The polarity of the ionic liquids [N 4111 ][Tf 2 N], [N 4441 ][Tf 2 N], and [choline][Tf 2 N], as well as their binary mixtures with ethanol has been investigated using solvatochromic dyes and expressed in terms of the Reichardt parameter, E T N , and the Kamlet−Taft parameters (π*, α, and β), at 298.15 K. The synergetic behavior revealed essentially by the ethanol + [N 4441 ][Tf 2 N] system for the acidity of the solvent shows the potential importance of this solvent media for some chemical applications. Even though the preferential solvation model of Bosch and co-workers has been applied with some statistical success to the whole composition range of EtOH + [N 4111 ]-[Tf 2 N] and EtOH + [N 4441 ][Tf 2 N] binary mixtures, the interpretation of the results is somehow difficult due to the competition of molecular interactions in the evaluation of transition energies for the pairs: dye−solvent (dye−cation and/or dye−anion, dye− EtOH, dye−complex entity) and cation−anion, depending on the composition range. Different patterns of solvent polarity behavior are shown along the entire ionic liquid (IL) composition. ■ INTRODUCTIONAlthough molecular solvents have been the usual media for physical and chemical processes in solution, 1 in the past decade, room temperature ionic liquids (RTILs) have shown to be very useful because of their very particular properties, such as nonflammability, thermal stability, very low volatility at ambient conditions, and high solubility in organic and inorganic compounds with a wide range of polarity. These characteristics make ionic liquids (ILs) promising compounds for use in industry and with a recent emphasis in the pharmaceutical industry. 2−8 When compared with molecular solvents, ILs often present higher dipolarity/polarizability characteristics which make them valuable "effective polarity" solvents. 9−11 In this perspective it will be crucial to obtain information regarding the relative polarity of ILs and to study the change in physicochemical properties produced by their addition to molecular solvents. It has been already identified that, when an IL is mixed with an other solvent such as water or an alcohol, the polarity properties of the system can be tunable depending on the combination of the cation and anion and on the careful choice of the cosolvent. 12−17 The solvent polarity is defined as "its overall solvation capability" depending on the action of all possible, specific (hydrogen bonds, electron pair donor/electron pair acceptor), and nonspecific (van-der-Waals and Coulombic interactions) intermolecular solute−solvent forces. 1,18 The inadequate way of characterizing the solvent polarity, merely based on macroscopic physical parameters, gave rise to the appearance of solvatochromic parameters. These parameters are based on molecular probes, the spectroscopic properties of which are strongly solvent-dependent and serve as appropriate model processes for the study of other solvent effects. 19−24 The purpose of this work is to measure the polarity of the ILs, butyl-trim...

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