Chemists take a closer look at the spot where water meets air

Structure of 20 water molecules surrounding a cesium ion (yellow, center). In th

Structure of 20 water molecules surrounding a cesium ion (yellow, center). In the water molecules, grey indicates hydrogen atoms, while oxygen atoms are colored orange or red. Grey dotted lines indicate hydrogen bonds between water molecules. Yang et al.. 2019

Water, despite its central place in so many processes vital to life on Earth, remains a chemical mystery in many respects. One of those mysteries is the nature of water at the exact point where it comes into contact with air.

A study published April 18 by researchers at Yale University and the University of Washington offers a new level of observation and analysis. They provide the first direct measurement of variations in frequency and complexity associated with bonded oxygen and hydrogen atoms perched on the surface of water - when one of these so-called "O-H” groups is sticking out of it. The researchers also offer the first measurement of how these O-H groups are coupled together on the surface plane of water.

"There is great interest in this for addressing fundamental science questions,” said Anne McCoy , a professor of chemistry at the UW. "People can use these results to test their models of how water behaves on larger scales and get a better sense of what’s going on in other types of water systems - such as looking at the structure and properties of bulk water or looking at the behavior of ions in water systems.”

A theoretical chemist, McCoy is co-corresponding author on the paper along with Mark Johnson, a professor of chemistry at Yale University. Johnson’s team used several spectroscopy techniques, some of which they developed in their laboratory, to collect this precise data on the behavior of O-H groups on the surface of a tiny drop of water - just 20 water molecules surrounding a cesium ion. McCoy performed calculations on the fundamental interactions among all of the atoms in the experimental setup.

Together, McCoy’s calculations and the Yale group’s measurements allowed them to create a physical model of the layout of the 20 water molecules around the cesium ion, including the orientation of O-H groups sticking up from the surface and the arrangement of water molecules at the surface.

"Our work is really a fundamental science contribution,” said Johnson. "Its importance lies in the fact that elementary mechanics and chemical properties of water are important in many fields, and many researchers are involved in simulating this behavior from first principles. We provide a quantitative benchmark upon which to calibrate such simulations.”

"There’s a lot about water that’s weird, yet it’s essential for life,” said McCoy. "Experiments like this let us get closer to understanding why.”

Lead author on the study is Nan Yang and co-authors are Chinh Duong and Patrick Kelleher - all from Yale.