A series of aliphatic amino acid ionic liquids (AAILs) composed of N7,N9-dimethyladeninium cation with amino acid anion as the functionalized ILs with dual groups of amine have been designed for CO2 capture. The most possible adsorption mechanisms were investigated and based on the obtained data, CO2 is chemically adsorbed on the both amine parts in carbamic acid and carbamate formation processes. There is more possibility for carbamic acid formation in amine group anion than the corresponding cation part. The effect of branching and length of the alkyl side chain of the amino acid anions on CO2 capture in the ionic liquids (ILs) was evaluated. Quantum theory of atoms in molecules analysis confirmed the covalent nature of the N–C bond formed between CO2 and N atom of the amine groups. Natural bond orbital analysis indicated that electric charge transfers from the nitrogen atom of –NH2 in AAILs to the carbon atom of CO2. Also, during the carbamate formation, proton transfer in the amine group of the anion part occurs more than the cation part. Finally, based on different analyses in this research, it is predicted that [dMA][AA] ionic liquids are better candidates for CO2 capture than the conventional ILs and alkanolamines.

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