Drastic influence of amide functionality and alkyl chain length dependent physical, thermal and structural properties of new pyridinium-amide cation based biodegradable room temperature ionic liquids

A series of new room-temperature ionic liquids (RTILs) based on biodegradable pyridinium carboxamide cation and conformationally flexible NTf₂ anion have been synthesised. The variation of thermo-physical properties of these RTILs with alkyl chain length size has been studied and compared with corresponding parental pyridinium NTf₂ ILs. A drastic change in thermophysical properties has been noted on the introduction of the amide functionality on pyridinium cation moiety. The crucial role of amide functionality is also important concerning biodegradability as it acts as an active site for an enzymatic attack, helpful toward biodegradation. Besides, the dependence of thermal and physical properties on alkyl chain length is also noteworthy. A detailed DFT based theoretical studies substantiate the experimental observation of chain length dependence melting point variation and successfully correlated that with stabilisation energy. Conformational dynamics of NTf₂ anion (between transoid and cisoid) has been elucidated by Raman spectroscopic studies. Interestingly, the data demonstrate that the conformation of NTf₂ anion depends on the chain length of the cation, clearly indicating anionic interaction with the hydrophobic region of the cation. This information is crutial as it vindicates the fact that multiple weak interaction do exist in ILs making it a complex nonostructured system composed of ions.