Recycling of metals, such as rare earths, 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 is therefore a crucial issue in order to optimise the separation processes. Although the aqueous solutions are nowadays well described, a lack remains in understanding the thermodynamics properties of ions in organic phases. Indeed, it is well known that 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. Therefore, to understand correctly the formation of such aggregates in organic phases, it remains crucial to understand the solvation properties of ions in both the organic and the aqueous phases. 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 lanthanide chloride (LnCl3) aqueous solutions [2]. This method, based on the calculation of effective ion-ion McMillan-Mayer pair potentials, allows for calculating pair association constants and activity coefficients in very good agreement with the experimental ones. Furthermore, this method has also proved its efficiency in the calculation of thermodynamics properties of other molecular electrolytes containing sulfate and nitrate anions, for example [3]. For the organic phases, the thermodynamics properties of ions, at the mesoscopic scale, have been deduced from umbrella-sampling molecular dynamics simulations (molecular scale). In addition, these results have been also used to determine the thermodynamics properties of solutes at the water/oil interface using the Helfich formalism [4,5]. 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. [1] Th. Zemb, C. Bauer, P. Bauduin, L. Belloni, Ch. 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] J. J. Molina, M. Duvail, J.-F. Dufrêche and Ph. Guilbaud. J. Phys. Chem. B 115, 4329 (2011) [3] M. Duvail, A. Villard, T. N. Nguyen and J.-F. Dufrêche. J. Phys. Chem. B 119, 11184 (2015) [4] M. Duvail, L. Arleth, J.-F. Dufrêche and Th. Zemb. Phys. Chem. Chem. Phys. 15, 7133 (2013) [5] M. Duvail, L. Arleth, Th. Zemb and J.-F. Dufrêche. J. Chem. Phys. 140, 164711 (2014)