Edward Matthijs

Electrochemical decomposition of choline chloride based ionic liquid analogues

Ionic liquids are studied intensively for electrochemical applications and more specifically for the electrodeposition of metals. In this paper the electrochemical stability of a deep-eutectic solvent based on choline chloride and ethylene glycol is studied over longer periods of electrolysis. The formation of several decomposition products such as 2-methyl-1,3-dioxolane was observed. Possible mechanisms for the formation of these products are given: some products involve a reaction at either the anode or the cathode, while others can be explained by consecutive reactions of reaction products formed at both electrodes. A range of chlorinated products like chloromethane, dichloromethane and chloroform could be detected as well. This is remarkable as evolution of chlorine gas at the anode is not observed. The formation of the chlorinated products is ascribed to the existence of the Cl3− ion in the solution. The presence of the Cl3− ion was observed photometrically. The presence of chlorinated products gives rise to a larger environmental impact and higher risks for health and safety, and it questions the “greenness” of these ionic liquid analogues. To reduce the decomposition of the solvent, water and easily oxidizable acids were added as ‘sacrificial agents’. Their influence on the formation of 2-methyl-1,3-dioxolane was quantified. However, the addition of the sacrificial agents did not improve the stability of the solvent. Addition of formic acid reduced the formation of 2-methyl-1,3-dioxolane but chlorinated products could still be detected. Water reduced the formation of chlorinated products.

Speciation of Copper(II) Complexes in an Ionic Liquid Based on Choline Chloride and in Choline Chloride/Water Mixtures

A deep-eutectic solvent with the properties of an ionic liquid is formed when choline chloride is mixed with copper(II) chloride dihydrate in a 1:2 molar ratio. EXAFS and UV-vis-near-IR optical absorption spectroscopy have been used to compare the coordination sphere of the cupric ion in this ionic liquid with that of the cupric ion in solutions of 0.1 M of CuCl(2)·2H(2)O in solvents with varying molar ratios of choline chloride and water. The EXAFS data show that species with three chloride ions and one water molecule coordinated to the cupric ion as well as species with two chloride molecules and two water molecules coordinated to the cupric ion are present in the ionic liquid. On the other hand, a fully hydrated copper(II) ion is formed in an aqueous solution free of choline chloride, and the tetrachlorocuprate(II) complex forms in aqueous choline chloride solutions with more than 50 wt % of choline chloride. In solutions with between 0 and 50 wt % of choline chloride, mixed chloro-aquo complexes occur. Upon standing at room temperature, crystals of CuCl(2)·2H(2)O and of Cu(choline)Cl(3) formed in the ionic liquid. Cu(choline)Cl(3) is the first example of a choline cation coordinating to a transition-metal ion. Crystals of [choline](3)[CuCl(4)][Cl] and of [choline](4)[Cu(4)Cl(10)O] were also synthesized from molecular or ionic liquid solvents, and their crystal structures were determined.

Challenges for recycling ionic liquids by using pressure driven membrane processes

Although the extremely low vapour pressure of ionic liquids prevents their emission to the atmosphere, they are at least partly miscible with water and will inevitably end up in the aqueous environment. One example where this can be expected to occur is the application of ionic liquids for the electrodeposition of metals. During industrial use, ionic liquids will also get mixed with other product streams and will have to be separated and recycled. Economically and ecologically it is important to recycle and re-use ionic liquids as efficiently as possible. The use of pressure driven membrane processes, nanofiltration, reverse osmosis and pervaporation, as a possibility to recycle ionic liquids from water was investigated. Ethaline200 (a deep eutectic formed between choline chloride and ethylene glycol) was used to perform these tests and the results were compared with those found in the literature. The osmotic pressure was determined experimentally to explain the results. High ion retentions (up to 0.95) were obtained, but the retention of the non-charged molecules was too low. For concentrating ionic liquids the osmotic pressure was found to be the limiting factor when using nanofiltration or reverse osmosis. Only a five-fold concentration of the ionic liquid was possible, to a maximum concentration of 20–25 vol% of ionic liquid. Pervaporation was investigated as an alternative. It was found to have limited usability for this application as the water content is too high. For low water contents, pervaporation is applicable although the flux is very low due to the presence of the ionic liquid, which decreases the activity of the water and thus the flux through the membrane. The necessary membrane area would be very high and makes pervaporation rather impractical.

The concept of planar diffusion zones. Theory of the potentiostatic transient for multiple nucleation on active sites with diffusion-controlled growth

Most theoretical models used to describe the potentiostatic transient for the case of multiple nucleation on active sites with diffusion-controlled growth are based on the concept of planar diffusion. It is shown that the definition of a planar diffusion zone is based on an analogy with the Cottrell equation but does not involve a flux balance, as is usually stated in the literature. The current to a growing nucleus can be written in a form that presents this analogy but it remains a hemispherical current, i.e. there is no change in the diffusional regime or the associated boundary conditions. This is the key feature of the concept of planar diffusion zones because it provides a simple way to describe the transition of hemispherical diffusion to planar diffusion when the diffusion fields interfere. In essence, a model based on the concept of planar diffusion zones can only work just because it is not based on a flux balance. The concept of a planar diffusion zone usually is associated with the growth of a single nucleus but this notion is not really needed to derive the equation of the potentiostatic transient. It is better to apply this concept to the ensemble of growing nuclei as a whole since this leads in a natural way to the definition of a uniform diffusion layer, which is connected with the idea of a mean concentration field. It is this notion of a uniform diffusion layer that reduces the interference of hemispherical diffusion fields to a true two-dimensional problem so that Avramis equation can be used correctly to describe the overlap of the planar diffusion zones.