Interfacial ordering in aqueous solutions: X-ray scattering studies

Electrostatic interactions play a major role in aqueous solutions containing ions. A variety of novel and counterintuitive interfacial behaviors have been predicted or observed in some systems. One such phenomenon is overcharging: wet charged surfaces may attract more ions than simple electrostatics would appear to allow, Hydrophobic (nonpolar) surfaces, while repelling water, may attract ions towards the interface. The interactions between ions and membranes can be strongly ion-specific, without an obvious reason, as can the interactions between ions and DNA molecules. Strong ion-dependent effects are also seen exploited in liquid-liquid separations processes used to extract lanthanides and actinides from solution, but the origins are not understood. All these anomalous properties have, at least in principle, structural origins. As with many topics involving liquids, there is considerable theoretical and modeling activity, but few experimental tools to test the assumptions used or predictions made. It has been shown in recent years that X-ray scattering can give us a subnanoscale view of what happens at liquid-solid and liquid-molecule interfaces. Therefore, X-ray studies of interfaces are proposed where one side of the interface is a solution containing ions. These studies will seek to characterize the distributions normal to the interface, in particular to understand how and when they deviate from classic Gouy-Chapman-Stern theory; and they will seek to understand whether and under what conditions the ions order laterally, leading to condensed interface phases with modified properties. Intellectual Merit: This proposal addresses fundamental questions regarding the behavior of ions near interfaces, which is crucial to a variety of real-world processes. Gouy-Chapman-Stern theory, which treats charge distributions as continuous and water as a dielectric medium. While this approach has been very successful, there is increasing evidence that it must fail, especially as one moves away from the dilute limit. However, there is no consensus about why they fail and how to develop a more universal picture. Looking directly at how ions are arranged near real interfaces as the surface, the ion, the concentration, the temperature, the pH, etc. are varied will help verify or refute theoretical predictions and develop knowledge on which future theoretical work can be based. Broader Impacts: Ions in solution affect the behavior of colloids (including commercial products ranging from salad dressings to paint). The solid-solution interface is a crucial part of batteries and other devices. Ions affect the physical properties of membranes and modify the behavior of proteins and DNA. Understanding their structure and properties at interfaces is crucial to extracting them from water, whether for desalinization, removal of lead, arsenic or radioactive elements from groundwater or wastewater, or increasing the supply or rare earths. All these processes would benefit from a better fundamental understanding of how ions behave at interfaces, and that is what this proposal seeks to provide through experimental studies.