Recycling of metals, such as rare earths, into valuable material relies on ion specific separation, basis of the hydrometallurgy [1]. Most of efficient methods known for separating ions are based on equilibria between complex fluids, typically between aqueous and organised organic phases. Indeed, ions migrate from the aqueous to the organic phase thanks to surfactant or extractant molecules in the organic phase, and then are captured in reverse micelles. Understanding the driving forces of the ion transfer is therefore a crucial issue to understand the properties of liquid-liquid interfaces between organic and aqueous phases, but also to assess the chemical potentials of the compounds involved. Here, we propose multi-scale approaches for calculating the thermodynamics properties of ions in aqueous and organic solutions directly comparable to the experimental ones and calculated only by taking into account the molecular properties of the solutes in solutions with no adjustable parameters. Based on the osmotic equilibrium method, activities and activity coefficients for aqueous electrolyte solutions composed of nitrate lanthanide salts have been successfully calculated [2]. Simulating vapour-liquid interfaces of mixtures and pure solvents by means of molecular dynamics yield activities and activity coefficients of concentrated solutions in good agreement with experimental findings [3]. In the meantime, thermodynamics properties of solutes in organic phase have been deduced from umbrella-sampling molecular dynamics simulations. We demonstrated that molecular complexes formed in such phase during solvent extraction self-assemble as reverse micelles, and therefore induce a supramolecular organization of this medium. In most of the cases, water molecules play an essential role in the organization of this non polar medium [4]. Coupling these solute molecular properties with a mesoscopic water/oil interface model 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., 2015, 293, 1–22. [2] M. Bley, M. Duvail, Ph. Guilbaud and J.-F. Dufrêche, J. Phys. Chem. B, 2017, 121, 9647–9658. [3] M. Bley, M. Duvail, Ph. Guilbaud, Ch. Penisson, J. Theisen, J.-C. Gabriel and J.-F. Dufrêche, Mol. Phys., 2018, DOI: 10.1080/00268976.2018.1444209. [4] Y. Chen, M. Duvail, Ph. Guilbaud and J.-F. Dufrêche, Phys. Chem. Chem. Phys., 2017, 19, 7094–7100. [5] M. Duvail, S. van Damme, Ph. Guilbaud, Y. Chen, Th. Zemb and J.-F. Dufrêche, Soft Matter, 2017, 13, 5518–5526.