Exploring the interaction of amino acid-based ionic liquids in water and organic solvents: Insight from MD simulations and QM calculations

Molecular dynamic (MD) simulations and quantum mechanics calculations were performed on 1-ethyl-3-methylimidazolium ([C2mim])-based ionic liquids having the amino acid anion. The corresponding mixtures with water, methanol, and chloroform were conducted to investigate the effect of interactions between the cation and anions of the amino acid ionic liquids (AAILs). The MD simulation results indicate that the interaction of the AAILs is affected by the mole ratio of chloroform, methanol, and water. The AAILs are accumulated with increasing the mole ratio of chloroform to 0.9. These results show that the strength of the interaction in AAILs increases in the presence of chloroform, which is gradually affected by the mole ratios higher than 0.5 in methanol and water. Therefore, the polarity and mole ratio of the solvent plays a major role in ion aggregation and cross complex formation of the AAILs. Density Functional Theory (DFT) calculations were performed to investigate the interaction of AAILs at M06-2X/6–311++G(d,p) level of theory. The obtained results indicate that the highest interaction energy between the cation and anion of the ILs belongs to the gas phase. Also, it is demonstrated that the interaction energy in methanol and water is lower than chloroform, which confirms that the structure of the AAILs is affected in the presence of the polar solvent. The natural bond orbital (NBO) and non-covalent interaction (NCI) analyses confirm that the H-bond formation is an important interaction between the ion pairs of the AAILs. Finally, MD simulations and DFT calculations are in good agreement with the experimental results.