Understanding the Complex Surface Interplay for Extraction: A Molecular Dynamics Study

By means of classical molecular dynamics simulation the interfacial properties of methanol and n-dodecane, which are two potential candidate solvents for use in non-aqueous liquid-liquid extraction, were assessed. The question of how the interface changes depending on the concentration of extractant (tri-n-butyl phosphate) and salt (LiCl) is addressed. Two different models to represent systems were used to evaluate how LiCl and tri-n-butyl phosphate affect mutual miscibility, and how the last-named behaves depending on the chemical environment. Tri-n-butyl phosphate increases the mutual solubility of the solvents, whereas LiCl counteracts it. The extractant was found to be mostly adsorbed on the interface between the solvents, and therefore the structural features of the adsorption were investigated. Adsorption of tri-n-butyl phosphate changes depending on its concentration and the presence of LiCl. It exhibits a preferential orientation in which the butyl chains point at the n-dodecane phase and the phosphate group points at the methanol phase. For high concentrations of tri-n-butyl phosphate, its molecular orientation is preserved by diffusion of the excess molecules into both the methanol and n-dodecane phases. However, LiCl hinders the diffusion into the methanol phase, and thus increases the concentration of tri-n-butyl phosphate at the interface and forces a rearrangement with subsequent loss of orientation.