Structural and Thermal Properties of Choline Chloride- and Thymol-Based Deep Eutectic Solvents: Experimental and COSMO-RS Modeling

Abstract

Abstract Deep eutectic solvents (DESs) were prepared by mixing choline chloride (ChCl) and thymol (Thy) at molar ratios ranging from 1:1 to 1:10. Among the prepared mixtures, ChCl:Thy ratios of 1:4 to 1:10 formed homogeneous liquids and remained stable for more than 744 h. These seven DESs were characterized using Fourier-transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR) spectroscopy, and thermogravimetric analysis (TGA) to evaluate their structural characteristics and thermal stability. FTIR and NMR analyses confirmed the formation of strong hydrogen-bonding interactions, particularly between chloride ions and the hydroxyl group of thymol (Cl···H–O), which disrupt the crystal lattices of the parent components and facilitate eutectic formation. The decomposition temperature decreased from 239.9 °C for ChCl:4Thy to 142.3 °C for ChCl:6Thy with increasing thymol content. Compared with pure ChCl (319.2 °C) and thymol (101.9 °C), the DESs exhibited modified thermal stability due to the newly established hydrogen-bonding network. Furthermore, COSMO-RS-derived sigma profiles and sigma potentials supported the strong Cl···H–O interactions and confirmed the favorable formation and stability of ChCl:Thy DESs over the 1:4 to 1:10 molar-ratio range.