Surface tension of concentrated electrolyte solutions.

The surface tension σ of inorganic electrolyte aqueous solutions at a given concentration c follows the Hofmeister series. The explanation of this phenomenon was sought in the increased adsorption of certain ions due to specific ion-surface interactions. However, the ion-specific dependence of the activity coefficient γ(±) on c also influences σ, and its contribution to the ion-specificity of σ prevails. Thus, the surface tension of potassium salts follows the order σ(KOH)>σ(KCl)>σ(KNO3), which turns out to be a direct corollary of the corresponding activity coefficients series: γ(KOH)>γ(KCl)>γ(KNO3). In fact, the adsorption of NO(3)(-) at the water surface is lower than that of OH(-) and Cl(-)! If the bulk ion-specific effects are correctly evaluated, Schmutzer's classical model predicts accurately the surface tension of a large number of inorganic salt solutions in a wide concentration range, without adjustable parameters. This model accounts for image and hydration forces. Comparison with tensiometric data shows that other ion-surface interactions play a role only in the adsorption of ions of bare radius larger than a threshold value of about 1.95 Å (e.g. HCOO(-), I(-), SCN(-)).