Linker-length dependence of the reorientational dynamics and viscosity of bis(imidazolium)-based ionic liquids incorporating bis(trifluoromethanesulfonyl)amide anions

A series of bis(imidazolium)-based ionic liquids (ILs) with alkylene linkers of various lengths (carbon number = 5–12) was characterised with respect to viscosity and reorientational dynamics with bis(trifluoromethanesulfonyl)amide as the counteranion. From the correlation times obtained by 13C NMR spectroscopy, it was confirmed that a linker-length dependence of the viscosities of the series was closely related to the reorientational motion of the entire cations. The variation of such motion with elongation of the linker length results from a balance of multiple interactions, namely, coulombic repulsion between the linked cations and van der Waals interactions between the linkers.