Structure, Microheterogeneity, and Transport Properties of Ethaline Decoded by X-ray/Neutron Scattering and MD...

Ethaline, a deep eutectic solvent (DES) composed of choline chloride (ChCl)-ethylene glycol (EG) in a 1:2 molar ratio, is garnering significant interest for its wide potential applications. The nature of liquid formation and the structure of H-bonds within ethaline were investigated by X-ray scattering (XRS), neutron scattering (NS), and MD simulations. The sum of the dissociation energy barriers of Ch-EG (3.31 kJ·mol^-1) and EG-Cl (4.28 kJ·mol^-1) exceeds that of Ch-Cl (5.97 kJ·mol^-1). This results in a more pronounced solvation of ChCl by EG compared to ChCl association, facilitating the solubilization of ChCl crystals by EG to form a DES. A partial radial distribution function (PDF) reveals that Cl^- solvation is dominated by the hydroxyl group of EG, while the methylene group dominates Ch⁺ solvation. The spatial distribution function (SDF) shows that the distribution of EG and Cl^- around Ch⁺ partially overlaps with that of the quaternary ammonium group. However, the center of mass distance of Ch-Cl (4.95 Å) is significantly lower than that of Ch-EG (5.65 Å), suggesting a favorable advantage for Cl^- in this competition. Chain and ring structure distributions provide direct evidence of the microheterogeneity of ethaline. Hydroxyl groups on the EG promote the formation of a chain structure in ethaline, while methylene groups favor a ring structure. H-bond, carbon H-bond, and Cl^- bridge bond restrict Cl^- diffusion. This new understanding is crucial for a deeper comprehension of the microstructure of ethaline and for elucidating its mechanisms in applications.