Deep eutectic solvents : investigation of solvent structure and its applications

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Deep eutectic solvents (DESs) represent an alternative class of ionic fluids closely resembling room-temperature ionic liquids (RTILs), they are distinguished by the fact that they also contain an organic molecular component (typically, a hydrogen-bond donor like a urea, amide, acid, or polyol), frequently as the predominant constituent. DESs possess several advantages over RTILs while being less expensive, synthetically accessible, nontoxic, and biodegradable. In this work we have probed into liquid structure of DES using quantum chemical calculations and neutron scattering experiments to elucidate the molecular interactions, charge transfer interactions, thermodynamics and mass transport associated with these systems. The DESs studied comprise 1:2 choline chloride/urea (reline), 1:2 choline chloride/ethylene glycol (ethaline), 1:2 molar ratio of choline chloride to glycerol (glyceline) and 1:1 choline chloride/malonic acid (maloline). The DESs were found to be stabilized by both conventional hydrogen bonds and C-H...[pi] interactions with significant charge transfer from choline and chloride to the hydrogen-bond donors, further confirmed by density of states analysis. Consequently, it was found that the sum of the bond orders of various choline-C1[minus] interactions in the DESs correlates directly with the melting temperatures of the DESs. From macroscopic measurements of glyceline, it was observed that the long-range translational diffusion of the larger cation (choline) is slower compared to the smaller glycerol molecule. However, the diffusion dynamics analyzed on the subnanometer length scale revealed that the displacements associated with the localized diffusive motions are actually larger for choline. This is due to ability of glycerol to form stronger hydrogen bonds with chloride anions compared to choline cation. Quantum chemical simulations performed on ASCs paired reline and ethaline DES systems revealed that ASCs interacted with the choline and HBD components of DESs through multiple unconventional non-covalent interactions i.e., CH...[pi], C=O...[pie], O-H...[pi] and N-H...[pi] interactions. Oxidation of ASCs improved interaction with DES components due enhanced ability to form multiple hydrogen bonds. ASCs exhibited significant improvement in change free energy of solvation upon oxidation indicating that oxidation could aid in enhancing selective extraction of oxidized ASCs from liquid fuel using DES.