Synergistic Effects and Correlating Polarity Parameters in Binary Mixtures of Ionic Liquids Vijay Beniwal and Anil Kumar* [a] 1. Introduction Chemical transformations in the solution phase cannot be fully understood without having sufficient knowledge of the medium of a reaction/process. [1] This effect or, in other words, the solvent effect on a chemical process can be defined as the sum of solvent–solvent interactions, solvent–solute interac- tions, and solute–solute interactions. One well-established method used to measure these interactions is solvent polari- ty. [2, 3] According to Reichardt, solvent polarity is a complex issue that arises during the solvation of the reactants, activated complex, and products as a consequence of various specific or nonspecific intermolecular interactions between solutes and solvent. [4] Due to the combinatorial nature of these interac- tions, the quantification of “solvent polarity” by a single method is not possible. Reichardt also pointed out that the macroscopic measurement of the dielectric constants alone does not fully reflect the polarity of the solvent. [1] Diverse sol- vent-sensitive references have been assessed to define the po- larity of the solvent. A variety of scales are available to define solvent polarity based on their e.g. polarity scale’s dependence on the specific and nonspecific interactions of the solvent. [2, 5–13] Most of these scales are constructed by measuring a single spectroscopic data set for the probe molecule. The scale that is most commonly used to measure solvent polarity is the electronic transition energy parameter E T (30) scale. [2, 3, 14] However, according to Reichardt, the dye molecule is not capable of interacting specifically and significantly with EPD (electron pair donor) solvents; that is, the Lewis basicity of the solvents is not registered by this probe. [3] To account for the effect of basicity and other interactions on a physical prop- erty, a number of expressions have been developed. [3, 10–12] The polarity of the solvent medium can be studied in terms of E T (30), [3, 14] hydrogen-bond-donor acidity (a), [3, 11] hydrogen- bond-acceptor basicity (b), [10] and dipolarity/polarisability pa- rameter (p*). [15] The E T (30) parameter is generally used in its normalised form; that is, E N T (normalised electronic transition energy). [14] Ionic liquids (ILs) are greener alternatives to volatile organic compounds because of their recyclability, thermal and chemi- cal stability, nontoxicity, and very low vapour pressure. [16, 17] In view of this, ILs are increasingly being used as replacements for organic solvents. [18–21] However, the high viscosities of ionic liquids present constraints during chemical transforma- tions. [22, 23] To overcome this constraint, the addition of a cosol- vent has been applied to ionic liquids, which can result in dras- tic changes in viscosities. [24] These changes are accompanied by variations in solute–solvent interactions. [25] To better under- stand these solute–solvent interactions in ionic liquids, many studies have been undertaken to describe the polarity parame- ters in pure ionic liquids [14, 26, 27] and their binary mixtures. [28–34] Fletcher et al. reported that the polarity of a mixture of ionic liquids is greater than those of pure ionic liquids. [35] This obser- vation was described as hyperpolarity by these authors. An in- crease in E T (30) values for the [Bmim][PF 6 ]–ethanol system (see Table 1 for acronyms) compared with those of the pure com- ponents can be ascribed to the hydrogen-bond-donating abili- ty and dipolarity/polarisability of the system. [36] In an another study, it has been shown that the local solvation environment of the dye molecule, rich in water, in the water–ethanol– [Bmim][PF 6 ] system acted as a principal factor that accounts for the increased E T (30) values in the system when compared with aqueous ethanol or pure ionic liquid. [37] A similar study Understanding how a chemical reaction proceeds in solution requires precise knowledge of solute–solvent interactions. Cer- tain issues involved in ionic liquid binary mixtures are still not clearly understood, including: 1) the effects of hydrogen-bond- acceptor basicity (b) of solvents on the “synergistic effects” found in ionic liquid–alcohol mixtures, 2) the interrelation be- tween the polarity parameters in binary mixtures, and 3) the application of a model for the description of normalised elec- tronic transition energy for all the binary mixtures. Here, a de- tailed analysis of polarity parameters in both hydrophilic and hydrophobic ionic liquids has been carried out. A three-param- eter model developed by Roses et al. [J. Chem. Soc. Perkin Trans. 2 1997, 1341—1348] was found to be applicable to all the binary mixtures, and synergistic effects are generated as a result of high b values of alcohols compared with those of the ionic liquids. A strong correlation was observed in the b values and hydrophobicities of pure ionic liquids, suggesting that b values can play a role in describing synergism. [a] V. Beniwal, Dr. A. Kumar Physical & Materials Chemistry Division CSIR-National Chemical Laboratory Pune 411008 (India) E-mail : a.kumar@ncl.res.in Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201402825. ChemPhysChem 0000, 00,0–0  0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1 & These are not the final page numbers! ÞÞ These are not the final page numbers! ÞÞ Articles DOI: 10.1002/cphc.201402825