Structure-property relationships in ionic liquids: Chain length dependence of the vaporization enthalpies of imidazolium-based ionic liquids with fluorinated substituents

Molar vaporization enthalpies of fluoroalkyl-substituted imidazolium-based ionic liquids were derived from two concurring quartz crystal microbalance (QCM) and thermogravimetry (TGA) methods. For comparison, enthalpies of vaporization measured at elevated temperatures have been adjusted to the reference temperature 298 K and tested for consistency. It was found that vaporization enthalpies of fluorine substituted families are significantly higher compared to the analogous ILs with the alkyl-substituted cation. This is in agreement to molecular solvents, where fluorination typically increases vaporization enthalpy relative to hydrocarbon analogues. A useful group contribution for the incremental CF2 fragment in the alkyl chain was recommended for the quick estimation of vaporization enthalpies of various substituted IL cations (e.g., imidazolium, ammonium, pyridinium, etc.).