Coupling between the mesoscopic dynamics and shear stress of a room-temperature ionic liquid.

Molecular dynamics (MD) simulation is performed on two imidazolium-based ionic liquids, one of which possesses a long alkyl chain whereas the other does not, and the coupling between the liquid structure and the shear stress is examined by means of cross-correlation analysis. The former liquid exhibits a prepeak representing the polar-nonpolar domain structure whereas the latter does not. The relaxation of the intermediate scattering function at the prepeak is slower than those of other microscopic structures. Nonetheless, the principal part of the viscoelastic relaxation is ascribed to the relaxation of the charge-alternation mode in both liquids. In particular, the dynamics of the domain structure hardly seems to affect the viscosity of the ionic liquid with a long alkyl chain, although the cross correlation between the shear stress and the domain mode exists and the relaxation of the cross correlation with the domain mode is far slower than that with the charge alternation mode. The absence of the domain dynamics in the viscoelastic relaxation is ascribed to the decoupling between the relaxations of the short- and the long-range structures.