First-Principles Molecular Dynamics Study of a Deep Eutectic Solvent: Choline Chloride/Urea and Its Mixture with Water

Abstract

First-principles molecular dynamics simulations in the canonical ensemble at temperatures of 333 and 363 K and at the corresponding experimental densities are carried out to investigate the behavior of the 1:2 choline chloride/urea (reline) deep eutectic solvent and its equimolar mixture with water. Analysis of atom-atom radial and spatial distribution functions and of the H-bond network reveals the microheterogeneous structure of these complex liquid mixtures. In neat reline, the structure is governed by strong H-bonds of the trans- and cis-H atoms of urea to the chloride ion. In hydrous reline, water competes for the anions, and the H atoms of urea have similar propensities to bond to the chloride ions and the O atoms of urea and water. The vibrational spectra exhibit relatively broad peaks reflecting the heterogeneity of the environment. Although the 100 ps trajectories allow only for a qualitative assessment of transport properties, the simulations indicate that water is more mobile than the other species and its addition also fosters faster motion of urea.

The peculiar effect of water on ionic liquids and deep eutectic solvents

“…Specifically, when water is added in a small amounts (w H 2 O o 5%), water gets absorbed in the structure of DES by forming H-bonds with the ions and HBDs; beyond that water amount (w H 2 O 4 5%), water also dampens the intramolecular interactions in the DES, leading to drastic changes of properties. Fetisov et al 214 investigated the behaviour of ChCl/urea (1 : 2) and its equimolar mixture with water (x H 2 O = 0.5 or w H 2 O = 17%) by performing first-principles MD simulation. Their results show that the addition of H 2 O breaks the strong H-bonds between the H atoms of urea and [Cl] À in the pure ChCl/urea (1 : 2) by preferentially hydrating [Cl] À and forming urea-water H-bonds.…”

Strong Microheterogeneity in Novel Deep Eutectic Solvents

“…[2,3,[18][19][20] Calculations on DESs are rather sparse compared to experiments and often concentrate on the understanding of the hydrogen bonding behavior since a plethora of hydrogen bonds are available, examples are Refs. [5,18,[21][22][23][24].A very important kind of nanostructuring, namely, molecular ionic clusters within a complex and disordered hydrogen bonding network was observed by Edler and coworkers. [25][26][27] Biswas and coworkers found various type of aggregats depending on the anion and temperature.…”

“…[2,3,[18][19][20] Calculations on DESs are rather sparse compared to experiments and often concentrate on the understanding of the hydrogen bonding behavior since a plethora of hydrogen bonds are available, examples are Refs. [5,18,[21][22][23][24].…”

Water distribution at the electrified interface of deep eutectic solvents

“…4,31,35,36 Extensive experimental and computational studies have been dedicated to rationalize the correlation between melting point depression and the hydrogen bond strength and its inuence on the thermo-physical properties of DESs, with special emphasis on 1 : 2 choline chloride-urea (Reline). [4][5][6][7][8][9]35,[37][38][39] Thus, it has been proposed that the DES mixture melting point depression depends on the lattice energies of the DESs, the anion-HBD interaction, and the change in entropy due to formation of the liquid phase. 9 Following this, Perkins et al 40 provided the rst molecular structural insights using infrared spectroscopy (IR) experiments and molecular dynamics (MD) simulations.…”

“…6 This was also conrmed by an ab initio molecular dynamics study. 39 On the other hand, more recently, it has been shown that the presence of small amounts of residual or intentionally added water in Reline could be benecial for controlling the size, shape and distribution of deposited nanoparticles. 5,24,26,29,49,51,52 Also, in a carbon dioxide gas capture experiment, it has been shown that the solubility of carbon dioxide in a mixed water-Reline solvent is strongly dependent on the amount of water, and that increasing the amount of water leads to a decrease in carbon dioxide solubility.…”