Study on surface tension and surface phase of molten MCl-CeCl₃ (M = Li, Na, K, Cs): A comparison of Butler's equation and a newly-proposed model

The thermodynamic optimization of NaCl-CeCl₃ has been carried out through CALPHAD method, and a set of reliable parameters has been obtained to supplement the thermodynamic database. Based on Butler's equation and thermodynamic database established by CALPHAD method, the calculated surface tensions of molten salt mixtures MCl-CeCl₃(M = Li, Na, K, Cs) agree quite well with experimental results reported in the literature. The model parameter β was determined to be 0.95, 1.04, 1.09 and 1.12 for LiCl-CeCl₃, NaCl-CeCl₃, KCl-CeCl₃ and CsCl-CeCl₃, respectively, by fitting the experimental data of surface tension of these mixtures, since it is out of the question to get a universal β applicable to all molten salt mixtures. The new model proposed lately by Santos et al. to describe the thermodynamics in surface phase of liquid solution has been reduced to a much simpler form which is nearly numerically equivalent to it by eliminating the derivative of molar surface area with respect to pressure. The surface compositions and surface activity coefficients from Butler's equation were compared with those from Santos model. For LiCl-CeCl₃ and NaCl-CeCl₃, the results from the two models are almost the same due to the similar surface tensions of LiCl, NaCl and CeCl₃. Whereas for KCl-CeCl₃ and CsCl-CeCl₃, the mole fraction of CeCl₃ in surface phase from Butler's equation is overestimated compared with that from Santos model, leading to a slight underestimation of surface activity coefficients of CeCl₃ and also a slight overestimation of those of KCl and CsCl, since the surface tensions of KCl and CsCl are much smaller than that of CeCl₃.