(368f) Exploring Phase Modifier Interactions in Solvent Extraction Separations Systems

Solvent extraction processes involving the separation of aqueous Lanthanide cations into an organic phase utilize specialized surfactants to selectively complex with the cations. The resulting solvation free energy change of the complex causes it to be extracted into the organic phase, allowing for further processing. Certain surfactants known for their high cation selectivity, such as tetra-octyl diglycolamide (TODGA), can induce phase splitting at high extraction concentrations. This newly formed phase is a highly viscous and concentrated organic phase, making it difficult to process and transport. The high concentration of this phase can also lead to criticality events in the presence of radioactive species. Separations chemists have utilized secondary surfactants, called phase modifiers (PMs), to prevent the formation of this third phase.

Phase modifier development has progressed mainly by an "artistic" approach, where individual chemists use their intuition in choosing and creating phase modifiers. This quantifies molecular-level surfactant interactions (sterics vs. hydrogen bond strength) and their impact on microemulsion mesostructure using molecular dynamics (MD) simulations to allow for a data-driven approach to phase modifier development.

By modifying MD force field parameters, it is possible to separate the effects of sterics and polar head hydrogen bond strength on the microemulsion structure and local composition. We find that changes in polar head hydrogen bond strength destabilize the structure and reduce the size of the polar core. We also find that phase modifier sterics significantly affect polar core adsorption. This adsorption behavior is significantly modified by changes to polar head interaction strength of as little as 2 kcal/mol. We further quantify the steric differences between TODGA, TBP, and DHOA in terms of these changes in hydrogen bond strength.