Selective Lithium–Sodium Separation Using a DBM-TOPO Deep Eutectic Solvent: Experimental and Molecular Insights

As alkali metals have similar physicochemical properties, efficient separation of lithium and sodium is a common challenge in the fields of salt lake brine processing and lithium battery resource recycling. In this work, a hydrophobic deep eutectic solvent (HDES) composed of dibenzoylmethane (DBM) and trioctylphosphine oxide (TOPO) was investigated for the selective extraction of lithium ions. The solid–liquid equilibrium phase diagram confirmed eutectic formation. By adjusting the molar ratio of DBM-TOPO, the solubility of the HDES in the diluent and its lithium ion extraction performance were enhanced, ultimately yielding a single-stage Li⁺ extraction efficiency of 91.1% and a Li⁺/Na⁺ separation selectivity of 92.6. Density and viscosity were measured across a temperature range and correlated using a linear equation and the Vogel–Fulcher–Tammann equation, respectively. Spectroscopic analyses (FT-IR, ¹H NMR) and density functional theory (DFT) calculations demonstrated that hydrogen bonding between enol-form DBM and TOPO drives HDES formation and preferential coordination with Li⁺. These findings provide both experimental data and molecular-level understanding of DBM-TOPO HDESs, supporting their application in high-performance lithium extraction systems.