Surface tensions of molten alkali-metal halides

Surface tensions of molten alkali-metal chlorides, bromides, and iodides have been measured by the capillary-rise method. The surface tension cell, made of quartz, has two different capillaries, and the melt is sealed in it under vacuum. The surface tension is determined through the height difference of menisci in the capillaries and is highly reproducible. Surface properties of molten alkali-metal halides vary systematically with the radii of the anions and cations except for the lithium ion; i.e., lithium halides show considerably lower surface energies and surface entropies. Internal pressures of lithium halides, determined through the surface energies, show very low values. Hypothetical surface energies have been calculated by taking the internal pressures of the melts into consideration and assuming no ionic interactions other than long-range Coulombic energy. It is suggested that a considerable part of the Coulombic energy of the molten halides with small cation is canceled by short-range interactions.