Molten salts theory

Functions of hydration, 203 Furth model, in molten salt theory, 638 Fused oxides, and the structure of liquid water, 726... 

These are typical of ionic liquids and are familiar in simulations and theories of molten salts. The indications of structure in the first peak show that the local packing is complex. There are 5 to 6 nearest neighbors contributing to this peak. More details can be seen in Figure 4.3-3, which shows a contour surface of the three-dimensional probability distribution of chloride ions seen from above the plane of the molecular ion. The shaded regions are places at which there is a high probability of finding the chloride ions relative to any imidazolium ion. 

The popular and well-studied primitive model is a degenerate case of the SPM with = 0, shown schematically in Figure (c). The restricted primitive model (RPM) refers to the case when the ions are of equal diameter. This model can realistically represent the packing of a molten salt in which no solvent is present. For an aqueous electrolyte, the primitive model does not treat the solvent molecules exphcitly and the number density of the electrolyte is umealistically low. For modeling nano-surface interactions, short-range interactions are important and the primitive model is expected not to give adequate account of confinement effects. For its simphcity, however, many theories [18-22] and simulation studies [23-25] have been made based on the primitive model for the bulk electrolyte. Ap-phcations to electrolyte interfaces have also been widely reported [26-30]. 

Equation (9.83) is also the basis for the compound energy model. The excess energy of the mixture is here represented by any type of equation, for example a power series [15, 16], Equation (9.83) has also been derived using the conformal solution theory after Blander [14] and as an extension of the molten salts models presented by Flood, Fprland and Grjotheim [17],... 

Conformal ionic solution theory was the first theory applied to molten salts which rigorously took coulomb interactions into... 

Reciprocal molten salt systems are those containing at least two cations and two anions. We shall deal with the simplest member of this class, that containing the ions A+, B+, X-, and Y-. The four constituents of the solution, AX, BX, AY, and BY, will be designated by 1, 2, 3, and 4 respectively. There are four ions in the system and one restriction of electroneutrality. Consequently, of the four constituents, there are only three which are independent components. In order to calculate the Helmholtz free energy of mixing conveniently, we must (arbitrarily) choose the three components. Here we choose BX, AY, and BY. This choice requires that in order to make mixtures of some compositions a negative quantity of BY must be used. This presents no difficulty in the theory and is thermodynamically self-consistent. One mole of some arbitrary composition (XA, XB, Yx, XY) can be made by mixing Arx moles of BX (component 2), XA moles of AY (component 3), and (XY — XA) moles of BY (component 4). ... 

Stillinger, F. H., Equilibrium Theory of Pure Fused Salts in Molten Salt Chemistry, M. Blander, Ed., Interscience Publishers, New York, 1964. 

This comprehensive survey of the title topic is in three parts, the first dealing with the theoretical background and laboratory studies, with 29 references. The second part, with 21 references deals with case histories and experimental studies of industrial vapour explosions. These involved the systems molten titanium-water, molten copper-water, molten aluminium-water, smelt-water, water-various cryogenic liquids, molten salt-water and molten uranium dioxide-liquid sodium. In the third part (with a further 26 references) is discussion of the various theories which abound, and the general conclusion that superheated liquids most likely play a major role in all these phenomena [1]. A further related publication covers BLEVEs and pressure let-down explosions [2],... 

A modified superheat theory was proposed by Shick to explain molten salt (smelt)-water thermal explosions in the paper industry (see Section IV). (Smelt temperatures are also above the critical point of water.) In Shick s concept, at the interface, salt difiuses into water and water into the salt to form a continuous concentration gradient between the salt and water phases. In addition, it was hypothesized that the salt solution on the water side had a significantly higher superheat-limit temperature and pressure than pure water. Thicker, hotter saltwater films could then be formed before the layer underwent homogeneous nucleation to form vapor. 

