Molecules (Apr 2024)
A Computational and Spectroscopic Analysis of Solvate Ionic Liquids Containing Anions with Long and Short Perfluorinated Alkyl Chains
Abstract
Anion-driven, nanoscale polar–apolar structural organization is investigated in a solvate ionic liquid (SIL) setting by comparing sulfonate-based anions with long and short perfluorinated alkyl chains. Representative SILs are created from 1,2-bis(2-methoxyethoxy)ethane (“triglyme” or “G3”), lithium nonafluoro-1-butanesulfonate, and lithium trifluoromethanesulfonate. Molecular dynamics simulations, density functional theory computations, and vibrational spectroscopy provide insight into the overall liquid structure, cation–solvent interactions, and cation–anion association. Significant competition between G3 and anions for cation-binding sites characterizes the G3–LiC4F9SO3 mixtures. Only 50% of coordinating G3 molecules form tetradentate complexes with Li+ in [(G3)1Li][C4F9SO3]. Moreover, the SIL is characterized by extensive amounts of ion pairing. Based on these observations, [(G3)1Li][C4F9SO3] is classified as a “poor” SIL, similar to the analogous [(G3)1Li][CF3SO3] system. Even though the comparable basicity of the CF3SO3− and C4F9SO3− anions leads to similar SIL classifications, the hydrophobic fluorobutyl groups support extensive apolar domain formation. These apolar moieties permeate throughout [(G3)1Li][C4F9SO3] and persist even at relatively low dilution ratios of [(G3)10Li][C4F9SO3]. By way of comparison, the CF3 group is far too short to sustain polar–apolar segregation. This demonstrates how chemically modifying the anions to include hydrophobic groups can impart unique nanoscale organization to a SIL. Moreover, tuning these nano-segregated fluorinated domains could, in principle, control the presence of dimensionally ordered states in these mixtures without changing the coordination of the lithium ions.
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