Density and Viscosity of Aqueous Mixtures of N‑Methyldiethanolamines (MDEA) and Ionic Liquids R. Yusoff,* M. K. Aroua, Ahmad Shamiri, Afshin Ahmady, N. S. Jusoh, N. F. Asmuni, L. C. Bong, and S. H. Thee Department of Chemical Engineering, Faculty of Engineering, University Malaya, 50603 Kuala Lumpur, Malaysia ABSTRACT: The density and viscosity of aqueous mixtures of N- methyldiethanolamine (MDEA) and the ionic liquids (ILs) 1-n-butyl-3- methylimidazolium tetrafluoroborate ([bmim][BF 4 ]), 1-butyl-3-methylimi- dazolium dicyanamide ([bmim][DCA]), and 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([emim][OTf]) were determined. The measure- ments were carried out at 1 atm pressure and temperatures ranging from (303.15 to 363.15) K. The concentration of MDEA was fixed at (2.0 and 4.0) M, whereas the IL concentration was varied from (0.5 to 2.0) M. Both densities and viscosities were increased with increasing IL concentration. Correlation equations of density and viscosity for pure substances and for MDEA and ILs aqueous mixtures as a function of temperature and concentration of MDEA and ILs were also determined. The linear correlation for density had an excellent accuracy with less than 0.98 % deviation for all aqueous mixtures of MDEA and ILs. Meanwhile, the extended Arrhenius equation for viscosity achieved acceptable precision with less than 30 % of deviation from experimental data except for 2.0 M MDEA and 1.5 M [bmim][DCA] mixtures. ■ INTRODUCTION The emissions of carbon dioxide (CO 2 ) from the combustion of fossil fuels cause more and more environmental problems, especially the greenhouse effect. CO 2 , the primary greenhouse gas, is one of the main causes for the global warming, which results in a rise of the atmospheric temperature. Alkanolamine solutions using, for example, monoethanolamine (MEA), diethanolamine (DEA), N-methyldiethanolamine (MDEA), and 2-amino-2-methyl-1-propanol (AMP) as solvents are widely used to capture CO 2 from gas streams. However, this method has shown some disadvantages such as high energy consumption, serious equipment corrosion, and high amine loss and degradation, which pose environmental problems. Hence, alternative solvents are required, and in this regard, ionic liquids (ILs) show great potential as alternative solvents. IL performance in absorbing CO 2 can be severely improved through incorporating an amine function in the structure of the IL. 1 The unique characteristics of ILs make them interesting as the next generation of solvents for industrial applications. Brennecke and Maginn 2 presented several potential applications of ILs. Gordon, 3 Holbrey and Seddon, 4 Welton, 5 and Zhao et al. 6 reported that ILs are good solvents for a wide range of both organic and inorganic materials, polar and nonpolar. ILs have negligible vapor pressures, which reduces emissions to the environ- ment and working exposure hazards. Other advantageous properties include high electric conductivity, the ability to be recycled, high thermal stability, and nonflammability. A number of investigations by different groups such as Blanchard et al., 7 Anthony et al., 8 Cadena et al., 9 Camper et al., 10 Scovazzo et al., 11 and Ahmady et al. 12 have shown that CO 2 is remarkably soluble in conventional ILs. A wide range of literature is available on the absorption of CO 2 in ILs. 13,14 The most widely studied class of ILs to date is based on alkylimidazolium cation with tetrafluoroborate and hexaflourophosphate. Despite all of these works, in industry to date, conventional amines are still preferable because of their lower price, higher CO 2 absorption capacity, and lower solution viscosity. The use of mixtures of alkanolamines and ILs can be considered as a possible alternative since it combines the advantages of both systems. Generally, all tested room-temperature ILs show a very low CO 2 loading capacity in comparison to amine-based solvents as in Keskin et al. 15 Hence, researchers attempt to determine other alternatives which have the green properties of ILs and the high productivity of amines as well. MDEA is one of the commercial amine solutions usable for CO 2 absorption, and 1-n-butyl-3-methylimidazolium tetrafluor- oborate ([bmim][BF 4 ]) is an IL which is water-soluble. 13 According to Aki et al., 16 the solubility of CO 2 in ILs with anions containing fluoroalkyl groups is high due to a fluoroalkyl group which is “CO 2 philic”. ILs such as 1-butyl-3-methylimidazo- lium dicyanamide ([bmim][DCA]), 1-n-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF 4 ]), and 1-ethyl-3-methylimidazolium tri fluoromethanesulfonate ([emim][OTf]) are also reported to be potential solvents for CO 2 capture. 12,17 In recent works, aqueous solutions of MDEA mixed with different types of ILs were used by Feng et al. 18 and Ahmady et al. 1 to absorb carbon dioxide. Received: June 10, 2012 Accepted: December 30, 2012 Published: January 11, 2013 Article pubs.acs.org/jced © 2013 American Chemical Society 240 dx.doi.org/10.1021/je300628e | J. Chem. Eng. Data 2013, 58, 240-247