Relationship between mesoscale dynamics and shear relaxation of ionic liquids with long alkyl chain.

The shear relaxation spectra of three imidazolium-based ionic liquids, 1-methyl-3-octylimidazolium chloride ([C(8)mim][Cl]), 1-methyl-3-octylimidazolium hexafluorophosphate ([C(8)mim][PF(6)]), and 1-dodecyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C(12)mim][TFSA]) were measured and compared with the intermediate scattering functions determined with neutron spin echo (NSE) spectroscopy. The shear relaxation is slower than that predicted from the relaxation of the main peak of the structure factor that is common to other molecular liquids, whereas it is faster than that from the relaxation of the pre-peak, that corresponds to the correlation length of about 10 nm specific to ionic liquids with an intermediately long alkyl chain. The role of the pre-peak structure in the mechanism of shear viscosity of ionic liquids is discussed based on the comparison between NSE and shear relaxations.