The Complex Story Behind a Deep Eutectic Solvent Formation as Revealed by <scp>l</scp>-Menthol Mixtures with Butylated Hydroxytoluene Derivatives

Abstract

High Resolution Image Download MS PowerPoint Slide An in-depth study of the hydrophobic eutectic mixtures formed by l -menthol (MEN) with the butylated hydroxytoluene (BHT), 2- tert -butyl- p -cresol (TBC), and p -cresol (PC) compounds has been carried out, where TBC and PC are analogous to the BHT species but with a different degree of steric hindrance around the hydroxyl group. Thermal characterization evidenced that the BHT/MEN system can be classified as an ideal eutectic, while the TBC/MEN and PC/MEN mixtures behave as type V deep eutectic solvents (DESs) for a wide range of compositions around the eutectic point. As shown by an array of experimental and theoretical methods, in the BHT/MEN mixtures the establishment of hydrogen-bond (H-bond) interactions between the components is dramatically hampered because of the steric hindrance in the BHT molecule, so that the achievement of a liquid phase at room temperature for the eutectic composition is driven by apolar–apolar attractions among the alkyl functional groups of the constituents. Differently, the TBC-MEN donor–receptor H-bond is the main driving force for the formation of a type V DES and derives from a concurrence of electronic and steric factors characterizing the TBC molecule. Finally, the absence of steric hindrance around the hydroxyl group allows the self-association among PC molecules through H-bonded networks already in the pristine compound, but the replacement with the more favorable PC-MEN H-bond provides a type V DES upon mixing of these components. Our combined approach, together with the peculiarity of the inspected systems, delivered an archetypal study able to shed light onto the various contributions ruling the structure–properties relationship in DESs and possibly deepening the currently accepted view of these inherently complex media.