Liquid-liquid extraction is a key hydrometallurgical process used to selectively separate the lanthanide cations, such as in the DIAMEX process (DIAMide EXtraction) [1] developed by CEA for the recycling of spent nuclear fuels. In this process, the lanthanide cations are transferred from an acid-enriched aqueous phase to an organic phase containing malonamide extractants, like DMDOHEMA, diluted in a mixture of alkanes. This ion transfer between the two phases is governed by the Gibbs energy of transfer ∆Gtr which is measured experimentally by the distribution coefficients of the ions [2]. This work deals with the prediction of the Gibbs energies of transfer of lanthanide cations using Steered Molecular Dynamics (SMD), an out-of-equilibrium simulation technique for Molecular Dynamics. The SMD methodology uses a moving biasing harmonic potential to steer the lanthanide cations from the aqueous phase to the organic phase, allowing for accurate sampling of the free energy landscape of the interface [3,4]. We simulated water–octanol interfaces as a model system for liquid-liquid extraction. In nuclear hydrometallurgy, octanol (octan-1-ol) is used as a phase modifier to prevent the formation of a heavy organic third phase associated with criticality risks. Calculating the molecular orientation revealed that the octanol molecules at the interface organize themselves in a rigid bilayer structure, as previously observed [5] (Fig. 1), preventing the water transfer toward octanol. SMD simulations will be used to determine the Gibbs energy barrier of this interface, allowing the calculation of the water solubility in octanol. [1] Madic, C. et al. PARTNEW. CEA Report 6066. 2004 [2] Špadina, M. et al. Langmuir 2021, 37 (36), 10637–10656. [3] Jami, L. et al. J. Chem. Phys. 2022, 157 (9), 094708. [4] Jarzynski, C. Phys. Rev. E 1997, 56 (5), 5018–5035. [5] Benay, G.; Wipff, G. J. Phys. Chem. B 2013, 117 (4), 1110–1122.