COSMO-RS prediction, liquid-liquid equilibrium experiment and quantum chemistry calculation for the separation of n-butanol and n-heptane system using ionic liquids

•[EMIM][DCA], [BMIM][DCA] and [HMIM][DCA] with low viscosity were selected as the appropriate extractants to separate n-butanol from n-heptane with the application of the COSMO-RS model.•The liquid–liquid equilibrium data of the ternary system n-heptane + n-butanol + [EMIM][DCA]/ [BMIM][DCA]/ [HMIM][DCA] were measured at 298.15 K and 101.3 kPa.•The liquid–liquid experimental data were well correlated by the NRTL model.•The interactions between ILs and n-butanol were analyzed through the RDG function and QTAIM analysis based on DFT calculations.