Molecular dynamics simulation of the interfacial structure of [C_nmim][PF₆] adsorbed on a graphite surface: effects of temperature and alkyl chain length

Abstract

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.

On the temperature dependence of the double layer capacitance of ionic liquids

“…Numerous dependences of differential capacitance curves on temperature have been reported for RTILs [19][20][21][22][23][24][30][31][32][33][34][35][36][37][38][39][40][41]. The calculated differential capacitance from our MD simulations, shown in Fig.…”

“…One of the first theories that reproduced the overscreening EDL structure of RTILs was the Bazant-Storey-Kornyshev theory [7]. That theory [7], however, cannot fully reproduce the charge density profile of atomistic molecular dynamics simulations [19][20][21][22][23][24] and force-balance measurements [25][26][27][28][29], as the resulting oscillating profiles in that theory [7] decay with the distance from the electrode very rapidly, exhibiting just one or two oscillations [7]. Development of theories that better describe the EDL has attracted much attention [13][14][15][16][17].…”

“…One aspect of the EDL in RTILs that has been studied numerous times with experimental [30][31][32][33][34][35][36][37] and computational/theoretical [19][20][21][22][23][24][38][39][40][41] methods is the temperature dependence. No coherent view has been reached on the dependence and its origin, however; the topic remains to be controversial.…”

“…No coherent view has been reached on the dependence and its origin, however; the topic remains to be controversial. Within the computational/theoretical studies [19][20][21][22][23][24][38][39][40][41], where a variety of RTILs on different electrodes have been investigated, both a positive and negative dependence of linear response capacitance on temperature, and transitions in shapes of differential capacitance curves, have been reported. The former, so-called anomalous temperature dependence of linear response capacitance, has attracted much attention of theorists [42][43][44][45][46][47][48].…”

Computer simulations of ionic liquids at electrochemical interfaces

“…of the ionic liquid film was commensurate with that of the aluminium surface, with different patterning depending on the metallic face oriented towards the liquid 28 . The question of the orientational ordering of the ionic liquid has also been addressed in numerous computer simulations 19,56,[82][83][84][85][86] .…”