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. Although the aqueous solutions are nowadays well described in terms of structural, dynamical and thermodynamics properties, 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 the presence of surfactant or extractant molecules in the organic phase, and then are captured in reverse micelles. Therefore, in order 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, since it is well known that such micelles contain water molecules. 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 association constant of complex UO2Cl+ and the osmotic coefficients have been estimated, and pointed out a good agreement with the experimental data available in the literature. 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. Concerning the calculation of the thermodynamics properties of ions in organic phases, almost the same approach as the one developed for the aqueous solutions has been used. The thermodynamics properties at the mesoscopic scale have been deduced from umbrella-sampling molecular dynamics simulation. In addition to what has been performed in aqueous solutions, these thermodynamics properties have been then used in order to determine the thermodynamics properties of the solutes between the aqueous and organic phase using the Helfich formalism [4]. 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, M. Duvail and J.-F. Dufrêche. Isr. J. Chem. 53, 108 – 112 (2013) [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. Submitted (2014) [4] M. Duvail, L. Arleth, J.-F. Dufrêche and Th. Zemb. Phys. Chem. Chem. Phys. 15, 7133 – 7143 (2013)