High-Melting Salts

The structural picture of molten salts emerging from the data dealt with in Sect. 3.2, consists of intertwining quasi-lattices of cation sites and anion sites. In charge-symmetrical salts 1 1 (e.g. NaCl) and 2 2 (e.g. CaO) a majority of the sites are occupied by the itms, with some of them empty. However, charge-unsymmetrical salts, namely 1 2 (e.g. CaFa), 1 3 (e.g. LaCls), 2 1 (e.g. LiaCOa), etc., have a substantial number of unoccupied sites, whether of sizes commensurate with those of the ions or collapsed to smaller sizes. It is expedient to deal with charge-symmetrical molten salts separately from the charge-unsymmetrical ones. 

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Thus the occupancy of the valence orbitals is the clue we shall follow in our attempt to predict when to expect a substance to be a high-melting, salt-like crystal, when to expect a metal, when to expect a low-melting, molecular crystal. This is an ambitious program. Let s see how far we can go, beginning with the pure elements. 

The solid-state reactivity of the carboxylic function was demonstrated with oxalic acid dihydrate (346) and o-phenylenediamine (46a) (Scheme 54). A 100% yield of quinoxalinedione (347) is easily obtained upon cogrinding of the components and heating of the high-melting salt thus formed in a vacuum to 150 °C for 8 h, to 180 °C for 30 min, or to 210-220 °C for 10 min [104]. Compound 347 is ready for further interesting condensation reactions [104]. 

The melts and melt mixtures that are discussed in this chapter are limited to those classified in categories (a) and (b) because these intrinsically conductive ionic melts are the most widely used electrochemical solvents. For the purpose of this chapter, low-melting salts are defined as those melting below 100°C, moderatemelting salts encompass those melting below 300°C, and all others are considered to be high-melting salts (>300°C). 

An ionic liquid is a liquid that consists only of ions. However, this term includes an additional special definition to distinguish it from the classical definition of a molten salt. While a molten salt generally refers to the liquid state of a high-melting salt, such as molten sodium chloride, an ionic liquid exists at much lower temperatures (approx. < 100 °C). The most important reported ionic liquids are composed of the following cations and anions ... 

It is also relatively easy to functionalise imidazolium cations at the 2-position.[88] For example, the phosphine derivatised salts shown in Figure 2.7 illustrate such a substitution pattern and they are easily prepare by virtue of the acidity of the 2-proton.[74] Substitution of the 2-proton tends to yield relatively high melting salts instead of ionic liquids. Despite this limitation the imidazolium-phosphine compounds are good ligands for catalysis improving the immobilisation potential of complexes dissolved in ionic liquids. 

Similar colour changes occur, if partially polymerized samples are heated (Fig. 7c) or treated with a solvent able to dissolve the monomer and unable to dissolve the polymer (Fig. 7 d) Colour changes upon heating of the samples are reversible, unless the annealing temperature exceeds the melting point of the residual monomer. Reversible thermochromism is especially observed for high melting salts as for example the lithium salt 6b... 

Another advantage to the lithium-iron disulphide system is the absence of high-melting salt phases such as CaCL-KCl. The cell can thus operate close to 352°C. While calcium-calcium chromate may be used with homogeneous electrol5rte-depolarizer blends, iron disulphide must be separated from the anode by a distinct electrolyte layer. Otherwise, the iron disulphide, which is a fairly good conductor, will be electronically shorted to the anode. 

The molar enthalpies of melting, A j/f, and of vaporization, Av// are important quantities characterizing molten salts. The enthalpies of melting, A , shown in Table 3.8 for uni-univalent salts pertain to the melting point temperature, T, because most high melting salts do not supercool appreciably nor can retain crystalline states above T. The vapor pressures of molten salts are expressed as either the simpler or as some extended equatimi ... 

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