Theoretical Predictions of Thermochemical Properties

Theories for the prediction of thermochemical properties of molten salts have been reviewed a number of times over the years [131-134]. The main approaches that have been used are hole theory [135-137], corresponding states theory [138-140], perturbation theories [114, 141], and the significant structure theory [142-144]. Empirical correlation expressions have also been developed for the various properties as dealt with in the appropriate sections below. 

The numerical coefficient has been changed from that in [145] to account for the average deviation of the calculated from the experimental values for the alkali metal halides. Cavity formation in molten salts is compatible with their restricted primitive model (RPM), Eq. (3.8) [146]. For a molten salt having N ions of mean diameter d = (r+ + r ) in a volume V, the size distribution of cavities with radii r 0.5r/ is  

The surface tension of an RPM fluid is therefore a = r)IAnr, in fair agreement with the results of Monte Carlo computation with this model. The main point is that there are a good number of cavities with sizes commensurate with those of the ions in a molten salt. 

The corresponding states theory was developed by Reiss, Mayer and Katz [138] to deal with molten salts. It employs the primitive model with a single distance parameter d (=2ri in Eq. 3.8) to which all the inter-ionic distances in the entire volume of the molten salt V are proportional = 4ijd for all ions I and J, whether 1 = J or 1 / J. Reduced thermodynamic quantities are then defined as  

Fairly constant values of the reduced vapor pressures P corresponding to deviations of 2 % in the enthalpy of vaporization, resulted at corresponding temperatures of r= 1.30 I m and 1.55 T. Fairly constant values of the reduced surface tension of uni-univalent salts also resulted at T= 1.00 and 1.10 T. Agreement with experimental data was obtained for charge-symmetrical molten salts except for the lithium halides, and for charge-unsymmetrical salts only for the alkaline earth metal fluorides, whereas for other salts of this class some degree of covalent bonding was supposed to account for the deviations. 

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