Surface tension and surface Δχ-potential of concentrated Z+:Z- electrolyte solutions

Schmutzer's model for the surface of aqueous electrolyte solutions is generalized to Z+:Z- salts. The thickness of the ion-free layer is calculated from the thickness of the "hydrophobic gap" at the water surface (1.38Å) and the radii of the ionic hydration shells. The overlap between the adsorption and the diffuse double layers is accounted for. The proposed model predicts the dependence of the surface tension σ and the surface Δχ-potential on the electrolyte concentration c(el) in agreement with the available data, without adjustable parameters. The Hofmeister effect on σ for salts of the same valence type is explained with their ion-specific activity coefficients. The negative value (toward air) of the Δχ-potential of most 1:1 electrolytes originates from the dipole moment of the water molecules at the surface. The negative χ-potential due to water dipoles is inversely proportional to the dielectric permittivity ε of the solution. Since ε diminishes as c(el) increases, most 1:1 electrolyte solutions exhibit a more negative χ-potential than pure water (Δχ<0). The Hofmeister series of Δχ of 1:1 salts (Δχ(LiCl) ≈ Δχ(NaCl)<Δχ(KCl)<Δχ(KF)) follows the corresponding series of ε (ε(LiCl) ≈ ε(NaCl)<ε(KCl)<ε(KF)). The theory allows the estimation of the surface potential χ0 of pure water from the experimental data for electrolyte solutions; the result, χ0 ≈ -100 mV, confirms the value currently accepted in the literature.