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

• The surface tension of aqueous 2:1 electrolytes follows Schmutzer’s model.
• The chi-potential of brines is more negative due to reorientation of water dipoles.
• The Hofmeister effect on surface tension is due to the activity coefficient.
• Ion specificity of the chi-potential follows the permittivity of the solution.
• The surface potential of pure water is χ0 ≈ −100 mV.

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 (1.38 Å) 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 cel 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 cel 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 (ΔχLiCl ≈ ΔχNaCl < ΔχKCl < ΔχKF) follows the corresponding series of ε (εLiCl ≈ εNaCl < εKCl < εKF). 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.

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