Temperature-Dependent Interaction Studies of Choline–Levulinic Acid Deep Eutectic Solvent with Methanol and Ethanol: Combined Experimental and Density Functional Theory Study

Abstract Deep eutectic solvents have gained significant attention in recent years as sustainable alternatives to conventional solvents, owing to their eco-friendly nature and advantageous properties over traditional organic solvents and ionic liquids. This study evaluates the physicochemical and thermodynamic properties of a choline chloride:levulinic acid DES prepared at a 1:2 molar ratio. Densities, speed of sound, and refractive indices of pure DES and its binary mixtures with methanol and ethanol were measured across the entire mole composition range of DES (xDES = 0.0000:1.0000) at different temperatures ranging from 293.15 to 313.15 K. The experimental data was modeled using the Lorentz–Lorenz equation, confirming its accuracy in predicting the measured properties and excess molar volumes. The excess molar volumes (VmE), intermolecular free length (Lf), isentropic compressibilities (κs), deviations in isentropic compressibilities (Δκs), and deviation refractive indices (ΔnD) were calculated to elucidate the intermolecular interactions between the DES and alcohols. The density functional theory (DFT) approach was used to support and evaluate the intermolecular interactions between the DES mixtures with selected alcohols and strong interactions were also observed. The DFT calculations, supplemented by noncovalent interactions (NCI) and reduced density gradient (RDG) analyses, were performed to elucidate the intermolecular interactions within DES-alcohol systems.