Anatomy of Microscopic Structure of Ethaline Deep Eutectic Solvent Decoded through Molecular Dynamics Simulations

Atomistic molecular dynamics simulations have been performed to investigate the microscopic structure of ethaline deep eutectic solvent (DES), a mixture of choline chloride ([Ch][Cl]) and ethylene glycol (EG) in molar ratio of 1:2, respectively. As much as the structure of a DES is derived by the composition of the species present in it, the chemical nature of the hydrogen bond donor species involved also plays a crucial role in laying down the microscopic structure of DESs. By virtue of its inherent chemical structure, EG renders both intra- and intermolecular hydrogen bonds. Therefore, the molecular level structural landscape of DESs containing EG as hydrogen bond donor is reckoned to be a bit complex. In the present study, we aim to understand the structural morphology of ethaline using optimum force-field parameters for EG recently proposed by our group. After an initial assessment of the refined force-field parameters for ethaline DES, we have presented an in-depth analysis of the arrangement and ordering of its components at the molecular level. Simulated X-ray scattering structure function and its partial components reveal the presence of short-range as well as long-range interactions in ethaline. The role of hydrogen bonding interactions among all the three species [Ch](+), [Cl](-), and EG was predominantly observed through radial and radial-angular distribution functions and substantiated by spatial distribution functions. The observation of the competitive nature of [Ch](+) and EG to form a hydrogen bond with the anion is one of the major outcomes of the present study. Also, weaker intra- and intermolecular hydrogen bonding interactions between EG molecules were seen along with their simultaneous involvement with the ammonium group of the choline cation.