Phase behavior of CO2/Organic Solvent/Poly(ethylene glycol) ternary systems

• Phase diagrams of CO 2 /EtOH/PEG and CO 2 /DMSO/PEG were systematically measured. • Cosolvent species strongly influenced ternary liquid–liquid phase separation. • Free volume fraction explained cosolvent effects on Px phase boundaries. • Dipole and quadrupole moments clarified mutual solubility among components. • Findings advance prediction of CO 2 /cosolvent/polymer phase behavior. Phase diagrams of CO 2 /cosolvent/polymer systems are critical for polymer processing applications, yet most studies have focused on pressure–temperature diagrams, leaving pressure–composition ( Px ) diagrams less understood. In particular, the influence of cosolvent species on ternary Px phase behavior and its molecular-level origin has not been systematically investigated. This study examined the phase behavior of CO 2 /ethanol (EtOH) or dimethyl sulfoxide (DMSO)/poly(ethylene glycol) (PEG) ternary systems to evaluate the effect of cosolvent species on the phase diagram. Measurements were performed using a synthetic method combined with laser displacement and turbidity detection. Bubble points (vapor–liquid (VL) phase separation) were determined from piston displacement changes, and cloud points (liquid–liquid (LL) phase separation) were identified from turbidity variations. Phase boundaries were obtained for CO 2 mass fractions of 0.148–0.680 while varying the cosolvent/PEG mass ratio. The polymer did not participate in VL phase separation, reflecting the affinity between CO 2 and the cosolvent. By contrast, LL phase separation required consideration of the mutual affinities among all three components. To support this interpretation, the free volume fraction difference ( Δf ) estimated from the Sanchez–Lacombe equation and the dipole and quadrupole moments obtained from quantum chemical calculations were analyzed. The Δf values explained the cosolvent-dependent shifts in the LL line, whereas the μ and Q values clarified the order of mutual solubility (EtOH > DMSO > Tol) in agreement with experimental observations. These findings provide a more comprehensive understanding of the CO 2 /cosolvent/polymer phase behavior and facilitate the prediction of phase diagrams.