5824
J. Phys. Chem. B 2006, 110, 5824
Reply to “Comment On ‘How Polar Are Ionic Liquids? Determination of the Static Dielectric Constant of an Imidazolium-based Ionic Liquid by Microwave Spectroscopy’” C. Wakai, A. Oleinikova, and H. Weinga¨rtner* Physical Chemistry 2, Ruhr-UniVersity of Bochum, D-44780 Bochum, Germany ReceiVed: January 11, 2006; In Final Form: February 2, 2006 In a recent paper1 we have demonstrated that by microwave dielectric spectroscopy one can determine dielectric constants, S, of room-temperature ionic liquids (RTILs). Because the electrical conductivity of RTILs prevents measurements of S by conventional techniques, many workers have resorted to indirect methods. Essentially, these alternative methods use nonionic solvents to establish correlations between S and various spectroscopic, thermodynamic, or kinetic polarity probes. These scales are then used to interpret experiments for RTILs. As a major result of our study,1 the dielectric constants of RTILs are substantially lower than most estimates from alternative methods. Roughly, we found for some popular RTILs S = 10. According to the opinion expressed in most papers, S should fall into the range between 20 and 40. Note that, from a practical point of view, the determination of reliable physicochemical parameters depends crucially on the preparation of samples. With regard to dielectric constant measurements this point has carefully been addressed in ref 1, and it is very unlikely that any contaminations may be responsible for these gross effects. In a comment to our paper2 Baker and Bright point out that S = 10 has already been found in their study of solvatochromic shifts of pyrenes in RTILs3 and in other work.4,5 Inspecting their work, we confirm their comment and regret not having cited their paper. However, we would like to stress that indirect probes have provided a broad band of S values, classifying RTILs from hydrocarbon-like (S ∼2) to strongly polar (S ∼40) with most values at the upper end of this scale. Without knowledge of the actual values of dielectric constants, it is certainly difficult to assess which probe best mimics dielectric behavior. Moreover, the results of two other studies4,5 noted in the comment to yield S ∼10 are not as unambiguous, as it may appear from the arguments given in the comment. First, the authors of the comment draw attention to the work of Gra¨tzel and co-workers.4 This paper has indeed been cited by us (ref 2f in our paper1). However, it may not the best example to prove the reliability of dielectric constant estimates: Graetzel et al. observe for pyrenecarboxaldehyde as an indicator in [emim]+ [Tf2N]- “...features very similar to those observed for n-hexane...which are typical for a solvent of very low S (
