Orientational wetting and dynamical correlations toward glass transition on the surface of imidazolium-based ionic liquids

Surface induces many fascinating physical phenomena, such as dynamic acceleration, surface anchoring, and orientational wetting, and, thus, is of great interest to study. Here, we report classic molecular dynamics simulations on the free-standing surface of imidazolium-based ionic liquids (ILs) [C(4)mim][PF6] and [C(10)mim][PF6]. On [C(10)mim][PF6] surface, a significant orientational wetting is observed, with the wetting strength showing a diverging tendency. Depth of the wetting was captured from the density and orientational order profile by a static length, which remarkably increases below the temperature T-stat upon cooling down. The dynamical correlation length that measures the distance of surface-dynamics acceleration into the bulk was characterized via the spatial-dependent mobility. The translational correlation exhibits a similar drastic increment at T-stat, while the rotational correlation drastically increases at a lower temperature T-rot. We connect these results to the dynamics in bulk liquids, by finding T-stat and T-rot that correspond to the onset temperatures where the liquids become cooperative for translational and rotational relaxation, respectively. This signifies the importance of collective dynamics in the bulk on the orientational wetting and surface dynamics in the ILs. Published under an exclusive license by AIP Publishing.

Automated summary

Surface-induced phenomena, such as dynamic acceleration, surface anchoring, and orientational wetting, were studied in this report through classic molecular dynamics simulations on the free-standing surface of imidazolium-based ionic liquids. The presence of significant orientational wetting on the [C(10)mim][PF6] surface was observed, with the wetting strength showing a diverging tendency. The study also connected the results to the dynamics in bulk liquids, highlighting the importance of collective dynamics in the bulk on orientational wetting and surface dynamics in the ionic liquids.