Predicting the Thermal Behavior in the Design of Type V Deep Eutectic Solvents: The Combined Role of Polarity and Steric Asymmetry

The thermal behavior of the hydrophobic eutectic mixtures formed by l-menthol (MEN) with 4-methoxyphenol (4-Met), 2-tert-butyl-4-methoxyphenol (BHA), 2-tert-butyl-4-ethylphenol (TBEP), and tert-butylhydroquinone (TBHQ) has been investigated to elucidate the different factors governing the solid–liquid equilibrium (SLE) at the molecular level. Our comparative study highlights the contribution of hydrogen-bonding (H-bonding) asymmetry of deep eutectic solvent (DES) precursors, i.e., their ability to act as strong H-bond donors and weak H-bond acceptors or vice versa. When deriving from resonance effects, this concept was previously presented as “polarity asymmetry” and introduced to explain the thermal behavior of type V DES. Here, we expand this concept to demonstrate that steric factors led by the insertion of ad hoc functional groups in the precursor molecules also have a dominant role in the deviation from thermodynamic ideality. In this way, an increasing temperature depression is observed starting from the 4-Met/MEN mixtures up to the BHA/MEN, TBHQ/MEN, and TBEP/MEN ones. The SLE prediction has been carried out through an innovative approach combining the COSMO-RS solvation model and density functional tight-binding molecular dynamics simulations, benchmarked against experimental data, and presented here for the first time. The impact of this work is that of providing new tools for a more conscious understanding and design of hydrophobic type V DES as new sustainable media for applicative purposes.