Ionic liquids-further progress on the fundamental issues

Ionic liquids continue to challenge conventional descriptions of liquids and their behaviour. Indeed, the ever-increasing variety of ionic liquid compounds has generated a need for multiple descriptions of the different molecular families, including protic, aprotic, solvate, and metal coordination complex families of ionic liquids, that exhibit very different behaviours. Within families, the balance of long-range electrostatic and short-range dispersion forces plays out in nanoscale heterogeneity that also impacts markedly on properties. In this perspective, we highlight some of the issues in the field that continue to deserve further investigation and development at both the experimental and fundamental levels. We also propose a set of nomenclature abbreviations in an attempt to systematise the plethora of confusing abbreviations that appear in the field. The distinction between ionic liquids, ionic liquid-solvent mixtures, and deep eutectic solvents is also discussed.