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 specific 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 find evidence supporting a recently proposed scheme for control of solvent polarity. Finally, we show that nonspecific electrostatic solute−solvent interactions are screened by the ionic solvent, dictating that the thermodynamic properties of solvation are determined by local interactions. However, thermal fluctuations create transient long-ranged solute−solvent interactions that could be important for chemical kinetics.

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