Unveiling molecular interactions in glycerol-based deep eutectic solvents†

Abstract

Elucidating the liquid structure of deep eutectic solvents (DES) is crucial to understand how local interactions determine their properties. In this work, the impact of the anion on the liquid structure and local interactions was investigated for mixtures of tetrabutylammonium chloride and bromide ([N₄₄₄₄]Cl and [N₄₄₄₄]Br) with glycerol (Gly). The phase behavior was explored across various compositions using differential scanning calorimetry (DSC) showing that these mixtures form a (meta)stable liquid at room temperature and x_salt = 0.25. At this composition, infrared spectroscopy (IR) revealed strong hydrogen bonding between glycerol and the anion that is more pronounced for chloride than bromide. This finding is supported by the enthalpy of mixing measurements and by quantum chemical calculations. Molecular dynamics (MD) simulations demonstrated that intermolecular hydrogen bonds between glycerol molecules persist, maintaining a long-range liquid structure even in the presence of salt. Far-infrared spectroscopy (FIR) combined with MD simulations revealed changes in local intermolecular dynamics due to a confinement effect caused by the strong anion–glycerol interactions. These results highlight the critical influence of local interactions driven by the anion on DES properties.