Effect of ion to ligand ratio on the aqueous to organic relative solubility of a lanthanide–ligand complex†

In the solvent extraction of rare earth elements, mechanistic aspects remain unclear regarding where and how extractant molecules coordinate metal ions and transport them from the aqueous phase into the organic phase. Molecular dynamics simulations were used to examine how unprotonated di(2-ethylhexyl)phosphoric acid (DEHP⁻) ligands that coordinate the Gd³⁺ ion can transfer the ion across the water–organic interface. Using the umbrella sampling technique, potential of mean force profiles were constructed to quantify the relative solubility of the Gd³⁺ ion coordinated to 0–3 DEHP⁻ ligands in either water, 1-octanol, or hexane solvents and at the water–organic interfaces. The simulations show the Gd–DEHP⁻ complexes, at varying Ln–ligand ratios, preferentially solvate on water–organic interfaces. While the Gd(DEHP⁻)₃ complex will diffuse past the aqueous–organic interface into the octanol solvent, it is thermodynamically preferred for the Gd(DEHP⁻)₃ complex to remain in the water–hexane interface when there is no amphiphilic layer of excess ligand.