Eutectic Point Determination of Type V Hydrophobic Octanoic Acid-Based Solvents by “Lock-Free” ¹H and ¹³C NMR Self-Diffusion Experiments

Deep eutectic solvents (DESs) constitute a peculiar class of solvents with specific properties and applications. DESs are classified into five different types (I-V). Unlike types I-IV, type V deep eutectic solvents are a novel class of sustainable solvents prepared by physically mixing nonionic components. Currently, no techniques are available to characterize hydrophobic type V DESs without causing denaturation. When the components of the DES are in a solid state, it is visually obvious that the DES exists because a liquid is formed due to the decrease in the melting point. However, the complexity increases significantly when the constituents of the DES are in liquid form or are a combination of solids and liquids. New "lock-free" 1H and 13C NMR diffusion ordered spectroscopy (DOSY) experiments proved the formation of eutectic mixtures in two cases: neat liquid octanoic acid mixed with either solid L-menthol or neat liquid propane-1,3-diol without resorting to deuterated solvents. We observed maximum interaction at 5:5 and 4:6 molar ratios, respectively. These results confirmed the predicted ratio for the eutectic point from COSMO-RS quantum mechanical calculations and agreed with the experimental differential scanning calorimetry (DSC) results.