Probing Pre-Hydrate Cage Formation at MID
Scientists have used high energy (23 keV) X-ray pulses to characterize the onset of water cage formation in the presence of THF molecules (clathrate hydrates). These are present in a variety of environments, from the depths of the ocean to distant planets and comets.
Greenhouse gases trapped in icy cages
In many cold environments on Earth, such as in the very deep sea, water molecules can take on a special structure: cages that can trap other molecules. These cage-like structures can bind together to form compounds known as clathrate hydrates, crystalline water-based solids that resemble ice, which can ‘capture’ small other molecules, particularly gases. Perhaps the best-known example of such a clathrate hydrate is the methane hydrate, which traps the greenhouse gas methane in icy reserves. Some scientists are even hoping to utilise such trapping mechanisms to help in the fight against global warming.
“With climate change, and the melting of these clathrate hydrates, some of this methane might be released, so we need to know as much about them as possible,” says Felix Lehmkühler, scientist using European XFEL’s Materials, Imaging and Dynamics (MID) instrument to investigate clathrate hydrates. “We were looking specifically into their formation, a process that’s still not entirely understood.”
To understand the formation process of the hydrates, the scientists used a molecule known as Tetrahydrofuran or “THF” which is known to form a clathrate hydrate with water. The scientists mixed THF and water together and then cooled the mixture down from room temperature to watch the cages form. They looked at the interaction between neighbouring water molecules, watching how the intermolecular forces changed as the hydrate cages began to develop. The high intensity of X-ray flashes at the MID instrument, as well as their ultrashort duration, is what enabled the scientists to make such detailed measurements of these icy structures.
The team also stressed the importance of MID’s geometry to unravel the structure of clathrate hydrates. They used the pair distribution function (PDF) method, which allows scientists to quantitatively measure the structure of extremely small compounds. It involves looking at the ways in which incoming, short X-ray flashes delivered by the European XFEL are bent or ‘scattered’ by clathrate hydrates. This technique is only possible in certain experimental set-ups with very specific configurations and detectors, making MID the perfect place for the research.
The structure of clathrate hydrates
“MID has a specific geometry, where we can get incredibly close to the sample with a sensitive detector, around 17 cm away,” Lehmkühler continues. By getting so close to the sample, the team can measure the X-rays bent at a wide range of angles, giving them more information about the samples themselves. “This gives us outstanding imaging resolution”
Learning about the formation of clathrate hydrates could potentially have profound consequences for society. Their formation in pipelines can have disastrous consequences, such as in the Deepwater Horizon incident, but they can also be of use in trapping greenhouse gases. By learning more about them, researchers hope to be able to implement and understand clathrate hydrates, giving us better insights into the chemistry of our planet.