Measurement and Correlation of Liquid–Liquid Equilibrium for the Ternary System 1-Octyl-3-methylimidazolium Tetrafluoroborate + 1-Ethyl-3-methylimidazolium Tetrafluoroborate + Water

Mixed ionic liquids have excellent physicochemical properties; therefore, they are increasingly used in academic research. The separation of mixed ionic liquids is a key problem for the recycle of ionic liquids, and liquid–liquid extraction is an effective process to separate mixed ionic liquids. The COSMO-SAC model was used to screen solvents to separate the ionic liquid mixture 1-octyl-3-methylimidazolium tetrafluoroborate ([C8mim][BF4]) + 1-ethyl-3-methylimidazolium tetrafluoroborate ([C2mim][BF4]) by liquid–liquid extraction, and water is selected as solvent. The liquid–liquid equilibrium (LLE) data for the ternary system of [C8mim][BF4] + [C2mim][BF4] + water were measured at temperatures of 298.15, 308.15, and 318.15 K under atmospheric pressure. The experimental LLE data were well correlated by the nonrandom two-liquid model. The experimental results show that water is an effective solvent for the separation of the ionic liquid mixture of [C8mim][BF4] and [C2mim][BF4].