Unravelling the influence of water: Investigating the physicochemical properties of deep eutectic solvents based on choline chloride and pyruvic acid/α-keto glutaric acid

In the vast and intricate world of solution chemistry, deep eutectic solvents (DESs) have established a unique standing due to the easy accessibility of their components, along with their non-hazardous nature, biodegradability, recyclability, and cost-effectiveness. The main objective of this study is to investigate the key physicochemical properties of DESs, particularly those composed of choline chloride (ChCl) and pyruvic acid (referred to as pyruline), as well as those formulated with α-keto glutaric acid (referred to as glutaline). The initial investigation encompasses a comprehensive analysis of the thermal stability of the proposed DESs at 1:1, 1:2, and 2:1 M ratio, which were systematically assessed using TGA and DSC. The findings indicated that all eutectic mixtures exhibit a glass transition phenomenon instead of conventional melting and undergo a two-stage decomposition process. Following the thermal analysis of the neat DESs, the other physicochemical properties, including density, ionic conductivity, and electrochemical stability, were exclusively evaluated for the aqueous pyruline (1:1) and aqueous glutaline (2:1) pseudo-binary mixtures. The density studies of both proposed DES/H2O pseudo-binary mixtures across the temperature range of 293.15–343.15 K show a decrease with increasing temperature, accompanied by a corresponding increase in the thermal expansion coefficients (αP). Furthermore, the nonideal behaviour of the aqueous DESs was investigated by calculating the excess molar volume (VE) derived from density values across various temperature range and differing H2O mole fractions. The obtained VE demonstrated an excellent correlation with the Redlich–Kister polynomial equation. Additionally, the temperature dependent ionic conductivity of DES/H2O pseudo-binary mixtures was studied over the temperature range of 298.15–333.15 K. The cyclic voltammetry analysis offers a comprehensive insight into the electrochemical stability of DESs. Ultimately, the examination of binding energy (BE) and frontier molecular orbitals (FMOs) through density functional theory (DFT) offers valuable insights into the intermolecular interactions, molecular structures, and stability of DES clusters at the molecular level. © 2024 Elsevier B.V.