We are forced to reflect that the failure of so many attempts to improve on the DH theory can be attributed to a premature rejection of the DH approach, and a tendency to include extra parameters without proper theoretical foundation. It is surprising that although ionic polarization is emphasized in studies of solvation (36), molten salts (37), and chemistry in general (38), the phenomenon has received little attention in interionic theory. In particular, our attention is drawn to the early work of Fajans and co-workers (39), who first noted the effects of concentration on the ionic molar refractivities of solutions, which were interpreted in terms of a distorting effect on the ions. For various reasons the significance of this work has not been appreciated in the field of electrochemistry. 

At one time solutions of metals in their molten salts were thought to be colloidal in nature, but this has been shown not to be true. However, no completely satisfactory theory has been advanced to account for oil the properties of these solutions. One hypothesis involves reduction of the cation of the molten salt to a lower oxidation state. For example, the solution of mercury in mercuric chloride undoubtedly involves reduction ... 

Linear and cyclic sweep stationary electrode voltammetry (SEV) play preeminent diagnostic roles in molten salt electrochemistry as they do in conventional solvents. An introduction to the theory and the myriad applications of these techniques is given in Chapter 3 of this volume. Examples of the linear and cyclic sweep SEV current-potential responses expected for a reversible, uncomplicated electrode reaction are shown in Figures 3.19 and 3.22, respectively. The important equation of SEV, which relates the peak current, ip, to the potential sweep rate, v, is the Randles-Sevcik equation [67]. For a reversible system at some temperature, T, this equation is... 

There are several approaches for obtaining spectral data for low-abundance transition metal ions, rare minerals and crystals of small dimensions. Data for a transition element in its chemical compounds, such as hydrates, aqueous solutions, molten salts or simple oxides, may be extrapolated to minerals containing the cation. Such data for synthetic transition metal-doped corundum and periclase phases used to describe principles of crystal field theory in chapter 2, appear in table 2.5, for example. There is a growing body of visible to near-infrared spectral data for transition metal-bearing minerals, however, and much of this information is reviewed in this chapter and the following one. These results form the data-base from which crystal field stabilization energies (CFSE s) of most of the transition metal ions in common oxide and silicate minerals may be estimated. 

This principle serves as the basis for a number of models of fused salt systems. Perhaps the best known of these is the Temkin model, which uses the properties of an ordered lattice to predict thermodynamic quantities for the liquid state [79]. However, certain other models that have been less successful in making quantitative predictions for fused salts may be of interest for their conceptual value in understanding room temperature ionic liquids. The interested reader can find a discussion of the early application of these models in a review by Bloom and Bockris [71], though we caution that with the exception of hole theory (discussed in Section II.C) these models are not currently in widespread use. The development of a general theoretical model accurately describing the full range of phenomena associated with molten salts remains a challenge for the field. 

We have fitted the viscosity of ionic liquids using hole theory [123]. The theory was developed for molten salts but has been shown to be very useful for ionic liquids. It was shown that the value of En is related to the size of the ions and the size of the voids present in the liquid [103]. The viscosity of ionic liquids is... 

A theory of molten salts must be able to predict thermodynamic and transport properties. The theory must also give a relation between state properties, pressure, temperature and volume. Such theories have been obtained by several methods. 

The second group of theories is based on a general approximation and consists of the use of a molten salt model to obtain a partition function from the molecular motion. This group includes the following theories the hole theory, the theory of significant structures and other structural models. The theories of the first group are mathematically more difficult but lead to good results for the molten salt structure. 

Structural models for molten salts have been proposed by several authors Bockris [1-3], Stillinger [4], Zarzycki [5], Janz [6], Kleppa [7], Blander [8], and others. These structural models are based on the older theories of the liquid state, which were applied for molecular liquids, liquified gases, molten metals, etc. Some of these models will be treated in the following. 

The enthalpy of mixing of some mixtures of molten salts with monovalent ions could be calculated in terms of the quasi-chemical theory given by Guggenheim [19] as follows ... 

Until now there has not been any general theory which explains the experimental results of electrical conductivity in molten salts. Some attempts at conductivity calculations following structure models of liquids have been made, but the results are not satisfactory. 

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