Surrounded by Water

Water molecules can organize around protons from acids, influencing how the positive charge behaves in batteries, power plants, and waste sites.

water molecules
Simulations quantify the electric fields (blue) on the hydrogen atom sites using 20 water molecules surrounding a hydronium (H3O+) ion, which is shown on the top of the cluster. The red and green surfaces represent positive (from the nuclei) and negative (from the lone pairs of electrons) voltages acting as local electrodes. Image courtesy of Fournier et al., Journal of Physical Chemistry A 119, 9425 (2015).

The Science

Whether producing new types of power or cleaning old waste sites, the reaction between water and positively charged particles from acids is crucial. However, scientists do not understand in detail the behavior and the underlying structures formed by adding water molecules to interact with particles, or protons, at the microscopic level. To gain insight, scientists isolated certain structures of a proton being surrounded by an increasing number of water molecules. The team’s findings suggest that the ion is overall hydrophobic (it “fears water”).

The Impact

Understanding water’s behavior towards important dissolved substances is crucial to influencing water’s role in chemical reactions. These reactions occur in power plants, manufacturing sites, and the earth’s environment. To this end, interpreting the information hidden in microscopic interactions can provide indispensable information for these topics.

Summary

Scientists understand the macroscopic behavior of the interaction between water and acid, yet the microscopic behavior and the specific structures that form are largely a mystery. With water being termed “the universal solvent,” understanding the structures it forms at the microscopic level with acids, and other solutes, could aid the scientific community in studies as diverse as renewable energy generation and waste site cleanup. A team of scientists at Pacific Northwest National Laboratory, Yale University, and the University of Pittsburgh used cryogenic clusters to unravel the microscopic mechanics responsible for the interaction of an excess proton in bulk water. Using experiment and theory, they examined the structures that water forms around an excess proton, ranging from two-dimensional arrangements to cages. The research helps to correlate known spectral features with microscopic interactions. For example, scientists can trace the position of the OH stretch most associated with the excess proton to large increases in the electric fields exerted on the hydronium (H3O+) ion upon formation of the first and second shells of water. The correlation between the underlying local structure and the observed spectral features provides valuable information to the large community studying the nature of an excess proton in water.