Influence of the Alkyl Chain Length on the Low Temperature Phase Transitions of Imidazolium Based Ionic Liquids

The effects induced by alkyl chain length on the thermal phase behavior for a series of imidazolium based ionic liquids (ILs) containing the anion bis(trifluoromethanesulfonyl)imide (TFSI) were investigated by infrared spectroscopy combined with density functional theory (DFT) calculations. In agreement with previous results on pyrrolidinium and ammonium based ionic liquids sharing the TFSI anion, our study shows that with increasing the length of the alky chain only the less stable cis-TFSI is retained in the solid state, while for shorter chain the trans-TFSI predominates in the crystalline phase. Also, we examined the remarkable effect on the packing efficiency due to the addition of 2-hydroxyethyl group on the imidazolium cation ring, reporting that the absence of crystallization is observed in correspondence with the presence of both the conformers of TFSI in the whole temperature range (150-325 K). Moreover, a detailed study of 1-ethyl-2,3-dimethylimidazolium-TFSI (EDMIM-TFSI) reveals the existence of cation rotational isomerism. In the liquid phase both planar (P) and non-planar (Np) conformers are present in the equilibrium, while a conformational change for the CNCC dihedral angle of the EDMIM cation leads to stabilize the formation of the more stable non-planar geometry in the crystalline phase.