Recycling of metals, such as rare earths, lanthanides or actinides, into valuable material relies on ion specific separation, basis of the hydrometallurgy. Most of efficient methods known for separating ions are based on equilibria between complex fluids, typically between aqueous and organised organic phases [1]. Understanding the thermodynamics in both phases remains therefore a crucial issue in order to optimise the separation processes. In organic phases, molecular complexes formed in the organic phase during solvent extraction may self-assemble as reverse micelles, and therefore induce a supramolecular organization of this phase. In most of the cases, water molecules play an essential role in the organization of this non polar medium. Here, we propose a multi-scale approach developed for calculating the thermodynamics properties of ions in organic phases based on molecular dynamics simulations and mesoscopic modelling. In such approach, the thermodynamics properties (mesoscopic scale) are calculated only by taking into account the molecular properties of the solutes in solutions, and no adjustable parameters have been added to the models. First, the speciation of the aggregates formed in the organic phase during solvent extraction has been investigated in order to assess their stability as a function of the number of water molecules included in their polar core [2]. We focused on malonamide extractants, namely DMDOHEMA, that are generally used for extracting lanthanide cations. Several stoichiometries of reverse micelles in the organic phase have been studied by means of classical molecular dynamics simulations allowing for calculating equilibrium constants and reaction free energies corresponding to association/dissociation pathways of water molecules in the aggregates. In the meantime, bending rigidities of small reverse aggregates involved in such process has been determined by molecular dynamics simulations [3]. We pointed out that elastic film bending energy that is needed for mesoscopic modelling of transfer of species between complex fluids is harmonic in terms of curvature (Helfrich formalism) and packing parameter only if the solvent is explicitly taken into account. In terms of packing parameter of real molecular film constituting the reverse water in oil aggregates and taking into account (i) molecular volume, (ii) area and (iii) film thickness, the bending rigidity is calculated to be about 16 kBT per extractant molecule, which is smaller than the free energy of transfer from an isolated “monomer” molecule to a weak aggregate, but of the order of magnitude of the free energy of transfer used in liquid–liquid extraction processes. 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. Recycling metals by controlled transfer of ionic species between complex fluids: en route to “ienaics”. Colloid Polym. Sci. 2015, 293, 1-22 [2] Y. Chen, M. Duvail, Ph. Guilbaud and J.-F. Dufrêche. Stability of reverse micelles in rare-earth separation: a chemical model based on a molecular approach. Phys. Chem. Chem. Phys. 2017, 19, 7094-7100. [3] M. Duvail, S. van Damme, Ph. Guilbaud, Y. Chen, Th. Zemb and J.-F. Dufrêche. The role of curvature effects in liquid–liquid extraction: assessing organic phase mesoscopic properties from MD simulations. Soft Matter 2017, 13, 5518-5526