Surface Tensions of Basic NaCl-AlCl₃ and LiCl-AlCl₃ Binary Melts

Surface tensions of basic NaCl-AlCl₃ and LiCl-AlCl₃ binary melts were measured by using capillary-rise method up to 1200 K precisely. It was found that the adding very small amount of AlCl₃ into NaCl or LiCl decreases the surface tension very steeply to less than half value of NaCl or LiCl. The surface enthalpy and surface entropy also decreases steeply. These phenomena are similar to the action by surface active agent although the phenomena are rare in molten salt systems. This is due to the formation of the compound of NaAlCl₄ and LiAlCl₄. The compound is considered to be concentrated in surface phase due to their lower coulombic energy, and resulted the decrease of surface tension and surface energy, and furthermore the increase of ordering on surface phase. Based on present results and the thermodynamic consideration, the concentrations of NaAlCl₄ and LiAlCl₄ in the surface phase were calculated.