Temperature-dependent anomalous viscosity of aqueous solutions of imidazolium-based ionic liquids

Ionic liquids (ILs) are organic salts with a low melting point compared to inorganic salts. Room temperature ILs are of great importance for their widespread potential industrial applications. The viscosity of aqueous solutions of two imidazolium-based ILs, investigated in the present study, exhibits an anomalous temperature variation. Unlike conventional molecular fluids, the viscosity of 1-methyl-3-octyl imidazolium chloride [OMIM Cl] and 1-methyl-3-decyl imidazolium chloride [DMIM Cl] solutions is found to increase with temperature and then depress. The Small Angle X-ray Scattering (SAXS) data suggest that the lattice parameter of the body-centered cubic lattice formed by the spherical micelles of these ILs, and the morphology of the micelles remain intact over the measured temperature range. The molecular dynamics simulation shows the micelles to be more refined with their integrated structure on increasing the temperature. On further increase of the temperature, the structure is found to be loosened, which is corroborated by the simulation work. The ionic conductivity of these IL solutions shows a trend that is opposite to that of the viscosity. The observed anomalous nature of the viscosity is attributed to the trapped dissociated ions in the network of the micellar aggregates.