Characterization of hydrophobic and metal-based deep eutectic solvents

This study investigates the characterization of hydrophobic and metal-based deep eutectic solvents (DESs), focusing on the relationship between the molecular structure of their components and the resulting properties. The research identifies structural properties as crucial for understanding deviations from ideality in these solvents. Various systems, including L-menthol (L-MEN) mixed with butylated hydroxytoluene (BHT), tert-butyl-p-cresol (TBC), and p-cresol (PC), were explored. The results demonstrate that steric hindrance significantly affects the thermal behavior of the mixtures. Specifically, the BHT:L-MEN system was classified as an ideal eutectic, whereas TBC:L-MEN and PC:L-MEN were categorized as type V DESs. Further analysis of different systems with varying functional groups in the \textit{para} position relative to the hydroxyl group underscored the combined role of steric and polarity asymmetry. Eutectic mixtures of L-MEN with compounds such as 4-methoxyphenol (4-Met), 2-tert-butyl-4-methoxyphenol (BHA), tert-butylethylphenol (TBEP), and tert-butylhydroquinone (TBHQ) were examined. The TBEP:L-MEN system exhibited the highest deviation from ideality, attributed to its pronounced structural and electronic asymmetry. The impact of precursor chirality was subsequently investigated by analyzing BHT:L-MEN and Thymol (TYM):L-MEN eutectic mixtures as a function of MEN enantiomers. Thermal and structural characterization revealed that in systems with significant deviations from ideality, such as TYM:L-MEN, it is possible to switch MEN enantiomers without affecting the DES properties. However, in ideal eutectic solvents like BHT:L-MEN, the inherent properties of the pure compounds dominate, limiting the interchangeability of mixtures with different enantiomers. Structural changes in the hydrophobic eutectic mixture BHT:L-MEN (1:3) were then examined upon the addition of methanol (MeOH) and ethanol (EtOH) as cosolvents. The results showed that the cosolvents disrupted the primary interaction in the system which is the hydrogen bond between L-MEN molecules. Additionally, the study explored the effects of n-hexane (HEX) on the nanostructure of various eutectic mixtures (BHT:L-MEN 1:3, TYM:L-MEN 1:2, ChCl:TYM 1:7). HEX was found to disrupt molecular aggregation in BHT:L-MEN and TYM:L-MEN mixtures, while inducing nanoscale inhomogeneities in the ChCl:TYM system at high HEX concentrations. Lastly, the role of water in DES formation was investigated by characterizing a nickel chloride hexahydrate and urea metal-based DES (MDES). The findings highlighted the critical influence of water on the structural organization and thermal behavior of the MDES, particularly in facilitating the packing of Ni2+ ion clusters, which is fundamental to eutectic formation. Collectively, this study underscores the importance of structural factors, electronic effects, and the complex interplay of molecular interactions in the formation and behavior of DESs. The overall results, obtained through a combination of experimental and computational approaches, provide valuable insights into the design and application of new DESs.