Modeling of ionic liquid solubility in supercritical carbon dioxide + co-solvent phase predicted by ε*-modified Sanchez-Lacombe equation of state

Abstract

Supercritical fluid deposition (SCFD) using supercritical carbon dioxide (CO₂) has environmental advantages and enables the uniform and precise impregnation of ionic liquids (ILs) into porous supports. In general, the solubility of ILs in CO₂ is extremely low, but it can be greatly increased by adding a co-solvent. However, due to the diversity of IL and co-solvent combinations, models for predicting IL solubility remain limited. This study employs the ε*-modified Sanchez-Lacombe equation of state (ε*-mod SL-EoS) to predict IL solubility in supercritical CO₂ + co-solvent systems. Compared to the Peng-Robinson equation of state (PR-EoS) previously used, ε*-mod SL-EoS, derived from lattice fluid theory, has superior predictive capabilities, particularly for systems involving heavy molecules like ILs. Pure component parameters for ILs, CO₂, and co-solvents were determined through high-pressure density and vapor pressure correlations. Binary interaction parameters for IL + CO₂, CO₂ + co-solvent, and IL + co-solvent systems were fitted using available phase equilibrium data. Ternary phase equilibrium predictions using ε*-mod SL-EoS showed improved accuracy, achieving an average logarithmic AARD of 11.0%, outperforming PR-EoS (13.8%). The results highlight the ε*-mod SL-EoS as a robust predictive tool for IL solubility in CO₂-rich phases, even under dilute conditions with co-solvents, offering valuable insights for optimizing the SCFD processes.