Systematic and Comparative Study on the Green Separation of Toluene-Alkanol Azeotropic Mixtures Using a Sustainable Deep Eutectic Solvent via Liquid–Liquid Equilibria

Abstract In this study, a deep eutectic solvent composed of choline chloride and propylene glycol (ChCl:PG, 1:4 molar ratio) was prepared for extracting alkanols from toluene via liquid–liquid equilibrium (LLE) experiments at 298.2 K and 101 kPa. A homologous series of alkanols (methanol to 1-pentanol) was selected to systematically investigate the influence of alkyl chain length and polarity on intermolecular interactions and the resulting LLE behavior of {toluene + (C1–C5) alkanols} binary azeotropic mixtures. The ChCl:PG DES exhibits good chemical stability, low toxicity, high biodegradability, and aligns with green chemistry principles, making it a sustainable, environmentally friendly, and economically viable solvent for industrial applications. Phase diagrams were constructed and the experimental tie-line data were correlated using the NRTL thermodynamic model, and the binary interaction parameters were determined accordingly. The consistency of the obtained parameters was validated through topological analysis based on the Gibbs tangent plane criterion. The reliability of the experimental tie-line data was verified using the Othmer-Tobias correlation. Solute distribution coefficients and selectivity values were subsequently calculated to evaluate the separation efficiency of the prepared DES. This work provides the first experimental LLE data for {toluene + C1–C5 alkanols + ChCl:PG} ternary systems.