Macroscopic Differentiators for Microscopic Structural Nonideality in Binary Ionic Liquid Mixtures

Abstract

Combining two ionic liquids to form a binary ionic liquid mixture is a simple yet effective strategy to not only expand the number of ionic liquids but also precisely control various physicochemical properties of resultant ionic liquid mixtures. From a fundamental thermodynamic point of view, it is not entirely clear whether such mixtures can be classified as ideal solutions. Given a large number of binary ionic liquid mixtures that emerge, the ability to predict the presence of nonideality in such mixtures a priori without the need for experimentation or molecular simulation-based calculations is immensely valuable for their rational design. In this research report, we demonstrate that the difference in the molar volumes (ΔV) of the pure ionic liquids and the difference in the hydrogen-bonding ability of anions (Δβ) are the primary determinants of nonideal behavior of binary ionic liquid mixtures containing a common cation and two anions. Our conclusion is derived from a comparison of microscopic structural properties expressed in terms of radial, spatial, and angular distributions for binary mixtures and those of the corresponding pure ionic liquids. Molecular dynamics simulations of 16 binary ionic liquid mixtures, containing a common cation 1-n-butyl-3-methylimidazolium [C4mim]+ and combinations of (less basic) fluorinated {trifluoromethylacetate [TFA]−, trifluoromethanesulfonate [TFS]−, bis­(trifluoromethanesulfonyl)­imide [NTf2]−, and tris (pentafluoroethyl) trifluorophosphate [eFAP]−} versus (more basic) nonfluorinated {chloride Cl–, acetate [OAC]−, methylsulfate [MeSO4]−, and dimethylphosphate [Me2PO4]−} anions, were conducted. The large number of binary ionic liquid mixtures examined here enabled us to span a broad range of ΔV and Δβ values. The results indicate that binary mixtures of two ionic liquids for which ΔV > 60 cm3/mol and Δβ > 0.4 are expected to be microscopically nonideal. On the other hand, ΔV < 60 cm3/mol and Δβ < 0.4 will lead to molecular structures that are not differentiated from those of their pure ionic liquid counterparts.

Screening Ionic Liquids Based on Ionic Volume and Electrostatic Potential Analyses

“…More recently, Kapoor and Shah found that the mixing behavior of binary IL mixtures could be inferred from the difference in the molar volumes of the pure ILs, in conjunction with the difference in the H-bond ability of the anions. These nonideal issues in IL mixtures were also recently investigated by Sappidi et al As mentioned in the Introduction, the H-bond basicity of IL pairs has been reported in some previous experimental investigations − and using COSMO-RS approaches. , Here, we correlate the anion IPI with the H-bond basicity, as illustrated in Figure b and with the corresponding data listed in Table S6.…”

Molecular Modelling of Ionic Liquids: Perfluorinated Anionic Species with Enlarged Halogen Substitutions

“…This chemical modification could incur variations in various properties of ILs, e.g., molar volume, viscosity, thermal stability and gas permeability. [34][35][36][37][38][39][40][41][42][43] A practical example is the decrease of the 1 H chemical shift of C2-H (hydrogen) of imidazolium cations upon the F-to-C2F5 mutation, suggesting a weakening effect on acidities. 44 Also, the low hydrolytic stability of [PF6]-involved ILs is overcome by incorporating the long-chain derivative [FAP].…”

The structural and hydrogen bonding properties of ionic liquid–co-solvent binary mixtures: the distinct behaviors of two anions

“…Room temperature ionic liquids (RTILs) as a new type of green solvent are developing vigorously and becoming a research hotspot. RTILs are salts composed entirely of ions with a melting point below 373 K. [1][2][3] Compared with traditional organic solvents, they have many advantages such as non-volatility, high thermal stability, excellent solubility, and recyclability, [4][5][6] and have been widely used in extraction, 7,8 catalysis, 9,10 and electrochemistry. [11][12][13] The combination of ions in ILs can affect their characteristics, and the use of different anions can result in varying properties and applications of ILs.…”