Ministry 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.
Phys Chem Chem Phys. 2022 Jul 27;24(29):17792-17808. doi: 10.1039/d2cp01990f.
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.
研究了胆碱氨基酸([Cho][AA])离子液体(IL)与水分子的微观结构和相互作用。当水加入[Cho][AA]时，-COO 基团的不对称和对称振动峰分别移向较低和较高的波数。随着水的加入，[Cho][AA]-水混合物的电导率值增加，粘度值降低。这些特征与物理图像一致，即[Cho][AA]可以逐渐解离成水合紧密离子对和水分离离子对，然后形成自由和溶剂化离子。对于不同的阴离子(赖氨酸胆碱盐，[Cho][Lys]和天门冬氨酸胆碱盐，[Cho][Asp])，阴离子结构对[Cho][AA]-水相互作用具有显著的调节作用。Asp 的侧链较短且极性较强的-COOH 基团赋予[Cho][Asp]更强的阳离子-阴离子相互作用和更少的水分子解离。因此，[Cho][Asp]-水混合物的频率位移程度和电导率值较低，粘度值较高，而[Cho][Lys]-水混合物的电导率值和粘度值则相反。而且，[Cho][Lys]可以在 w ：IL≥7 ：3 时完全解离为自由水合离子，而[Cho][Asp]的自由水合离子仅在 w ：IL 比达到 8 ：2 时才出现。这些结果可以减轻实验工作量，提高[Cho][AA]IL 的应用效率。
