Viscosity prediction of CO₂-saturated imidazolium-based ionic liquids using the <i>ε</i>*-modified Sanchez–Lacombe equation of state and free volume theory with a new correction term

Ionic liquids (ILs) have gained attention as solvents for carbon dioxide (CO₂) separation and as catalysts for CO₂ fixation. Viscosity is a fundamental transport property of ILs because it plays a significant role in their process efficiency. In this study, a viscosity prediction model for CO₂-saturated ILs was developed by combining the <i>ε</i>*-modified Sanchez-Lacombe equation of state (<i>ε</i>*-mod SL-EoS) and free volume theory (FVT) with a new correction term, <i>βx</i>', where <i>β</i> and <i>x</i>' are the correction factor and molar ratio of CO₂ to IL, respectively. The viscosities were predicted by incorporating the mixture density, calculated using the <i>ε</i>*-mod SL-EoS, into the FVT, and the parameters for <i>ε</i>*-mod SL-EoS and FVT were calculated by correlating the high-pressure densities and viscosities of the ILs. Although the initial deviations, which are due to the affinity between ILs and CO₂, were considerable for the five imidazolium-based ILs studied in this work, they were improved by introducing <i>βx</i>'. <i>β</i> could be calculated using the solubility parameters of the ILs and CO₂, without requiring correlation. The average absolute relative deviations were 6.05-35.3% in the range of <i>x</i>' < 1.0, sufficiently predicting the viscosity of the IL + CO₂ mixtures.