Burba CM, Feightner K, Liu M, Hawari A. The Effect of Fluorinated Solvents on Physicochemical Properties, Ionic Association, and Free Volume of a Prototypical Solvate Ionic Liquid.
Chemphyschem 2022;
23:e202100548. [PMID:
34982854 DOI:
10.1002/cphc.202100548]
[Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/21/2021] [Revised: 01/03/2022] [Indexed: 11/09/2022]
Abstract
Solvate ionic liquid synthesis and properties depend on a delicate balancing of cation-solvent and cation-anion interactions to produce materials containing only cation-solvent complexes and solvent-separated anions. Most SILs meeting these characteristics fall within the paradigm of oligomeric ethylene oxides and lithium salts. Targeted functionalization of solvent molecules to achieve desired properties is a relatively unexplored avenue of research. We explore solvent fluorination for a prototypical SIL based on lithium bis(trifluoromethylsulfonyl)imide (LiNTf2) and triethylene glycol (TEG). In the first experiment, TEG is partially substituted with 2,2,4,4,5,5,7,7-octafluoro-3,6-dioxaoctane-1,8-diol (FTEG). This leads to a decrease in ionic conductivity and proliferation of Li(NTf2)2- species. Both results suggest FTEG does not readily coordinate Li+ ; a conclusion that is reinforced by computational studies of [(TEG)1Li]+ and [(FTEG)1Li]+ cation stabilities. A second experiment adds FTEG as a diluent to [(TEG)1Li]NTf2. This places FTEG and TEG in competition to coordinate a limited number of Li+ ions. The resulting mixtures exhibit conductivity and viscosity enhancements over the parent SIL and minimal changes in ion speciation due to the poor Li+ binding by FTEG. Positron annihilation lifetime spectroscopic studies point to increased amounts of free volume upon dilution of FTEG. This likely explains the origin of the conductivity enhancement.
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