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Published on: March 24, 2018
Evolution of microstructures and hydrogen bond interactions within choline amino acid ionic liquid and water mixtures
Jin Chen1, Xixi Zeng1, Ling Chen1
1Ministry of Education Engineering Research Center of Starch & Protein Processing, Guangdong Province Key Laboratory for Green Processing of Natural Products and Product Safety, School of Food Science and Engineering, South China University of Technology, Guangzhou 510640, China. felchen@scut.edu.cn.
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.
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