Suitability prediction and affinity regularity assessment of H2O + imidazolium ionic liquid working pairs of absorption cycle by excess property criteria and UNIFAC model

• Improved the working pair evaluation method by UNIFAC model.
• Proposed new group interaction parameters for H2O + IL systems.
• Clarified the affinity regularity of working pairs of H2O and imidazolium ILs.
• Proposed two sets of preferred working pairs, H2O + [DMIM]DMP and H2O + [DMIM]Cl.

The aim of this work is to investigate the approach for selecting alternative working pairs of absorption cycle which consists of refrigerant H2O and absorbent short carbochain imidazolium ionic liquids (ILs). The studied ILs are composed by short carbochain imidazolium cations, such as 1,3-dimethylimidazolium ([DMIM]), 1-ethyl-3-methylimidazolium ([EMIM]), 1-butyl-3-methyl-imidazolium ([BMIM]), and 1-hexyl-3-methyl-imidazolium ([HMIM]), with seven kinds of anions, Cl, Br, tetrafluoroborate (BF4), dimethylphosphate (DMP), dibutylphosphate (DBP), ethyl sulfate (EtSO4), and trifluoromethanesulfonate (OTf). For the H2O + IL binary systems without experimental data, the UNIFAC model was used to predict the vapor-liquid equilibrium (VLE) data and excess property. Based on the available experimental data of H2O + IL binary systems, 28 new group interaction parameters for the UNIFAC model were obtained by means of correlating the activity coefficients of systems, which extended the evaluation and prediction range of H2O + IL systems for developing novel short carbochain imidazolium ILs working pairs based on seven kinds of anions, Cl, Br, BF4, DMP, DBP, EtSO4 and OTf. Then the extreme values of the excess Gibbs function (GmaxE) of H2O + IL systems were used as criteria to evaluate the suitability of the H2O + IL systems. It was found by the assessment that for short carbochain imidazole ILs, the effects of different anions on the GmaxE are more significant than the alkyl chain length of the cations. Moreover, halogen and phosphate anion ILs are suitable for working pairs of absorption cycles, such as 1,3-dimethylimidazolium chloride ([DMIM]Cl) and 1,3-dimethylimidazolium dimethylphosphate ([DMIM]DMP).