Thermodynamics and Rheology of Imidazolium-Based Ionic Liquid-Oil Mixtures: A Molecular Simulation Study

Conventional lubricants decrease the wear and friction between rolling and sliding surfaces while raising environmental concerns. The thermodynamics and flow behavior of lubricants that contain environmentally friendly additives such as ionic liquids (ILs) are of interest to industries that require lubricants with superior tribological properties. Noncorrosive ILs are promising additives for conventional oil that not only further decrease friction but also create a less hazardous alternative to existing lubricants. In this study, thermodynamics, dynamics, and rheology of IL-oil mixtures are studied by using atomistically detailed molecular dynamics (MD) simulations. A combination of different imidazolium-based cations with linear ([CnC1Im]+[NTf2]-, n = 3, 7) and branched chains ([(n - 2)mCn-1C1Im]+, n = 3, 7) and bis[(trifluoroethane)sulfonyl]imide ([NTf2]-) anion are selected as the model IL which is suspended in bulk hexadecane. The effects of IL content, architecture, size of cation, and temperature are examined on the thermorheological and dynamics of the IL-hexadecane mixture. At equilibrium, ILs self-assemble and create clusters with different shapes and sizes and affect the dynamics and rheology of the mixtures. Mixtures show a significant deviation from ideal solutions behavior. Shear and extensional rheology simulations show that mixtures mostly show Newtonian behaviors at low and moderate rates which is followed by a weak thinning behavior at high rates. It is shown that a small addition of ILs (x=2% mole fraction) to oil significantly increases both the shear and extensional viscosity of the mixture by a factor of 3.