Kumar Nitesh, Clark Aurora E
Department of Chemistry, Washington State University Pullman Washington 99164 USA.
Pacific Northwest National Laboratory Richland Washington 99354 USA
Chem Sci. 2021 Sep 29;12(41):13930-13939. doi: 10.1039/d1sc04004a. eCollection 2021 Oct 27.
Liquid/liquid extraction is one of the most widely used separation and purification methods, where a forefront of research is the study of transport mechanisms for solute partitioning and the relationships that these have to solution structure at the phase boundary. To date, organized surface features that include protrusions, water-fingers, and molecular hinges have been reported. Many of these equilibrium studies have focused upon small-molecule transport - yet the extent to which the complexity of the solute, and the competition between different solutes, influence transport mechanisms have not been explored. Here we report molecular dynamics simulations that demonstrate that a metal salt (LiNO) can be transported a protrusion mechanism that is remarkably similar to that reported for HO by tri-butyl phosphate (TBP), a process that involves dimeric assemblies. Yet the LiNO out-competes HO for a bridging position between the extracting TBP dimer, which in-turn changes the preferred transport pathway of HO. Examining the electrolyte concentration dependence on ion-pair transport unexpectedly reveals an inverse correlation with the extracting surfactant concentration. As [LiNO] increases, surface adsorbed TBP becomes a limiting reactant in correlation with an increased negative surface charge induced by excess interfacial NO , however the rate of transport is enhanced. Within the highly dynamic interfacial environment, we hypothesize that this unique cooperative effect may be due to perturbed surface organization that either decreases the energy of formation of transporting protrusion motifs or makes it easier for these self-assembled species to disengage from the surface.
液/液萃取是应用最广泛的分离和纯化方法之一，目前研究的前沿领域是溶质分配的传输机制以及这些机制与相界面处溶液结构的关系。迄今为止，已报道了包括凸起、水指和分子铰链在内的有组织的表面特征。许多这些平衡研究都集中在小分子传输上——然而，溶质的复杂性以及不同溶质之间的竞争对传输机制的影响程度尚未得到探索。在此，我们报告分子动力学模拟结果，结果表明金属盐（LiNO）可以通过一种与磷酸三丁酯（TBP）对水的传输机制非常相似的凸起机制进行传输，该过程涉及二聚体组装。然而，LiNO在萃取TBP二聚体之间的桥连位置上比水更具竞争力，这反过来又改变了水的首选传输途径。研究电解质浓度对离子对传输的依赖性意外地发现其与萃取表面活性剂浓度呈负相关。随着[LiNO]的增加，表面吸附的TBP成为限制反应物，这与过量界面NO诱导的表面负电荷增加相关，然而传输速率却提高了。在高度动态的界面环境中，我们推测这种独特的协同效应可能是由于表面组织受到扰动，这要么降低了传输凸起基序形成的能量，要么使这些自组装物种更容易从表面脱离。
