Preparation and physical and thermodynamic characteristics of proline-based natural ternary deep eutectic solvents: A combined experimental and theoretical investigation

Natural deep eutectic solvents (NADESs) are known for their superior properties, such as stability, ease of preparation, recyclability, biodegradability, availability of raw materials, low toxicity, and unique applications. In this study, six new three-component NADES samples based on the amino acid proline were prepared. Three of the samples contain water, proline and ascorbic acid, while the other three consist of glycerin, proline, and ascorbic acid, with different molar ratios. The formation of NADESs was confirmed using DSC and FT-IR analyses. The basic physical properties of the prepared NADESs including refractive index, density, conductivity, and viscosity were experimentally measured at different temperatures ranging from 288.15 to 328.15 K. The pH measurements of the NADESs at ambient temperature showed that all samples are acidic (pH ≈ 3–4). It was found that the conductivity and refractive index of the NADES samples increase with rising temperature, while their viscosity and density decrease. The temperature dependence of the physical properties of the prepared solvents was analyzed using suitable linear (simple or polynomial) models. Moreover, theoretical studies based on density functional theory (DFT) were conducted to better understand the structure of the NADES compounds, their intermolecular interactions, stability and thermodynamic characteristics. The DFT calculations indicate that the changes in enthalpy, interaction energy and Gibbs free energy of the NADESs are negative, demonstrating high stability and the spontaneous nature of the interactions at room temperature.