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Chemistry at the Gas/Liquid Border

Since the mid-2000s, the David Nesbitt group has conducted experiments aimed at better understanding the interactions of atoms and molecules at the interface between a gas and a liquid. Initial experiments explored what happened when a beam of molecules (cooled down into a single quantum state) was directed toward a rotating disk with a freshly made liquid surface inside a vacuum. Inside the vacuum, a CD-sized disk rotated through a liquid trough, dragging the liquid on it. A “razor blade” then scraped off most of the liquid, leaving a liquid surface about 0.5 mm thick.

The researchers found that some of the molecules in the beam dissolved in the liquid; others went for a swim across the surface; and some helicoptered right back off the liquid surface. Some molecules were very hot when they came off the liquid; others came off at room temperature.

This series of experiments led to consideration of some profound questions about the nature of the gas/liquid interface, particularly when distances are measured in nanometers. At this scale, liquids don’t have a sharp edge; rather the transition to a gas occurs slowly and is often “wavy.” Tiny “capillary” waves create a dynamic surface on liquids that look a lot like a rough sea to the molecules on both sides of the interface. And, the shorter the length scale under scrutiny, the rougher things get. To a molecule, a “placid” liquid surface can look like a snapshot of water boiling!

This state of perpetual motion has led to such questions as (1) What is the pH of water at the surface? (2) Do anions or cations prefer to be on the surface of a liquid? (3) What kinds of experiments can reveal the physics of what’s occurring at the liquid-gas interface?
The group decided to learn more about this interface by throwing molecules at it and seeing what happened. They’ve tossed hydrochloric acid molecules (HCl) and its deuterated analog DCl at a liquid surface to see which end of the molecules hit first. They’ve investigated what happens if molecules are rotationally excited before impact and discovered that the ions on the liquid surface influence what part of the molecule hits first! They’ve tried using sodium chloride (NaCl), potassium chloride (KCl), and LiCl and seen no difference between these salts and the HCl acid. But when they tried sodium bromide (NaBr) and potassium bromide (KBr), things were different.

These investigations are perfect for the use of molecular beams. They allow the researchers to see what is happening inside a 5–10 angstrom slice of the gas-liquid interface. It’s a beautiful, simple experiment with many possibilities.

In related work, the group is investigating gas-liquid interfaces of room-temperature ionic liquids and molten metals. It’s particularly interesting to bounce molecules off the interface of metals with a gas. Metals have a huge electron density, and these experiments are allowing the researchers to look at chemistry from the perspective of encouraging electron transfer. Recent experiments have monitored electron transfer between nitric oxide (NO) and a liquid gallium surface. The researchers observed electrons leaping from the metal to the molecule and back in 1 picosecond. It was like witnessing chemistry in action.

The gas-liquid interface experiments have been so rewarding, the group has launched a new series of experiments to investigate chemical reactions that occur when molecules are tossed at a self-assembled monolayer. Self-assembled monolayers consist of hydrocarbon chains (made of carbon and hydrogen) with a thiol (SH) group on one end. The thiols react chemically with atoms in a sheet of solid metal such as gold. The other ends of the hydrocarbon chains form a forest-like canopy that can be probed with molecular beams. The concept is like investigating a rain forest by flying over it and shooting beach balls at the canopy.
In the lab, however, the canopy can have different reactive molecules covering its surface. The researchers plan to throw hydrochloric acid (HCl) molecules to see what kind of chemistry takes place when they hit canopies consisting of different kinds of reactive groups. They plan to make forest tops from acid groups, aldehydes, alcohols, or amines and then monitor the chemistry that takes place in the canopy when it’s bombarded with HCl molecules.  This ingenious setup has opened the door to exploring chemistry from the perspective of a physicist.