Department of Chemistry, Washington State University, Pullman, WA, USA.
Phys Chem Chem Phys. 2019 Feb 6;21(6):2866-2874. doi: 10.1039/c8cp06450d.
Liquid/liquid extraction (LLE) is one of the most industrially relevant separation methods, successfully leveraging the variable solubility of solutes (or their complexes) between two immiscible solvents. Independent of the relative solubilities of those solutes and complexes (which determine their distribution between phases), the dynamics of phase transfer processes are impacted by the molecular interactions and structure of those species at the interface. A simple example includes the formation and extraction of water-extractant adducts observed in the ternary water/organic/tri-n-butyl phosphate (TBP) system. Despite its implications for LLE, a detailed description of the structural and dynamic mechanisms by which such adducts are formed at the interface is not established. Describing that process requires connecting the evolving interfacial molecular organization in the presence of surfactants to dynamic surface fluctuations and interfacial heterogeneity. Herein, molecular dynamics simulation is combined with state-of-the-art network theory analysis to reveal features of interfacial structures and their relationship to the extraction of water in the water/n-hexane/TBP system. Surfactant adsorption enhances interfacial roughness which in turn causes directly interfacial water to become less connected through hydrogen bonding to subjacent layers, particularly upon formation of the water-bridged TBP dimer adduct. Furthermore, heterogeneity within the interface itself is enhanced by surfactant adsorption and serves as the basis for the formation of protrusions of water into the organic phase at the extremes of surface fluctuations. These features disproportionately incorporate the water-bridged TBP dimer and are the primary means by which water is transferred to the organic phase. This work presents for the first time a holistic understanding of how interfacial heterogeneity and spatial fluctuations become amplified in the presence of surfactants, enabling water extraction into the organic phase. It further affords the opportunity to study how solution conditions can control interfacial behavior to create more efficient solvent extraction systems.
液/液萃取（LLE）是最具工业相关性的分离方法之一，成功地利用了溶质（或其配合物）在两种不混溶溶剂之间的可变溶解度。独立于那些溶质和配合物的相对溶解度（决定它们在相之间的分布），相转移过程的动力学受到那些物种在界面处的分子相互作用和结构的影响。一个简单的例子包括在三元水/有机/三丁基膦（TBP）体系中观察到的水萃取剂加合物的形成和萃取。尽管它对 LLE 有影响，但尚未建立形成界面加合物的结构和动态机制的详细描述。描述该过程需要将表面活性剂存在下界面上不断演变的分子组织与动态表面波动和界面异质性联系起来。在此，分子动力学模拟与最先进的网络理论分析相结合，揭示了界面结构的特征及其与水在水/正己烷/TBP 体系中萃取的关系。表面活性剂吸附增强了界面粗糙度，这反过来又导致直接界面水通过氢键与下层连接减少，特别是在形成水桥接 TBP 二聚体加合物时。此外，表面活性剂吸附增强了界面本身的异质性，并为在表面波动的极端情况下水向有机相中的突起形成提供了基础。这些特征不成比例地包含水桥接的 TBP 二聚体，是水转移到有机相的主要途径。这项工作首次全面了解了在存在表面活性剂的情况下界面异质性和空间波动如何放大，从而实现水向有机相的萃取。它进一步提供了研究溶液条件如何控制界面行为以创建更有效的溶剂萃取系统的机会。
