Dottorato in SCIENZE CHIMICHE

Titolo della tesi: Characterization of Hydrophobic and Metal-based Deep Eutectic Solvents

A comprehensive characterization of hydrophobic and metal-based deep eutectic solvents (DESs) has been conducted, focusing on how the molecular structure of the components influences the properties of the mixtures. Various systems were analyzed, including L-menthol (L-MEN) combined with butylated hydroxytoluene (BHT), tert-butyl-p-cresol (TBC), and p-cresol (PC). The results demonstrate that the degree of steric hindrance around the hydroxyl group significantly impacts the thermal behavior of these solvents, leading to distinct deviations from ideality. Subsequently, the analysis of systems with various functional groups in the para position relative to the hydroxyl one emphasized 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. Among these, the TBEP:L-MEN system exhibited the greatest deviation from ideality, primarily due to its pronounced structural and electronic asymmetry. The impact of the precursors 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 select 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 both the cosolvents disrupt 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 eutectics with different degree of hydrophobicity (BHT:L-MEN 1:3, TYM:L-MEN 1:2, ChCl:TYM 1:7). HEX was found to disrupt molecular aggregations 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 DESs 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 was found to be fundamental for the 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 approach, provide valuable insights into the design and application of new DESs.