Development of Heavy Element Chemistry at Interfaces: Observing Actinide Complexes at the Oil/Water Interface in Solvent Extraction by Nonlinear Vibrational Spectroscopy

Abstract

Understanding the chemistry of elements at the bottom of the periodic table is a challenging goal in chemistry. Observing actinide species at interfaces by using interface-selective second-order nonlinear optical spectroscopy, such as vibrational sum frequency generation (VSFG) spectroscopy, is a promising route for developing heavy element chemistry; however, such attempts are scarce. Here, we investigated the phase transfer mechanism of uranyl ions (UO 2 2+ ) in solvent extraction using the di(2-ethylhexyl)phosphoric acid (HDEHP) extractant dissolved in the dodecane organic phase by probing the oil/water liquid–liquid interface using VSFG spectroscopy. The POO – symmetric stretch vibrational signals of the HDEHP ligands clearly demonstrated that uranyl ions form interfacial complexes with HDEHP at the oil/water interface. The interfacial uranyl–HDEHP complexes were formed with uranyl ions coming from both the aqueous and oil phases, strongly suggesting that the interfacial complex is an intermediate to cross the oil/water interface. Density functional theory calculations proposed the molecular structure of the interfacial uranyl–HDEHP complex.

Tracking Molecular Transport Across Oil/Aqueous Interfaces: Insight into “Antagonistic” Binding in Solvent Extraction

“…These studies reveal interfacial speciation and kinetics of self-assembly that can be tuned by changing the aqueous subphase compositions, such as pH, supporting ionic species, − or via the presence of nanoparticles . Similarly, Kusaka and Watanabe have recently probed the interfacial binding of uranium bearing species at L/L interface using SFG and support the notion that relevant binding takes place at interfaces . Theory and simulation work often compliment these experimental efforts; for instance, Benjamin and colleagues used molecular dynamic (MD) simulations to understand the structural and dynamical factors that influence ion transfer across L/L interfaces. , Furthermore, studies have discussed how L/L interfaces create ordered structures extending away from the interface. , For instance, Benjamin and Michael demonstrated that the time-dependent solvent response at the water/octanol interface is sensitive to the location of a newly formed charge and to the intrinsic width and structure of the interface .…”

“…20 Similarly, Kusaka and Watanabe have recently probed the interfacial binding of uranium bearing species at L/L interface using SFG and support the notion that relevant binding takes place at interfaces. 21 Theory and simulation work often compliment these experimental efforts; for instance, Benjamin and colleagues used molecular dynamic (MD) simulations to understand the structural and dynamical factors that influence ion transfer across L/L interfaces. 15,22 Furthermore, studies have discussed how L/L interfaces create ordered structures extending away from the interface.…”

Aqueous Interfaces in Chemical Separations

“…Copyright 2023 American Chemical Society. (b) Reproduced with permission from ref . Copyright 2022 American Chemical Society.…”

“…Even seemingly “simple” systems, such as trivalent ion adsorption to a soft charge interface , or water organization near graphene under potential control, − remain active topics of debate. To progress in separations, it is essential to develop model experimental ,− and computational ,,,− systems that represent important aspects of solute–amphiphile–solvent interactions at the interface. As discussed below, some of these model systems are more realistic than others, but they all offer valuable insights as long as their limitations are carefully considered.…”

The Unexpected Role of Cations in the Self-Assembly of Positively Charged Amphiphiles at Liquid/Liquid Interfaces

“…Of relevance to the current work is the liquid/liquid (L/L) interface between the oil and aqueous phases, which host a range of unique phenomena that are essential to both synthetic and natural systemse.g., unique solvation environments and chemical asymmetry that define the surface. The partitioning of amphiphiles, such as lipids, surfactants, − proteins, − extraction reagents, − nanoparticles, − and polymers − to these L/L interfaces provide an opportunity to leverage synergistic interactions between organized molecules localized in between the two bulk phases to achieve function. These phenomena also represent key steps in chemical separations, − phase transfer catalysis, and even applications in soft matter electronics. , …”