Ionic eutectic-based

Many of these problems may be overcome by using ionic liquids based on sugars [35] or deep eutectic mixtures. Deep eutectic mixtures such as that derived from choline chloride and urea (m. pt. 12°C [36]) or carboxylic acids [37] can be liquids and have very low vapour pressure. They have been successfully used as electrochemical solvents, but their use in catalysis remains little explored. Urea is a fertiliser and choline chloride (Vitamin B4) is a component of chicken feed so the mixture is environmentally acceptable. 

Ionic liquids with discrete anions have a fixed anion structure but in the eutectic-based liquids at some composition point the Lewis or Bronsted acid will be in considerable excess and the system becomes a solution of salt in the acid. A similar scenario also exists with the incorporation of diluents or impurities and hence we need to define at what composition an ionic liquid is formed. Many ionic liquids with discrete anions are hydrophilic and the absorption of water is found sometimes to have a significant effect upon the viscosity and conductivity of the liquid [20-22], Two recent approaches to overcome this difficulty have been to classify ionic liquids in terms of their charge mobility characteristics [23] and the correlation between the molar conductivity and fluidity of the liquids [24], This latter approach is thought by some to be due to the validity of the Walden rule... 

The effect of the quaternary ammonium cations is quite complex because the smaller cations depress the freezing point more because the halide salts of the smaller cations also have a higher freezing point the net result is that all of the eutectic mixtures will have reasonably similar freezing points. Hence the cation is observed to have little effect on the absolute freezing point of the eutectic-based ionic liquids. 

The ability to vary the composition of Lewis or Bronsted acid adds an additional dimension to the tuneability of the eutectic-based ionic liquids. It has been shown that the Lewis acidity of the liquid affects not only the physical properties of the liquids but also the electrochemical behavior. Type I ionic liquids are also clearly... 

The conductivities of Type I ionic liquids based on anhydrous zinc and iron salts tend to be lower than those of the corresponding aluminum ionic liquids. This is due largely to the higher viscosity of the former, primarily because of the large size of the ions and the availability of suitably sized holes in the ionic liquids for the ions to move into. This has been quantified by the application of hole theory as is explained in Section 2.3.4. In general imidazolium-based liquids have lower viscosities and higher conductivities than the corresponding pyridinium or quaternary ammonium eutectics formed under the same conditions. 

This chapter shows that eutectic-based ionic liquids can be made in a variety of ways. The above description of liquids falling into three types is by no means exclusive and will certainly expand over the coming years. While there are disadvantages in terms of viscosity and conductivity these are outweighed for many metal deposition processes by issues such as cost, ease of manufacture, decreased toxicity and insensitivity to moisture. The high viscosity of some of these liquids could be ameliorated in many circumstances by the addition of inert diluents. 

The physical principles underlying eutectic-based ionic liquids are now relatively well understood, however, the liquids described above have tended to be less academically fashionable and have received comparatively little attention. Concerted effort with these types of liquids could lead to optimization of their properties such that they would be suitable for commercial deposition processes. 

In this chapter we will concentrate on the deposition of metals from eutectic-based ionic liquids. These have been developed since the end of the 1990s, primarily by our group and that of Sun. Figure 4.10 shows just some of the metals that... 

A significant number of studies have characterized the physical properties of eutectic-based ionic liquids but these have tended to focus on bulk properties such as viscosity, conductivity, density and phase behavior. These are all covered in Chapter 2.3. Some data are now emerging on speciation but little information is available on local properties such as double layer structure or adsorption. Deposition mechanisms are also relatively rare as are studies on diffusion. Hence the differences between metal deposition in aqueous and ionic liquids are difficult to analyse because of our lack of understanding about processes occurring close to the electrode/liquid interface. 

In eutectic-based ionic liquids, the chloride ions act as strong ligands for the oxidized metal ions, forming a range of chlorometallate anions. The free chloride ions are present in very low concentrations as they are complexed with the Lewis acidic metal ions and so the dissolution of metal ions must lead to a complex series of equilibria such as... 

In principle, there is no difference between the pretreatment that a metal should undergo before immersion in an ionic liquid or in an aqueous solution. The sole difference is that the workpiece must be dry before immersion in the ionic liquid. The sensitivity of the ionic liquid to water content is dependent upon the ionic liquid. Eutectic-based ionic liquids are less sensitive to water content than liquids with discrete anions. This is thought to be due to the ability of the chloride anions in the former interacting strongly with the water molecules, decreasing their ability to be reduced. Especially with AICI3-based ionic liquids water has to be strictly avoided. 

