CO₂ solvation regimes in deep eutectic solvents revealed by internal-pressure profile

The thermodynamic internal pressure quantifies the balance of molecular-scale attractions and repulsive volume-exclusion effects, providing a basis for interpreting CO₂ solvation in deep eutectic solvents (DESs). By precisely correlating high-pressure pvTx data for representative type III ethylene glycol-based DESs (choline chloride, tetrabutylammonium bromide, tetrabutylphosphonium bromide) under CO₂-saturated conditions with a Hybrid-Tait equation, the resulting internal-pressure profiles allow identification of three thermodynamic response regimes: (i) progressive occupation of high-affinity CO₂ interaction environments, (ii) growth of CO₂-enriched regions associated with available free volume, and (iii) transition from supercritical gas-like to liquid-like behavior near the Widom-line pressure of pure CO₂. Unlike typical liquid solutions, CO₂-DES systems show a secondary rise in internal pressure at elevated CO₂ loadings, consistent with coupling between a non-volatile associative phase and an increasingly liquid-like supercritical phase. Internal-pressure profiles offer a quantitative framework for relating CO₂ solvation behavior, cohesive energy density, and thermodynamic response in DESs, with broader relevance to non-volatile associative liquid systems.