The recycling of metals, such as rare earths, into valuable materials relies on ion-specific separation, which is the basis of the hydrometallurgy [1]. Most of the efficient methods known for separating ions are based on equilibria between complex fluids, typically between aqueous and organized organic phases. Understanding the driving forces of the ion transfer is, thus, a critical issue to understand the properties of liquid-liquid interfaces between organic and aqueous phases, but also to assess the chemical potentials of the species involved. Here, we propose a multiscale approach for calculating the thermodynamic properties of ions in the organic phase taking into account the molecular properties of the solutes with no adjustable parameters. We demonstrated that molecular complexes formed during solvent extraction self-assemble as reverse micelles, and therefore induced a supramolecular organization. In most of cases, water molecules play an essential role in the stability of such aggregates in non-polar media [2]. We also pointed out that the length of the solvent’s aliphatic chains has a minor effect on the elastic properties of the polar core of the aggregate, i.e., the spontaneous packing parameter and the effective bending rigidity [3,4]. Coupling these molecular properties with a mesoscopic water/oil interface model based on the microemulsion theory allows for accessing all the thermodynamic properties needed for chemical engineering, e.g., activity coefficients, association constants, ternary phase diagrams [5]. References [1] Th. Zemb, C. Bauer, P. Bauduin, L. Belloni, C. Déjugnat, O. Diat, V. Dubois, J.-F. Dufrêche, S. Dourdain, M. Duvail, C. Larpent, F. Testard, and S. Pellet-Rostaing. Colloid Polym. Sci. 293, 1 (2015). [2] Y. Chen, M. Duvail, Ph. Guilbaud, and J.-F. Dufrêche. Phys. Chem. Chem. Phys. 19, 7094 (2017). [3] M. Duvail, S. van Damme, Ph. Guilbaud, Y. Chen, Th. Zemb, and J.-F. Dufrêche. Soft Matter 13, 5518 (2017). [4] S. Stemplinger, M. Duvail, J.-F. Dufrêche, J. Mol. Liq. 348, 118035 (2022). [5] S. Gourdin-Bertin, J.-F. Dufrêche, M. Duvail, Th. Zemb, Solv. Extr. Ion Exch. 40, 28 (2022).