Exfoliation of Graphene in Ionic Liquids: Pyridinium versus Pyrrolidinium

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 <i>N</i>-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 engineer more advanced solvents. The reported results support development of new higher-performance RTILs for use in solvent-assisted graphene exfoliation