Ions at aqueous interfaces: X-ray fluorescence and scattering studies

Aqueous solutions play essential roles in our bodies, in food and drink, in the ocean, and elsewhere in everyday life. They are fascinating soft matter systems that contain many surprises, and a variety of novel and counterintuitive interfacial behaviors have been predicted and/or observed in such systems. Some of these behaviors are known as "specific ion effects": different ions affect polymer and protein molecules in completely different ways even when the ions would seem to be quite similar. Another unexpected phenomenon is known as overcharging or charge inversion: under certain conditions, wet charged surfaces appear to attract more counterions than simple electrostatics would justify. X-ray scattering and fluorescence spectroscopy studies, which will give us a subnanoscale view of what happens at molecule-aqueous solution interfaces, are proposed. These studies will measure the numbers and types of ions near molecular interfaces, determine the liquid density profiles near the interfaces and their dependence on ion concentration, and identify the conditions under which ions form interfacial condensed phases with different types of lattice structures. The overall objective is to understand the physical mechanisms that give rise to the unusual behaviors described above.