Density scaling of structure and dynamics of an ionic liquid

Room temperature ionic liquids are salts with low melting points achieved by employing bulky and
asymmetrical ions. The molecular design leads to apolar and polar parts as well as the presence of
competing Coulomb and van der Waals interactions giving rise to nano-scale structure, e.g. charge
ordering. In this paper we address the question of how these nano-scale structures influence transport
properties and dynamics on different timescales. We apply pressure and temperature as control parameters
and investigate the structure factor, charge transport, microscopic alpha relaxation and phonon dynamics
in the phase diagram of an ionic liquid. Including viscosity and self diffusion data from literature we find
that all the dynamic and transport variables studied follow the same density scaling, i.e. they all depend on
the scaling variable G = rg/T, with g = 2.8. The molecular nearest neighbor structure is found to follow a
density scaling identical to that of the dynamics, while this is not the case for the charge ordering,
indicating that the charge ordering has little influence on the investigated dynamics.