Speciation, Conductivities, Diffusivities, and Electrochemical Reduction as a Function of Water Content in Mixtures of Hydrated Chromium Chloride/Choline Chloride

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

The metal-based deep eutectic solvent (MDES) formed by NiCl₂·6H₂O 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 Ni²⁺ 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 Ni²⁺ 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 NiCl₂·6H₂O 1:3.5 MDES, the Ni²⁺ 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.

The metal-based deep eutectic solvent formed by NiCl₂·6H₂O and urea in 1:3.5 a molar ratio was prepared for the first time, and its aqueous mixtures were characterized from a structural point of view, highlighting the fundamental role of water in the eutectic formation.

Hygroscopicity of 1:2 Choline Chloride:Ethylene Glycol Deep Eutectic Solvent: A Hindrance to its Electroplating Industry Adoption

Deep eutectic solvents have been established as feasible metal electroplating solvent alternatives over traditional toxic aqueous plating baths. However, water, either added intentionally or unintentionally, can significantly influence the solvent’s physical properties and performance, thereby hindering its industry application. In this study, the hygroscopicity, or the ability to absorb moisture from the environment, of synthesized ethaline (1:2 choline chloride:ethylene glycol) was investigated. The kinematic viscosity, electrical conductivity, electrochemical window, and water content of ethaline were monitored over a 2-week period. Karl Fischer titration tests showed that ethaline exposed to the atmosphere displayed significant hygroscopicity compared to its unexposed counterpart. 1H NMR spectroscopy revealed that water vapor was readily absorbed at the surface due to the hydrophilic groups present in the ethaline molecule. Water uptake resulted in the decrease in viscosity, increase in electrical conductivity and narrowing of the electrochemical window of ethaline. Solution heating at 100°C removed the absorbed moisture and allowed the recovery of the solvent’s initial properties.

Mechanistic Study on Surface Tension of Binary and Ternary Mixtures Containing Choline Chloride, Ethylene Glycol and Water (Components of Aqueous Solutions of a Deep Eutectic Solvent, Ethaline)

The paper reports the results of the measurement of surface tension in binary liquid mixtures containing choline chloride, ethylene glycol and water and ternary mixtures of ethaline (a eutectic mixture of ethylene glycol and choline chloride, the so-called deep eutectic solvent) with water. The surface tension is determined in a wide range of components’ concentration for the temperatures of 25, 30, 40, 50, 60 and 70 °C. The dependences of surface tension on the concentration of liquids are treated by means of Gibbs adsorption isotherm and linear form of Langmuir adsorption isotherms. The values of the equilibrium constant of adsorption at the interface liquid solution/air as well as the standard adsorption Gibbs energy, enthalpy and entropy are calculated for the binary and ternary mixtures under consideration. The obtained results are interpreted in the light of intermolecular interactions in fluids. Our findings show that ethylene glycol and choline cation interact with each other in the adsorbed surface layer formed at the interface between air and diluted solutions of ethaline.

Combining MCR-ALS and EXAFS as tools for speciation of highly chlorinated chromium(iii) in mixtures of deep eutectic solvents and water

Spectral signatures of Cr(iii) complexes in deep eutectic mixtures were remarkably different from aqueous solutions due to exchange of the water ligands with ethylene glycol.

Microstructure and corrosion behavior of Cr and Cr–P alloy coatings electrodeposited from a Cr(iii) deep eutectic solvent

Cr and Cr–P coatings were electrodeposited on Fe substrates from non-aqueous deep eutectic solvent-based electrolytes containing Cr(iii).