Ionic liquids-Conventional solvent mixtures, structurally different but dynamically similar.

Abstract

In more than one way pure ionic liquids (ILs) can be seen as mixtures. By definition they are comprised of cationic and anionic components and they also possess dual charge and apolar characteristics. We recently uncovered interesting dynamical behavior [Araque et al., J. Phys. Chem. B 119(23), 7015 (2015)] that can be ascribed to this duality. For small neutral solutes local friction can be high in certain regions and much lower in others. It is only reasonable to ask whether this interesting behavior is unique to ILs or is also common in certain conventional solvent mixtures such as dimethylsufoxide/glycerol for which the viscosity can be tuned to be similar. We make the case that the latter scenario is correct and that whereas viscous conventional solvent mixtures are structurally very different from ILs, dynamically they are not. From the perspective of a solute that is small, both ILs and viscous conventional solvent mixtures display frictionally stiff and soft regions associated with cage and jump diffusive regimes. In the case of ILs these are associated with charge-enhanced and charge-depleted liquid regions, whereas in the case of the conventional solvents by the distinct frictional properties of the two components.