Engineering Thermodynamics, Process & Energy Department, Faculty of Mechanical, Maritime and Materials Engineering, Delft University of Technology, Leeghwaterstraat 39, 2628CB Delft, The Netherlands.
J Phys Chem B. 2021 Nov 11;125(44):12303-12314. doi: 10.1021/acs.jpcb.1c07796. Epub 2021 Oct 31.
Hydrophobic deep eutectic solvents (DESs) have recently gained much attention as water-immiscible solvents for a wide range of applications. However, very few studies exist in which the hydrophobicity of these DESs is quantified. In this work, the interfacial properties of hydrophobic DESs with water were computed at various temperatures using molecular dynamics simulations. The considered DESs were tetrabutylammonium chloride-decanoic acid (TBAC-dec) with a molar ratio of 1:2, thymol-decanoic acid (Thy-dec) with a molar ratio of 1:2, and dl-menthol-decanoic acid (Men-dec) with a molar ratio of 2:1. The following properties were investigated in detail: interfacial tensions, water-in-DES solubilities (and salt-in-water solubilities for TBAC-dec/water), density profiles, and the number densities of hydrogen bonds. Different ionic charge scaling factors were used for TBAC-dec. Thy-dec and Men-dec showed a high level of hydrophobicity with negligible computed water-in-DES solubilities. For charge scaling factors of 0.7 and 1 for the thymol and decanoic acid components of Thy-dec, the computed interfacial tensions of the DESs are in the following order: TBAC-dec (ca. 4 mN m) < Thy-dec (20 mN m) < Men-dec (26 mN m). The two sets of charge scaling factors for Thy-dec did not lead to different density profiles but resulted in considerable differences in the DES/water interfacial tensions due to different numbers of decanoic acid-water hydrogen bonds at the interfaces. Large peaks were observed for the density profiles of (the hydroxyl oxygen of) decanoic acid at the interfaces of all DES/water mixtures, indicating a preferential alignment of the oxygen atoms of decanoic acid toward the aqueous phase.
疏水型深共晶溶剂（DESs）作为一种与水不混溶的溶剂，在广泛的应用中引起了人们的极大关注。然而，很少有研究定量描述这些 DESs 的疏水性。在这项工作中，使用分子动力学模拟计算了不同温度下水不混溶性 DESs 与水的界面性质。考虑的 DESs 是摩尔比为 1:2 的四丁基氯化铵-癸酸（TBAC-dec）、摩尔比为 1:2 的麝香草酚-癸酸（Thy-dec）和摩尔比为 2:1 的 dl-薄荷醇-癸酸（Men-dec）。详细研究了以下性质：界面张力、水在 DES 中的溶解度（TBAC-dec/水的盐在水中的溶解度）、密度分布和氢键的数密度。TBAC-dec 使用了不同的离子电荷标度因子。Thy-dec 和 Men-dec 表现出很高的疏水性，计算得到的水在 DES 中的溶解度可以忽略不计。对于胸腺酚和癸酸成分的电荷标度因子分别为 0.7 和 1 的情况，DES 的计算界面张力按以下顺序排列：TBAC-dec（约 4 mN m）<Thy-dec（20 mN m）<Men-dec（26 mN m）。对于 Thy-dec 的两组电荷标度因子并没有导致不同的密度分布，但由于界面处癸酸-水氢键的数量不同，导致 DES/水界面张力有很大的差异。在所有 DES/水混合物的界面处，观察到癸酸（羟基氧）的密度分布出现大峰值，表明癸酸的氧原子优先排列朝向水相。
