Understanding the Properties of Ionic Liquids: Electrostatics, Structure Factors, and Their Sum Rules

Abstract

The properties of room-temperature ionic liquids (ILs) may be viewed as resulting from a balance of electrostatic interactions that can be tuned at short range but constrained to satisfy universal, asymptotic screening conditions. Short-range interactions and ion packing provide ample opportunity for chemical tunability, while asymptotic sum rules dictate that the long-range structure and charge oscillation be similar to those of molten alkali halide salts. In this work, we study the structure factors and long-range electrostatic interactions in six ILs. The cation in all cases is 1butyl-3-methylimidazolium (BMIM + ), and we study six anions, namely, tetrafluoroborate (BF 4 − ), hexafluorophosphate (PF 6 − ), nitrate (NO 3 − ), triflate (CF 3 SO 3 − ), bisfluorosulfonylimide [(FSO 2 ) 2 N − ], and bistriflimide [(CF 3 SO 2 ) 2 N − ]. To gain insight, we perform similar computer simulations of a primitive molten salt model with and without electronic polarization. We emphasize universal similarities among ionic liquids and molten salts in the long-range ion ordering and the influence of electronic polarization on the screening conditions while also characterizing important differences in the short-range electrostatic interactions. We show that polarization systematically reduces charge oscillations by as much as ∼0.5−1 ion per radial shell, which we argue is general to all room-temperature ILs as well as molten salts. We suggest that a fundamentally important distinction among BMIM-based ionic liquids (with different anions) is the nature of the midrange, ∼1 Å −1 peak in the charge-correlation structure factor; while this correlation is straightforward to analyze in computer simulations, it may often be hidden in X-ray and/or neutron scattering structure factors.

Tuning Water Networks via Ionic Liquid/Water Mixtures

“…Figure S10 for [BMIM + ][TFSI − ]/water mixtures. All systems show a peak at ∼0.6-0.7 nm lengthscale, which reflects the characteristic anion separation distance in the corresponding neat IL [111]. This peak is the closest anion separation distance in the mixtures and is present at all concentrations.…”

“…This peak is the closest anion separation distance in the mixtures and is present at all concentrations. For neat ILs, it has been shown that this 0.6-0.7 nm lengthscale corresponds to oscillations of shells of counterions within the liquid [111]. It is notable that addition of water does not give rise to a closer distance anion/anion peak, which might be expected for solvent separated anion pairs.…”

“…Such trends may be difficult to infer from the collection of previously cited work, due to the high sensitivity of predicted water clustering/phase behavior to force field details [96][97][98]106]. McDaniel and coworkers have recently developed the ab initio, polarizable SAPT-FF force field for a variety of ILs [109,110], with property prediction benchmarks for neat ILs [111] as well as a variety of IL solvent mixtures [112]. For IL/water mixtures, the force field employs an ab initio description of cross-interactions [113] and water-water interactions are described by the well-benchmarked SWM4-NDP water model [114].…”

“…Explicit polarization is expected to be important for accurate predictions for two primary reasons. First, polarization has a universal and systematic influence on the structure of neat ionic liquids due to fundamental screening conditions [111,115] and this effect is expected to modulate the structure in low water content IL systems. Additionally, it is expected that polarization is important for accurately describing IL/water interfaces and thus phase behavior in hydrophobic IL mixtures [96][97][98].…”

“…− 3 ]/water mixtures which were run at 320 K so as to be sufficiently above the melting point of the IL. The IL/water mixtures were modeled utilizing the SAPT-FF force field for IL/IL and IL/water interactions [111,113] and SWM4-NDP water model [114] for water/water interactions. This explicitly polarizable, force field combination has been previously demonstrated to predict accurate conductivities of IL/water mixtures [112] and excess chemical potentials of water in ILs [28].…”