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

Abstract

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.

Influence of Hydration on the Structure and Interactions of Ethaline Deep‐Eutectic Solvent: A Spectroscopic and Computational Study

“…Though this is slightly true in Complex A, it is clearly persuasive in Complexes B, C, and D. Further, in Complexes E, F, and I, a water molecule dissociates the strongest H‐bond by inserting and bridging the anion and cation with a pair of H‐bonds. These results are in agreement with a recent work using molecular dynamics simulations, where Cl − was found no longer to act as a bridge between Ch + and EG, as it forms strong H‐bond with water at very high hydration level …”

“…Accordingly, the negative excess peak is best attributed to neat ETH which is represented by Complex B (Figure ), and the deconvoluted positive excess peaks are attributed to the appearing EG dimer and EG trimer upon mixing. The results are consistent with a recent work on the same system using molecular dynamics simulations, where the strengthening of H‐bonding interactions among the EG molecules was reported, and thereupon, segregation of EG was predicted at high hydration level . In addition, it should be noted that the AWs of hydroxyls in 1 : 1 to 1 : 4 molar ratios of ETH‐D 2 O mixtures are close to that of neat ETH.…”

Do Deep Eutectic Solvents Form Uniform Mixtures Beyond Molecular Microheterogeneities?

“…ethylene glycol) in the pool of the aqueous mixture. 9 This is supported by the existence of multimers composed of strongly H-bonded ethylene glycol molecules in aqueous solution of ethaline as demonstrated by infrared spectroscopy and DFT quantum calculations. 45 According to the above-mentioned predictions from MD simulation, the mesoscopic structural ordering of ethylene glycol molecules during water addition should be evidenced by the raising of a low-Q component (0.1 ≤ Q ≤ 0.25 Å −1 ) for the EG-EG partial structure factor, but it could not be seen in the total S(Q) due to cancellation effects.…”

“…For practical reasons, aqueous solutions of DESs have been formulated and their physical properties, including excess properties, viscosity, acoustic, structural and optical properties have been studied thoroughly. [9][10][11][12][13][14][15][16][17][18][19][20][21] These works have also raised the question of the resilience of the specific local arrangement of DES during dilution with water and its evolution towards an aqueous solution of its constituents. 9-11, 19, 22 The structure of DESs has attracted a lot of interest during the last four years.…”

“…45 According to the above-mentioned predictions from MD simulation, the mesoscopic structural ordering of ethylene glycol molecules during water addition should be evidenced by the raising of a low-Q component (0.1 ≤ Q ≤ 0.25 Å −1 ) for the EG-EG partial structure factor, but it could not be seen in the total S(Q) due to cancellation effects. 9 Experimentally, conventional X-rays and neutron diffraction methods provide a weighted sum of all partial structure factors, which is likely also affected by the cancellation effects. Small angle neutron scattering experiment with isotopic labelling offers a unique possibility to measure the partial structure factors and detect this low-Q component.…”