A combined molecular dynamic simulation and experimental study of thermo-physical properties of the new synthesized amino acid-based ionic liquids

Three new amino acid ionic liquids (AAILs), 1‑decyl‑3‑methylimidazolium glycinate ([C10mim]Gly), 1‑decyl‑3‑methylimidazolium alaninate ([C10mim]Ala) and 1‑decyl‑3‑methylimidazolium valinate ([C10mim]Val), were prepared by the neutralization method and characterized by 1H NMR spectroscopy. Precise values of density, molar volume, crystal energy, thermal expansion coefficients and standard entropy of the synthesized AAILS in their pure state were measured experimentally at different temperatures (293.15, 298.15, 303.15, 308.15, 313.15, 318.15, 323.15, 328.15 and 333.15 K). In the next part of this work, molecular dynamics (MD) simulation was carried out to theoretical determination of densities of the synthesized AAILS and the results were compared with the experimental density data and good agreements were reached. Microstructure and correlations between the imidazolium cation and amino acid anions of the synthesized AAILs were also investigated by computing radial distribution functions (RDFs) and spatial distribution functions (SDFs) and it was found that, the strong interactions between the anions and cation were caused the anions to be well distributed around the imidazolium cation. The results for the cation-cation interactions showed that the intensity of tail aggregation was enhanced in the position far away from the imidazolium ring. From the calculated combined distribution functions (CDFs), the convenient sites for the hydrogen bond interactions were determined and it was found that the strong hydrogen bond interactions are formed between the nitrogen and oxygen atoms of the anions and the hydrogen (H1) atom of the cation. In order to investigate the microscopic dynamic behavior of the systems, the mean-square displacements (MSDs) curves were plotted as a function of time and then the diffusion coefficients of the synthesized AAILS were determined using the slope of the mean-squared displacement curves and from the obtained diffusion coefficients, the values of the transference numbers were determined. It was found that, the diffusion coefficients of the anions are higher than that of the cation and among the investigated anions, the lowest mobility was obtained for valinate.