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The electrodeposition of cobalt from alternative electrolytes, including ionic liquids and deep eutectic solvents (DES), has become a topic of great interest to the scientific community, with a significant impact in both academic circles and in the development of commercial industrial electrochemical processes. However, very few studies have considered the effects of water on the electrodeposition of metals from deep eutectic solvents. In this work, the electrodeposition of Co from a choline chloride (ChCl) ethylene glycol (EG)-based deep eutectic solvent (DES) containing 10%, 20% and 30% water has been studied, and for the first time a uniform and bright Co deposit has been obtained when the deposition was achieved from an electrolyte containing 20% water. The speciation of Co in a mixed 1:2 ChCl:EG-based liquid (Ethaline 200) has been studied in both the absence and presence of water. The conductivities of the Co electrolyte were increased with increasing amounts of water. The electrochemical properties of the Co electrolytes have been studied using cyclic voltammetry, where it was found that the redox peak current gets larger and shifts in a positive direction when water was included in the Co solution. The resultant surface morphologies, topography, and roughness of the Co deposits were revealed by scanning electron microscopy (SEM) and atomic force microscopy (AFM), which demonstrated that a highly uniform and smooth cobalt coating had been produced when the deposition occurred in Ethaline 200 containing 20% water.

Most studies of metal electrodeposition in ionic liquids dry the electrolyte thoroughly, as water is thought to be detrimental. In some cases, water has a beneficial effect on deposit morph...

We report herein the electrochemical polymerisation of aniline from a choline chloride (ChCl)-based deep eutectic solvent (DES) at room temperature in both the presence and absence of deionised water in the solvent. The current study used Propeline as the DES electrolyte, which we found to be rich in protons (i.e. similar to protic ionic liquids) and which allowed the polymerisation of aniline in this system. The morphology of films was characterised by scanning electron microscopy (SEM) and atomic force microscopy (AFM), which showed the polymer film to have a compact structure and rough surface. The electrochemical quartz crystal microbalance (EQCM) was used for the first time to investigate the mass changes that occur during the electrochemical deposition of polyaniline (PANI) in situ, and was used to monitor cation, anion, and solvent transport when PANI films were immersed in pure Propeline and 0.5 M H2SO4 solution (monomer free) at 25 °C and 50 °C. In the case of the PANI exposed to pure Propeline at room temperature, the mass change increased continuously in both oxidation and reduction switching due to anion transfer dominating the mass change, while the PANI exposed to pure Propeline at 50 °C was found to form due to a two-step combined mechanism, showing excellent redox switching stability with changing mass. These variations in mass tend to be dominated by cation expulsion in the first oxidative process (the later reductive process being cation insertion) and by anion insertion in the second oxidative stage (the early reductive process being anion expulsion). In aqueous systems, the mechanism of mass change was dominated by anion movement associated with solvent transfer. The movement of neutral species in Propeline and water is in the reverse direction to those of cation or anion transfer.

The interfacial nanostructure of the three most widely-studied Deep Eutectic Solvents (DESs), choline chloride:urea (ChCl:Urea), choline chloride:ethylene glycol (ChCl:EG), and choline chloride:glycerol (ChCl:Gly) at a Pt(111) electrode has been studied as a function of applied potential and water content up to 50 wt%. Contact mode atomic force microscope (AFM) force–distance curves reveal that for all three DESs, addition of water increases the interfacial nanostructure up to ∼40 wt%, after which it decreases. This differs starkly from ionic liquids, where addition of small amounts of water rapidly decreases the interfacial nanostructure. For the pure DESs, only one interfacial layer is measured at OCP at 0.5 nm, which increases to 3 to 6 layers extending ∼5 nm from the surface at 40 or 50 wt% water. Application of a potential of ±0.25 V to the Pt electrode for the pure DESs increases the number of near surface layers to 3. However, when water is present the applied potential attenuates the steps in the force curve, which are replaced by a short-range exponential decay. This change was most pronounced for ChCl:EG with 30 wt% or 50 wt% water, so this system was probed using cyclic voltammetry, which confirms the interfacial nanostructure is akin to a salt solution.

Despite the importance of ionic liquids in a variety of fields, little is understood about the behaviour of protons in these media. The main difficulty arises due to the unknown activity of protons in non-aqueous solvents. This study presents acid dissociation constants for nine organic acids in deep eutectic solvents (DESs) using standard pH indicator solutes. The pKIn value for bromophenol blue was found by titrating the DES with triflic acid. The experimental method was developed to understand the acid-base properties of deep eutectic solvents, and through this study it was found that the organic acids studied were slightly less dissociated in the DES than in water with pKa values between 0.2 and 0.5 higher. pKIn values were also determined for two ionic liquids, [Bmim][BF4] and [Emim][acetate]. The anion of the ionic liquid changes the pH of the solution by acting as a buffer. [Emim][acetate] was found to be more basic than water. It is also shown that water significantly affects the pH of ionic liquids. This is thought to arise because aqueous mixtures with ionic liquids form heterogeneous solutions and the proton partitions into the aqueous phase. This study also attempted to develop an electrochemical pH sensor. It was shown that a linear response of cell potential vs. ln aH+ could be obtained but the slope for the correlation was less than that obtained in aqueous solutions. Finally it was shown that the liquid junction potential between two reference electrodes immersed in different DESs was dependent upon the pH difference between the liquids.

Deep eutectic solvents (DESs) have been used for the purification of oils and the extraction of active ingredients from natural products but little is known about the mechanism of the extraction process. In this study a variety of molecular solutes are dissolved in alkanes and the thermodynamics of transfer into six DESs have been quantified. It is shown that the transfer of most solutes into the DES is endothermic and driven by entropy. The largest partition coefficients were demonstrated by the liquids with the lowest surface tensions and this is thought to arise because the enthalpy of hole formation controls the rate of solute transfer. Accordingly, it was shown that the size of the solute has an effect on the partition coefficient with smaller solutes partitioning preferably into the DES. As expected, solutes capable of strongly hydrogen bonding partitioned much better into the DES as the enthalpy of transfer was negative.

Correction for ‘Molecular and ionic diffusion in aqueous – deep eutectic solvent mixtures: probing inter-molecular interactions using PFG NMR’ by Carmine D'Agostino et al., Phys. Chem. Chem. Phys., 2015, 17, 15297–15304.

Physical properties of pure DESs and DESs mixed with different amounts of water were investigated by different techniques, including, rotational viscometer, Quartz Crystal Microbalance (QCM) and conductivity meter. In order to study heterogeneity of those liquids in nature. Analysis of these data showed some characteristics of heterogeneity, the extent of which depends on the number of hydrogen bond donors in the pure DES.