Static Dielectric Constant of Room Temperature Ionic Liquids: Internal Pressure and Cohesive Energy Density Approach

Measurements of the static dielectric constant (epsilon) of ionic liquids (ILs) are very difficult because of the decay of field by the ionic conductivity of ILs. Herein, we describe an easy method for the prediction of epsilon of various imidazolium-based ILs [C(n)mim] from n, i.e. the ratio of internal pressure (P-i) and cohesive energy density (ced). A calibration curve of n vs epsilon for conventional organic solvents (mainly the linear alcohols) has been used to estimate the E of the ILs. Estimated epsilon values for ILs having the anions [Cl](-), [BF4](-), [PF6](-), [TfO](-), and [Tf2N](-) showed a very good comparison with the literature results, whereas ILs having the anions [CnOSO3](-) tend to deviate from such correlation. Also, for a series of ILs having a common anion, the - is shown to follow a very good correlation with the molecular volumes. Predicted values show that both the nature of the anion and alkyl chain length of the cation contribute significantly to the epsilon of the ILs. The method developed makes use of properties which can be either experimentally determined or estimated with good accuracy and can be extended to the other categories of ILs with ease and reasonable accuracy.