Ionic liquids (ILs) have captured the imagination of a large and steadily growing community of scientists due to their applications as reaction media, [1,2] in batteries and supercapacitors, [3] in solar and fuel cells, [4] for electrochemical deposition of metals and semiconductors, [5] for protein extraction and crystallization, [6] in nanoscience, [7] in physical chemistry, [8] and many others. By choosing different combinations of ions, or by modifying the chemical structures of the constituent ions, the physical properties of an IL can be significantly altered. However, the number of possible modifications is huge, and one can envisage an enormous number of salts that have the potential to form ILs-some say as many as 10 12 to 10 18 . [9, 10] Since the vast majority of these have yet to be synthesized, it is imperative to develop methods to predict the physical properties of unknown ILs in order to facilitate the design of new materials and reduce the need for time-consuming trial-and-error syntheses. The "Holy Grail" is the full characterization of an unknown IL prior to its laboratory synthesis.Previous attempts to make quantitative predictions of the physical properties of ILs by using quantitative structureproperty relationships (QSAR), molecular mechanics (MM) simulations, as well as modifying older ideas, such as the concept of "hole theory" or the "Parachor", have had some success. [10][11][12] However, these methods all have significant drawbacks which limit their application for predicting the properties of unknown salts. These include the need for large experimental datasets to derive correlations, time-consuming computational methods, or the need for at least some experimental data from the IL under study.We recently showed that the relatively low melting points of ILs can be understood by a simple thermodynamic cycle based on lattice and solvation energies. [13,14] This model also allowed the prediction of the melting points (and dielectric constants) of ILs with good accuracy. Subsequently, we noticed a strong relationship between the molecular volumes V m of ILs and their fundamental physical properties: viscosity, conductivity, and density. Herein we describe these simple relationships and show that it is possible for nonspecialists to predict the physical properties of even unknown ILs with very good accuracy from only their molecular volumes and an anion-dependent correlation.The molecular volume V m (or formula-unit volume) of a salt is a physical observable and is defined as the sum of the ionic volumes V ion of the constituent ions. For example, for a binary IL V m is given by Equation (1).The ionic volume is a measure of the size of an ion, similar to the traditional ionic radius.[15] However, ionic radii are poorly defined and arguably not physically meaningful for nonsymmetrical ions such as those found in many ILs. In contrast, ionic volumes are well defined and equally valid for symmetrical and nonsymmetrical ions. The ionic volume can be derived from crystal structures (e.g., the...

In continuation of recent work on the dielectric response of imidazolium-based ionic liquids (ILs) (J. Phys. Chem. B, 2006, 110, 12682), we report on the effect of cation variation on the frequency-dependent dielectric permittivity up to 20 GHz of ionic liquids. The salts are comprised of pyrrolidinium, pyridinium, tetraalkylammonium, and triethylsulfonium cations combined with the bis-((trifluoromethyl)sulfonyl)imide anion. The dielectric spectra resemble those observed for imidazolium salts with the same anion. In all cases, the major contribution results from a diffusive low-frequency response on the time scale of several 100 ps, which shows a broadly distributed kinetics similar to that of spatially heterogeneous states in supercooled and glassy systems rather than that observed in fluid systems. There is evidence for a weak secondary process near 10-20 ps. Perhaps the most interesting difference to imidazolium salts is founded in the missing portions of the spectra due to processes beyond the upper cutoff frequency of 20 GHz. These are lower than that observed for imidazolium-based salts and seem to vanish for tetraalkylammonium and triethylsulfonium salts. As for imidazolium salts, the extrapolated static dielectric constants are on the order of epsilon(S) congruent with 10-13, classifying these ILs as solvents of moderate polarity.