1-propyl-3-methylimidazolium cation with glycinate, alaninate, valinate, leucinate, cysteinate, and seri- nate as amino acid anions were selected as the counter ions to study the effect of the side chain group of anions on the absorption of SO2. One can see that the enlargement of the alkyl group from glycinate to leucinate leads to a slight decrease in gas absorption interaction energy; changing the side chain group to the methyl thiol (cysteinate) and methyl alcohol (serinate), drastically changes gas nteraction energy with the same trend in absorption Gibbs energy. In addition, the charge transfer between the imidazolium-based amino acid ionic liquid and captured gas as well as the distance between anion and the absorbed SO2 gas in glycinate is the lowest which can be the main reason for being the most favorable absorbent in the present study. A deep insight into the nature of interactions sheds light on the difference in the distribution of van der Waals (vdW) interaction that plays the essential role in the SO2 capturing; in other words, vdW distribution in the glycinate system is concentrated and clear while in the other systems, this distribution is observed in a scattered manner.

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