MD simulation of xenon in ionic liquids: Disentangling the cationic and anionic cage effects on the structural and dynamic properties

We present a computational and theoretical study of the microscopic structure of two representative ionic liquids as probed by using xenon. Trajectories obtained from classical molecular dynamics simulations of xenon dissolved in [bmim][Cl] and [bmim][PF6] have been used to define the cage of xenon following a theoretical modeling introduced some years ago for simple fluids. The separate contribution of cations and anions to the caging of xenon has been disentangled, showing a major contribution from the cations. Moreover the coupled dynamics of the probe and the associated cage have been analyzed. The distribution of librational frequencies for the putative motion of the probe within the cage of the two systems shows clear, though not large, differences. The diffusion coefficients of cations, anions and xenon support the validity of hydrodynamic theory.