An experimental route to Hofmeister

The puzzling effects of the Hofmeister series is the result of a subtle balance of several competing evenly matched interactions. The complex interplay of electrostatics, dispersion forces, thermal motion, fluctuations, hydration, ion size effects and the impact of interfacial water structure make it hard or even impossible to identify an universal law. The diversity of specific ion effects as manifested by the Hofmeister series of ions is a direct consequence of the subtle interplay of forces. The decisive information is completely contained in the ion distribution. We discuss a simple approach developed in our laboratory providing first insights in the prevailing ion distribution at a charged interface based on ellipsometry. The distribution of ions is transformed in a layer with effective surface charge density and a Poisson–Boltzmann distribution within the diffuse layer. Under these assumptions Fresnelʹs equations can be simplified to an analytical expression, which directly relates the effective surface charge density and the ellipsometric signal. The formula does not contain free parameters and can be used to interpret ellipsometric isotherms at charged interfaces.