Thermophysical properties for the binary mixtures of 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [hmim][Tf2N] + N-methyldiethanolamine (MDEA) at temperatures (303.15 to 323.15) K
Highlights
► Densities, viscosities and refractive indices for [hmim][Tf2N] + MDEA are measured. ► Excess properties, thermal expansion coefficients and deviations are deduced from the experimental data. ► Redlich–Kister type of equations are used to correlate the excess properties.
Introduction
Ionic liquids (ILs) are organic salts, having a bulky asymmetrical organic cation and an inorganic or organic anion. The most common cations are pyridinium, quaternary ammonium, tetraalkylphosphonium, pyrrolidinium and imidazolium. The anions can be nitrate, halide, acetate, trifluoroacetate, tetrafluoroborate, triflate, alkylsulfate, bis(trifluoromethylsulfonyl)imide and the others [1]. ILs possess some unique properties like low melting point, negligible vapour pressure, non-volatility, non-flammability, stability at high temperature and good solvency power for both organic and inorganic substances (polar and non-polar) [2]. ILs have wide commercial applications namely as lubricants, as catalysts, as electrolytes, as solvents for reactions, in azeotropic and extractive distillations and absorption media for gases [3]. Traditionally aqueous amine solutions are used for CO2 removal. Aqueous amine solutions react rapidly with CO2 to form carbamate. It is an energy intensive process but still widely practiced due to its effectiveness. In United States 95% of gas sweetening operations are based on amine scrubbing process. However amine solutions have serious environmental concerns like high volatility, corrosiveness and degradation during regeneration [4]. Due to these disadvantages now the researchers are focusing on the development of energy efficient and environment friendly solvents for CO2 removal. Recent studies indicated that ILs could be an effective alternative to conventional solvents used for CO2 capture [5], [6], [7]. But due to their high cost of synthesis and high viscosity, the commercial application of ILs is not very attractive. By incorporating the advantages of both ILs as well as amines for the development of hybrid solvent for efficient capture of CO2 has drawn great attention [8]. Hybrid solvents are meant to take advantage of good attributes of parent solutions and to minimize the drawbacks associated with these. So, high priced and high viscous IL could be mixed with less priced and less viscous amine solution. By mixing these targeted ILs with different amines, a large number of possible and workable binary mixtures could be prepared. Such mixtures are expected to encompass environment friendly characteristics of ILs coupled with the reactivity of amines. Camper et al. [4] showed that ILs and amines, when combined together gave a better performance than amine functionalized ILs and CO2 capturing occurred rapidly and could be readily reversed. Similarly Feng and co-workers [9] prepared binary mixtures of four different ILs with N-methyldiethanolamine (MDEA) for CO2 capture, and found that the presence of IL in the mixture enhanced the solubility. Knowledge of physical properties is essential for the design and scaling up of process equipments required for the absorption and scrubbing operations [3]. Even though the physical properties of hydroxylammonium based ILs with monoethanolamine (MEA) are available [8], the physical properties data for the binary mixtures of imidazolium based IL with amines are not readily available. It is noted that among the available ILs, imidazolium based ILs have the higher capacity for CO2 absorption [5]. For the case of amines, MDEA is prominent among others due to its unique properties like less corrosive behaviour, resistance to chemical and thermal degradation and low vapor pressure [10]. Hence in the present work it is proposed to mix 1-hexyl-3-methylimidazolium bis(trifluromethylsulfonyl)imide [hmim][Tf2N] with N-methyldiethanolamine (MDEA) at different mole fractions to form binary mixtures, which could be further used for CO2 capture. For the purpose of characterization of the developed new solvent, the physical properties namely density ρ, dynamic viscosity η, and refractive index nD were measured. Based on the experimental data excess molar volumes VE, excess refractive indices nDE, coefficients of thermal expansions α and excess coefficients of thermal expansion αE were calculated. Viscosity values of binary mixtures were correlated by McAllister three body interaction model.
Section snippets
Materials
Ionic liquid, 1-hexyl-3-methylimidazolium bis(trifluromethylsulfonyl)imide [hmim][Tf2N] (having 99% purity) and N-methyldiethanolamine (MDEA) having 99.5% purity were purchased from Merck.
Purity of the chemicals was rechecked by chromatographic methods. Ionic liquid was analyzed by using HPLC (Agilent-1100) with C-18 column and methanol as solvent in accordance with the reported methods [11]. The ionic liquid was found to be 98.9% pure (assay, HPLC, area %). It was dried under vacuum at 80 °C
Results and discussions
Physical properties namely densities ρ, refractive indices nD, and dynamic viscosities η, for the binary mixtures of 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [hmim][Tf2N] with N-methyldiethanolamine (MDEA) were measured over a temperature range of (303.15 to 323.15) K. Excess molar volumes VE, excess refractive indices nDE, coefficients of thermal expansions α and excess coefficients of thermal expansion αE, were calculated from the experimental data. Table 1 presents the
Conclusions
The physical properties namely densities ρ, dynamic viscosities η and refractive indices nD, for the binary system 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [hmim][Tf2N] with N-methyldiethanolamine (MDEA) were measured and reported. In mixtures the density values increased with increase in mole fraction of [hmim][Tf2N], whereas refractive index values increased with a decrease in the mole fraction of [hmim][Tf2N]. For the present binary mixture, the viscosity values
Acknowledgements
The authors are thankful to the Ministry of Science, Technology and Innovation (MOSTI), Malaysia (e Science Fund 03-02-02-SF004) and Universiti Teknologi PETRONAS for financial support.
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