Studies on the Surface Adsorption of Binary Molten Salts

The surface adsorption of eight binary molten salts, AgNO₃-M₁NO₃(M₁ = Li, Na, K, Rb), NaNO₃-M₂NO₃ (M₂ = K, Rb), Ca(NO₃)₂-CsNO₃, and Cd(NO₃)₂-NaNO₃, has been investigated. It is found that the surface tension and temperature of molten salts at constant pressure and mole fraction satisfy the same equation as that for pure liquid compounds reported in our previous works. The heats of phase transition from the bulk to the surface phase for eight molten salts are determined. The heats per unit area are all at the order of -10^-2 J/m². The phase transition is exothermic because the entropy in the surface phase is smaller than the entropy in the bulk phase. The ratio of the solute surface concentration to the solute bulk concentration is approximated as the first-order polynomials of the solute bulk concentration. Then, curves of the surface tension vs the solute bulk concentration are well fitted. The ratio (Δc_B^s/Δc_B^α) is used to interpret the changing trend of the surface tension with bulk concentrations of solute. It is also found that the surface tension of molten salts decreases linearly with the solute surface concentration.