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

Northwestern University, Evanston IL

Investigators

Abstract

Non-technical Description: Solutions of salts in water play essential roles in our bodies, in food and drink, in the ocean, and elsewhere in everyday life. These are fascinating soft matter systems that contain many surprises. The behavior of dissolved ions is quite complex, and some remarkable unexpected behaviors have been observed in such systems. For example, dissolved ions that are thought to be very similar, such as sodium and potassium, affect polymer and protein molecules in completely different ways ----this is known as a ‘specific ion effect’. Another unexpected phenomenon is known as overcharging or charge inversion: under certain conditions, molecules or oil droplets in water will change the sign of their electric charge, and thus start moving in the ‘wrong’ direction in the presence of an electric field. In this project, X-ray beams from synchrotron radiation facilities are used in order to gain a subnanoscale quantitative view of these systems and thus shed light on the origins of these novel phenomena. Technical Description: This proposal addresses fundamental questions regarding the behavior of ions near aqueous solution interfaces. Gouy-Chapman-Stern theory describes the distribution of such ions, but does not properly account for the discreteness of ionic charge, and there is increasing evidence that it fails in many cases. However, there is no consensus about why it fails and how to develop a more universal picture. The goal of this project is to study the nanoscale ionic structures and processes underlying the nonintuitive phenomena of charge inversion (also known as overcharging), and specific ion effects (such as the Hofmeister series). The primary experimental probes used in this research are X-ray scattering and fluorescence spectroscopy, using synchrotron radiation. Ions in aqueous solution affect the behavior of colloids, polymers, batteries and some capacitors, and many other everyday materials and devices. Dissolved ions affect the physical properties of membranes and modify the behavior of proteins and DNA; indeed they are crucial to life. All these and other processes benefit from a better nanoscale understanding of how ions in aqueous solutions arrange themselves near interfaces. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.