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

Schmutzer's model for the surface of aqueous electrolyte solutions is generalized 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 angstrom) 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 sigma and the surface Delta chi-potential on the electrolyte concentration c(el) in agreement with the available data, without adjustable parameters. The Hofmeister effect on sigma for salts of the same valence type is explained with their ion-specific activity coefficients. The negative value (toward air) of the Delta chi-potential of most 1:1 electrolytes originates from the dipole moment of the water molecules at the surface. The negative chi-potential due to water dipoles is inversely proportional to the dielectric permittivity a of the solution. Since epsilon diminishes as c(el) increases, most 1:1 electrolyte solutions exhibit a more negative chi-potential than pure water (Delta chi < 0). The Hofmeister series of Delta chi of 1:1 salts (Delta chi(LiCl) approximate to Delta chi(NaCl) < Delta chi(KCl) < Delta chi(KF)) follows the corresponding series of epsilon (epsilon(LiCl) approximate to epsilon(NaCl) < epsilon(KCl) < epsilon(KF)). The theory allows the estimation of the surface potential chi(0) of pure water from the experimental data for electrolyte solutions; the result, chi(0) approximate to -100 mV, confirms the value currently accepted in the literature. (C) 2013 Elsevier Inc. All rights reserved.