Bulk and Liquid–Vapor Interface of Pyrrolidinium-Based Ionic Liquids: A Molecular Simulation Study

Abstract

Using molecular dynamics simulations, we have studied the structure of three 1-butyl-1-methylpyrrolidinium ionic liquids whose anions are triflate, bis(trifluoromethanesulfonyl)imide, and tris(pentafluoroethyl)trifluorophosphate. The structure of the bulk phase of the three ionic liquids has been interpreted using radial and spatial distribution functions and structure factors that allows us to characterize the morphology of the polar and nonpolar domains present in this family of liquids. The size of the polar regions depends on the anion size, whereas the morphology of the nonpolar domains is anion-independent. Furthermore, the surface ordering properties of the ionic liquids and charge and density profiles were also studied using molecular simulations. The surface tension of the liquid-vapor interfaces of these ionic liquids was also predicted from our molecular simulations. In addition, microscopic structural analysis of orientational ordering at the interface and density profiles along the direction normal to the interface suggest that the alkyl chains of the cation tend to protrude toward the vacuum, and the presence of the interface leads to a strong organization of the liquid phase in the region close to the interface. In the interfacial area, the polar regions of the ionic liquids are more structured than those in the bulk phase, whereas the opposite behavior is observed for the nonpolar regions.

Potential‐Dependent Adlayer Structure and Dynamics at the Ionic Liquid/Au(111) Interface: A Molecular‐Scale In Situ Video‐STM Study

“…In the bulk of RTILs such separation (albeit without long‐range order) is well‐established by experimental and simulation studies (see Ref. 15 for a detailed overview). On the metal surface, this separation may be assisted by epitaxial effects, for example, a preference of the molecules for specific adsorption sites of the Au(111) lattice.…”

The ionic liquid–vacuum outer atomic surface: a low-energy ion scattering study

“…13,[25][26][27] The techniques that have been used to investigate the ionic liquid-vacuum surface (the region within which the properties of the ionic liquid are signicantly different from that of the bulk ionic liquid) can be separated into four different areas: spectroscopy, [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] scattering, [46][47][48][49][50][51][52][53][54][55][56][57][58][59] sticking probability/temperature programmed desorption, [60][61][62] and molecular dynamics (MD) simulations. 45,[63][64][65][66][67][68][69][70][71]<...>…”

“…66 Unlike for cations, there is no contradictory evidence in the literature about the anion orientation for the ions nearest the vacuum. The anion orientation reported is the most probable: for [Tf 2 N] À the CF 3 groups are near the vacuum and the (SO 2 ) 2 N groups are towards the bulk, 29,52,65,68,70,73,77 for [C n OSO 3 ] À the alkyl groups are near the vacuum and the OSO 3 groups are towards the bulk ([C n OSO 3 ] À ¼ alkylsulfate), 29,30,67 and for [CF 3 SO 3 ] À the CF 3 groups are near the vacuum and the SO 3 groups are towards the bulk. 34 Experimental reports on the possibility of more than one anion orientation are limited.…”

Room temperature ionic liquids: A simple model. Effect of chain length and size of intermolecular potential on critical temperature

“…Calculation of the structural properties and surface tension of three RTIL at their liquid-vapor equilibrium interphase has been recently reported. 8 The conductivity has been measured 9 for two different RTILs doped with lithium. Other area of interest is the behavior of imidazolium-based RTIL in the presence of nanoporous carbon or nanostructures [10][11][12] to produce electrical double layer capacitors.…”