Voids in Ionic Liquids─What Simulations, Hard-Sphere Theories, and Gas Solubilities Indicate About Positronium Lifetime Estimates

Molecular solubility and transport are central to applications of ionic liquids in areas such as chemical synthesis, separation and extraction, and electrochemical processing. Because solubility and transport are closely tied to the availability of unoccupied spaces or voids in liquids, both computer simulations and positron annihilation lifetime spectroscopy (PALS) have been used to probe the void structure in these liquids. However, a serious disagreement exists between the size distributions of voids reported by these two approaches, at least near room temperature. Herein, classical molecular dynamics (MD) simulations of void structure and solvation free energies of light gases in six 1-butyl-3-methyl imidazolium ionic liquids previously studied with PALS spectroscopy are used to highlight this disagreement and suggest reasons for it. This comparison, together with a survey of PALS results in conventional liquids, suggests that positronium atoms play an active role in determining the cavities in which they reside, rather than reporting on the inherent ionic liquid structure as previously assumed.