Eutectic-based Ionic Liquids

Eutectic mixtures have been used extensively for applications of molten salts to reduce the operating temperature and this is where the significant area of ionic liquids developed from i.e. the quest to find aluminum-based salt mixtures. While the development of aluminum-containing ionic liquids is technologically very important for the field of metal deposition it is clear that there are many other issues that also need to be addressed and hence methods need to be developed to incorporate a wide range of other metals into ionic liquid formulations. 

The systems so far described can be expressed in terms of the general formula Cat+X z Y, where Cat+ is in principle any ammonium, phosphonium or sulfo-nium cation, X is generally a halide anion (usually Cl ). They are based on equilibria set up between X- and a Lewis or Bronsted acid Y, z refers to the number of Y molecules which complex X-. The ionic liquids described can be subdivided into three types depending on the nature of the complexing agent used. 

An extensive range of metal salts [96] have been studied but the only ones which produce ionic liquids (i.e. liquid below 100°C) with pyridinium, imidazolium and quaternary ammonium halides are FeCh, ZnC, SnC, CuCl [105], InCh [100] andAuCb [106,107], 

For the zinc chloride choline chloride mixtures the eutectic is observed at a 2 1 composition, whereas for the tin chloride choline chloride mixtures it is observed at 2.5 1. This is presumably because SnCl2 is less Lewis acidic than ZnCl2 and hence more SnCl2 is require to push the equilibrium for the reaction SnCl2 + SnCl3 5 S Cls- to the optimum Sn2Cl5 composition. 

The ZnCl2 system has probably been studied in the most detail. Fast atom bombardment mass spectrometry (FAB MS) has been used to identify the species present. It was found that ZnCl3, Zn2Cl5 and Zn3Cl7 species are all present in the liquids. The relative proportions of anionic species depend on the ionic liquid composition. Lecocq et al. [108] used electrospray ionization to look at the various species present and found that in Lewis basic liquids x(ZnCl2) 0.5 ZnC -whereas the di- and tri-metallate species were more prevalent in Lewis acidic liquids. 

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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... 

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. 

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. 

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. 

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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