Tracing Local Nanostructure of the Aqueous Solutions of the Biocompatible [Cho][Gly] Ionic Liquid: Importance of Hydrogen Bond Attraction between Like-Charged Ions

Abstract

The neat and aqueous solutions of the cholinium glycinate ionic liquid (IL), [Cho]­[Gly], at different water mole fractions, x ws, are studied by molecular dynamics simulations. The changes in the local nanostructure of systems with composition have been determined by calculation of various structural distribution functions. Hydrogen bond (H-bond) attractions determine the major relative orientations of the oppositely and like charged nearest neighbors. The cation–anion H-bonds mainly form between the hydrogen of the hydroxyl or methyl groups of the cation and the carboxylate oxygen of the anion. A preferred (antiparallel) arrangement between adjacent [Cho]+ cations is due to the effective H-bond between the hydroxyl oxygen and the methyl hydrogen sites that promotes the like-charge cluster formation. Adding water decreases the occurrence probability of the [Cho]+···[Gly]−···[Cho]+ bridge structure in the aqueous solutions due to the formation of the [Gly]−···HOH···[Gly]− structure via H-bonding. Observed density trend versus x w is interpreted based on an interstice model and investigating the water cluster size distribution. Finally, the effect of x w on the infrared (IR) vibrational spectra were studied and blue and red shifts were observed for the stretching and bending vibrational modes of the hydroxyl group of [Cho]+, respectively. Current findings will improve the efficient engineering design and task-specific applications of aqueous solutions of bio-ILs consist of [Cho]+ and amino acid anions.

Liquid Nanostructure of Cholinium Argininate Biomass Solvents

“…This apparent contradiction is resolved by the SDFs in Figure D, which show that the probability surfaces for these interactions overlap. This is in agreement with a recent molecular dynamics study on the aqueous solutions of [Ch]­[Gly], in which the authors describe an overall increased repeat spacing between cation–cation, cation–anion, and anion–anion upon the addition of water, while the first peak position of hydrogen bonding between the species remain unchanged …”

“…This is in agreement with a recent molecular dynamics study on the aqueous solutions of [Ch][Gly], in which the authors describe an overall increased repeat spacing between cation−cation, cation−anion, and anion−anion upon the addition of water, while the first peak position of hydrogen bonding between the species remain unchanged. 28 RDFs between anion groups (guanidine−guanidine and guanidine−carboxylate), in Figure 5B, reveal weak correlations. The guanidine−carboxylate RDF has a peak at 2.2 Å, but the corresponding coordination numbers of 0.04−0.05 are low.…”

“…Recent molecular dynamics studies showed that the water content in [Ch][Gly] has an impact on the local nanostructurelike ion repeat spacing and hydrogen bond interactions between cation−cation, anion−anion and cation−anion. 27,28 This emphasizes the importance to study the [Ch][Arg] system with water as the added molecular solvent.…”

“…Thus, it is open to question whether a cation charge group is located in position to polarize hydrogen bonds with the guanidine nitrogen in the bulk bio-IL. Recent molecular dynamics studies showed that the water content in [Ch]­[Gly] has an impact on the local nanostructurelike ion repeat spacing and hydrogen bond interactions between cation–cation, anion–anion and cation–anion. , This emphasizes the importance to study the [Ch]­[Arg] system with water as the added molecular solvent.…”

Cholinium amino acid-based ionic liquids

“…Le Donne et al ( , 2019 using AIMD have found a rough but evident correlation between the association energies of the dimeric anion-anion structures and the measured viscosities of the corresponding ChAAILs. A recent work by Khorrami and Kowsari (2020) based on classical MD has further extended the study of these like-charge interactions to the mixtures of ChAAILs with water.…”