Liquid microjet

Liquid microjet, with a diameter of around 10 micrometres © W. Pokapanich

Wandared Pokapanich, of Nakhon Phanom University in Thailand, has helped to develop a pioneering technique that explores liquids at the atomic level. Her discoveries have implications in wide-ranging fields, from atmospheric and climate science to biomedical research and clinical therapy.

Water supports all life and is the most abundant substance on Earth. Other liquids are also important in biological systems, and are necessary for many industrial and chemical processes. Knowing the exact properties of water and other liquids therefore reveals much about the world around us.

While solids have very regular molecular arrangements and gases have no regularity at all, liquids are in the middle. Their global arrangements are fairly irregular, giving them overall fluidity. But at a local level, the molecules are constantly interacting with one another. Studying this arrangement in pure liquids and solutions is important for fully understanding their properties.

gas liquid solid states

Schematic figure shows electron arrangement in gas, liquid and solid. © W. Pokapanich

An ideal way to study this is a technique called photoelectron spectroscopy. This involves firing X-rays or ultraviolet light at a material, which causes it to release electrons. Measuring the energy of these electrons reveals information about the molecular organisation and properties of the material being studied. By controlling the energy of the radiation used, specific elements can be targeted.

This method works well for solids and gases, but is problematic for liquids. The procedure must be carried out in a vacuum to avoid interference, but liquids vaporise in these conditions, making the standard technique impossible. Pokapanich worked at Uppsala University in Sweden on developing an adapted system that allows photoelectron spectroscopy to be used with liquids.

The specialised technique uses a liquid microjet, first introduced in the early 1990s and developed by Pokapanich and her colleagues into a method that has already provided some unexpected insights. “The liquid sample is pumped through a narrow nozzle into the vacuum and forms a microjet of about 10 micrometre diameter,” explains Pokapanich,

“This means that a relatively small area of liquid is exposed to vacuum.” This low surface area decreases the amount of vaporation, and measurements can be taken by firing X-rays at the microjet.

Pokapanich has used the technique to study a variety of systems, with some surprising results. The traditional idea that ions – the components of dissolved substances – are absent from the surface of solutions has been overturned, and it seems that they may even be enriched. In particular, analysis of seawater-like solutions has revealed that bromide ions are enriched at the surface. “[This] could explain why bromine is much more important in atmospheric chemistry than expected from its relative abundance in seawater,” explains Pokapanich.

Other insights include details about interactions between water molecules and ions in solutions. By exciting the ions and seeing how the surrounding molecules respond, different behaviour has been revealed with different ions, and the distances between interacting particles may be deduced. Not only important for the fundamental understanding of solutions, this work is also relevant to medicine, since it shows how water molecules interact with ions after exposure to radiation.

“It is well known that our body contains 70 per cent of water and a few essential salts, such as bromide, chloride, fluoride, and iodide,” says Pokapanich,

“It is valuable to investigate how the [ions] behave after exposure to X-rays, which can apply to the radiation treatment in cancer patients.”

While it is tempting to think that there is little more to learn about water, Pokapanich says otherwise:

“Do you know that the water surface is acidic or that in sea spray, which contains water and salt, there is a competition between ions? Water is more complicated and interesting than we expected.”

It is not just water that this photoelectron spectroscopy can shed light on either. The system has also been used to study formamide, a mixture of methanol and ethanol, and can help us to understand all the liquids around us. According to Pokapanich, this work is fundamental to understanding natural and anthropogenic processes in the biosphere.

For further information contact:

Dr Wandared Pokapanich
Faculty of Liberal Arts and Science
Nakhon Phanom University, Thailand