Fatty Acid-Based (Deep) Eutectic Solvents Thermodynamic Behavior and Thermal Properties for Low-Temperature Thermal Energy Storage

Abstract

The mismatch between heat supply and heat demand can be resolved by better utilization of low temperature waste heat with the help of low-temperature thermal energy storage (TES). In this work, binary mixtures of thymol (THY) with decanoic acid (C10), undecanoic acid (C11), and dodecanoic acid (C12) were prepared across a broad composition range and evaluated as phase-change material (PCM) candidates. Differential scanning calorimetry (DSC) was used to determine melting points (Tm), heat of melting (ΔHm), and short-term thermal-cycling stability, while Fourier Transform Infrared (FTIR) and NMR measurements were used as supporting evidence for intermolecular interactions. Phase diagrams were constructed from replicate measurements and compared with ideal-liquidus reference curves. The lowest average Tm values were observed near XTHY = 0.40 for THY:C10, XTHY = 0.40 for THY:C11, and within XTHY = 0.5 for THY:C12. Near-eutectic compositions were evaluated for cycle stability. The results demonstrate retained ΔHm over 100 melt/freeze cycles. The calculated ideal-liquidus curves reproduced the overall Tm depression but they systematically overpredicted the experimental Tm, indicating modest nonideal interactions. Excess Gibbs free energy (GE) data support favorable heteromolecular interactions and the stabilization of eutectic composition and qualification of THY-fatty acid–based mixtures as deep eutectic solvents.