Transport in ionic liquids

SELF-DIFFUSION AND IONIC TRANSPORT IN IONIC LIQUIDS... 

M. Liang, S. Khatun, E.W. Castner, Communication unusual structure and transport in ionic liquid-hexane mixtures, J. Chem. Phys. 142 (12) (2015) 121101/1-121101/4. 

Transport numbers are intended to measure the fraction of the total ionic current carried by an ion in an electrolyte as it migrates under the influence of an applied electric field. In essence, transport numbers are an indication of the relative ability of an ion to carry charge. The classical way to measure transport numbers is to pass a current between two electrodes contained in separate compartments of a two-compartment cell These two compartments are separated by a barrier that only allows the passage of ions. After a known amount of charge has passed, the composition and/or mass of the electrolytes in the two compartments are analyzed. Erom these data the fraction of the charge transported by the cation and the anion can be calculated. Transport numbers obtained by this method are measured with respect to an external reference point (i.e., the separator), and, therefore, are often referred to as external transport numbers. Two variations of the above method, the Moving Boundary method [66] and the Eiittorff method [66-69], have been used to measure cation (tR+) and anion (tx ) transport numbers in ionic liquids, and these data are listed in Table 3.6-7. 

Table 8.2 lists the conductivities, transport numbers and molar conductivities of the electrolyte A = olc, and ions Xj = t+A for a number of melts as weU as for 0.1 M KCl solution. Melt conductivities are high, but the ionic mobilities are much lower in ionic liquids than in aqueous solutions the high concentrations of the ions evidently give rise to difficulties in their mutual displacement. 

Motion in ionic liquids is predominantly collective. The strength of interion Coulomb interactions makes independent motion of ions impossible, and transport modes are collective. Further, solvation dynamics involve... 

What is dear from this introduction is that the journey into the area of metal deposition from ionic liquids has tantalizing benefits. It is also dear that we have only just begun to scratch the surface of this topic. Our models for the physical properties of these novel fluids are only in an early state of devdopment and considerably more work is required to understand issues such as mass transport, spedation and double layer structure. Nudeation and growth mechanisms in ionic liquids will be considerably more complex than in their aqueous counterparts but the potential to adjust mass transport, composition and spedation independently for numerous metal ions opens the opportunity to deposit new metals, alloys and composite materials which have hitherto been outside the grasp of electroplaters. 

One issue is that most metal complexes formed in ionic liquids are anionic and these will have a significant effect on viscosity and mass transport. The effect of metal ion concentration on reduction current will therefore not be linear. Relative Lewis acidity will affect mass transport, ionic strength and speciation and accordingly the nucleation and growth mechanism of metals would be expected to be concentration dependent. 

Mass transport may influence material growth in ionic liquids. 1-Butyl-l-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide, for example, is, at room temperature, about 60 times more viscous than water. At temperatures above 150°C its viscosity is similar to most molecular solvents at ambient conditions. Indeed, temperatures between 150 and 200 °C were best to deposit tantalum from TaFs in the presence of LiF. One has to keep in mind that the deposition of tantalum from TaFs or an anionic complex delivers one Ta atom and 5-7 fluorides. If the deposition is too fast F may not diffuse rapidly enough from the surface to the bulk of the solution and may be trapped in the deposit. This might explain why we only got crystalline tantalum layers at low current densities. 

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... 

Diffusion in Ionic Liquids and Correlation with Ionic Transport Behavior... 

DIFFUSION IN IONIC LIQUIDS AND CORRELATION WITH IONIC TRANSPORT BEHAVIOR... 

In this chapter we deal with the ionic diffusion coefficient, the ionic association, and the interaction between ions in ionic liquids. We employ self-diffusivity to interpret the ionic transport behavior and aim at evaluating the ionicity and the ionic states of the ionic liquids. 

Fig. 5). Although the reasons for this effect were not perfectly clear, they concluded that small amounts of impurities do have a significant effect on transport phenomena in ionic liquids. 

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