Melting Behavior and Ionic Conductivity in Hydrophobic Ionic Liquids

Four room-temperature ionic liquids (RTILs) based on the N-butyl-N-methyl pyrrolidinium (Pyr(14)(+)) and N-methyl-N-propyl pyrrolidinium cations (Pyr(13)(+)) and bis(trifluoromethanesulfonyl)imide (TFSI(-)) and bis(fluorosulfonyl)imide (FSI(-)) anions were intensively investigated during their melting. The diffusion coefficients of (1)H and (19)F were determined using pulsed field gradient (PFG) NMR to study the dynamics of the cations, anions, and ion pairs. The AC conductivities were measured to detect only the motion of the charged particles. The melting points of these ionic liquids were measured by DSC and verified by the temperature-dependent full width at half-maximum (FWHM) of the (1)H and (19)F NMR peaks. The diffusion and conductivity data at low temperatures gave information about the dynamics at the melting point and allowed specifying the way of melting. In addition, the diffusion coefficients of (1)H (D(H)) and (19)F (D(F)) and conductivity were correlated using the Nernst-Einstein equation with respect to the existence of ion pairs. Our results show that in dependence on the cation different melting behaviors were identified. In the Pyr(14)-based ILs, ion pairs exist, which collapse above the melting point of the sample. This is in contrast to the Pyr(13)-based ILs where the present ion pairs in the crystal dissociate during the melting. Furthermore, the anions do not influence the melting behavior of the investigated Pyr(14) systems but affect the Pyr(13) ILs. This becomes apparent in species with a higher mobility during the breakup of the crystalline IL.