Preparation of ferric chloride-based deep eutectic solvents and characterization of their physicochemical properties

We report FeCl<inf>3</inf>·6H<inf>2</inf>O-based deep eutectic solvents (DESs) formed from FeCl3·6H2O with acetic (Fe–AC), butyric (Fe–BA), and lactic acid (Fe–LA) over hydrogen bond acceptors (HBA) to hydrogen bond donors (HBD), HBA:HBD = 2:1–1:4. Polarized optical microscopy confirmed that the systems form homogeneous, non-birefringent liquids. For cross-system comparisons, a fixed 2:1 (FeCl<inf>3</inf>·6H<inf>2</inf>O:acid) composition was selected. FTIR shows broadened, red-shifted absorption corresponding to O-H stretching vibration, carbonyl stretching absorption band perturbation, and low-frequency features associated with Fe-O, evidencing extended H-bond networks interlaced with Fe³⁺-O coordination. Differential scanning calorimetry presents broad endotherms without sharp melting peaks, consistent with amorphous/glass-like behavior driven by dehydration and H-bond reorganization. The reduced arc radii for Fe–BA and Fe–LA indicate lower bulk resistance and higher conductivity than for Fe–AC, consistent with direct conductivity measurements (s). Broadband dielectric spectroscopy reveals a large low-frequency permittivity due to electrode polarization, with convergence at high frequencies to the dipolar baseline. Dielectric modulus and loss-tangent analyses indicate faster relaxation in Fe-LA and enhanced polarization in Fe-BA. Together, Fe-LA and Fe-BA emerge as the most capable ion-transport media among the three systems under study. At the same time, Fe-AC offers a more compact and less polarizable matrix. These results connect structure (H-bond/coordination), dynamics (relaxation), and transport (s, e', tan d) to guide application of Fe-based DESs in electrochemical and catalytic processes.