(383ax) Modeling Lanthanide-Ligand Complexes across Aqueous-Organic Interfaces

Much remains to be studied in the solvent extraction of rare earth elements about the mechanism by which extractant molecules coordinate lanthanide ions and transport them from the aqueous phase into the organic phase. Molecular dynamics simulations were used to study how di(2-ethylhexyl)phosphoric acid (HDEHP) ligands coordinate and transfer Gd³⁺ ions across water-organic interfaces. Potential of mean force profiles were constructed, using the umbrella sampling technique with classical molecular dynamics simulations, to quantify the solubility of Gd³⁺ coordinated to HDEHP ligands in either water, octanol, or hexane solvents and at the water-organic interfaces. The computational protocol was tested with solutes with known water-octanol partition coefficients, in order to compare the predicted relative solubilities with experiment. The simulations show the Gd-HDEHP complexes, at varying lanthanide-ligand ratios, preferentially solvate on water-organic interfaces. While the Gd(HDEHP)₃ complex will diffuse past the aqueous-organic interface into the octanol solvent, it is thermodynamically favored for the Gd(HDEHP)₃ complex to remain in the water-hexane interface.