Exfoliation of Graphene in Ionic Liquids: Pyridinium versus Pyrrolidinium

Abstract

Exfoliation is an elegant physical technique to directly obtain graphene from graphite. A search for high-performance exfoliation solvents is actively pursued nowadays. We report potentials of mean force and free energies, characterizing steered separation of graphene sheets and their subsequent solvation by room-temperature ionic liquids (RTILs). The exfoliation performance of N-butylpyridinium bis(trifluoromethanesulfonyl)imide [BPY][TFSI] is compared to that of 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide [PYR][TFSI]. Both RTILs are shown to exhibit comparable exfoliation performance, which is superior to the performance of 1-ethyl-3-methylimidazolium tetrafluoroborate by ∼20 kJ mol–1 nm–1. The sulfur atom of TFSI– contributes an essential portion of the solvation enthalpy due to its dispersive attraction to carbon atoms of graphene. Both RTILs maintain a single, moderately defined shell around the graphene surface. [BPY][TFSI] and [PYR][TFSI] are suitable starting compounds to engi...