Solid State Physics Division , Bhabha Atomic Research Centre , Mumbai 400085 , India.
ISIS Facility, Science and Technology Facilities Council , Rutherford Appleton Laboratory , OX11 0QX Didcot , U.K.
J Phys Chem B. 2020 Feb 27;124(8):1509-1520. doi: 10.1021/acs.jpcb.9b11137. Epub 2020 Feb 17.
Over the last couple of decades, deep eutectic solvents (DESs) have emerged as novel alternatives to ionic liquids that are extensively used in the synthesis of innovative materials, metal processing, catalysis, etc. However, their usage is limited, primarily because of their large viscosity and poor conductivity. Therefore, an understanding of the molecular origin of these transport properties is essential to improve their industrial applicability. Here, we present the report of the nanoscopic diffusion mechanism of acetamide in a DES synthesized with lithium perchlorate as studied using neutron scattering and molecular dynamics (MD) simulation techniques. A diffusion model is constructed with the help of MD simulation data comprising two distinct processes, corresponding to long-range jump diffusion and localized diffusion within a restricted volume. This diffusion model is validated through the analysis of neutron scattering data in the acetamide based DES (ADES) and molten acetamide. Although ADES has a remarkably lower freezing point compared to pure acetamide, the molecular mobility is found to be enormously restricted in the former. Particularly, the long-range jump diffusion process of acetamide is slower by a factor of 3 in ADES in comparison with molten acetamide. Further, the geometry of localized diffusion is found to be unaltered, but the dynamics is observed to be slightly slower in ADES. The diffusion model is found to be consistent over a wide temperature range for the ADES. Both long-range and localized diffusion show Arrhenius dependence with temperature in ADES. MD simulation analysis reveals that the long-range diffusion in ADES is restricted mainly due to the formation of hydrogen bond mediated complexes between the ionic species of the salt and acetamide molecules. Hence, the origin of higher viscosity observed in ADES can be attributed to the complexation in the ADES. The complex formation also explains the inhibition of the crystallization process while cooling and thereby results in depression of the freezing point of ADES.
在过去的几十年中，深共晶溶剂（DESs）作为离子液体的替代品出现，它们被广泛应用于新型材料的合成、金属加工、催化等领域。然而，它们的应用受到限制，主要是因为它们的高粘度和低电导率。因此，了解这些输运性质的分子起源对于提高它们的工业适用性至关重要。在这里，我们报告了使用中子散射和分子动力学（MD）模拟技术研究以高氯酸锂为合成原料的 DES 中乙酰胺的纳米扩散机制。通过 MD 模拟数据构建了一个扩散模型，该模型包含两个不同的过程，分别对应于长程跳跃扩散和受限体积内的局部扩散。通过对乙酰胺基 DES（ADES）和熔融乙酰胺的中子散射数据的分析，验证了该扩散模型。尽管 ADES 的冰点明显低于纯乙酰胺，但前者的分子迁移率受到了极大的限制。特别是，与熔融乙酰胺相比，ADES 中乙酰胺的长程跳跃扩散过程慢了 3 倍。此外，局部扩散的几何形状没有改变，但动力学速度稍慢。在 ADES 中，该扩散模型在很宽的温度范围内是一致的。长程和局部扩散都表现出与温度的阿伦尼乌斯关系。MD 模拟分析表明，ADES 中长程扩散的限制主要是由于盐的离子物种和乙酰胺分子之间形成氢键介导的复合物。因此，ADES 中观察到的较高粘度可以归因于复合物的形成。复合物的形成也解释了在冷却过程中结晶过程的抑制，从而导致 ADES 冰点的降低。
