Deep eutectic solvents (DES) composed of ionic liquids as hydrogen bond acceptors (HBAs) and amines as hydrogen bond donors (HBDs) are promising candidates for CO₂ capture due to their tunability, low volatility and low cost. Despite progress in developing DES formulations, the molecular mechanism underlying CO₂ capture and regeneration processes remain poorly understood. These processes involve multiple steps, including physisorption, chemisorption, proton transfer and desorption. A key step is proton transfer from the HBA cation or HBD to the HBA anion, which helps activate nucleophilic sites for CO₂ chemisorption. This step along with the final binding energy of chemisorbed CO₂ are both thought to influence DES performance. However, how the thermodynamics and kinetics of these steps vary with HBD composition remains poorly understood. This knowledge gap limits the optimization of DESs. As a step towards addressing this, we use density functional theory (DFT) to investigate the free energy landscapes of CO2 capture and regeneration in DES comprising choline as the HBA cation and imidazole as the HBA anion and either mono-ethanolamine (MEA), di-ethanolamine (DEA), or methyl-diethanolamine (MDEA) as the HBD. These model HBDs were selected to compare how amine structure affects chemisorption, as MEA and DEA can donate protons, while MDEA, being a tertiary amine, cannot. Our results show that chemisorption of CO₂ to the HBD is thermodynamically more favorable than HBA in the DES system based on MEA and DEA; however, CO₂ chemisorption is favorable on HBA in the DES system based on MDEA. This difference arises from the availability of the amine (–NH₂) group in MEA and DEA, which enables proton transfer from the HBD to the HBA, thereby facilitating chemisorption. MDEA, lacking an amine group and possessing a more basic hydroxyl group than choline, instead promotes CO₂ binding at the HBA site. These molecular level insights show how HBD structure influences proton transfer, CO₂ binding strength, and chemisorption pathways, offering guidance for designing more effective DES for CO₂ capture and release.
