[en] This thesis presents a set of models and methods for the structural and thermodynamic description of organic solutions and interfacial systems encountered in the context of liquid-liquid extraction. The models and methods are based on an approach that is essentially molecular. It has a strong numerical component. A study based on a molecular dynamics approach was used to investigate the phase separation of a water-oil mixture. It has also been used to simulate organic solutions whose supramolecular organization has been verified by comparisons between the experimental and the molecular simulations signals associated with small angle X-ray scattering. The supramolecular organization has been characterized more finely during studies devoted to the aggregation in organic phase in the presence of extractant malonamide molecules (DMDOHEMA) and of europium nitrate salts thanks to advanced numerical treatments presented in this thesis. These numerical treatments allowed the calculation of the mean distributions of the chemical species formed in the organic solutions. From these distributions, thermodynamic models of the aggregation phenomena in the organic phase based on numerical and analytical approaches have been developed. These models allowed the calculation of the energies of formation of the species in solution according to their composition, and the determination of the mean aggregation numbers in very good agreement with the experimental data, the study of the mechanisms associated with the phenomenon of 'third phase formation' thanks to a super-species percolation model and the calculation of quantities associated with the kinetics of formation of aggregates in organic phase. (author)