Polarizable MD simulations of ionic liquids: How does additional charge transfer change the dynamics?

Abstract

Both experimental and computational evidence exist that Coulomb interactions between the molecular ions in ionic liquids are significantly damped by almost a factor of two. This circumstance is often used to justify charge scaling. However, as polarizable MD simulations are also capable of explaining the reduced Coulomb interaction between the ionic liquid ions [C. Schröder, Phys. Chem. Chem. Phys., 2012, 14, 3089], the question arises, if the reduced Coulomb interactions are due to a charge transfer between the molecules or due to an overall effect of induced dipolar interactions. We aim to contribute to this discussion using polarizable MD simulations of 1-ethyl-3-methylimidazolium tetrafluoroborate including a new model for treating charge transfer between the cations and anions. The diffusion time scales are not changed significantly with the inclusion of charge transfer, but individual ions show a strong dependence on charge transfer amounts. Ions which have transferred more charge, and have a charge with a smaller magnitude, diffuse slower. The charge transfer model shows a slightly larger conductivity, despite having smaller charges, and shows a much stronger contribution of the anions to the conductivity. With charge transfer, the anions become the dominant species for charge transport, while the polarizable models show a roughly equal contribution from the anions and the cations.