Collectivity in ionic liquids: A temperature dependent, polarizable molecular dynamics study

András SZABADI¹, Philipp HONEGGER^1,3, Esther HEID^1,2, Othmar STEINHAUSER¹, Christian SCHRÖDER^1
1University of Vienna, Vienna, Austria
²Technical University of Vienna, Vienna, Austria
³Harvard Medical School, Boston, United States

The physico-chemical properties of ionic liquids are usually explained in terms of singular ions, e.g.
"The cation has a large surface-to-volume ratio". The individual molecules are assumed to be innocent,
that is non-interacting, resulting in (at least) two separate species existing in a mixture.
Collectivity, however, plays a major role in the evolution of both structural and dynamic properties,
such as coordination numbers, viscosity, diffusivity and conductivity.
Using polarizable molecular dynamics simulations, the infuence of single-particle and collective variables on the dynamics of the model compounds 1-ethyl-3-methylimidazolium dicyanamide as well as 1-ethyl-3-methyl-imidazolium trifluoromethanesulfonate has been investigated at five different temperatures from 260 to 340 K. The frequency-dependent dielectric spectrum indicates that cage dynamics accurately describe the behavior of the ions, making the definition of ion pairs somewhat arbitrary.
A parameter-free Voronoi analyis also casts doubt on the concept of alternating ion shells. Nearest-neighbor
analysis and potentials of mean force reveal that the cation used in this study is the preferred contact ion
for both cations and anions.