Structural study of a eutectic solvent reveals hydrophobic segregation and lack of hydrogen bonding between the components

An in-depth study of the hydrophobic eutectic solvent formed by butylated hydroxytoluene (BHT) and L-menthol (MEN) in a 1:3 molar ratio has been carried out using an integrated approach that combines differential scanning calorimetry (DSC), attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, small- and wide-angle X-ray scattering (SWAXS), and molecular dynamics (MD) simulations. The obtained data have been step-by-step compared to those collected on the eutectic formed by 3,5-di-tert-butyltoluene (TBT) and MEN in the same molar ratio, where TBT is analogous to the BHT without the hydroxyl group. The DSC characterization showed comparable results between the two systems, evidencing that the hydroxyl group of the BHT has little or no impact on the thermal behavior of the BHT:MEN eutectic. Both the FTIR and MD results agree in finding that no hydrogen bond (H-bond) interactions are played by the BHT because of the high steric hindrance suffered by its hydroxyl group so that the only established H-bonds are those between MEN molecules. The incompatibility between the components in terms of H-bonds formation results in hydrophobic segregation promoting the MEN-MEN interactions, which are even more intense than in the pure compound. The three-dimensional arrangement between the components showed a remarkable degree of structural order among the alkyl functional groups, suggesting that the apolar-apolar attraction might be the driving force of the eutectic formation. This picture is translated into the establishment of an intermediate-range organization in solution, as evidenced by the SWAXS data. The overall impact of this study is that of pushing a little bit further the definition of these eutectics, indicated until now as extensively H-bonded systems.