Evolution of microstructures and hydrogen bond interactions within choline amino acid ionic liquid and water mixtures

Abstract

The microstructures and interactions of choline amino acid ([Cho][AA]) ionic liquid (IL) and water molecules were investigated. When water was added to [Cho][AA], the asymmetric and symmetric vibration peaks of the –COO⁻ group shifted to lower and higher wavenumbers, respectively. The increase of water addition also resulted in increased conductivity values and decreased viscosity values of [Cho][AA]–water mixtures. These features are consistent with the physical picture that [Cho][AA] could gradually dissociate into hydrated tight ion pairs and water-separated ion pairs and then into free and solvated ions. When it comes to different anions (choline lysine, [Cho][Lys], and choline aspartic acid, [Cho][Asp]), the anion structure has a significant regulation on [Cho][AA]–water interactions. The shorter side chain length and strong polar –COOH group of Asp⁻ endow [Cho][Asp] with stronger cation–anion interactions and less dissociation by water molecules. As a result, the frequency shift degrees and conductivity values of [Cho][Asp]–water mixtures were lower, and the viscosity values were higher than those of [Cho][Lys]–water mixtures. And, [Cho][Lys] could completely dissociate as free hydrated ions at w : IL ≥ 7 : 3, while the free hydrated ions of [Cho][Asp] only occurred when the w : IL ratio reached 8 : 2. These results can ease the experimental effort and improve the application efficiency of [Cho][AA] ILs.