< p>In this work, a modifed UNIFAC model was proposed to investigate the solubility of acid gases, i.e., CO2 and H2S in nine imidazolium-based ionic liquids (ILs). The influence of effective parameters including temperature, the nature of anions and cations of the ILs, and the length of alkyl chain
on the solubility of acid gases in the ILs were studied. The interaction parameters between the new functional subgroups of the fragmented ILs such as [IM], [BF4], [PF6], [Tf2N], CH3 and CH2 with CO2 and H2S molecules were reported using the original UNIFAC model. In the proposed model, the
segment fraction in the Flory-Huggins term of the UNIFAC combinatorial part was modifed considering the free-volume differences between the ionic liquid and the acid gas molecules. While the free-volume parameter represents the free-volume percent ratio of ionic liquid to that of the acid gas, it was regressed for each system using the extensive VLE experimental data from literature. It was verfed that the free-volume parameter can be a linear fuction of the molecular weight of the ionic liquids and, thus, only the values for the molecular weight of the ionic liquids are required to estimate the free-volume parameters. The modeling results were compared with those of the original UNIFAC model. The results showed that the proposed model accurately correlated the VLE experimental data for the systems containing ILs in the presence of CO2 and/or H2S at pressures up to 150 bar.