A Molecular Dynamics Study of Polarity in Room-Temperature Ionic Liquids

In this work, we use molecular dynamics simulation to explore the physical principles governing the polarity of room-temperature ionic liquids. We use the calculated absorption spectrum of the solvatochromic dye molecule betaine-30 as a proxy for polarity, and characterize the solute-solvent interactions responsible for the solvatochromic shift. We analyze specic solute-solvent interactions and discuss the complications posed by the proximity of counterions in solution. We also explore the question of competition between solute-solvent and solvent-solvent interactions, and nd evidence supporting a recently-proposed scheme for control of solvent polarity. Finally, we show that nonspecic electrostatic solute-solvent interactions are screened by the ionic solvent, dictating that the thermodynamic properties of solvation are determined by local interactions. However, thermal uctuations create transient long-ranged solute-solvent interactions that could be important for chemical kinetics