On the Role of Water in the Formation of a Deep Eutectic Solvent Based on NiCl₂·6H₂O and Urea

Abstract

The metal-based deep eutectic solvent (MDES) formed by NiCl 2 ·6H 2 O and urea in 1:3.5 molar ratio has been prepared for the first time and characterized from a structural point of view. Particular accent has been put on the role of water in the MDES formation, since the eutectic could not be obtained with the anhydrous form of the metal salt. To this end, mixtures at different water/MDES molar ratios ( W ) have been studied with a combined approach exploiting molecular dynamics and ab initio simulations, UV–vis and near-infra-red spectroscopies, small- and wide-angle X-ray scattering, and X-ray absorption spectroscopy measurements. In the pure MDES, a close packing of Ni 2+ ion clusters forming oligomeric agglomerates is present thanks to the mediation of bridging chloride anions and water molecules. Conversely, urea poorly coordinates the metal ion and is mostly found in the interstitial regions among the Ni 2+ ion oligomers. This nanostructure is disrupted upon the introduction of additional water, which enlarges the Ni–Ni distances and dilutes the system up to an aqueous solution of the MDES constituents. In the NiCl 2 ·6H 2 O 1:3.5 MDES, the Ni 2+ ion is coordinated on average by one chloride anion and five water molecules, while water easily saturates the metal solvation sphere to provide a hexa-aquo coordination for increasing W values. This multidisciplinary study allowed us to reconstruct the structural arrangement of the MDES and its aqueous mixtures on both short- and intermediate-scale levels, clarifying the fundamental role of water in the eutectic formation and challenging the definition at the base of these complex systems.

Solubilization and coordination of the HgCl2 molecule in water, methanol, acetone, and acetonitrile: an X-ray absorption investigation

“…As a consequence, the amplitude of the EXAFS signal associated with the solvent molecules is quite low and it is mainly detected in the low-k region of the spectrum. 35,43,56 This circumstance was already evidenced in an XAS study on HgCl 2 in aqueous solution 16 and is in line with the high rate of exchange events observed between the coordinating and the outer-sphere water molecules around the HgCl 2 molecule obtained by ab initio molecular dynamics simulations. 25 3.2.2 XANES data analysis.…”

The Complex Story Behind a Deep Eutectic Solvent Formation as Revealed by l-Menthol Mixtures with Butylated Hydroxytoluene Derivatives

“…The majority of the currently available DESs, classified in the type I–IV categories, is formed by at least one ionic component, so that the origin of the MP depression is typically attributed to complexation of a hydrogen-bond (H-bond) acceptor from a H-bond donor. ,,− More recently, type V DESs have been presented and are formed by neutral molecular components only, very often based on terpenoid compounds like menthol and thymol. , Being nonionic, they typically display lower viscosities, are chloride-free, and often hydrophobic, so that they are suited for the removal of target analytes from aqueous solutions through liquid–liquid biphasic extractions. − However, also, this category deals with species possessing H-bonding capabilities. For these reasons, DESs are often indicated as extensively H-bonded media, which, although not being a strict definition for these systems, has become a common way to identify them.…”

Unveiling the Influence of Hydrated Deep Eutectic Solvents on the Dynamics of Water–Soluble Proteins

“…Depending on the nature of their constituents, DESs are classified into five main types (I–V). Most of them, including “metal-based deep eutectic solvents” (MDESs), fall into types I–IV, while non-ionic DESs belong to the recently proposed fifth type …”

“…7 Depending on the nature of their constituents, DESs are classified into five main types (I−V). Most of them, including "metal-based deep eutectic solvents" (MDESs), 8 fall into types I−IV, 4 while non-ionic DESs belong to the recently proposed fifth type. 9 Type III DESs, formed by choline chloride and hydrogenbond donors, are certainly the most studied so far, mainly due to the ease of preparation, low cost, and high biodegradability of the starting materials.…”