Simple molten salts, structure

The structure of ionic liquids in the liquid state is determined by the interplay of two interactions. Firstly, a general Coulombic interaction that, in the absence of any others, would result in a concentric shell structure of ions similar to those observed for simple molten salts. Secondly, directional interactions between ions arising from charge distribution over the... 

Then the rate at which transport, viscous flow, diffusion, and conduction occur is controlled by either the rate at which the opportunities for escape occur or the ease with which the ion jumps into the new open structure. Of course, these statements apply only to molten salts such as sodium chloride, simple molten salts as they are called, and those for which the log D versus l/Tline is straight (Fig. 5.49). If the molten salt forms complexes (e.g., ZnCl , which is formed in NaCl-ZnClj), then it is rather different the control of transport rate in these substances will be discussed in a later section. 

So far, there have been few published simulation studies of room-temperature ionic liquids, although a number of groups have started programs in this area. Simulations of molecular liquids have been common for thirty years and have proven important in clarifying our understanding of molecular motion, local structure and thermodynamics of neat liquids, solutions and more complex systems at the molecular level [1-4]. There have also been many simulations of molten salts with atomic ions [5]. Room-temperature ionic liquids have polyatomic ions and so combine properties of both molecular liquids and simple molten salts. 

A number of molten salt systems [e.g., the simple ionic system Ca(N03)2-KN0j], have the property of being able to be supercooled, i.e., to remain liquid at temperatures below the melting point down to a final temperature. This is called the glass transition temperature, and at this temperature the salts form what is called a glass. This glass is only apparently solid. It is a highly disordered substance in which a liquid structure... 

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 ... 

From the standpoint of this comparison (Fig. 5.54), it is seen that the concept of a complex ion in a molten salt is at least as tenable as that of an ion with a primary solvation sheath (Section 2.4) in aqueous solutions. Whatexperimental evidence exists for complex ions in fused salt mixtures To anwer this question, one must discuss some results of investigating the structure of mixtures of simple ionic liquids. 

To summarize Unlike fused salts, mixtures of fused oxides are associated liquids, with extensive bonding between the individual molecules or ions. In fused oxides, hole formation occurs but it is not the step that determines the rate of transport processes. It is the rate of production of individual small jumping units that controls them. This conclusion makes it essential to know what (possibly different) entities are present in fused oxides and what are the kinetic entities. In simple fused salts, the jumping particles are already present (they are the ions themselves) the principal problem is the structure of the empty space or free volume or holes, and the properties of these holes. In molten oxides, the main problem is to understand the structure of the macrolattices or particle assemblies from which small particles break off as the flow units of transport. 

The Structure of inorganic melts is, in spite of their relative simplicity, not completely understood. The earlier calculations and simple models of molten salts were built up rather on intuition. However, they were the necessary first step for more sophisticated approaches. 

Theoretical interpretation of the concentration dependence of equivalent conductivity for simple binary mixtures was first presented by Markov and Shumina (1956). It should be emphasized that this theory, even when considering simple structural aspects, represents rather a method of interpretation of the experimental data than a genuine picture of the structure of the melt. In molten salts generally only ions and not molecules are present, hence the conception of Markov and Shumina (1956) is to be considered also from this aspect. Their theory is based on the assumption that the electrical conductivity of a mixture of molten salts varies with temperature like pure components. In this respect, general character of the electrical conductivity dependence on composition, indicating the interaction of components in an ideal solution, could be expected. 

We should expect similar results for ionic liquid simulations, and that greater accuracy will come with improved treatment of inter- and intramolecular energetics. Second, the simulations have yielded a tremendous amount of quantitative and qualitative information, which has enabled us to understand molten salt systems much better than would have been possible using only experiment or theory. We know that the Coulombic forces lead to order over a much longer length scale than is present in simple liquids [112] and that these forces can lead to the appearance of small voids having lifetimes on the order ofps[112,113]. This is in contrast to normal molecular liquids, where the steeply repulsive part of the potential dominates fluid structure and leads to a more close-packed structure. 

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