The relationship between solvent polarity and molar volume in room-temperature ionic liquids

Solvent polarity is a subject of great interest to chemists. A significant component of a solvent's polarity is its capacity for nonspecific electrostatic interactions, which is often parameterized using the dielectric constant epsilon or the Kamlet-Taft dipolarity/polarizability parameter pi*. Recent theoretical work has established a connection between the molar volume of an ionic liquid and its capacity for nonspecific electrostatic interactions with a neutral dipolar solute. In this work, we make use of a recently-developed theoretical method to estimate the molar volume of a series of ionic liquids, and explore the variation of experimentally-measured epsilon and pi* values with molar volume. Both variables are found to vary with molar volume, and we observe an anomaly in the behavior of pi* that offers insight on the nanoscale inhomogeneity of ionic liquids. An important outcome of this work is a simple scheme for the estimation of the relative polarities of ionic liquids; while not quantitatively accurate, the scheme permits prediction of the change in solvent polarity on ionic substitution or derivitization. The approach is sufficiently simple that for most commonly-used ionic liquids it can be implemented on a pocket calculator in a matter of minutes, making it a practical aid to researchers seeking to design task-specific ionic liquids.