BTP-based ligands and their complexes with Eu3+ at “oil”/water interfaces. A molecular dynamics study

Abstract

Heterocyclic N-donor ligands based on the bistriazinylpyridine (BTPs) skeleton have been recently developed to separate trivalent actinides from lanthanides by liquid–liquid extraction from nuclear solutions. In this paper, we report molecular dynamics investigations on BTPs in water–“oil” biphasic systems (oil = hexane + octanol vs. hexane vs. nitrobenzene vs. chloroform) and compare different BTP derivatives, their neutral vs. protonated forms, and their 1 : 3 complexes with Eu^(III). Neutral BTPs are found to be weakly surface active and to display multiple orientations at the interface, depending on time and on their lateral and para substituents. This contrasts with their protonated forms that strongly adsorb at interfaces with neutral or acidic water. Remarkably, the protonated cyMe₄-BTPH⁺ and, to a lesser extent, ⁱPrBTPH⁺ ligands adopts at the interface an “inversed orientation”, where NH⁺ points towards oil, instead of water. The [Eu(BTP)₃]³⁺ complexes are also found to be highly surface active: in spite of Eu^(III) shielding by the three ligands, these complexes remain strongly attracted by water at the aqueous side of the interface. Taken together, the MD results suggest that ion complexation by BTPs occurs right at the interface, from the protonated BTPH⁺ forms. They may explain why extraction is improved upon increase of the aqueous phase acidity, though with slow kinetics. They also open perspectives for designing new derivatives for efficient separation of trivalent actinides from lanthanides.