According to molecular dynamics simulations, uncomplexed Cyanex-301-like ligands L- and LH (derived from the dithiophosphinic acid R2PS2H) and their EuL3 complexes are surface active. They adsorb at a water/"oil" interface, where "oil" is modeled by chloroform. It is shown that addition of neutral phosphoryl ligands such as TBP (tri-n-butyl phosphate) is necessary to promote the extraction of the complex, via two complementary mechanisms. First, gradual co-complexation of TBP, forming EuL3(TBP)(n) species, enhances the interfacial activity and the hydrophobicity of the lanthanide complex. Second, high concentrations of TBP are needed to saturate and modify the interface, leading to the migration of the EuL3(TBP)(3) complex to the oil phase. The effect of oil/water ratio is also investigated. At high ratio, one observes the formation of water-in-oil microdroplets onto which TBPs adsorb, while the complexes are fully extracted to the oil phase. These features make it possible to better understand synergistic and solvation effects in the assisted liquid-liquid extraction of lanthanide or actinide cations.