Hydrogen bond kinetics and their temperature dependence of biocompatibility-based choline amino acid ionic liquids

Choline amino acid ionic liquids ([Ch][AA]) are green solvents with high thermal stability, good biodegradability and low toxicity. In this work, the hydrogen bond kinetic properties of the four [Ch][AA] and the effect of temperature on them were investigated using density functional theory (DFT) calculations and molecular dynamics (MD) simulations. By atom-in-molecule (AIM) and noncovalent interaction (RDG) analysis, it was found that the hydroxyl hydrogen (H_c1) atom and the methyl hydrogen (H_c2) atom of the cation form hydrogen bonds with the carboxylate oxygen (O_a) atom of the anion (O_c-H_c1···O_a hydrogen bond, C_c-H_c2···O_a hydrogen bond), respectively. However, the O_c-H_c1···O_a hydrogen bond interaction was stronger than the C_c-H_c2···O_a hydrogen bond interaction. Simultaneously, radial distribution functions (RDFs) analysis further confirmed that the O_c-H_c1···O_a hydrogen bond interaction was stronger. As chain length of anion increases, the strength of the O_c-H_c1···O_a and C_c-H_c2···O_a hydrogen bond interactions increase. Furthermore, with the increase of temperature, the interactions between the particles weaken, while the diffusion coefficient and the ionic conductivity increase. Box snapshots and Spatial distribution functions (SDFs) analysis indicated that aggregation phenomenon occurs between anions in [Ch][AA], forming clusters, which was mainly due to the side chain interaction between anions. As chain length of anion increases, this aggregation phenomenon became more evident. In addition, it was found that hydroxyl groups can participate in the formation of more hydrogen bonding networks, which enhances the intermolecular forces between the ionic liquids and makes the system more viscous.