Published: February 16, 2011 r2011 American Chemical Society 1494 dx.doi.org/10.1021/je101184s | J. Chem. Eng. Data 2011, 56, 1494–1499 ARTICLE pubs.acs.org/jced Static Relative Dielectric Permittivities of Ionic Liquids at 25 °C Mian-Mian Huang, † Yanping Jiang, † Padmanabhan Sasisanker, †,§ Gordon W. Driver, ‡ and Hermann Weing € artner* ,† † Department of Physical Chemistry II, Faculty of Chemistry and Biochemistry, Ruhr-University Bochum, D-44780 Bochum, Germany ‡ Department of Chemical Engineering, Laboratory of Analytical Chemistry, Åbo Akademi University, Biskopsgatan 8, Turku/Åbo Finland ABSTRACT: For understanding solvation by ionic liquids, it is mandatory to characterize their static relative dielectric permittivities ε (“static dielectric constants”). Exploiting the definition of ε in terms of the zero-frequency limit of the frequency-dependent dielectric dispersion curve, the static dielectric constant of an electrically conducting liquid can be extrapolated from dielectric relaxation spectra in the microwave regime. On the basis of this method, we report dielectric constants of 42 ionic liquids at 25 °C. ’ INTRODUCTION The static relative dielectric permittivity ε of a solvent, 1 usually called “static dielectric constant”, is a key property for under- standing its solvation capability and forms an important input parameter of many modelings of solvation processes. The past decade has seen the advent of ionic liquids (ILs) 2 —organic salts which are liquid at or near room temperature—as innovative solvents which can revolutionize chemical methodologies. For assessing solvation by ILs it is mandatory to characterize and understand their static dielectric constants. 3-5 The concept of the static dielectric constant of an electrically conducting liquid is subtle. Capacitance methods for measuring ε fail because the sample cell is short-circuited by the electrical conductance of the sample. In contrast to a widely held opinion 6 the static dielectric constant of a conducting liquid is by no means ill-defined or infinite. Dielectric theory defines ε as the zero-frequency limit (ν f 0) of the dielectric dispersion curve ε 0 (ν), which forms the real part of the frequency-dependent complex dielectric permittivity ε*(ν). 1,7 For ILs of low viscosity, up to 500 mPa 3 s (500 cP) say, the relevant part of ε 0 (ν) falls into the MHz/GHz regime covered by microwave di- electric relaxation spectroscopy (DRS). 8-10 For a long time this method has been used for determining static dielectric constants of highly conducting electrolyte solutions. 11 Because of the huge number of possible cation-anion combinations, 12 it is impossible to determine the static dielectric constants of even a small fraction of the ILs of interest. The present study aims at establishing a representative database, which elucidates the most important trends imposed by cation and anion variation. For this purpose we have measured the static dielectric constants of 42 ILs at 25 °C. Because progress in understanding dielectric processes in ILs suggests a modi fication of the dielectric model used in our data evaluation, 13,14 we also revise some data published earlier. ’ METHODS Materials. Table 1 documents sources of the ILs used. 1-H-3- methylimidazolium chloride (1) and the corresponding bromide (2) were synthesized according to a modified literature proce- dure, 15 where single equivalents of HCl or HBr were employed to yield 1:1 salts. The synthesis of some ILs followed literature procedures specified in Table 1: ILs comprising the bis- (trifluoromethylsulfonyl)imide anion ([NTf 2 ] - ), 16 1-alkyl- 2,3,4,5-tetramethylimidazolium salts, 17 ethylammonium nitrate, 8 and OH-functionalized ILs. 18 All other salts were purchased from Iolitec (Heilbronn, Germany) or Solvent-Innovation (Cologne, Germany), as marked in Table 1. Most ILs were dried up to 48 h at 60 °C and 1 mbar to achieve a weight fraction of water of less than 10 -4 , as determined by Karl Fischer titration. The Cl - content, which is of concern in many applications, is uncritical in our experiments up to at least a weight fraction of 10 -3 because Cl - is dielectrically inactive and affects dielectric properties only indirectly via its effect on the IL structure. Hydrolysis of [PF 6 ] - and [BF 4 ] - ions 19 and decomposition of ammonium-based protic ILs into amines and acids 20 were of special concern to us. We dried the relevant ILs for up to one week at 25 °C and 1 mbar and surveyed decomposition by 1 H and 13 C NMR spectroscopy. Because drying at elevated tem- peratures enhanced decomposition, impurities and water were tolerated in these cases up to weight fractions of 10 -3 each. Measurements of aqueous mixtures of the formates (32) and (37) over the complete composition range 21 show that at high IL concentrations, for example at weight fractions above 0.75, the static dielectric constant is rather insensitive to the water content. Dielectric Relaxation Spectroscopy. Our coaxial method 9 is able to probe the complex dielectric function ε*(ν) in the frequency range 0.3 MHz e ν e 20 GHz. The method requires three-point calibration at each frequency, made with an open circuit, a short circuit, and water or benzonitrile 22 as a calibration Special Issue: John M. Prausnitz Festschrift Received: November 3, 2010 Accepted: January 27, 2011