Adsorption of ions to the surface of dilute electroyte solutions: The Jones-Ray effect revisited

The controversial observation of a minimum in the surface tension of dilute aqueous electrolyte solutions by Jones and Ray in the 1930s is confirmed by new resonance-enhanced second harmonic generation (SHG) experiments demonstrating surface enhancement of simple inorganic anions in the same concentration range. New experiments show that the quadruply charged ferrocyanide, Fe(CN)(6)(4-), anion is not surface active at high concentrations, as expected, but at dilute concentrations, the anion is strongly attracted to the interface with a Gibbs free energy of adsorption of -6.8 kcal/mol. Using this value, the original Jones and Ray data are fit to a simple model of the surface tension with qualitative agreement, although better agreement is found for all 13 Jones and Ray salts with an even stronger surface adsorption.