Relative adsorption excess of ions in binary solvents determined by grazing-incidence X-ray fluorescence

We demonstrate a model-independent method for experimental determination of the relative surface excess of inorganic ions in binary liquid mixtures, based on grazing-incidence X-ray fluorescence. For this purpose, we probe the ion density profiles in a mixture of water and 2,6-dimethylpyridine containing a hydrophilic salt, potassium chloride. Thereby we demonstrate that the proposed method quantifies in a direct manner the difference between cation and anion excess adsorption in binary solvents with a resolution of one excess ion per 200nm² or better.