Structure of the Liquid−Vacuum Interface of Room-Temperature Ionic Liquids:  A Molecular Dynamics Study

Abstract

Molecular dynamics simulations for the liquid-vacuum interface of the ionic liquid 1-ethyl-3-methylimidazolium nitrate (EMIM+/NO3-) were performed for both electronically polarizable and nonpolarizable potential energy surfaces. The interfacial structural properties, such as the oscillation in the number density profile, the orientational ordering, and the local clustering in the interfacial region, were calculated. The simulations with both the polarizable and nonpolarizable model demonstrate the existence of an inhomogeneous interfacial structure normal to the surface layer. It was found for both models that the ethyl tail group on EMIM+ is likely to protrude outward from the surface. In the outmost surface layer, the cation is likely to lie on the surface with the imidazolium ring parallel to the interface, while there is a second region with enhanced density from that in the bulk where the cation preferably slants with the imidazolium ring tending to be perpendicular to the surface. The results also reveal that the electronic polarization effect is important for the ionic liquid interface. It is found that the cation is likely to be segregated at the ionic liquid surface for the polarizable model, while for the nonpolarizable model, the anion is found to be more likely to exhibit such behavior. The surface tension of the polarizable model (58.5 +/- 0.5 mN/m) is much smaller than that of the nonpolarizable model (82.7 +/- 0.6 mN/m), in better agreement with extrapolated experimental measurements on similar ionic liquid systems.

Spotlight on ionic liquids

“…26 Similar gauche defect reduction was seen in SFG studies of short-to long-chain 1-alkyl-3-methylimidazolium salts at the SiO 2 interface. 116,117 The structure of the IL-vacuum interface for ͓emim͔ + / NO 3 − was reported by Yan et al 119 Surface tension and thickness of IL-vapor interfaces have been reported from both simulations 120 and experiment. 121 By accounting for both capillary waves and zero-point motions, the underlying structure of the IL surface has been determined by comparing MD simulations with x-ray reflectivity experiments.…”

“…A wide variety of experimental, theoretical, and simulation methods have been applied to the study of ILs at interfaces. [107][108][109][110][111][112][113][114][115][116][117][118][119][120][121][122] Santos and Baldelli 112 have used interfacial sumfrequency generation ͑SFG͒ to show that at IL interfaces with gases or vacuum, alkyl chains on either the cations or the anions extend from the surface into the gas or vacuum. Rutherford backscattering spectra for the ͓bmim͔ + / ͓PF 6 ͔ − -vacuum interface also show the cation butyl group protruding from the bulk liquid into vacuum.…”

Theoretical reconstruction and elementwise analysis of photoelectron spectra for imidazolium-based ionic liquids

“…However, using the extremely surface sensitive metastable induced electron spectroscopy (MIES) 1 we have recently also found significant hints that the alkyl-chains in [OMIm]Tf 2 N do not completely cover the surface, but the [Tf 2 N] À anion may be still present there. The former result has been supported by parallel MIES studies of T. Iwahashi et al 39 Molecular dynamics simulations of the interface between imidazolium-based ionic liquids and vacuum/air by A. S. Pensado et al, 40 T. Yan et al, 41 and C. D. Wick et al 42 have found the alkyl-chain sticking out of the surface and have found the anion, e.g. 26 Consequently, a single ion pair seems to be better applicable for larger anions than for smaller ones, which might require another approach and a single ion pair costs only adequate calculation times.…”

New quantum chemistry-based descriptors for better prediction of melting point and viscosity of ionic liquids

“…In general, these models are either Quantitative Structure Property Relationship (QSPR) correlations [6][7][8] , group contribution (GC) type [9][10][11][12][13][14][15][16] , or molecular dynamics (MD) simulations. [17][18][19][20][21][22][23] The main drawback of the QSPR and GC models is the narrow restrictions in choice of applicable compounds while the main limitation of MD simulations is the high computational demands and the need for starting geometry. 24 In this paper, we present two empirical correlations to predict melting point and viscosity of ILs in a way that does not need experimental input or tedious simulations, but rely on inputs from simple quantum chemical calculations.…”