As explained previously, electrodissolution in ionic liquids is a simple and efficient process, particularly in chloride-based eutectics. Type III eutectics based on hydrogen bond donors are particularly suitable for this purpose. However, it has been noted that the polishing process only occurs in very specific liquids and even structurally related compounds are often not effective. It has been shown that 316 series stainless steels can be electropolished in choline chloride ethylene glycol eutectics [19] and extensive electrochemical studies have been carried out. The dissolution process in aqueous solutions has been described by two main models the duplex salt model, which describes a compact and porous layer at the iron surface [20], and an adsorbate-acceptor mechanism, which looks at the role of adsorbed metallic species and the transport of the acceptor which solubilises... 

No systematic study of inert electrode materials has taken place to date and nothing is known about the anodic processes taking place in ionic liquids. It is probable that noble metal oxide coatings should be suitable but processes such as chlorine evolution will clearly have to be avoided for eutectic-based ionic liquids. The breakdown products of most cations are unknown but it is conceivable that some of them could be potentially hazardous. 

Ionic liquids can be compared to any other liquid in that the reactivity of a species will depend upon its relative activity in solution. To this end it is important to consider the relative Lewis and Bronsted acids that can interact with the solutes to affect their activity. It is also important to remember that ionic liquids with discrete anions have wider potential windows and what we therefore hope to achieve with them is more susceptible to the presence of reactive species. The influence of impurities on the electrochemical behavior of an ionic liquid will depend upon the relative Lewis acidity/basicity of the liquid and of the redox process in question. Eutectic-based ionic liquids behave very differently from ionic liquids with discrete... 

The main contaminants in an ionic liquid will be introduced from the synthesis, absorbed from the atmosphere or produced as breakdown products through electrolysis (see above). The main contaminants for eutectic-based ionic liquids will be from the components. These will be simple amines (often trimethylamine is present which gives the liquid a fishy smell) or alkyl halides. These do not interfere significantly with the electrochemical response of the liquids due to the buffer behavior of the liquids. The contaminants can be effectively removed by recrystallization of the components used to make the ionic liquids. For ionic liquids with discrete anions the major contaminants tend to be simple anions, such as Li+, K+ and Cl-, present from the metathesis technique used. These can give significant difficulties for the deposition of reactive metals such as Al, W and Ti as is demonstrated below with the in situ scanning tunnelling microscope. 

The absorption of species from the atmosphere is common to all electrolyte solutions and clearly the absorption of water is the biggest issue. This is not solely confined to ionic liquids, however, as all electroplaters who deal with aqueous solutions of acids know, if the solution is not heated then the tank will overflow from absorption of atmospheric moisture after some time. In the aqueous acid the inclusion of water is not a major issue as it does not significantly affect the current efficiency or potential window of the solution. Water absorption is also not such a serious issue with eutectic-based ionic liquids and the strong Lewis acids and bases strongly coordinate the water molecules in solution. The presence of up to 1 wt.% water can be tolerated by most eutectic-based systems. Far from having a deleterious effect, water is often beneficial to eutectic-based liquids as it decreases the viscosity, increases the conductivity and can improve the rate of the anodic reaction allowing better surface finishes. Water can even be tolerated in the chloroaluminate liquids to a certain extent [139] and it was recently shown that the presence of trace HQ, that results from hydrolysis of the liquid, is beneficial for the removal of oxide from the aluminum anode [140]. 

Impurities are a lot less problematic for eutectic-based ionic liquids. The strong acid-base nature of these systems leads to predominantly halometallate species which tend to be unaffected by simple salts or other impurities such as water. The strong Lewis acids and bases coordinate well to water and even in the chloroa-luminate systems low amounts of water do not significantly affect voltammetric behavior or have a deleterious effect on deposit morphology. 

