Design and characterization of novel hydrophobic eutectic solvents based on metal-extracting ligands

Abstract

In the search for efficient and sustainable liquid–liquid extraction systems, some examples of hydrophobic deep eutectic solvents (HDES) recently emerged as promising alternatives due to their lower volatility, higher stability and extraction performances compared to conventional systems with organic diluents. However, the novelty of HDES has so far limited their study to a small number of systems, precluding further optimization and a deeper understanding. In the present study, we propose the design and full characterization of new non-ionic HDES using a combined experimental and simulation-based approach. These HDES are formulated with extractant molecules that are well known in the field of hydrometallurgy. Specifically, neutral extractants never explored in HDES, such as Tributyl phosphate (TBP), N,N′-dimethyl,N,N′-dioctylhexylethoxymalonamide (DMDOHEMA) or N,N,N′,N′ tetraoctyl diglycolamide (TODGA), were selected as Hydrogen Bond Acceptors (HBA), and associated with decanoic acid (DecA), as a Hydrogen Bond Donor (HBD). A detailed characterization study, using complementary techniques such as FT-IR spectroscopy, small angle X-rays scattering (SAXS) and molecular dynamics (MD), allowed us to elucidate the structural features and intermolecular interactions governing HDES formation. FT-IR and MD revealed quantitatively how the solvent properties are related to hydrogen bond interactions. MD results were successfully exploited to reproduce the experimental SAXS signals, which allowed for the accurate interpretation of the HDES structure. A physicochemical characterization study was further applied to demonstrate the possible application of these HDES as media for liquid–liquid extraction.