The properties of residual water molecules in ionic liquids: a comparison between direct and inverse Kirkwood–Buff approaches

Abstract

We study the properties of residual water molecules at different mole fractions in dialkylimidazolium based ionic liquids (ILs), namely 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM/BF4) and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM/BF4) by means of atomistic molecular dynamics (MD) simulations. The corresponding Kirkwood-Buff (KB) integrals for the water-ion and ion-ion correlation behavior are calculated by a direct evaluation of the radial distribution functions. The outcomes are compared to the corresponding KB integrals derived by an inverse approach based on experimental data. Our results reveal a quantitative agreement between both approaches, which paves a way towards a more reliable comparison between simulation and experimental results. The simulation outcomes further highlight that water even at intermediate mole fractions has a negligible influence on the ion distribution in the solution. More detailed analysis on the local/bulk partition coefficients and the partial structure factors reveal that water molecules at low mole fractions mainly remain in the monomeric state. A non-linear increase of higher order water clusters can be found at larger water concentrations. For both ILs, a more pronounced water coordination around the cations when compared to the anions can be observed, which points out that the IL cations are mainly responsible for water pairing mechanisms. Our simulations thus provide detailed insights in the properties of dialkylimidazolium based ILs and their effects on water binding.

Microphase separation and the formation of ion conductivity channels in poly(ionic liquid)s: A coarse-grained molecular dynamics study

“…as the standard definition for a Kirkwood-Buff integral. 40,43,44 In accordance with Refs. 43 and 44, Kirkwood-Buff integrals can be interpreted as excess volumes and can be further evaluated in order to estimate the excess number of particles…”

“…The local composition of a PIL system at different length scales can be studied through partial structure factors, which were already established for molecular ionic liquid/water mixtures 24,40 according to the relation 41,42…”

Structural Elucidation of the Binary Mixtures of [EMIM][BF4] and [BMIM][BF4] in Ethyl‐Substituted Solvents by Isothermal Titration Calorimeter

“…The MD simulation studies have shown that more number of water molecules can accommodate around [BMIM][BF 4 ] as compared to the [EMIM][BF 4 ] due to the larger molecular size of the BMIM + cation . This study revealed that the water binding to BMIM cation is more preferable as compared to the EMIM cation . This process governs interactions between the cation possessing alkyl group and the solvents have been described in details by Frank and Evan which fits nicely with the present study.…”

“…It has also been reported that the [EMIM][BF 4 ] shows higher mass density as compared to the [BMIM][BF 4 ]. The MD simulation studies have shown that more number of water molecules can accommodate around [BMIM][BF 4 ] as compared to the [EMIM][BF 4 ] due to the larger molecular size of the BMIM + cation . This study revealed that the water binding to BMIM cation is more preferable as compared to the EMIM cation .…”

Properties of Ion Complexes and Their Impact on Charge Transport in Organic Solvent-Based Electrolyte Solutions for Lithium Batteries: Insights from a Theoretical Perspective

“…It is worth noting that KB theory can also be applied to experimental data to evaluate the underlying radial distribution functions via an indirect technique [34,161,162]. A recent study on the convergence of experimental and simulation data for water in ionic liquids reveals the benefits of combined experimental and computational approaches [163].…”