Tailoring ionic liquid properties via amino acid anions: structural influence of isoleucine, methionine, and serine anions

Abstract

Choline amino acid ([Ch][AA]) ionic liquids have recently gained considerable attention due to their biocompatibility, low toxicity, and structural tunability, which makes them promising electrolytes for sustainable applications in electrochemistry. In the present study, classical molecular dynamics (MD) simulations were carried out to investigate the structural and dynamical behaviour of three choline-based amino acids ILs [Ch][Ile], [Ch][Met], and [Ch][Ser] at 300 K. Atomic charges are derived using density functional theory (DFT) calculations using the density derived electrostatic and chemical (DDEC) method. Radial distribution functions (RDFs) helped to probe the role of the carboxylate group and amino group in the local structural organisation, and electrostatic interactions were found to be dominant for the carboxylate group. The hydroxyl group in [Ser]⁻ anions shows strong hydrogen bonding interaction with the [Ch]⁺ cation’s hydroxyl group. Spatial distribution function (SDF) analysis revealed the preferential hydrogen bonding patterns between the [Ch]⁺ cation’s tail and the amino acid anions. Structure factor is explored to obtain real-space correlation lengths and validate amino acid-driven experimental reported structural features of pre-peaks and molecular peaks. The mean square displacement plot is computed to calculate diffusion coefficients, and anionic mobility is found to be very similar for all three anions. However, there is a significant effect of counter amino acid anions on cationic mobility and the [Ch][Ser] IL exhibits the highest cation diffusion.