Surface Tensions of MgCl₂-KCl and KNO₃-NaNO₃ Binary Melts

Abstract

Surface tensions of MgCl₂-KCl and KNO₃-NaNO₃ binary melts have been measured by means of the maximum bubble pressure method over the temperature ranges of 650∼875°C and 270∼405°C respectively.
The variation of the constituent ionic species with composition for the MgCl₂-KCl binary system has been estimated by comparing the results obtained with those determined for the KNO₃-NaNO₃ binary system having relatively strong ionic characters. The results obtained are summarized as follows:
(1) For both systems, the surface tension shows an approximately linear decrement with increasing temperature at each composition. The surface tension and its temperature coefficient of molten MgCl₂ are considerably low as compared with those of other chlorides but these are markedly increased by the addition of KCl. These trends are considered to be indicating the associating property as in the case of molten ZnCl₂ previously studied by the authors.
(2) The variation of the surface tension at the melting point with composition is well corresponding to the phase diagram for the KNO₃-NaNO₃ system, but not for the MgCl₂-KCl system, particularly on the MgCl₂-rich side. The temperature coefficient of surface tension corresponding to the surface entropy per unit area and the surface enthalpy per unit area vary monotonously with composition in the KNO₃-NaNO₃ system, but show notable inflections at about 30 and 80 mol%KCl in the MgCl₂-KCl system. The surface heat of mixing per unit area for the MgCl₂-KCl system shows a maximum and a minimum at about 30 and 90 mol%KCl.
(3) The surface tensions calculated from a modified Guggenheim equation agreed very well with the experimental values for KNO₃-NaNO₃ binary melts, indicating that this system can be treated as a nearly ideal solution.
(4) It is also found that the variations of the surface properties with composition are well corresponding to the changes of the other physicochemical properties in the MgCl₂-KCl system.