Computer Simulations of Rare Earth Erbium Ion-Extractant Binding at Liquid-Liquid Interfaces

This thesis focuses on understanding the molecular mechanisms underlying the solvent extraction of trivalent lanthanide ions in bulk water and at the aqueous-organic interface. Solvent extraction is a critical process in the separation of rare earth elements, which are essential for technologies like the nuclear fuel cycle, electronics, and renewable energy. The process involves the binding of extractants to metal ions in aqueous solution and transfers to the organic phase. We focus on the interaction of erbium (Er³⁺) with the organophosphorus extractant bis(2-ethylhexyl) phosphoric acid (HDEHP). Despite its industrial importance, it is not well understood how extractants bind to lanthanide ions at the molecular level. This includes the removal of water molecules upon ion binding and the geometry and dynamics of binding. Our work addresses this knowledge gap by employing classical molecular dynamics (MD) simulations to investigate the binding mechanism of HDEHP to Er³⁺ in two separate environments: bulk water and the aqueous-organic interface. In our first study, we investigated the binding of a single DEHP⁻ (deprotonated HDEHP) extractant to fully hydrated Er³⁺ in bulk water. Using MD simulations, we discovered that the binding process is remarkably fast, despite the strong hydration energy of Er³⁺. As the DEHP⁻ headgroup approaches Er³⁺, water molecules in the first hydration shell undergo collective rotational motions, creating space for the incoming extractant. This concerted motion leads to the ejection of a water molecule located 180° opposite to the incoming DEHP⁻ oxygen. When the headgroup binds, it positions its oxygen atom closer to Er³⁺ than the water molecules in the ion's first hydration shell. This study provided the first detailed molecular-level understanding of how DEHP⁻ binds to Er³⁺ in bulk water, highlighting the importance of collective water dynamics and geometric specificity in the binding process. Building upon the first study, we extended our research to investigate the binding of multiple DEHP⁻ extractants to Er³⁺, both in bulk water and at the aqueous-organic interface. This second work aimed to understand how sequential binding events occur and how the presence of multiple extractants affects the binding kinetics and hydration shell structure. The binding probability decreases as more DEHP⁻ molecules bind to Er³⁺, with an especially significant decrease at the interface, likely due to steric constraints. The highly charged ion Er3+ is surrounded by a well-ordered hydration shell of water molecules. When a DEHP- ligand binds to Er3+, it can partially or fully displace water molecules from this strong hydration shell. Bound DEHP- occupies space that can make it more difficult for additional DEHP- molecules to approach and bind to Er3+. At the interface, this steric constraint becomes more relevant because DEHP- tails are preferentially solvated by the organic phase. This further reduces the available space for additional DEHP- to approach and bind. When we investigate the ejection of water molecules by the incoming DEHP-, we find that water can be ejected before or after the DEHP- is fully bound to the Er3+ ion. However, in the case of binding of the third DEHP⁻, water ejection occurs mostly after the binding event, which may be related to the significant drop in binding probability for the third DEHP-. These simulations reveal how the multi-step binding and hydration shell dynamics are influenced by the number of bound extractants and the different environments of bulk water and water-dodecane interface.