The Role of Hydrogen Bonds in Thermodynamic and Structural Properties in Binary Mixtures of Amino Acid Ionic Liquid–H₂O: A Combined Experimental and Computational Study

Amino acid ionic liquids (AAILs) have attracted attention due to their excellent biocompatibility and degradability. In order to better understand the effect of water on the thermodynamic properties and nanomicrostructures of AAILs, we synthesized choline alanine ([Ch][Ala]) and measured the density of [Ch][Ala]-H₂O with different water mass fractions at 298.15 K-348.15 K. The excess molar volumes (<i>V</i>_m^E) were calculated from the density data and compared with 3-aminopropyl-tri-<i>n</i>-butylphosphonium alanine aqueous solutions ([aP₄₄₄₃][Ala]-H₂O), and it was found that the <i>V</i>_m^E of both AAILs-H₂O mixtures were negative. The transport properties and nanomicrostructures of AAILs-H₂O mixtures were then investigated by MD simulations and DFT calculations. The results showed that the microstructures and transport properties of both AAILs and H₂O mixtures changed significantly upon the addition of water. Hydrogen bonds between anions slow down the dynamic properties of the mixtures. With the addition of water molecules, the anions and cations of both AAILs form hydrogen bonds with H₂O, reducing the structural correlation of the anion-cation and destroying the polar ionic network and nonpolar structural domains. In addition, there were different aggregation states of water molecules in AAILs at different water concentrations.