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. Here, we propose multi-scale approaches developed for calculating the thermodynamics properties of ions in aqueous and organic solutions directly comparable to the experimental ones, such as the ion pair association constants and the activity coefficients. In such approaches, 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 to fit the experimental data. In aqueous solutions, a multi-scale approach based on molecular dynamics and Monte Carlo simulations has been first developed on simple electrolytes, such as alkali chloride (MCl) and lanthanide chloride (LnCl3) aqueous solutions [2]. This method allows for calculating pair association constants and activity coefficients in very good agreement with the experimental ones. This approach has been then extended to molecular electrolytes such as uranyl chloride (UO2Cl2) electrolytes [3]. Here, the thermodynamic properties have been calculated from the effective ion-ion McMillan-Mayer pair potentials. The pair association constants and the osmotic coefficients of molecular electrolytes (containing uranyl cations, nitrate and sulphate anions) have been estimated, and pointed out a good agreement with the experimental data available in the literature. In organic phases, the physical (rigidity) and thermodynamics (stability of reverse micelle) properties at the mesoscopic scale have been deduced from umbrella-sampling molecular dynamics simulation. These results have been then used in order to determine the thermodynamics properties of the organic phase at the macroscopic scale using the Helfich formalism. This approach allows for calculating the ternary phase diagrams and the scattering functions as a function of the rigidity of the interface and the shape of the extractant molecule [4]. [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), 1 (2015) [2] J. J. Molina, M. Duvail, J.-F. Dufrêche and Ph. Guilbaud J. Phys. Chem. B 115(15), 4329 – 4340 (2011) [3] T. N. Nguyen, M. Duvail, A. Villard, J. J. Molina, Ph. Guilbaud and J.-F. Dufrêche J. Chem. Phys. 142, 024501 (2015) [4] M. Duvail, L. Arleth, Th. Zemb and J.-F. Dufrêche J. Chem. Phys. 140(16), 164711 (2014)