Investigations of Concentrated Salt and Acid Solutions Using Ultrafast Nonlinear Spectroscopy

With support from the Chemical Structure, Dynamics, and Mechanisms-A (CSDM-A) program in the Division of Chemistry, Professor Michael Fayer of Stanford University is developing sophisticated ultrafast spectroscopies to study the dynamics and structure of solvated ions and protons. Salt and acid solutions are common in nature and important in technological applications. The challenge in directly investigating ions in water is that the ion/water dynamics occur on exceedingly short time scales, trillionths of a second, or a billion times faster than a camera shutter speed. Professor Fayer and his students will advance ultrafast infrared and visible spectroscopic methods and use them to study concentrated salt and acid solutions. Their discoveries will enhance basic knowledge of important ion/water systems and lead to better understanding of complex electrolytes, such as those being developed for use in batteries for energy storage. In addition to the direct scientific impact through the sharing of information and technologies with academia, government laboratories, and industry, the project will provide research opportunities for the next generation of top scientists and contribute to the Nation's science, technology, engineering, and mathematics (STEM) workforce. The Fayer group will employ ultrafast infrared nonlinear spectroscopic methods, particularly two-dimensional infrared (2D IR), polarization selective pump-probe (PSPP), and ultrafast optical heterodyne detected optical Kerr effect (OHD-OKE) experiments to probe highly concentrated salt and acid solutions. The salt solutions are frequently referred to as “Water in Salt”. They have far too few water molecules to fully solvate the ions, e.g., one ion pair for four water molecules. The acid solutions will also have very high concentrations. In the acid solutions, hydronium replaces the cation found in salt solutions. While hydronium is a cation, it can change its identity to a water molecule by transferring a proton. The salt and acid solutions are composed of extended ion/water networks rather than hydrated ions. The experiments will be conducted as a function of ion concentration and temperature. In addition, the differences between monatomic and polyatomic ions and the effects of their relative sizes will be investigated. The methods mentioned above are a set of interrelated tools that will be used to examine different aspects of the same systems, enabling a fundamental understanding of the relationships among dynamics, structure, and bulk properties. 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.