Interfacial Properties of an Ionic Liquid by Molecular Dynamics

We studied the influence of a liquid-vapor interface on dynamic properties like reorientation and diffusion as well as the surface tension of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) by molecular dynamics simulations. In the interfacial region, reorientation of a short molecular axis is slightly faster than that in the central layer, while that of the longer molecular axis is retarded. The molecular reorientation is well-described by a stretched exponential decay modeled by the Kohlrausch-Williams-Watts equation. Analysis of the average translational diffusion coefficient of molecules in a central layer shows consistency with the Vogel-Fulcher-Tamann equation in a temperature range from 300 to 380 K. A first-passage time analysis of the system at 380 K yields a more refined spatial characterization of translational diffusion perpendicular to the liquid-vapor interfaces. In the central region of the slab, the diffusion coefficient of cations is only marginally higher than that of anions, but close to an interface, this difference is much higher, up to 50%. Apparent activation energies for rotational and diffusional dynamics, respectively, were estimated assuming Arrhenius behavior. They indicate that reorientation of the long molecular axis depends on the diffusion ability, whereas for the reorientation of the short axis, no such correlation is observed. The results are in general agreement with the literature, with slightly overestimated absolute values. This applies as well for the surface tension, where, however, a dependence on the treatment of the electrostatics was found. Particle-mesh Ewald (PME) or reaction field (RF) and the treatment of bonds by constraints have an influence. If no bond constraints are applied, the results are consistent for both methods for the description of the electrostatics.