X-ray Standing Waves and Molecular Dynamics Studies of Ion Surface Interactions in Water at a Charged Silica Interface

The adsorption of monovalent ions in millimolar concentrations at the silica solution interface has been investigated using X-ray standing waves, a method which combines element sensitivity and subnanometer depth resolution. Results were analyzed using a model free analysis and a Gouy-Chapman-Stern model giving access to interfacial concentration profiles. We find the concentration of potassium cations in the Stern layer to be slightly higher than that of cesium cations. They also come closer to the interface, in agreement with our molecular dynamics simulations which suggests that our experimental results can be explained by the higher ability of smaller cations to share their hydration shell with the surface. Extension of the method to different systems can provide the necessary experimental basis to benchmark ion surface potentials, a necessary step to predict interfacial behavior of ions which is of high relevance to several areas of science and technology.