Electrolytes at Uncharged Liquid Interfaces: Adsorption, Potentials, Surface Tension, and the Role of the Surfactant Monolayer.

The article summarizes the results of our research on the behavior of ions at uncharged fluid interfaces, with a focus on moderately to highly concentrated aqueous electrolytes. The ion-specific properties of such interfaces have been analyzed. The ion-specificity series are different for water|air and water|oil; different for surface tension σ , surface Δ χ potential and electrolyte adsorption, and they change with concentration. A methodology has been developed that allows to disentangle the multiple factors controlling the ion order. The direct ion-surface interactions are not always the most significant factor behind the observed ion sequences: indirect effects stemming from conjugate bulk properties are often more important. For example, the order of the surface tension with the nature of the anion ( σ KOH > σ KCl > σ KNO 3 for potassium salts) is often the result of bulk nonideality and follows the order of the bulk activity coefficients ( γ KOH > γ KCl > γ KNO 3 ) rather than that of a specific ion-surface interaction potential. The surface Δ χ potential of aqueous solutions is, in many cases, insensitive to the ion distribution in the electric double layer but reflects the orientation of water at the surface, through the ion-specific dielectric permittivity ε of the solution. Even the sign of Δ χ is often the result of the decrement of ε in the presence of electrolyte. A whole new level of complexity appears when the ions interact with an uncharged surfactant monolayer. A method has been developed to measure the electrolyte adsorption isotherms on monolayers of varying area per surfactant molecule via a combination of experiments-compression isotherms and surface pressure of equilibrium spread monolayers. The obtained isotherms demonstrate that the ions exhibit a maximum in their adsorption on monolayers of intermediate density. The maximum is explained with the interplay between ion-surfactant complexation, volume exclusion and osmotic effects.