Since 1980s, a large number of studies on the electrodeposition of metals have been reported. Historically, the water-sensitive chloroaluminate first-generation ILs are the most intensively studied. However, in future, the focus will rather be on air-and water-stable ionic liquids due to their variety and the less strict conditions under which they can be handled. Several review articles, books, and book chapters on the electrodeposition of metals and alloys from ILs have already been published [27-183], Ionic liquids can be classified as water/air sensitive (first-generation ionic liquids based on AIX3 (X=C1, Br)) and water/air stable (discrete anions based and eutectic solvents/ionic liquids such as ZnCl, urea, ethylene glycol, and choline chloride). A selection of published examples of the electrodeposition of metals and alloys from ionic liquids is listed in Table 5.3 the original work for each metal can be found in Refs. [23, 29-183], In this section, we mainly focus on the electrodeposition of active metals such as Al, Mg, and Ti in ionic liquids. 

As follows from Part 1, the ionic melts based on molten alkali metal halides are referred to the solvents of the Second Kind (Kind II), and, therefore, the acid-base ranges for these media are half-open (see Fig. 1.1.1, scheme N3). Therefore, to form an idea of the relative oxoacidic properties of the studied chloride melts it is enough to know their oxobasicity indices. The necessary experimental parameters obtained at 600 °C are presented in Table 1.3.1. The data in this Table show that the KCl-LiCl eutectic melt possesses appreciable acidic properties, the corresponding oxobasicity index being equal to 3.2. 

A number of our works are devoted to the investigations of different kinds of acid-base equilibria in the ionic melts based on alkali metal halides in order to determine their oxobasicity indices at 700 and 800 °C. Unfortunately, none of the necessary experimental data have been published by other investigators. The equimolar KCl-NaCl mixture has been chosen to be the reference melt at these temperatures, although its oxoacidic properties differ by less than 0.1 from the CsCl-KCl-NaCl eutectic (see below), i.e. they are practically coincident. The solubilities of 11 metal oxides in the equimolar KCl-NaCl mixture are reported in Ref. [175]. Similar investigations in the molten CsCl-KCl-NaCl eutectic [188] allow us to conclude that the solubility products of the same oxide (in molar fraction scale) in both melts are close. This leads to the conclusion that both melts are suitable as reference ones, not only at 700 °C but also at other temperatures at which these media exist in the liquid state. 

Scionix has developed an alternative concept to forming eutectic-based ionic liquids which is to complex the anion of choline chloride with a hydrogen-bonding compound rather than a metal halide [21,22]. The ionic liquids allow electropolishing with high current efficiency (>80%), improved surface finish and improved corrosion resistance [23]. 

Abbott, A. R, Cullis, R M., Gibson, M. J., Harris, R C., and Raven, E. (2007]. Extraction of glycerol from biodiesel into a eutectic based ionic liquid. Green Chem., 9, pp. 868-872. 

Abbott AP, Ryder KS, Koenig U (2008) Electrofinishing of metals using eutectic based ionic liquids. Trans hist Met Finish 86 196-204... 

The eutectic based ionic liquids are considerably less sensitive to water addition and sometimes its presence has a positive effect on the deposit morphology. 

Various ionic liquids based on choline chloride (noted as ChCl) - urea (symbolized IL) and choline chloride - ethylene glycol (symbolized ILEG) (1 2 molar ratio) eutectics have been synthesized involving various Ni salts, respectively NiCl2.6H20 and NiS04.7H20 with... 

This fact has been experimentally confirmed. For example for an ionic medium based on 0.2M Ni + in ChQ-urea eutectic mixture, a cathodic current of 0.881 mA/cm has been determined when the salt has been added as sulfate as compared with a value of 2.012 mA/cm2 in the case of metallic chloride utili2ation, for a working temperature of 40°C. The diffusive character of the electrode process control evidenced by the linear dependence between the cathodic limiting current and cation concentration was found to be the same regardless the metallic salt anion and regardless the type of the eutectic mixture (Florea, 2010). 

Abbott, A.P. Barron, J.C. Ryder, KS. Wilson, D. (2007), Eutectic-Based Ionic Liquids with Metal-Containing Anions and Cations, Chem. Eur.., Vol.l3, No.22, July 2007, pp. 6495 - 6501, ISSN 1521-3765... 

Abbott, A.P. Ttaib, KEl Ryder, KS. Smith, E.L. (2008), Electrodeposition of nickel using eutectic based ionic liquids, Trans.Inst Met. Finish., Vol.86, July 2008, pp. 234-240, ISSN 0020-2967... 

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