Different impact of menthol chirality on ideal and deep eutectic solvents: Thermal and structural insights

Although chiral molecules are common components of deep eutectic solvents (DESs), the impact of precursor chirality on the fundamental aspects of these solvents is far from being systematically explored. Here, the effect of chirality on the eutectic mixtures formed by the optically active menthol (MEN) with thymol (TYM) and butylated hydroxytoluene (BHT) has been investigated. To this purpose, we resorted to a combined thermal and structural analysis by means of differential scanning calorimetry measurements aided by the COSMO-RS solvation model, Raman spectroscopy, small- and wide-angle X-ray scattering, and molecular dynamics simulations. A slightly different thermal behavior and self-association are observed for the levorotatory enantiomer (L-MEN) as compared to the dextrorotatory one (D-MEN) and the racemate (D/L-MEN) in the pure MEN system. This behavior is preserved in the BHT:MEN 1:3 ideal eutectic as the interaction with the BHT molecules is hampered due to the presence of the bulky substituents. On the other hand, the precursor chirality has no detectable influence on the overall properties of the TYM:MEN 1:1 DES due to strong interactions with the non-chiral counterpart. These findings have important implications for the design and employment of chiral DESs for enantioselective purposes. • D/L-menthol melts at a significantly lower temperature than L-menthol and D-menthol. • Raman spectroscopy and MD simulations indicated stronger self-interaction among menthol molecules in L- compared to D/L- and D-menthol. • By mixing menthol and the butylated hydroxytoluene the properties of the pure components are preserved. • The thymol:menthol systems exhibit similar thermal and structural properties regardless of the menthol precursor's chirality.