Electrostatics of underscreening: concentration depth profiles of ions near the air-water interface at high salt concentrations

The classical Debye-Hückel theory of electrolytes is no longer valid at very high concentrations (> 1M). Recently, it has been shown that the range of electrostatic interactions is far greater than the Debye length in concentrated electrolytes, called "underscreening". The proposal is to map concentration depth profiles of ions in solutions of simple salts in the vicinity (0 - 30 nm) of (a) the air-water interface and (b) the interface decorated with a monolayer of charged or uncharged surfactants. These systems would allow the influence of concentration and surface potential on ion distribution to be investigated. Simultaneous X-ray reflectivity (XRR) and grazing incidence X-ray fluorescence (GIXF) at ID10 is highly suited for the determination of concentration depth profiles of different ions can be determined with Å resolution. Should such experiments be successful it is likely that this method would become the gold standard technique for future investigations of underscreening.