Surface tensions of inorganic multicomponent aqueous electrolyte solutions and melts

A semiempirical model is presented that predicts surface tensions (σ) of aqueous electrolyte solutions and their mixtures, for concentrations ranging from infinitely dilute solution to molten salt. The model requires, at most, only two temperature-dependent terms to represent surface tensions of either pure aqueous solutions, or aqueous or molten mixtures, over the entire composition range. A relationship was found for the coefficients of the equation σ = c₁ + c₂T (where T (K) is temperature) for molten salts in terms of ion valency and radius, melting temperature, and salt molar volume. Hypothetical liquid surface tensions can thus be estimated for electrolytes for which there are no data, or which do not exist in molten form. Surface tensions of molten (single) salts, when extrapolated to normal temperatures, were found to be consistent with data for aqueous solutions. This allowed surface tensions of very concentrated, supersaturated, aqueous solutions to be estimated. The model has been applied to the following single electrolytes over the entire concentration range, using data for aqueous solutions over the temperature range 233-523 K, and extrapolated surface tensions of molten salts and pure liquid electrolytes: HCl, HNO₃, H₂SO₄, NaCl, NaNO₃, Na₂SO ₄, NaHSO₄, Na₂CO₃, NaHCO ₃, NaOH, NH₄Cl, NH₄NO₃, (NH ₄)₂SO₄, NH₄HCO₃, NH ₄OH, KCl, KNO₃, K₂SO₄, K ₂CO₃, KHCO₃, KOH, CaCl₂, Ca(NO ₃)₂, MgCl₂, Mg(NO₃)₂, and MgSO₄. The average absolute percentage error between calculated and experimental surface tensions is 0.80% (for 2389 data points). The model extrapolates smoothly to temperatures as low as 150 K. Also, the model successfully predicts surface tensions of ternary aqueous mixtures; the effect of salt-salt interactions in these calculations was explored.