Structure of molten NaCl and the decay of the pair-correlations

What is it about?

New investigation of the archetypical molten salt NaCl confirms a fundamental discrepancy between the effect of long-ranged Coulomb forces probed by different experimental and theoretical methods

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Photo by Timo Volz on Unsplash

Photo by Timo Volz on Unsplash

Why is it important?

We explore the structure and thermodynamic properties of NaCl, an iconic member of the molten salt family of materials that is enjoying a resurgence of interest, partly because of the potential of these materials for energy storage applications. Molten salts have enjoyed a rich history as the test bed for developing force fields in view of the comparative simplicity of the inter-atomic interactions. There is, however, controversy regarding the asymptotic decay of the pair-correlation functions. The behaviour predicted by simple theory, which should also apply at large distances from the interface in an inhomogeneous fluid, is not in agreement with the results from surface force experiments: the decay lengths differ by an order of magnitude or more. We have therefore been motivated to re-examine the structure of molten NaCl, a prototypical ionic system, by combining state-of-the art neutron and x-ray diffraction with molecular dynamics. Simulations using a polarizable ion model for the interactions give a good account of the new diffraction results and reproduce a swathe of thermodynamic parameters. The results from experiment and simulation for the decay of the pair correlation functions are commensurate with the expectations of the restricted primitive model used in simple theory. The significant discrepancy in the asymptotic behaviour of the pair-correlation functions with surface force experiments does not, therefore, originate from the approximations made in the restricted primitive model, e.g., the omission of the ion polarizability.

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