Tab. The ADCH charge differences between the reactive sites of the C atom in CO2 and the O atom in water (1C-5O), and those of the N atom in amine and the H atom in water (7N-4H), as well as the relative activation Gibbs free energies of the transition state conformers in the MDEA-H2O system. The variation of amine substituents often has a great impact on amine-based CO2 absorption rate constants. For example, Jorgensen et al. report that using the electron-donating amine substituents (e.g., NH2CH3, etc.) or increasing the number of substituents on amines is able to improve the interaction between CO2 and amine. Xiao et al. study ten commercial tertiary amines and find that the ethyl group and the side carbon chain are able to promote the activity of tertiary amine, and the increase of hydroxyl group is able to reduce the reaction rate. The study by Muchan et al. shows that the increase in the number of amine groups is able to increase the absorption rate of CO2. This work also finds that the 𝑘 exp L of three structurally similar amines (DETA, AEEA, and DEA) increase with the decrease of their numbers of hydroxyl groups and the increase of their numbers of amine groups, as shown in Tab. 4.