Electrolytes\nat Uncharged Liquid Interfaces: Adsorption,\nPotentials, Surface Tension, and the Role of the Surfactant Monolayer

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