Pressure and shear rate effects on viscosity and structure of imidazolium-based ionic liquids

Abstract

A lubricant film can be subject simultaneously to very high pressures and shear rates under machine operation. Due to the promising applications of ionic liquids (ILs) as lubricants and as lubricant additives, it is crucial to know how the physical properties and molecular structure of these liquids change under these extreme conditions. Non-equilibrium molecular dynamics (NEMD) simulations were performed to determine the apparent viscosity as a function of the shear rate of the ILs 1-ethyl-3-methyl-imidazolium tetrafluoroborate ([EMIM][BF4]) and 1‑butyl‑3-methyl-imidazolium tetrafluoroborate ([BMIM][BF4]) under pressures of 0.101, 507 and 1013 MPa. For comparison purpose, a similar study was also conducted for benzene. The Carreau equation was used to describe the shear rate dependence of both the viscosity and the structure. We found that the zero shear viscosity and the relaxation time exhibit a remarkable pressure dependence. Under high pressure, the shear tinning behavior starts at lower shear rates, and that correlates with the weakening of the first coordination shell of anions around cations. Both imidazolium ring and benzene assume preferentially an orientation parallel to the flow at high shear rate, and this effect is intensified at high pressure.