Polarization Effects in Binary [BMIM⁺][BF₄^-]/1,2-Dichloroethane, Acetone, Acetonitrile, and Water Electrolytes

Organic electrolytes are unique in that their constituent solvents may be of much lower dielectric strength than water. This is because the ions of organic electrolytes are often those that comprise room-temperature ionic liquids, which are surprisingly miscible with many different organic solvents of low dielectric strength. Strong ion correlation results from the relatively low dielectric screening of the solvent, resulting in properties that can substantially deviate from Debye-Huckel descriptions; in addition remains the fundamental question of why the ionic liquids and low dielectric solvents are even miscible in the first place. In this work, we study electrolyte mixtures composed of the room-temperature ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM⁺][BF₄^-] mixed with four solvents of systematically varying dielectric strength, 1,2-dichloroethane, acetone, acetonitrile, and water. We show that miscibility with the lowest dielectric solvent, dichloroethane, is directly attributed to ion solvation enhancement mediated by both electronic and conformational polarization of the solvent molecules. A strong dielectric enhancement results from the ∼70-80% increase in solvent dipole moments at high ion content, providing significantly better solvation than predicted by the bulk solvent dielectric strength. This implies a general mechanism for miscibility of ionic liquids with low dielectric solvents, through which ions effectly create their own solvation dipoles by polarizing the local environment. From a purely computational perspective, our results imply that explicitly polarizable force fields are essential for modeling many of the organic electrolytes that are used in electrochemical applications.