Physicochemical and structural investigation of l-threonine/glycerol-based deep eutectic solvents using experimental and molecular modelling approaches

Abstract

One of the most popular topics in sustainable chemistry is the creation of new eco-friendly solvents. Deep eutectic solvents (DESs) have been established as accessible and affordable substitutes for ionic liquids. Herein, we studied the formation of a novel amino acid-based DES (AADES) considering L-threonine as a hydrogen bond acceptor and glycerol as a hydrogen bond donor. Owing to their complexity, a comprehensive understanding of DESs requires combined efforts that integrate experimental observations with computational approaches. The validation of the synthesized DES was initially performed through FTIR spectroscopy, where significant changes in bond-bending vibrations indicated strong non-covalent bond formation. In differential scanning calorimetry (DSC) analysis, the 1 : 3 Thr/Gly deep eutectic system displayed phase transitions marked by a pronounced peak at −22 °C, which was lower than the melting points of threonine (Thr) and glycerol (Gly). ¹H-NMR studies also revealed hydrogen bonding intermolecular interactions between glycerol and threonine. Deshielded chemical shifts of proton signals of both glycerol and threonine are due to the local changes in electron density induced by the closeness of electronegative oxygens via the inductive effect, which supports the formation of the conformer (3 : 1 glycerol/threonine DES). Molecular dynamics (MD) simulation was employed to acquire a comprehensive understanding of how these solvents form and function at both the molecular and macroscopic levels. RDF and CDF analyses revealed the non-bonding sites (C–O⋯H and CO⋯H), interaction intensity (2–4.7 Å), and predominant angles (135–180 and 0–30 degrees) governing the process by which hydrogen bonds originate in the DES. SDF uncovered the conformations of the liquid DES and highlighted its variability, particularly in terms of clusters. The bulk properties of DESs are of paramount significance because they are intricately linked to their diverse applications. Transport property values were obtained through specialized MD simulations, providing crucial insights into the behavior of the systems. Non-equilibrium molecular dynamics (NEMD) simulations were performed to assess the rheological properties of viscosity at three distinct temperatures (298 K, 313 K, and 328 K). The obtained viscosity, surface tension, and self-diffusion coefficient values appear practical and fall within a reasonable range when compared to those of other well-known DESs.