Novel acidic deep eutectic solvents: synthesis, phase diagram, thermal behavior, physicochemical properties and application

Three new binary acidic deep eutectic solvents (DESs) were derived from tetrabutylammonium bromide (TBAB) as a hydrogen bond acceptor (HBA) coupled with azelaic acid (AzA), benzilic acid (BeA) and boric acid (BoA) as hydrogen bond donors (HBD). In order to draw the complete phase diagram and find the molar ratios in which the DESs are formed, HBA and HBDs were mixed together in the full range of molar ratios and their thermal behavior (by differential scanning calorimetry method) and stability were investigated. The activity coefficient models including Wilson, NRTL and regular solution were applied to correlate the measured solid–liquid equilibrium data. FT–IR and NMR analysis were used to investigate the chemical structures, occurrence the hydrogen bond formation between the DESs constituents and purity of the prepared DESs. Physicochemical properties of these DESs including melting point, density, and speed of sound, conductivity and refractive index were measured at different temperatures. The performance of these new DESs were successfully evaluated for the dissolution of four metal oxides including PbO, CuO, Fe₂O₃ and ZnO and the obtained solubility values were compared with the reported literature data.