Interfacial ordering in aqueous solutions: X-ray scattering studies

Northwestern University, Evanston IL

Investigators

Abstract

Nontechnical abstract Ions dissolved in water determine the properties of colloids (including commercial products ranging from salad dressings to paint). The interface between an electrode and an ionic solution is a crucial part of batteries, supercapacitors and other devices. Surrounding ions affect the physical properties of membranes and modify the behavior of proteins and DNA. Understanding the structure and properties of ions 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 earth materials for modern electronics. All these systems and processes would benefit from a better nanoscale view of how ions actually arrange themselves near wet surfaces. This project uses synchrotron X-rays to look directly at how ions are arranged near wet surfaces, while varying the surface, the ion, the concentration, the temperature, etc. What is learned from this research will help verify or refute theoretical predictions, and may also ultimately help design systems with specific properties for practical applications. This project trains graduate students in an interdisciplinary environment and gives them experience in the use of synchrotron facilities. Project-related topics are being incorporated into undergraduate and graduate courses and web-disseminated teaching materials. Technical abstract A variety of novel and/or counterintuitive behaviors have been observed in aqueous solutions containing ions. For example, charged interfaces sometimes attract more counterions than simple electrostatics would predict ("charge inversion"). Though hydrophobic (nonpolar) surfaces repel water, they are thought to attract certain ions towards the interface. The effects of ions at interfaces and around macromolecules can be strongly ion-specific, e.g. different lanthanides behave very differently. Such effects are exploited, for example, in liquid-liquid separations processes used to extract lanthanides and actinides from solution, but the origins are not understood. These anomalous behaviors are likely to have structural origins. X-ray scattering studies of interfaces, where one side of the interface is a solution containing ions, are performed in this project. The goal is to characterize the distributions normal to the interfaces, and also to understand whether and under what conditions the ions order laterally to form condensed interfacial phases with modified properties. The project trains graduate students in an interdisciplinary environment and gives them experience in the use of synchrotron facilities. Liquids-related topics are being incorporated into undergraduate and graduate courses and web-disseminated teaching materials.