LAQV, REQUIMTE, Departamento de Química da Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal.
Institute of Physics, University of Rostock, Albert-Einstein-Str. 23-24, 18051 Rostock, Germany.
Phys Chem Chem Phys. 2021 Jan 21;23(2):1706-1717. doi: 10.1039/d0cp04992a.
The use of water as a component of deep eutectic systems (DES) has raised some questions regarding its influence on the nature of the mixture. Does it form a DES or an aqueous solution and what is the role of water? In this work, the nature of citric acid:l-arginine:water mixtures was explored through phase equilibria studies and spectroscopic analysis. In a first step, PC-SAFT was validated as a predictive tool to model the water influence on the solid liquid equilibria (SLE) of the DES reline using the individual-component approach. Hence, activity coefficients in the ternary systems citric acid:l-arginine:water and respective binary combinations were studied and compared using ePC-SAFT. It was observed that the water-free mixtures citric acid:l-arginine showed positive deviation from Raoult's law, while upon addition of water strong negative deviation from Raoult's law was found, yielding melting depressions around 100 K. Besides these strong interactions, pH was found to become acidic (pH = 3.5) upon water addition, which yields the formation of charged species ([H2Cit]- and [l-arg]+). Thus, the increased interactions between the molecules upon water addition might be caused by several mechanisms such as hydrogen bonding or ionic forces, both being induced by water. For further investigation, the liquid mixtures citric acid:l-arginine:water were studied by FTIR and NMR spectroscopy. FTIR spectra disproved a possible solubility enhancement caused by salt formation between citric acid and l-arginine, while NMR spectra supported the formation of a hydrogen bonding network different from the binary systems citric acid:water and l-arginine:water. Either being a DES or other type of non-ideal solution, the liquefaction of the studied systems is certainly caused by a water-mediator effect based on the formation of charged species and cross interactions between the mixture constituents.
水作为深共熔体系（DES）的组成部分，引起了一些关于其对混合物性质影响的问题。它是形成 DES 还是水溶液，水的作用是什么？在这项工作中，通过相平衡研究和光谱分析探索了柠檬酸：精氨酸：水混合物的性质。在第一步中，使用个体组分方法，通过 PC-SAFT 验证了其作为预测工具来模拟 DES 相关物的固液平衡（SLE）中水的影响。因此，使用 ePC-SAFT 研究并比较了三元体系柠檬酸：精氨酸：水和相应二元组合中的活度系数。观察到无水电解质柠檬酸：精氨酸混合物表现出对拉乌尔定律的正偏差，而添加水后则发现强烈的负偏差，导致熔点降低约 100 K。除了这些强烈的相互作用外，还发现添加水后 pH 值呈酸性（pH = 3.5），这导致形成带电物质（[H2Cit]-和[l-arg]+）。因此，添加水后分子之间相互作用的增加可能是由几种机制引起的，例如氢键或离子力，这两种力都是由水诱导的。为了进一步研究，通过傅里叶变换红外光谱（FTIR）和核磁共振光谱（NMR）研究了柠檬酸：精氨酸：水的液态混合物。FTIR 光谱证明了柠檬酸和精氨酸之间不可能形成盐而导致溶解度增加，而 NMR 光谱支持了不同于二元体系柠檬酸：水和精氨酸：水的氢键网络的形成。无论是 DES 还是其他类型的非理想溶液，所研究体系的液化肯定是由于基于带电物质的形成和混合物成分之间的交叉相互作用的水介导效应。
