Shelf life and time-resolved thermal, chemical, and thermodynamic characterization of four hydrophobic deep eutectic solvents

Abstract

Hydrophobic deep eutectic solvents (HDESs) are increasingly recognized as green alternatives to traditional organic solvents in extraction, catalysis, and polymer processing; however, their long-term stability remains poorly understood. This study investigated the critical gap in systematically characterizing the effects of long-term storage on four thymol-and tetraoctylammonium bromide (TOAB)-based HDESs with decanoic (C10) and dodecanoic (C12) over 20 weeks of storage. Weekly comprehensive measurements and characterization techniques were used to monitor changes in both bulk properties and molecular structures. Thermal stability was evaluated using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), which consistently showed onset decomposition temperatures between 136 °C and 205 °C across all four solvents, with a variation of ±2.77 °C over the 20-week storage period. TOAB- based systems exhibited superior thermal stability, likely due to ionic hydrogen bond networks. Thermodynamic analysis revealed that molar excess Gibbs free energy values satisfied the non-ideality criterion , and activity coefficients remained well below unity throughout the study, confirming strong intermolecular interactions and eutectic integrity. Fourier-transform infrared (FTIR) spectra showed consistent features with no signs of chemical degradation or moisture-induced structural changes. The C12-based systems undergo reversible room temperature crystallisation attributed to segregation of a fatty-acid-rich solid phase; this microphase separation modifies processability but not the underlying eutectic chemistry. Density (0.88–0.95 g cm⁻³), viscosity (18–85 mPa s @ 25 °C), and apparent pH remain within the acidic range expected for carboxylic acid-based HDESs, with observed fluctuations attributed mainly to the known limitations of glass electrodes in low conductivity media rather than genuine chemical change. These findings demonstrate that TOAB- and thymol-based HDESs are thermally and chemically stable during the storage period. This research contributes to an important knowledge gap in green solvent technology and supports their further development from laboratory research to industrial applications.