Thermodynamics of Molten Salts

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Abstract
The configurational thermodynamic properties of several molten tetraalkylammonium tetraalkylborate salts have been measured and are compared with the properties of fluids composed of their isoelectronic, electrically neutral hydrocarbons. These comparisons have been made over a 40C° temperature range and a 15% variation of their volumes in an attempt to determine the contribution of the internal Coulomb field of the salt to its thermodynamic properties. The pressure of the salt is of the order of 1000–2000 bars less than that of the nonelectrolyte when they are compared at the same temperature and volume. In examining the thermodynamic equation of state, the major contribution to the decreased pressure of the salt comes from the increase of (∂U/∂V)T by the Coulomb field and only a small contribution comes from a decrease of (∂S/∂V)T. For the several salts, the entropy derivatives (∂S/∂V)T scale onto one another with the same parameters that scale the neutral hydrocarbons. This scaling indicates that the small differences of (∂S/∂V)T between salt and nonelectrolyte arise from the softness and deformability of the ions and not primarily from the ordering by the Coulomb field. The relation between (∂U/∂V)T for the salt systems containing ions of different sizes is approximately the same as that for different crystalline salts having the same type of lattice. The present stage in the development of the theory of electrolytes is shown to predict the correct sign for all the effects as a nonelectrolyte is charged up to form a molten salts as well as their relative orders of magnitude, but it considerably underestimates the absolute magnitudes of the effects.