Ion binding to interfaces

A multitude of crucial chemical phenomena occur in the region where an aqueous medium meets a more apolar environment. The change in the character of the medium can be due to a macroscopic phase boundary, but it is much more common that it is due to the presence of an aggregate, a macromolecule or even an apolar molecule of moderate size. One aspect of such a transition region from polar to apolar is how small ions respond to such an environment. Based on a purely electrostatic point charge model one would expect the ions to be repelled by the apolar medium since it is less effective in accommodating the electric field emanating from the charge. This effect is often described as due to the repulsion between the source charge and its image and its consequences for the surface tension of aqueous electrolyte solutions were worked out by Onsager and Samaras. Cations like alkali metal ions and alkaline earth metal ions largely respond to apolar environments as one would expect from the simple electrostatics model, but for anions, like the halogen ions, there are large quantitative and even qualitative deviations.