Thermodynamic properties of water in aqueous amine solutions studied by energy-representation method

The use of aqueous amine solutions for CO₂ capture is considered as a promising strategy for mitigating global warming, and reducing the energy consumption of the capture process is essential for its widespread adoption. Understanding the mechanisms that govern the vapor pressure of water and enthalpy of water vaporization of CO₂-loaded solutions can contribute to lowering the energy cost of CO₂ capture. In this study, molecular dynamics simulations of aqueous amine solutions before and after CO₂ absorption were performed, and the solvation free energy of water was evaluated using the energy representation method. The results revealed that vapor pressure decreases by CO₂ absorption, particularly due to the formation of anions that strongly interact with water through hydrogen bonding. The contributions of each solvent species to the solvation free energy and solvation energy were also quantified. It was found that the enthalpy of vaporization increases with amine concentration both before and after CO₂ absorption. Before CO₂ absorption, this increase is attributed to enhanced energetic interactions, despite the destabilization in solvation free energy due to entropic contributions. Additionally, snapshots indicated water localization in certain systems. Analysis of concentration fluctuations showed that concentration regions with greater fluctuations tended to exhibit smaller changes in vapor pressure with respect to concentration, suggesting a correlation between microscopic structure formation and macroscopic thermodynamic properties.