Characterization of hydrophobic deep eutectic solvents to evaluate their application as a solvent in dispersive liquid-liquid microextraction in liquid environmental samples

This study reports the preparation, characterization, and application of three non-ionic hydrophobic deep eutectic solvents (HDES). The HDES prepared in this work were based on DL-menthol (MENT) as the hydrogen bond acceptor (HBA), combined with various carboxylic acids as hydrogen bond donors (HBDs), including acetic acid (AcA), decanoic acid (DCA), and dodecanoic acid (DDCA). Fourier Transform Infrared Spectroscopy analysis (FTIR) confirmed the formation of eutectic mixtures through characteristic shifts and broadening of O H and C O bands, as well as the disruption of carboxylic acid dimers. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TG/DTG) revealed that most mixtures formed eutectic systems with melting points lower than those of their individual components. However, the thermal behavior was complex, often exhibiting multiple crystallization or melting peaks. It suggests the formation of off-composition mixtures, polymorphic structures, or stable supercooled liquids depending on the specific molar ratio. After two years, the MENT: DCA and MENT: DDCA mixtures remained stable, with no significant changes observed in their FTIR spectra. In contrast, the MENT: AcA mixture exhibited significant spectral changes, indicating a weakening of intermolecular interactions. Evolved gas analysis (EGA) validated this by identifying the formation of menthyl acetate, indicating a slow esterification reaction over time. The MENT: AcA (1:1) HDES was successfully applied as an extraction solvent in a dispersive liquid-liquid microextraction (DLLME) method to preconcentrate contaminants from domestic sludge samples. The developed analytical methods demonstrated excellent performance. For parabens and bisphenol A analyzed by pyrolysis gas chromatography–mass spectrometry (Py-GC–MS), recoveries were above 90%, precision (CV%) was below 15%, and limit of detection (LODs) ranged from 25 to 50 μg L −1 . For polybrominated diphenyl ethers (PBDEs) analyzed by gas chromatography coupled to an electron capture detector (GC-ECD), recoveries ranged from 72.1% to 93.1%, precision was below 15%, and LODs ranged from 4 to 6 μg L −1 , highlighting the method's high sensitivity. These results demonstrate the effectiveness of HDES as robust and environmentally friendly solvents for environmental analysis. • The HDES exhibited eutectic behavior, characterized by a reduction in melting points observed by DSC. • The formation of the HDES was demonstrated by FTIR analysis, in which bands typically associated with hydrogen bonding were observed. • TGA analysis revealed variations in thermal stability among the HDES, allowing for improvements in methodologies for chromatographic applications. • The HDES remained physically and chemically stable, showing only minor changes in viscosity and density over 24 months. • The HDES proved effective in sample preparation, achieving recoveries above 90% using both Py-GC/MS and GC-ECD.