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Keywords:

  • amino acids;
  • hydrogen bonding;
  • ionic liquids;
  • molecular dynamics;
  • solvation

Abstract

The solvation of glycine in two ionic liquids (ILs), namely, 1-butyl-3-methylimidazolium acetate, [C1C4Im][OAc], and 1-butyl-3-methylimidazolium trifluoroacetate, [C1C4Im][TFA], was studied by a combination of experimental and theoretical methods. The solubility of glycine in both ILs was determined at 333.15 K to be (8.1±0.5) and (1.0±0.5) wt % in [C1C4Im][OAc] and [C1C4Im][TFA], respectively. By IR spectroscopy it was found that, when dissolved in the ILs, glycine was mainly present in its zwitterionic form. Structural and energetic aspects of the solvation of glycine in the ILs and in mixtures of ILs and water were investigated by ab initio calculations and molecular dynamic simulations. It was observed that the firstly solvation shell around glycine consisted predominantly of acetate or trifluoroacetate anions, which formed hydrogen bonds either with the carboxylic group of neutral glycine or with the protonated ammonium group of the zwitterionic form. When water is present in the solutions, hydrogen bonds between water and the anion prevail. The overall energy of the system was decomposed into its components between pairs of species. It was established that the dominant contribution to the interaction energy between glycine and the IL was due to hydrogen bonds with the anions and the statistics of hydrogen bonds were analysed.