Room temperature ionic liquids are an emerging class of solvents, which are mostly composed of bulky organic cations and inorganic anions. They have many properties distinguishing them from conventional solvents and molten salts, especially their heterogeneous and long-range structure. Many applications are emerging as a result of these properties including energy-storage, electrotunable nano-lubrication, and gas capture. However, there are many unanswered questions regarding their macro- and micro-scale properties. Molecular dynamics simulations have been used to understand their behavior in nano-scale systems. However, they are computationally expensive for large time- and size-scale. In this study, we developed coarse-grained force fields for imidazolium-based ionic liquids, which reproduce the thermodynamic (density and pressure) and structural properties of reference all-atom models for different alkyl chain length at different thermodynamic states. The coarse-grained force field is developed using the relative entropy method to obtain short-range van der Walls and long-range Coulombic interactions with an additional constraint in order to reproduce pressure. The method developed is generalizable to various ionic liquid classes, and it can pave the way for accurate study of large size- and time-scale systems, which, in turn, advances our fundamental understanding of ionic liquids behavior as solvents of future.
