Understanding the Effect of the C2 Proton in Promoting Low Viscosities and High Conductivities in Imidazolium-Based Ionic Liquids: Part I. Weakly Coordinating Anions

Abstract

1,3-Disubstituted imidazolium ionic liquids have been the subject of numerous theoretical and experimental studies due to their low viscosity-often the very lowest for any given cation/anion family. One of the mysteries in the imidazolium family of salts is the sharp increase in viscosity that is observed on methylating at the C2 position in the ring. In the nonmethylated case, the C2 proton is observed to be distinctly acidic and, where this is undesirable, substitution of the C2 position removes the problem, but produces an unexpected increase in viscosity. Methylation at other positions on the ring does not produce such a significant effect. In this study, two possible structural or energetic sources of the increased viscosity were investigated: (1) ion association, as probed by the Walden rule, and (2) differences in the potential energy surface profiles that favor ionic transport in the non C2-methylated imidazolium ionic liquids. The second hypothesis was investigated using high-level ab initio theory. The higher viscosity of C2-methylated imidazolium ionic liquids is shown to be a result of high potential energy barriers (significantly above the available thermal energy) between the energetically preferred conformations on the potential energy surface, thus restricting movement of ions in the liquid state to only small oscillations and inhibiting the overall ion transport.

Effects of C(2) Methylation on Thermal Behavior and Interionic Interactions in Imidazolium-Based Ionic Liquids with Highly Symmetric Anions

“…In the real liquid, a distribution of several cation/anion configurations will be present simultaneously [43], and the sketch must be seen as one example of what the data suggest exists in larger amounts. in computational studies [4,5,7,28,29,38]. This suggests that the above findings represent a rather universal phenomenon.…”

“…This is particularly true for the molecular anions, [BF 4 − ] and [PF 6 − ]. Consequently, the overall molecular system gains symmetry when the C (2) number of other dialkylimidazolium ILs, e.g., NTf 2 and other halogenide anions, and in computational studies [4,5,7,28,29,38]. This suggests that the above findings represent a rather universal phenomenon.…”

“…Noack et al [4] showed that electron density changes can alter the position and strength of interionic interactions and lead to reduced configurational variations. Izgorodina's dynamic theory [29] suggests that the free-energy barrier for the anion to move relative to the imidazolium cation increases the viscosity.…”

Molecular features contributing to the lower viscosity of phosphonium ionic liquids compared to their ammonium analogues

“…[34][35][36] Recent studies have shown that dispersion forces play an unexpectedly important role. [35][36][37] Additionally, functional parts of the molecule, such as hydrogen bonds 23,36,[38][39][40][41][42][43] or alkyl chain length, 6,23,29 may contribute to a low melting point or low viscosity.…”

Geometrical Isomerism of RuII Dye‐Sensitized Solar Cell Sensitizers and Effects on Photophysical Properties and Device Performances

“…Upon comparison electrolyte B with electrolyte A, which contains PMII, which is less viscous and transports ions faster than DMPII, [21] GuNCS (0.1 m), and LiI (0.5 m), we note that the efficiency of TFRS-2 a (h = 9.11 %) is higher than that reported for TFRS-52 a under identical conditions (h = 8.92 %). This trend is reversed if electrolyte A is used.…”