Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India.
J Chem Phys. 2021 Jul 28;155(4):044502. doi: 10.1063/5.0054699.
Hydrophobic deep eutectic solvents (HDESs) have gained immense popularity because of their promising applications in extraction processes. Herein, we employ atomistic molecular dynamics simulations to unveil the dynamics of DL-menthol (DLM) based HDESs with hexanoic (C6), octanoic (C8), and decanoic (C10) acids as hydrogen bond donors. The particular focus is on understanding the nature of dynamics with changing acid tail length. For all three HDESs, two modes of hydrogen bond relaxations are observed. We observe longer hydrogen bond lifetimes of the inter-molecular hydrogen bonding interactions between the carbonyl oxygen of the acid and hydroxyl oxygen of menthol with hydroxyl hydrogen of both acids and menthol. We infer strong hydrogen bonding between them compared to that between hydroxyl oxygen of acids and hydroxyl hydrogens of menthol and acids, marked by a faster decay rate and shorter hydrogen bond lifetime. The translational dynamics of the species in the HDES becomes slower with increasing tail length of the organic acid. Slightly enhanced caging is also observed for the HDES with a longer tail length of the acids. The evidence of dynamic heterogeneity in the displacements of the component molecules is observed in all the HDESs. From the values of the α-relaxation time scale, we observe that the molecular displacements become random in a shorter time scale for DLM-C6. The analysis of the self-van Hove function reveals that the overall distance covered by DLM and acid molecules in the respective HDES is more than what is expected from ideal diffusion. As marked by the shorter time scale associated with hole filling, the diffusion of the oxygen atom of menthol and the carbonyl oxygen of acid from one site to the other is fastest for hexanoic acid containing HDES.
疏水型深共晶溶剂 (HDES) 因其在萃取过程中的应用前景而备受关注。在此，我们采用原子分子动力学模拟揭示了以 DL-薄荷醇 (DLM) 为基础的 HDES 与己酸 (C6)、辛酸 (C8) 和癸酸 (C10) 作为氢键供体的动力学特性。特别关注的是随着酸尾长的变化，了解动力学的本质。对于所有三种 HDES，都观察到两种氢键弛豫模式。我们观察到酸的羰基氧和薄荷醇的羟基氧之间以及两种酸和薄荷醇的羟基氢之间的分子间氢键相互作用的氢键寿命更长。与酸的羟基氧和薄荷醇以及酸的羟基氢之间的氢键相比，我们推断它们之间存在较强的氢键，其特征是衰减速率更快且氢键寿命更短。随着有机酸尾长的增加，HDES 中各物种的平移动力学变得更慢。对于酸尾长较长的 HDES，也观察到稍微增强的笼蔽。在所有 HDES 中都观察到组分分子位移的动态异质性的证据。从 α-松弛时间尺度的值来看，我们观察到对于 DLM-C6，分子位移在较短的时间尺度内变得随机。自范霍夫函数的分析表明，在各自的 HDES 中，DLM 和酸分子的总覆盖距离超过了理想扩散的预期。正如与孔填充相关的较短时间尺度所标记的那样，对于含有己酸的 HDES，薄荷醇的氧原子和酸的羰基氧从一个位置到另一个位置的扩散最快。
