Mechanisms of CO₂ Absorption in Amino Acid-Based Deep Eutectic Solvents: Insights from Molecular Dynamics and DFT Calculations

This study explores the mechanisms of CO₂ absorption in two amino acid-containing deep eutectic solvents (DESs) through molecular dynamics (MD) simulations and density functional theory (DFT) calculations. The MD simulations, which focus mainly on physical absorption, reveal that alanine-based DES (Ala DES) exhibits higher CO₂ solubility than l-arginine-based DES (l-arg DES), attributed to stronger physical absorption. Furthermore, the hydrogen bond donor paired with the amino acids is identified as a critical factor for enhancing physical absorption efficiency. DFT calculations, which account for chemical absorption, investigate two reaction pathways: single-molecule reactions involving intramolecular proton transfer and two-molecule reactions involving intermolecular proton exchange. While Ala DES does not exhibit spontaneous chemical absorption, l-arg DES demonstrates such reactions, leading to the formation of carbamic acid or carbamate (Δ<i>G</i> < 0), indicative of CO₂ capture through chemical interactions. Consequently, Ala DES primarily relies on physical absorption, whereas l-arg DES utilizes multiple reactive sites for chemical absorption. These results are consistent with experimental findings, which show that l-arg DES achieves higher CO₂ solubility under atmospheric conditions. Overall, our study highlights the interplay between DES components and reactivity in enhancing CO₂ capture efficiency.