Mechanical spectroscopy study of ionic liquids with quaternary cations: Effect of conformational flexibility

Mechanical spectroscopy measurements were performed on ionic liquids (ILs) with quaternary cations, either ammonium or phosphonium, and two anions, bis(trifluoromethanesulfonyl)imide ([NTf2]-) and bis(trifluoromethanesulfonyl)methanide ([CHTf2]-), which have almost the same molecular weight and degree of fluorination but exhibit different flexibility. Furthermore, the effect of multiple ether functionalization was studied by measuring the mechanical spectrum of [P(2O2)31][CHTf2]. The measured data evidence the occurrence of a relaxation process in the liquid phase of the ILs with flexible [NTf2]- anion and of [P(2O2)31][CHTf2], which is analyzed by means of a modified Debye model relating the peak to the ion hopping between non-equivalent configurations. The analysis confirms the involvement of nonequivalent anion configurations as well as their energy separation. For other ILs having rigid ions, instead, a fast dynamic at local level in the liquid phase is rarely observed since a partial transition to a solid state is favored. For these liquids, the measurements suggest the formation of aggregates which is suppressed by the multiple ether functionalization.