Alkyl chain length affecting the structure and interactions of choline-based amino acid ionic liquids: Insights from a computational study

Abstract

The influences of the alkyl chain length of amino acid anions on the structure and interactions of choline-based amino acid ([Cho][AA]) ionic liquids were investigated based on density functional theory (DFT) and molecular dynamics (MD) simulations. The most stable configuration of the individual [Cho][AA] ion pair was determined, revealing negligible variations in the binding energy between choline cation ([Cho]+) and different amino acid anions ([AA]−). To further characterize the intermolecular interactions between charged entities, the atoms in molecules theory (AIM) and independent gradient model based on Hirshfeld partition (IGMH) were employed, which highlight the presence of robust hydrogen bonds between [Cho]+ and [AA] −, underscoring their significance in shaping the structure of [Cho][AA] ion pairs. The MD studies exhibited the aggregation of anions in [Cho][AA] ILs, which was attributed to side chain interactions of anions. This aggregation effect becomes more pronounced with the elongation of anionic alkyl chain. Simultaneously, radial distribution functions (RDF) analysis revealed a higher probability of hydrogen bonds occurring between [Cho]+ and [AA]− as the alkyl chain in [AA]− lengthened. Additionally, the hydrogen bond lifetimes roughly display an increasing trend as the alkyl chain of the [AA]− increases, which may be due to the reduced diffusion coefficients of ions. Hopefully, our results would contribute fundamental knowledge regarding the impact of alkyl chain length of anions on the structure and interactions in the choline-based amino acid ionic liquids.