Cation-specific effects on the attraction of anions to a hydrophobic surface

Halides such as bromide and iodide are known to accumulate near the free surface of an electrolyte solution. Solid hydrophobic surfaces in contact with water induce a density-depleted gap near the interface, creating a water density profile similar to the free surface. It is therefore possible that polarizable ions like halides are also attracted to hydrophobic surfaces, a notion that is supported by MD simulations. However, the buried solid-liquid interface is more difficult to access experimentally than the free surface of water and is thus much less studied. We present results of an x-ray reflectivity study of aqueous alkali metal-iodide solutions in contact with a hydrophobic self-assembled monolayer. A layer of enhanced anion density is observed at the SAM-water interface, but with an unexpected strong dependence on the cation present in the solution. This experiment attempts to provide insight into the less understood yet ubiquitous interactions between ions and real hydrophobic/non-polar materials, such as proteins and organic materials present in the atmosphere and soil.