Applications of ionic liquids (ILs) in electrified interfaces and electrochemical systems require insight into the molecular-level structure and properties of the interfacial ILs. Using atomistic molecular dynamics (MD) simulations, we show here that a new double-layer stacking formation of the [Bmim][PF(6)] IL can be triggered by the surface negative charge. We also found that the double-layer formation induced by the surface charge thoroughly extended into the bulk phase, implying a strong unscreened ion effect in our IL system. Further study indicated that the double-layer formation in the bulk phase was due to a rapid structural transition. Different IL formations, including the conventional adsorption layer and the double-layer formation, can be achieved in sequence by increasing the surface negative charge. Moreover, the diffusion ability of the new double-layer formation in the bulk phase is much lower when compared to that observed in its original uncharged condition. The structure and properties of the ILs formation may be attributed to the tail-tail aggregation hypothesis of the nonpolar domain in the IL.

The structures and diffusion behaviors of a series of ionic liquids [C(n)mim][PF(6)] (n = 1, 4, 8 and 12) on a graphite surface have been investigated by means of molecular dynamics simulation. It was found that three or four ordering layers of ionic liquids were formed near the graphite surface, and this layering structure was stable over the temperature range investigated. At the liquid/vacuum interface, the ionic liquid with a butyl chain had a monolayer ordering surface, while [C(8)mim][PF(6)] and [C(12)mim][PF(6)] exhibited a bilayer ordering with a polar domain sandwiched between two orientational nonpolar domains. More impressively, the simulated results showed that for the ionic liquids with alkyl chains longer than C(4), the adjacent alkyl chains in the whole film tended to be parallel to each other, with the imidazolium rings packed closely together. This indicated that the ionic liquids have a better regulated short-range structure than was previously expected. It was also found that both in the bottom layer and in the bulk region, the diffusion of the alkyl chains was much faster than that of the polar groups. However, as the alkyl chain length increased, the charge delocalization in the cation and the enhanced van der Waals interaction between the nonpolar groups contributed by reducing this difference in the diffusivity of major groups.

The electric double layer structure and differential capacitance of single crystalline Au(100) electrodes in the ionic liquid 1-butyl-3-methyl-imidazolium hexafluorophosphate are investigated using molecular dynamics simulations. Results show strong adsorption on the electrode surface. The potential of zero charge (pzc) and maxima of differential capacitance are strongly dependent on the adsorption layer structure. At potentials near the pzc, cations and anions adjacent to the electrode surface are coadsorbed on the same screening layer. This strong adsorption layer results in overscreening effects on the compact layer and induces both a bell-shaped differential capacitance curve and a positive pzc. Moreover, the potential required for transition from overscreening to overcrowding is about 4.0 V. This transition potential may be attributed to the higher interaction energy between the Au(100) electrode and ions compared with the binding energy in our cation-anion system.