Water is the most important liquid on Earth. It is involved in a vast number of technological applications as well as being fundamental to biology. Our understanding of water, at least in its pure form, is quite extensive. Water molecules interact with each other via hydrogen bonding, where a partially positively charged hydrogen atom on one molecule is attracted to a partially negatively charged oxygen on another molecule. Hydrogen bonding causes liquid water to have a significantly ordered structure, even ice-like in places. Most of the time in nature and human-conceived technologies, water has other chemicals in it, and when the concentration of these chemicals is high, much of what we know about pure water no longer applies. Instead of connecting with each other, water molecules become attracted to the positive and negative ions of the salt or acid species. In this project funded by the Chemical Structure, Dynamics and Mechanisms-A Program of the Division of Chemistry, Professor Michael Fayer and his students at Stanford University are employing a two-dimensional infrared laser spectroscopy technique (2D-IR) to explore the structure and dynamics of aqueous solutions containing high concentrations of salts and acids. Infrared spectroscopy reveals the vibrational motions of molecules. 2D-IR reveals how one molecule’s vibrations are affected by vibrations of nearby molecules, or the motions of nearby ions. This project uses ultrafast laser pulses, a tenth of a trillionth of a second, to make direct measurements on the dynamics and structure of salt and acid solutions. Ultrafast 2D-IR can also reveal how water molecules reorient themselves when they encounter other molecules or ions. Although the structure and dynamics of concentrated acid and salt solutions are very important to living systems and many industrial technologies, there is a great deal that remains to learn about them. Two graduate students are involved in this research project. In addition to gaining experience in advanced laser techniques, they are also being training in theory and computational techniques that aid in the interpretation of the experimental data.
Aqueous electrolyte solutions are important do to their ubiquity in chemistry, biology, and industrial applications such as fuel cells, water desalination and battery technology. Ion solvation structure, ion clustering and dynamics, and the dynamics of the water hydrogen bond networks are among the interesting aspects of salt solutions. Concentrated electrolyte solutions are sometimes referred to as “water-in-salt.†In these solutions, the hydrogen bond network among water molecules is severely disturbed. The ions cannot be fully solvated by water molecules and form pairs, solvent mediated pairs, and clusters. The crowded ionic environment, which produces strong electric fields, will restrain the motions of water molecules. This project involves experimental and theoretical investigations of the dynamics, structure, and interaction of ions and water in concentrated aqueous salt solutions and acid solutions using various types of new ultrafast two-dimensional infrared (2D IR) spectroscopies and ab-initio molecular dynamics simulations. Previous studies of water dynamics in concentrated ionic solutions used the oxygen-deuterium (OD) stretch of dilute HOD as the vibrational probe. However, the short vibrational lifetime (1.8 picoseconds) makes it impossible to observe relatively slow processes that occur in concentrated salt solutions. This study employs long-lived vibrational probes, e.g., the CN stretch of methylthiocyanate, whose relatively narrow absorption spectra makes it possible to obtain detailed information on acid and salt solutions. 2D IR experiments are being conducted on both concentrated salt and acid solutions. 2D IR chemical exchange spectroscopy directly measures the proton hopping time in acid solutions. Polarization selective 2D IR (PS2DIR) experiments measure water molecule orientational relaxation. The new experiments are being combined with high level ab initio molecular dynamics simulations, which are being performed in collaboration with Profs. Thomas Markland of Stanford University and Aurora Clark of Washington State University.
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.
