Layering at an Ionic Liquid−Vapor Interface:  A Molecular Dynamics Simulation Study of [bmim][PF₆]

Abstract

The structure of the planar liquid-vapor interface of a room-temperature ionic liquid, 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), is studied using atomistic molecular dynamics simulations. Layering of the ions at the interface is observed as oscillations in the corresponding number density profiles. These oscillations, however, are diminished in amplitude in the electron density profile, due to a near cancellation in the contributions from the anions and the cations. An enhancement by 12% in the electron density at the interface over its value in the bulk liquid is observed, in excellent agreement with X-ray reflectivity experiments. The anions are found to predominantly contribute to this increase in the interfacial electron density. The cations present at the interface are oriented anisotropically. Their butyl chains are observed to be preferentially oriented along the interface normal and to project outside the liquid surface, thus imparting a hydrophobic character. In the densest region of the interface, the imidazolium ring plane is found to lie parallel to the surface normal, in agreement with direct recoil spectroscopy experiments.

Hydrogen Bonds: A Structural Insight into Ionic Liquids

“…Further, by having water interact with the C 2 À H group, the C 2 À H bond will point into the bulk liquid and the C 4 À H and C 5 ÀH bonds will be directed into the vapor phase. [14,94] Theoretical calculations have confirmed the interaction between water and ILs. À1 at B3LYP/6-31 + + G** level after BSSE correction.…”

Specific distributions of anions and cations of an ionic liquid through confinement between graphene sheets

“…Therefore, it is important to have a fundamental understanding about the characteristics of the confined ILs. Investigations of the interfaces of the ILs, both solid and liquid, have been carried out using both experimental and simulation studies [11][12][13][14][15][16][17][18]. Recently, structure of the electrical double layer in IL 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF 6 ] near a charged and uncharged graphite surface has been studied [19].…”

Intrinsic Structure of the Interface of Partially Miscible Fluids: An Application to Ionic Liquids

“…in Refs. [6][7][8][9][10][11][12][13][14][15] Most of the computational effort has been focused on alkylimidazolium cations in combination with halides, BF − 4 , PF − 6 , and NTf − 2 anions. All these studies found considerable layering at the interface with ions strongly aligned leading to enhanced packing.…”