Molecular Dynamics Simulation Study on CO₂ Physical Absorption Mechanisms for Ethylene-Glycol-Based Solvents Using Free Energy Calculations

Molecular mechanisms of CO₂ physical absorption are significant for the development of higher-performance CO₂ absorbents. In this study, the free energy profiles for CO₂ approaching several types of ethylene glycol (EG)-based solvent molecules and their derivatives (diols, ethers, and alkanes, etc.) were evaluated using molecular dynamics simulations. A desolvation barrier appeared in the free energy profiles in the pure component solvent, corroborating the negative contribution of a void effect on CO₂ affinities. The profiles in vacuum were also estimated to investigate the functional group effects in a constituent solvent molecule. The profiles for the EG-based solvent molecules showed similar minimum values to their derivative diols, while the values for their derivative ethers and alkanes were markedly higher. CO₂ solubility is certainly determined by two major factors: the specific CO₂ affinities of the constituent hydroxyl groups and a steric void effect near an absorbed CO₂ molecule.