Extraction of Metal Ions Using Novel Deep Eutectic Solvents with Chelating Amine

Two novel hydrophobic deep eutectic solvents (HDESs), composed of alkyl (=Hexyl, Nonan) ethylenediaminium and menthol (Men), namely Hexen/Men and Nonen/Men, were synthesized. Hexen and Nonen primarily act as hydrogen bond acceptors, with Men serving as the principal hydrogen bond donor. After the formation of HDES, the IR absorption peaks of Hexen, Nonen's–NH₂, and Men–OH fused into a wider peak, the ¹H-NMR spectra of Men–OH, shifted to a lower field. Furthermore, a significant redshift approximately 300 cm⁻¹ was detected in the vibrational frequency of the Men–OH functional group when performing density functional theory (DFT) calculations for the HDESs. These results support the development of stronger O–H···N bonds between Hexen/Nonen–NH₂ and Men–OH, and the calculated sum of hydrogen bonding energy was approximately 56 mol·kg^–1, categorizing it as an intermediate-strength hydrogen bond. Both HDESs have ethylenediamine polar heads in their hydrogen bond acceptors, which have chelating characteristics that help them coordinate with transition metal ions. Metal ions such as Cu(II), Co(II), and Ni(II) were successfully extracted from aqueous solutions at a concentration of 10 mmol·L^–1using HDESs. The Cu(II) and Ni(II) extraction efficiencies exceeded 90%, indicating their effectiveness. Notably, even at higher metal ion concentrations (100 mmol·L^–1), the extraction efficiencies of all three metal ions remained consistently below 80%. This indicates that the HDESs can suitably collect trace metal ions.