How Hydration Affects the Microscopic Structural Morphology in a Deep Eutectic Solvent

Recently, it has been observed that choline chloride ([Ch][Cl]) based deep eutectic solvents (DESs) may possess nanostructures in which chloride ions play a major role by providing a backbone for the bridged hydrogen bond network. In this article, we present a fundamental study on the role of hydration on the nanostructure of a [Ch][Cl] based DES, ethaline. In this molecular dynamics investigation, we observe a nonlinear change in the structural morphology of ethaline on gradual addition of water. The initiation of disruption of the native structure of the DES at 40 mol % of water is clearly observed, after which the increasing dilution rapidly destructs the long-range as well as short-range intermolecular interactions existing between the constituent species of pure ethaline. Herein, we find that, at very high hydration level, chloride ion no longer acts as a bridge between the choline cation and ethylene glycol, as it forms strong hydrogen bond with water. Intriguingly, the strengthening of hydrogen bonding interactions among the ethylene glycol molecules is observed on increasing hydration level. Hence, it is predicted that segregation of ethylene glycol can occur in the pool of aqueous mixtures of [Ch][Cl] at very high hydration level.