Molten salts thermodynamic properties

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 stmcture and thermodynamics of neat liquids, solutions and more complex systems at the molecular level [1 ]. 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. 

Molten salt investigation methods can be divided into two classes thermodynamic and kinetic. In some cases, the analysis of melting diagrams and isotherms of physical-chemical properties such as density, surface tension, viscosity and electroconductivity enables the determination of the ionic composition of the melt. Direct investigation of the complex structure is performed using spectral methods [294]. 

A series of experiments have been undertaken to evaluate the relevant thermodynamic properties of a number of binary lithium alloy systems. The early work was directed towards determination of their behavior at about 400 °C because of interest in their potential use as components in molten salt batteries operating in that general temperature range. Data for a number of binary lithium alloy systems at about 400 °C are presented in Table 1. These were mostly obtained by the use of an experimental arrangement employing the LiCl-KCl eutectic molten salt as a lithiumconducting electrolyte. 

Good electrical conductance is one of the characteristics of many though not all molten salts. This characteristic has often been employed industrially. Various models have been proposed for the mechanism of electrical conductance. Electrolytic conductivity is related to the structure, although structure and thermodynamic properties are not the main subjects of this chapter. Electrolytic conductivities of various metal chlorides at the melting points are given in Table 4 together with some other related properties. "... 

Molecular dynamics and Monte Carlo simulations have been extensively applied to molten salts since 1968 to study structure, thermodynamic properties, and dynamic properties from a microscopic viewpoint. Several review papers have been published on computer simulation of molten salts. " Since the Monte Carlo method cannot yield dynamic properties, MD methods have been used to calculate dynamic properties. 

G. J. Janz, J. Phys. Chem. Ref Data 17, Supplement (1988) Thermodynamic and Transport Properties for Molten Salts Correlation Equations for Critically Evaluated Density, Surface Tension, Eleetrieal Conduetance and Viseosity Data, American Chemical Society-American Institute of Physics-National Bureau of Standards, Washington, DC, 1988. 

There are some density data for solid salts above ambient temperature which are given in the form of thermal expansion coefficients. These have been listed when they seemed reliable. Above the melting point, density data are scarce. Most are available for alkali halides but those available for salts are taken from the critically evaluated compilation Janz, G.J., Thermodynamics and transport properties for molten salts, correlation equations for critically evaluated density, surface tension, electrical conductance, and viscosity data,./. Phys. Chem. Reference Data, 17, Suppl. 2, 1988. 

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. 

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 formation of complex ions is an important problem for the study of the structure and properties of molten salts. Several physicochemical measurements give evidence of the presence of complex ions in melts. The most direct methods are the spectroscopic methods which obtain absorption, vibration and nuclear magnetic resonance spectra. Also, the formation of complex ions can be demonstrated, without establishing the quantitative formula of the complexes, by the variation of various physicochemical properties with the composition. These properties are electrical conductivity, viscosity, molecular refraction, diffusion and thermodynamic properties like molar volume, compressibility, heat of mixing, thermodynamic activity, surface tension. 

Blander M., Thermodynamic Properties of Molten Salt Solutions mMoltenSalt Chemistry, M. Blander, ed., Interscience, New York, 1964, p. 127. 

The noncrystalline sohds described here are amorphous and metastable. This specific thermodynamic property is because they aU originate from the liquid state. The corresponding glassy or vitreous materials are not very common in solid-state chemistry, and only a limited number of molten salts or molten alloys have the characteristics necessary to produce glasses when cooling. 

A large number of investigations have been reported on spectroscopic, thermodynamic, and other equilibrium properties of chalcogen tetrahalides (e.g., 158-162). They include vibrational spectroscopic analyses of SeCU and TeCU in the solid on the basis of the known structures 89, 373) and in the gas phase (37), equilibrium measurements of SeCU and TeCU in molten salts (f12,376,422), determination of enthalpies of formation 335, 339, 433), other equilibrium studies, and determination of thermodynamic data from vapor pressure measurements, mass spectrometric investigations, conductivity experiments, and thermal phase analysis in the solid (37, 39,203,275, 333, 337, 339, 340, 341, 342,379, 402, 403). 

Ionic fluids such as molten salts and electrolyte solutions have always been of central interest in Chemical Physics, Physical Chemistry and many applied fields such as electrochemistry, chemical engineering or the geosciences. It is the aim of this review to connect the knowledge about structure and thermodynamic properties of ionic fluids and electrolyte solutions with that of the ILs. Liquid-liquid phase transition of ionic solutions are the main topic of this paper. 

In the treatment of thermodynamic properties of mixed systems of molten salts or slags used as standard state pure components (Raoultian standard states), the use of single ion activity is to be avoided, as it leads to contradictions. 

Calorimetry constitutes a powerful tool to investigate materials. It is a measurement technique that enables us to obtain values of the thermodynamic quantities of substances. The methods used for the characterization of thermodynamic properties of molten salts include temperature, enthalpy, and heat capacity measurements as mixing enthalpy and phase diagram determinations for their mixtures. 

In 1963 Dr. Danbk joined the Institute of Inorganic Chemistry of the Slovak Academy of Sciences in Bratislava, of which he was the director in the period 1991-1995. His main field of interest was the physical chemistry of molten salts systems in particular the study of the relations between the composition, properties, and structure of inorganic melts. He developed a method to measure the electrical conductivity of molten fluorides. He proposed the thermodynamic model of silicate melts and applied it to a number of two- and three-component silicate systems. He also developed the dissociation model of molten salts mixtures and applied it to different types of inorganic systems. More recently his work was in the field of chemical synthesis of double oxides from fused salts and the investigation of the physicochemical properties of molten systems of interest as electrolytes for the electrochemical deposition of metals from natural minerals, molybdenum, the synthesis of transition metal borides, and for aluminium production. 

A preliminary examination of themodynamic data on silicates indicates that correlations developed for molten salts may be useful in understanding and ultimately in predicting magnitudes of the thermodynamic solution properties of silicates. 

Blander, M. Thermodynamic Properties of Molten Salt Solutions, Molten Salt Chemistry, Blander, M., Ed., Interscience, N.Y., 1964, pp. 127-237. 

Markov BE (ed) (1985) Thermodynamic properties of molten salt systems. Reference guide. Naukova Dumka, Kiev... 

References (i) Janz, G.J. (1967) Molten Salts Handbook. Academic Press, New York (ii) Lovering, D.G and Gale, R.J. (1983,1984, and 1990) Molten Salts Techniques, Vol. 1, 2, i and 4. Plenum Press, New York, (iii) Janz, G.J. (1988) Thermodynamic and Transport Properties for Molten Salts Correlation equations for critically evaluated density, surface tension, electrical conductance and viscosity data. Journal of Physical and Chemical Refrence Data. Vol. 17, Supplement 2, Published jointly by the American Chemical Society (ACS), the American Institute of Physics (AIP), and the National Bureau of Standards (NBS) and (iv) Barin, I., and Knacke, O. (1973) Thermodynamical Properties of Inorganic Substances. 

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