity

For the check of reliability of our experiment, the viscosity of pure Aceton was measured at 297.15K. Viscosity of pure Aceton was good agreement with literature data.  The viscosity values of Ionic liquids will be measured ov[eqn: BMImBF_4]er the temperature ranging from 297.15 to 323.15K . The viscosity of pure ionic liquids sharply went down as the temperature went up.

3. Refractive index

The measured refractive indices of ionic liquids were presented in Figure 2.. The measured data of Ionic liquids([eqn: BMImBr,~BMImI,~BMImBF_4])were measured over the temperature ranging from 297.15 to 323.15K The refractive index decreased a little as the temperature increased. A descending series is

Reference

[1]  J.D. Holbrey, K.R. Seddon, Ionic Liquid, Clean Products and Processes 1999, 223-236

[2] Rika Hagiwara, Yasuhiko Ito. Room temperature ionic liquids of alkylimidazolium cations and fluoroanions. J. Fluorine chemistry 2000, 105, 221-227

Figure 1. Densities of Ionic liquids

Figure 2. Refractive Index of Ionic liquids