Molten salts surface tension

For ionic solids interacting with Coulomb pair potentials, similar calculations can be carried out. However, this is a rather complex matter because Coulomb, van der Waals attraction and Pauli repulsion should all be taken into account. In addition, there are uncertainties in the choice of suitable pair-potential equation (many inter-atomic potential equations, including Lennard-Iones were tried), and the calculated Gf results are highly dependent on the particular choice of pair-potential model. As an example, Gf = 212m) m 2 was calculated theoretically for the NaCl (100) crystal, which is near to the experimental value of Gf = 190 m) m 2 from extrapolation of the molten salt surface tension values, but far away from Gf = 300 mj m 2, which was found from crystal cleavage experiments. 

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]. 

The National Institute of Standards and Technology (NIST) molten salts database has been designed to provide engineers and scientists with rapid access to critically evaluated data for inorganic salts in the molten state. Properties include density, viscosity, electrical conductance, and surface tension. Properties for approximately 320 single salts and 4000 multicomponent systems are included, the latter being primarily binary. Data have been abstracted from the literature over the period 1890-1990. The primary data sources are the National Bureau of Standards-National... 

The molten salt standard program was initiated at Rensselaer Polytechnic Institute (RPI) in 1973 because of difficulties being encountered with accuracy estimates in the NBS-NSRDS molten salt series. The density, surface tension, electrical conductivity, and viscosity of KNO3 and NaCl were measured by seven laboratories over the world using samples distributed by RPI. The data from these round-robin measurements of raw properties were submitted to RPI for evaluation. Their recommendations are summarized in Table 2. 

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. 

Equation (87) and analogous equations for AG , AHm, and for surface tensions apply to molten salt mixtures in which the interaction potential can be classed as conformal. These relations may also be used to test whether the ionic interaction potential in aqueous solutions may be considered as conformal. Thus, as will be shown in one simple example, the limits of usefulness of some interionic interaction potentials may be tested in ranges of concentration of salts in water too high to obtain absolute values for the partition functions. A similar test may be made for associations in salt vapors such as... 

By simple thermodynamic arguments Brown14 has shown that, consistent with the accuracy of this second-order approximation, one may obtain from the form of Eq. (87) the form of the excess Gibbs free energy of mixing (AG ), the enthalpy of mixing of a molten salt (AHm), and the deviation of the surface tension from linearity ... 

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. 

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. 

Morel, C. F. (1970) Surface Tensions of Molten Salts and Contact Angle Measurements of Molten Salts on Solids, EUR 4482 e, Commission of the European Communities-Euratom, Joint Nuclear Research Centre, Petten Establishment, Netherlands Nicholas, M. G. (1986a)7. Mater. Sci., 21, 3392 Nicholas, M. G. (1986b) Brit. Ceram. Trans., 85, 144... 

In fact, one can go further and make the following statement Molten salts look like water and not far above their melting points have viscosities, thermal conductivities, and surface tensions on the same orders ofmagnitude as those of water. In general, however, and with the important exception of some AlClj-complex organic systems, most fused salts are stable as liquids only at relatively high temperatures (500 to 1300 K) (Table 5.3). 

Several differing simple models of molten salts do indeed give reasonably close calculations of equilibrium properties, e.g., compressibility and surface tension. What these models do not do, however, is to quantitatively rationalize the data on the temperature dependence of conductance, viscous flow, and self-diffusion. The discovery by Nanis and Richards of the fact that simple liquids have heats of activation for all three properties given approximately by 3. lART presents a clear and challenging target for testing models of liquids. 

Calculate the work of hole formation in molten sodium chloride, using the Furth approach. The surface tension of NaCl, molten salt at 1170 K, is 107.1 dyn cm" and the mean hole radius of NaCl is 1.7 x 10" cm. (Contractor)... 

Surfaces of binary liquid mixtures are the simplest Gibbs monolayers that exist. We are considering the surface tension y as a function of the mole fraction X = x. It runs from the value y for pure component 1 to y for pure 2. that is from X = 0 to X = 1. Often such curves cae convex with respect to the x-cuds (as curves 2 and 3 in fig. 4.1), Implying the tendency of the Interface to be richer in the component with the lower y. At the same time this is the most volatile component. Such convex behaviour is the rule for mixtures of simple molecules, like liquid Ar, CH4, N2, CO, etc. but has also been observed for binary mixtures of the Kr-ethene-ethane triod ) and for molten salt mixtures ). Concave curves (2 in the figure) require the surface to be enriched by the component with the higher y, but such... 

The last method is similar in its principle to that developed by Kleppa (1960) in his pioneering and extensive calorimetric investigation of molten salts. In his arrangement, salt B was placed in an ampoule made of a pyrex tube fitted with a break-off tip (Figure 4.8b). In comparison with the break-off bubble method, a slight shift in composition due to the effect of surface tension and the wetting of the crucible and the ampoule... 

Density measurement by the method of maximum bubble pressure is essentially the same as the measurement of surface tension. However, the precision of this method in the density measurement of molten salts is far below the method of hydrostatic weighing and is used only exceptionally. On the other hand, this method is used with an advantage at higher temperatures to measure simultaneously density and surface tension of the oxide systems. 

One of the most important technological parameters in molten salt chemistry is surface tension, as the majority of important reactions take place at the interface of electrolytes or molten reacting media. In aluminum electrolysis, for instance, this parameter influences the penetration of the electrolyte into the carbon lining, the separation of carbon particles from the electrolyte, the coalescence of aluminum droplets and fog in the electrolyte, the dissolution rate of aluminum oxide in the electrolyte, etc. Similar is the effect of interface in aluminum recovery. 

The surface tension reflects the nature of chemical bonds between species of the system under question. Since molten salts are ionic in character, the surface tension is predominantly given by the chemical nature of the present ionic species. Due to different coulombic interactions between species, the ions more covalent in character are concentrated on the surface and become surface active. Therefore the concentration dependency of the surface tension of binary systems will be substantially influenced by the ionic composition. Due to the equilibrium between the bulk and the surface, the course of the... 

The measuring device used for measurement of the surface tension of molten salts is well described by Dangk and 0stvold (1995) and was applied to different molten salt systems. The apparatus consisted of a resistance furnace provided with an adjustable head fixing the position of the platinum capillary, the Pt-PtRhlO thermocouple and a platinum wire, serving as an electrical contact to adjust the exact contact of the capillary with the liquid surface. 

It is of some interest that even the presence of long-range forces, such as the Coulombic ones present in simple fused salts, does not force us to contradict the conjecture that the pair correlation function of a dense fluid is virtually equal to that of a fluid composed of the uncharged cores. Stillinger has aptly summarized the arguments in support of this contention for fused salts and we cannot afford the space to repeat these arguments here. In particular, he has shown that the extension of Fowler s approximate evaluation of liquid surface tension to molten salts of the symmetrical valence type yields ... 

Reiss and Mayer and Mayer - have also made extensive computations of values of the surface tension of molten ionic salts, liquid metals, and simple nonpolar and polar liquids using Eq. (97) or simple modifications thereof, obtaining results in remarkable agreement with experimental values. 

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