A new hydrophobic eutectic solvent based on trioctylamine and monoterpene alcohols: properties and application

Against the backdrop of increasingly stringent environmental regulations and the transition toward a circular economy, conventional hydrometallurgical methods are increasingly being superseded by innovative technological solutions. A critical bottleneck in this progression remains the selectivity and environmental safety of the reagents employed. In this context, the development of novel, highly efficient extractants capable of recovering target metals from aqueous media represents a priority objective in modern chemical engineering.

The aim of the present work is to synthesize novel eutectic solvents based on trioctylamine and monoterpene alcohols (linalool, citronellol, etc.), to fully characterize them, and to investigate their physicochemical and extraction properties.

Solid–liquid phase diagrams are presented for all the proposed systems, in which trioctylamine serves as a hydrogen bond acceptor, while the monoterpene alcohols act as hydrogen bond donors. The eutectic solvents, prepared at a 1:1 molar ratio of the components, were characterized using IR and NMR spectroscopy. The spectroscopic analysis confirmed intermolecular interactions between the amine and the monoterpene alcohol. The chemical stability of the eutectic solvents upon contact with mineral acid solutions was also demonstrated. Furthermore, the temperature dependences of density, viscosity, and refractive index for the prepared eutectic solvents were established.

The applicability of the proposed systems as extractants was demonstrated through the extraction of Fe(III) and Cu(II) from hydrochloric acid solutions. It was found that Fe(III) is efficiently extracted in the HCl concentration range of 1 to 3 mol/L, with extraction efficiencies exceeding 50 %. In contrast, the extraction of copper was found to be negligible under the same conditions.

Collectively, the obtained data suggest that eutectic solvents based on trioctylamine and monoterpene alcohols hold promise for use in hydrometallurgical processes targeting the treatment of metal-bearing waste streams.