Ionic liquids are the basis of a potential alternative to conventional processes based on aqueous amines currently used for carbon dioxide capture. Mixtures of ionic liquids offer an additional degree of tailoring over the intrinsic tunable properties of "single" ionic liquids, while totally maintaining the ionic liquid character. In this work, two mixtures of mutually miscible ionic liquids have been investigated, namely, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide + 1-ethyl-3-methylimidazolium ethylsulfate and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide + 1-butyl-3-ethylimidazolium ethylsulfate. Their ability to absorb carbon dioxide in the pressure range up to ca. 16 bar, at 298.2 K, was determined at different composition ratios of the mixtures; their thermal phase-transition temperatures and physical properties (density, viscosity, and surface tension) were determined over the entire compositional range, at atmospheric pressure and the same temperature. The absorption data were adequately correlated by the NRTL (Non-Random Two-Liquid) model, whereas different equations were successful in satisfactorily correlating the physical properties of the mixtures and in predicting them from knowledge of the properties of the pure ionic liquid components.

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