The [BMI][Tf₂N] Ionic Liquid/Water Binary System:  A Molecular Dynamics Study of Phase Separation and of the Liquid−Liquid Interface

Abstract

We report molecular dynamics (MD) simulations of the aqueous interface of the hydrophobic [BMI][Tf2N] ionic liquid (IL), composed of 1-butyl-3-methylimidazolium cations (BMI+) and bis(trifluoromethylsulfonyl)imide anions (Tf2N-). The questions of water/IL phase separation and properties of the neat interface are addressed, comparing different liquid models (TIP3P vs TIP5P water and +1.0/-1.0 vs +0.9/-0.9 charged IL ions), the Ewald vs the reaction field treatments of the long range electrostatics, and different starting conditions. With the different models, the "randomly" mixed liquids separate much more slowly (in 20 to 40 ns) than classical water-oil mixtures do (typically, in less than 1 ns), finally leading to distinct nanoscopic phases separated by an interface, as in simulations which started with a preformed interface, but the IL phase is more humid. The final state of water in the IL thus depends on the protocol and relates to IL heterogeneities and viscosity. Water mainly fluctuates in hydrophilic basins (rich in O(Tf2N) and aromatic CH(BMI) groups), separated by more hydrophobic domains (rich in CF3(Tf2N) and alkyl(BMI) groups), in the form of monomers and dimers in the weakly humid IL phase, and as higher aggregates when the IL phase is more humid. There is more water in the IL than IL in water, to different extents, depending on the model. The interface is sharper and narrower (approximately 10 A) than with the less hydrophobic [BMI][PF6] IL and is overall neutral, with isotropically oriented molecules, as in the bulk phases. The results allow us to better understand the analogies and differences of aqueous interfaces with hydrophobic (but hygroscopic) ILs, compared to classical organic liquids.

Transport of salicylic acid through supported liquid membrane based on ionic liquids

“…It is important to mention that Cammarata et al [42] reported the presence of water aggregates (clusters), using IR spectroscopy, in the ionic liquid [C 4 mim][PF 6 ], when this RTIL is saturated with water. The formation of reverse micelles or water aggregates in some ionic liquids was also observed experimentally in molecular simulation studies [43,44].…”

Ion Correlation and Collective Dynamics in BMIM/BF4-Based Organic Electrolytes: From Dilute Solutions to the Ionic Liquid Limit

“…Recently, solvation models have been proposed to predict 4 ion association constants, 45 and combined molecular simulations and quasi-elastic neutron scattering experiments have demonstrated correlation of ion association and ion dynamics with solvent dielectric strength. 46 While numerous studies exist for ionic liquid/water mixtures, 31,[47][48][49][50][51][52][53][54][55][56] these systems are deserving of separate discussion due to their unique phase behavior, and the properties of lower dielectric electrolytes may differ significantly (vide infra).…”

Fluid interfacial nanoroughness measurement through the morphological characteristics of graphene

“…4 Molecular dynamics (MD) simulation can provide an alternative solution to overcome these problems. Several studies have already proved the reliability of numerical methods for describing interfaces, including a liquid-vapor interface, [8][9][10][11] liquid-liquid interface, [12][13][14] and liquidsurfactant-liquid interfaces. 15,16 MD results show that an interface structure is strongly affected by molecular interaction; for example, the van der Waals parameter, e, significantly affected the thickness of a water and oil interface.…”