Effects of Lithium Salts on Shear Relaxation Spectra of Pyrrolidinium-Based Ionic Liquids

The shear relaxation spectra of the solutions of lithium
salts
in ionic liquids composed of N-methyl-N-propylpyrrolidinium cation paired with bis­(trifluoromethanesulfonyl)­amide
(TFSA–) or bis­(fluorosulfonyl)­amide (FSA–) anions are determined from 5 to 205 MHz at various concentrations
of lithium salts. The addition of lithium salt retards the shear relaxation,
together with the increase in the shear viscosity. The normalized
spectra reduce to a single curve when plotted against the product
of the frequency and zero-frequency shear viscosity, which indicates
that the increase in the shear viscosity by lithium salts is ascribed
to the increase in the relaxation time. The difference in the shear
viscosity of TFSA- and FSA-based ionic liquids is also elucidated
in terms of the shear relaxation time. The relationship with previous
studies on ionic mobility and liquid structure is also discussed.