Predicting the Thermal Behavior in the Design of Type V Deep Eutectic Solvents: The Combined Role of Polarity and Steric Asymmetry
Giorgia Mannucci, Gabriel Teixeira, Filipe H. B. Sosa, Matteo Palluzzi, Matteo Busato, João A. P. Coutinho, P. D’Angelo
Abstract
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.