A New Volume-Based Approach for Predicting Thermophysical Behavior of Ionic Liquids and Ionic Liquid Crystals

Volume-based prediction
of melting points and other properties
of ionic liquids (ILs) relies on empirical relations with volumes
of ions in these low-melting organic salts. Here we report an accurate
way to ionic volumes by Bader’s partitioning of electron densities
from X-ray diffraction obtained via a simple database approach. For
a series of 1-tetradecyl-3-methylimidazolium salts, the volumes of
different anions are found to correlate linearly with melting points;
larger anions giving lower-melting ILs. The volume-based concept is
transferred to ionic liquid crystals (ILs that adopt liquid crystalline
mesophases, ILCs) for predicting the domain of their existence from
the knowledge of their constituents. For 1-alkyl-3-methylimidazolium
ILCs, linear correlations of ionic volumes with the occurrence of
LC mesophase and its stability are revealed, thus paving the way to
rational design of ILCs by combining suitably sized ions.