Transport properties, ionic liquids

Watanabe reported on the concept of ionicity, which is a ratio between the ionic conductivity measured and that deduced from NMR diffteion coefficient data (that accounts for total mobility of ions, even in the form of associated pairs or clusters that do not contribute to charge transport). Ionicity is less than unity if potential charge carriers are not available for transport, and thus reflects the degree of ionic association in the liquid. Transport properties such as viscosity, diffusion coefficient and ionicity do not vary monotonically as the alltyl side chain increases, which is consistent with the appearance of nano-segregated structures at intermediate chain lengths [54]. 

Ionic liquids possess a variety of properties that make them desirable as solvents for investigation of electrochemical processes. They often have wide electrochemical potential windows, they have reasonably good electrical conductivity and solvent transport properties, they have wide liquid ranges, and they are able to solvate a wide variety of inorganic, organic, and organometallic species. The liquid ranges of ionic liquids have been discussed in Section 3.1 and their solubility and solvation in... 

Section 3.3. In this section we deal specifically with the electrochemical properties of ionic liquids (electrochemical windows, conductivity, and transport properties) we will discuss the techniques involved in measuring these properties, summarize the relevant literature data, and discuss the effects of ionic liquid components and purity on their electrochemical properties. 

The behavior of ionic liquids as electrolytes is strongly influenced by the transport properties of their ionic constituents. These transport properties relate to the rate of ion movement and to the manner in which the ions move (as individual ions, ion-pairs, or ion aggregates). Conductivity, for example, depends on the number and mobility of charge carriers. If an ionic liquid is dominated by highly mobile but neutral ion-pairs it will have a small number of available charge carriers and thus a low conductivity. The two quantities often used to evaluate the transport properties of electrolytes are the ion-diffusion coefficients and the ion-transport numbers. The diffusion coefficient is a measure of the rate of movement of an ion in a solution, and the transport number is a measure of the fraction of charge carried by that ion in the presence of an electric field. 

L. Copeland, Transport Properties of Ionic Liquids, Gordon and Breach, New York, 1974. 

Shobukawa, H. et al.. Ion transport properties of lithium ionic liquids and their ion gels, Electrochim. Acta, 50, 3872, 2005. 

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. 

In most cases, point defects constitute the mobile charge carriers of solid and liquid electrolytes. Several factors make the treatment of ionic solids more complicated, however electronic charge carriers frequently contribute to charge transport, nonstoichiometry often influences the defect concentrations, and internal interfaces such as grain boundaries or phase boundaries strongly affect the overall ionic and electronic transport properties. Moreover, each ionic solid represents a separate solvent , whereas liquid electrochemistry predominantly deals with only one solvent, namely water. Because of these intricacies, investigations of transport phenomena in electrolytes are more important in current solid state ionics research than in modern liquid electrochemistry. 

Figure 2.7 is a composite representation of the transport properties of ionic liquids of different types intended to show the relation between Walden behavior and the temperature dependence of conductivity. In Figure 2.7a we show, in this Walden representation, an alternative set of data emphasizing proton transfer salts (protic ELs). The plot in this case terminates at the universal high T limit for fluidity implied by Figure 2.3, 10" poise. 

The intercalation of ionic liquids containing paramagnetic anions into materials with layer (e.g., graphite) or porous (e.g., zeolite) structures and research concerning transport properties under the magnetic field are both promising areas for future exploration. 

Now that the viscous flow properties of an ionic liquid have been discussed, the next task is to derive an expression for the diffusion coefficient. The present modeling interpretation of the elementary act of a transport process consists of hole formation followed by a particle jumping into the hole. The focus in this elementary act has hitherto been the center of the hole. 

A measure of understanding has been gained on the structure and transport properties of simple ionic liquids. In practice, however, mixtures of simple liquid electrolytes are more important than pure systems such as liquid sodium chloride. One reason for their importance is that mixtures have lower melting points and hence provide the advantages of molten salts,but with a lessening of the difficulties caused by high temperatures. What happens when two ionic liquids, for example, CdClj and KCl, are mixed together ... 

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