From molten salts to ionic liquids: effect of ion asymmetry and charge distribution

We studied the influence of ion shape asymmetry and charge distribution on the liquid structure and transport properties of ionic liquids by the molecular dynamics of schematic models. The ion structure asymmetry results in a less compact packing, while the charge distribution gives potential wells with reduced depth with respect to the single-site centred charges. Both these aspects contribute to accelerate the dynamics of the melt. The liquids display a clearly detectable supercooled region, in which two different liquid structures appear to be present contemporarily. The diffusion process, as usual in supercooled liquids, proceeds through a cage-escaping process; the diffusion constants nevertheless show an opposite behaviour with respect to expectations from the cavity size distribution. The observed transient liquid structure can help to explain some surprising features of the diffusive behaviour of these systems.