Interfacial tensions between two liquid phases are discussed based on Dupre's and Good's equations. Thermodynamically, it is argued that the interfacial tension will always be less than the sum of the surface tensions of the two pure phases. The interfacial tensions between different metals and sodium fluxes were measured using the sessile drop technique combined with X-ray radiography. The metal-flux systems studied were Ag, Bi, Cu, Pb, and Sn with NaF, NaCl, Na2CO3, Na3AlF6, and Na2OSiO2. The interfacial tension decreased with temperature for all the systems studied. For a given flux, the highest value of the interfacial tension was obtained for the system with the largest value of the surface tension of the metal. The average value of Good's interaction parameter was 0.31 for metals and sodium fluxes. The lowest value of the interaction parameter was obtained when using cryolite as flux.
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