Molecular dynamics simulations of high-frequency sound modes in ionic liquids

Collective dynamics within the THz frequency range has been investigated by molecular dynamics (MD) simulations of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [C2C1im][TFSI], 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide, [C2C1im][FSI], and lithium solutions of Li[TFSI] and Li[FSI] in each of these liquids. We calculated time correlation functions of mass current fluctuations for longitudinal (LA) and transverse acoustic (TA) sound modes at different wave vectors, k. The dispersion curves of excitation energy, ω(k), calculated by MD simulations were compared with recent data for these systems reported by Fujii et al., J. Chem. Phys. 138, 151101 (2013), using inelastic X-ray scattering (IXS) spectroscopy. The reasonable agreement between experiment and simulation indicates that non-polarizable potential models for these ionic liquids provide satisfactory representation of the collective dynamics at least at high-(k,ω) range. Even though Li+ solution is more viscous than the pure ionic liquid, the high-frequency sound velocity of the solution is the same as the pure liquid.