On the Miscibility and Immiscibility of Ionic Liquids and Water

Abstract

Although the “like-dissolves-like” rule is often invoked to explain why sodium chloride dissolves in water, hidden behind this explanation is the delicate balance between the very large cohesive energy of the ionic crystal and large solvation energies of the ions. Room-temperature ionic liquids (ILs) are liquid analogues of ionic crystals and, as dictated by a similar energetic balance, may either fully mix with water or be immiscible with water depending on ion type and cation/anion combination. In this work, we study three hydrophobic and three hydrophilic ILs to examine whether a priori prediction of water miscibility is possible based on analysis of bulk properties alone. We find that hydrophilic and hydrophobic ILs exhibit distinct signatures in their (reciprocal space) Coulomb interactions that indicate predisposition to water mixing. Hydrophilic ILs exhibit a prominent peak in their electrostatic interactions at ∼5–8 Å length scale, largely due to repulsion between neighboring anion shells. When mixed with water, this peak is significantly reduced in magnitude, indicating that electrostatic screening by water molecules is an important driving force for mixing. In contrast, hydrophobic ILs show no such peak, indicating no predisposition to mixing. In addition to this analysis, we compute and compare solvation free energies of the six different anions in water, ion-pairing free energies at “infinitely” dilute concentration, and water absorption free energies in the different ILs. Analyzed within the context of empirical data, our calculations suggest that hydrophobicity trends of different ILs are very sensitive to precise water content at dilute conditions. For example, we predict that bis­(fluorosulfonyl)­imide-based ILs exhibit anomalously large water absorption free energies at zero water content, with increasing hydrophobicity as preferential absorption sites within the IL become saturated.

Tuning Water Networks via Ionic Liquid/Water Mixtures

“…Hydrophilic IL/water mixtures are investigated over systematically varying water content from 0 < φ H 2 O < 0.25 volume fraction water and hydrophobic IL/water mixtures are investigated from 0 < φ H 2 O < 0.13 volume fraction water, with the latter range (roughly) based on experimental phase coexistence data (see Methods). In this work, we use the terminology "hydrophobic" to refer to the [28]. We note that because the melting point of…”

“…For example, in ILs composed of 1-butyl-3-methylimidazolium (BMIM + ) cations, switching from similar-sized tetrafluoroborate (BF − 4 ) to hexafluorophosphate (PF − 6 ) anions qualitatively changes IL/water mixtures from fully-miscible and hydrophilic, to immiscible (hydrophobic) and phase-separated IL and water domains! This dramatic variation in IL/water miscibility from different anions is due to electrostatic screening energetics arising from the intrinsic electrostatic properties of the bulk ILs themselves [28]. In addition to the significant benefit of enhanced thermal stability, the ability to tune the water/ion nanostructure has motivated the incorporation of ILs into Nafion and/or other PFSA-like PEMs to potentially enhance performance [29][30][31][32][33].…”

“…For example, even miscibility trends are difficult to rationalize for certain ILs: Despite the similar nature of the BF 6 ] is hydrophobic and phase separates at significant water content. While rationalization of miscibility trends have primarily focused on the nature of the anion [23,24,47], recent work suggests that miscibility may depend on the net electrostatic interactions within the bulk IL resulting from a synergy of cation and anion contributions [28]. Understanding the physical principles dictating these trends is essential to enable tuning of water networks within IL/water mixtures and requires comprehensive investigation of a variety of systems and not just studies on a case-by-case basis.…”

“…Another study rationalized the significantly reduced water clustering 4 ] IL/water mixtures as due to formation of anion-water-anion wire structural motifs in the acetate IL [93]. A number of simulation studies have examined IL/water liquid/liquid [96][97][98], liquid/vapor [99] or other interfaces [100], with other studies focused on predicting thermodynamic properties of IL/water mixtures, with structural characterization not being the primary focus [28,[101][102][103][104][105][106][107]. Structural predictions from simulations may be validated through spectroscopic connection with experiment but such "first-principles" condensed phase spectroscopic predictions for IL/water mixtures are rare [108].…”

“…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]. MD simulations were conducted using the OpenMM software package [116], on Nvidia GTX-1080-Ti GPU cards.…”