Conformational Changes and Phase Behaviour in the Protic Ionic Liquid 1-Ethylimidazolium Bis(trifluoromethylsulfonyl)imide in the Bulk and Nano-Confined State

We report a Raman spectroscopic study of conformational changes to the TFSI anion in the protic ionic liquid 1-ethylimidazolium bis(trifluoromethylsulfonyl)imide, [C(2)HIm][TFSI], in its bulk and nano-confined state. We show that the TFSI anion is found as a mixture of cis and trans conformations at room temperature and in the liquid state, whereas this equilibrium shifts towards an increased cis population upon confinement in silica. In addition, the strong Raman signature at ca. 743 cm(-1) assigned to TFSI is found at systematically higher frequencies in the confined state. These findings suggest a higher packing efficiency, or density, for the ionic liquid at the silica surface. Moreover, the enthalpy of conformational change is only marginally affected upon confinement (4.32 +/- 1.30 vs. 5.27 +/- 1.09 kJmol(-1)), and the entropy is found to be a few Jmol(-1)K(-1) higher in the confined state. Raman spectra recorded upon heating from very low temperatures show that the phase behaviour of the ionic liquid is also affected by confinement, with crystallization upon cooling being frustrated in favour of an amorphous glassy phase. To summarize, our results indicate that the interaction established in the ionogels between the silica surface and the protic ionic liquid [C(2)HIm][TFSI] favours local structural disorder, in conceptual agreement with the slightly higher experimentally estimated entropy.