| Abstract
| - Molecular dynamics simulations for the liquid−vacuum interface of the ionic liquid 1-ethyl-3-methylimidazolium nitrate (EMIM+/NO3-) were performed for both electronically polarizable and nonpolarizable potentialenergy surfaces. The interfacial structural properties, such as the oscillation in the number density profile, theorientational ordering, and the local clustering in the interfacial region, were calculated. The simulations withboth the polarizable and nonpolarizable model demonstrate the existence of an inhomogeneous interfacialstructure normal to the surface layer. It was found for both models that the ethyl tail group on EMIM+ islikely to protrude outward from the surface. In the outmost surface layer, the cation is likely to lie on thesurface with the imidazolium ring parallel to the interface, while there is a second region with enhanceddensity from that in the bulk where the cation preferably slants with the imidazolium ring tending to beperpendicular to the surface. The results also reveal that the electronic polarization effect is important for theionic liquid interface. It is found that the cation is likely to be segregated at the ionic liquid surface for thepolarizable model, while for the nonpolarizable model, the anion is found to be more likely to exhibit suchbehavior. The surface tension of the polarizable model (58.5 ± 0.5 mN/m) is much smaller than that of thenonpolarizable model (82.7 ± 0.6 mN/m), in better agreement with extrapolated experimental measurementson similar ionic liquid systems.
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