Densities and Viscosities of Mixtures of Two Ionic Liquids Containing a Common Cation

Density and dynamic viscosity data of binary mixtures of ionic liquids (ILs) were determined in this work, at temperatures from 283.15 to 363.15 K and at 0.1 MPa. The mixtures of two ILs comprise a common cation and different anions, combining 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide with eight other ionic liquids, namely, 1-butyl-3-methylimidazolium thiocyanate, 1-butyl-3methylimidazolium dicyanamide, 1-butyl-3-methylimidazolium tricyanomethane, 1-butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium hexafluorophosphate, 1-butyl-3methylimidazolium acetate, 1-butyl-3-methylimidazolium trifluoromethanesulfonate, and 1-butyl-3-methylimidazolium dimethylphosphate. Five mole fractions (0.00, 0.25, 0.50, 0.75, 1.00) of each mixture were prepared and characterized in terms of density and dynamic viscosity. The temperature dependence of density was described using a linear model, while the Vogel−Tammann−Fulcher equation was used to describe the temperature dependence of viscosity. Ideal mixing rules were used to predict the molar volume and viscosity and to infer on the mixtures ideal/nonideal behavior. For the mixtures of ILs investigated almost null or small deviations were observed in the molar volumes, meaning that their mixing is remarkably close to linear ideal behavior when molar volumes of mixtures are considered. For viscosity, larger deviations were observed for some particular systems; yet, and in general, mixtures of ILs do not deviate in a significant extent from ideal behavior. Therefore, ideal mixture models can be used to predict the physical properties of mixtures of ILs and to a priori design mixtures with specific features. ■ INTRODUCTION Ionic liquids (ILs), known as salts with a melting temperature below a conventional temperature of 100 °C, have been largely explored in the past few years and are at last start reaching their place in industry. Ionic liquids are typically composed of an organic cation and an organic or inorganic anion, where a large number of potential fluids can be synthesized by simple chemical structural rearrangements either in the cation or in the anion. In an ideal situation, the combination of different ions allocates the tailoring of their properties and characteristics and allows them to be task specific fluids for particular applications. The ionic nature and the large array of cation−anion combinations of ILs are the main characteristics responsible for some of their outstanding properties, namely, a negligible vapor pressure, a high ionic conductivity, nonflammability, high thermal and chemical stabilities, and an enhanced solvation ability for a large variety of compounds. Due to the great interest in ILs from fundamental and applied point of views, and the wide number of ILs that is possible to obtain by the simple combination of the available cations and anions, the study of thermophysical properties of ILs mixtures is an important task given that the possibility of finding tailored fluids with target properties is largely increased. Recently, Niedermeyer et al. reported a critical review on the use of mixtures of two and three ILs as a way of further extending the ability to design ILs with tailored properties. The authors proposed a nomenclature for such mixtures, and here adopted, where mixtures of two ILs, [A][X] and [A][Y], bearing a common cation [A], is abbreviated to [A][X]x[Y](1−x), whereas for the [A][X] + [B][X] mixtures, bearing a common anion [X]−, it is abbreviated to [A]x[B](1−x), [X], where x and (1 − x) are the mole fraction of each IL. Within IL mixtures, Canongia Lopes et al. provided a pioneering work on their excess molar volumes (V), namely for the following mixtures: [C4C1im][PF6]x[NTf2](1−x), [C4C1im][BF4]x[NTf2](1−x), and [C4C1im][BF4]x[PF6](1−x). All of these mixtures were found to exhibit Special Issue: In Honor of Kenneth R. Hall Received: February 29, 2016 Accepted: July 4, 2016 Published: July 14, 2016 Article