Bonnett Brittany L, Rahman Tasnim, Poe Derrick, Seifert Soenke, Stephenson G Brian, Servis Michael J
Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL 60439, USA.
X-ray Science Division, Argonne National Laboratory, Lemont, IL 60439, USA.
Phys Chem Chem Phys. 2024 Jul 3;26(26):18089-18101. doi: 10.1039/d4cp01369g.
Structure at the nanoscale in the organic phase of liquid-liquid extraction systems is often tied to separation performance. However, the weak interactions that drive extractant assembly lead to poorly defined structures that are challenging to identify. Here, we investigate the mechanism of water extraction for a malonamide extractant commonly applied to f-element separations. We measure extractant concentration fluctuations in the organic phase with small angle X-ray scattering (SAXS) before and after contact with water at fine increments of extractant concentration, finding no qualitative changes upon water uptake that might suggest significant nanoscopic reorganization of the solution. The critical composition for maximum fluctuation intensity is consistent with small water-extractant adducts. The extractant concentration dependence of water extraction is consistent with a power law close to unity in the low concentration regime, suggesting the formation of 1 : 1 water-extractant adducts as the primary extraction mechanism at low concentration. At higher extractant concentrations, the power law slope increases slightly, which we find is consistent with activity effects modeled using Flory-Huggins theory without introduction of additional extractant-water species. Molecular dynamics simulations are consistent with these findings. The decrease in interfacial tension with increasing extractant concentration shows a narrow plateau region, but it is not correlated with any change in fluctuation or water extraction trends, further suggesting no supramolecular organization such as reverse micellization. This study suggests that water extraction in this system is particularly simple: it relies on a single mechanism at all extractant concentrations, and only slightly enhances the concentration fluctuations characteristic of the dry binary extractant/diluent mixture.
液 - 液萃取系统有机相中的纳米级结构通常与分离性能相关。然而，驱动萃取剂组装的弱相互作用导致结构定义不明确，难以识别。在此，我们研究了一种常用于锕系元素分离的丙二酰胺萃取剂的水萃取机制。我们在萃取剂浓度的精细增量下，用小角X射线散射（SAXS）测量了与水接触前后有机相中的萃取剂浓度波动，发现吸水后没有定性变化，这可能表明溶液发生了显著的纳米级重组。最大波动强度的临界组成与小的水 - 萃取剂加合物一致。水萃取对萃取剂浓度的依赖性在低浓度范围内与接近1的幂律一致，表明在低浓度下形成1∶1的水 - 萃取剂加合物是主要的萃取机制。在较高萃取剂浓度下，幂律斜率略有增加，我们发现这与使用弗洛里 - 哈金斯理论建模的活度效应一致，无需引入额外的萃取剂 - 水物种。分子动力学模拟与这些发现一致。随着萃取剂浓度增加，界面张力的降低显示出一个狭窄的平稳区域，但它与波动或水萃取趋势的任何变化均无关联，这进一步表明不存在诸如反胶束化之类的超分子组织。本研究表明，该系统中的水萃取特别简单：在所有萃取剂浓度下都依赖单一机制，并且仅略微增强了干燥二元萃取剂/稀释剂混合物特有的浓度波动。
