BACKGROUND The selection of competent entrainer is very important for separating azeotrope systems by extractive distillation. Ionic liquids (ILs) are outstanding entrainers due to their excellent properties. However, it is time-consuming to choose suitable ILs from the huge database of anions and cations. Herein, the selectivity and viscosity of ILs were combined as double standards to select suitable ILs for the separation of acetonitrile and ethanol mixture. RESULTS The selectivity and viscosity of 450 ILs (25 cations and 18 anions) were predicted based on Conductor-like Screening Model for Real Solvents (COSMO-RS). 1-Butyl-3-methylimidazolium acetate [BMIM][Ac] was selected as a potential entrainer due to its higher selectivity and lower viscosity and [BMIM][Cl] was chosen as a comparison item as ILs with [Cl](-) anion have separation performance next to [Ac](-)-based ILs. The vapor-liquid equilibrium (VLE) data of the acetonitrile + ethanol + ILs systems were determined at 101.3 kPa and correlated with the non-random two-liquid (NRTL) model. The result showed that the addition of the two ILs produced a salting-out effect on acetonitrile and enhanced the relative volatility of acetonitrile to ethanol. Furthermore, [BMIM][Ac] exhibited better separation performance than [BMIM][Cl] and our previous reported ILs, which are consistent with the predicted results. The reduced density gradient (RDG) method indicated that hydrogen bonding dominated the interactions between ethanol and IL. CONCLUSION [BMIM][Ac] is a superior entrainer for the separation of acetonitrile-ethanol azeotrope. Compared to the previously studied ILs, there is a stronger hydrogen bond between ethanol and [BMIM][Ac]. This makes ethanol 'bundled' and acetonitrile more volatile, resulting in the superior separation performance of [BMIM][Ac]. (c) 2019 Society of Chemical Industry