Structure of protic (HCnImNTf2, n = 0-12) and aprotic (C1CnImNTf2, n = 1-12) imidazolium ionic liquids: A vibrational spectroscopic study

The interactions of alkyl substituted imidazolium bis(trifluoromethanolsulfonyl)imide protic (PILs) HCnImNTf2 (n = 0-12) and aprotic ionic liquids (APILs) C1CnNTf2 (n = 1-12) were studied using vibrational spectroscopy (FT-IR/ATR and FT-Raman). The effect of alkyl substituent length (n = 0-12) on both polar and non-polar regions is elucidated. A sponge like structure is proposed for both systems. The spectral characteristics show that there are certain structural differences between PILs and APILs. The well defined hydrogen bonding in PILs strongly affects the local structure of both the polar head (i.e., induction effect) and the side alkyl chain. The spectral findings tentatively suggest that nanosegregation occurs at shorter alkyl chains. In APILs the data correlated with the polar and non-polar regions are discussed on the basis of the proposed sponge like structure. The trans/cis ratio of the anion conformers is related to the dispersion of the average position of the anion over the cation. Also in the oily phase the local structure of the side chain shows certain differences compared to PIL systems. A tail coupling occurred (up to eight carbon atom alkyls) followed by decoupling in higher length tails in APILs indicating an increase of van der Waals forces between the chains. Moreover, we show that the enthalpy of conformational isomerism of the anion is also a good indicator in the case of APILs, regarding the relative magnitude of these interactions.