In Silico Elucidation of Molecular Picture of Water-Choline Chloride Mixture

Choline chloride (ChCl) is a component of several deep eutectic solvents (DESs) having numerous applications. Recent studies have reported manifold promising use of aqueous choline chloride solution as an alternative to DES, where water plays the role of the hydrogen-bond donor. The characteristic physical properties of the DESs and aqueous DES originate from the "inter-" and intraspecies hydrogen-bond network formed by the constituents. However, a detailed molecular-level picture of choline chloride and water mixture is largely lacking in the literature. This motivates us to carry out extensive all-atom molecular dynamics simulations of the ChCl-water mixture of varying compositions. Our analyses clearly show an overall increase in the interspecies association with an increase in ChCl concentration. At higher concentrations, the trimethylammonium groups of choline are stabilized by a nonpolar interaction, whereas the hydroxyl groups preferentially interact with water. Chloride ions are found to be involved in two types of interactions: one where chloride ions intercalate two or more choline cations, and the other one where they are surrounded by five to six water molecules forming solvated chloride ions. However, the relative fractions of these two types of associations depend on the concentration of ChCl in the mixture. Another important structural aspect is the disruption of the hydrogen-bonded water network due to the presence of both choline cations and chloride ions. However, chloride ions participate to partially restore the tetrahedral arrangement of partners around water molecules.