The team of scientists was led by Franziska Emmerling from Germany’s Federal Institute for Materials Research and Testing (BAM) in Berlin (Germany), and included Jonathan Wright from the European Synchrotron Radiation Facility (ESRF) in Grenoble (France), where the experiments took place. The results were published 18 July 2011 as an advanced online publication by Chemical Communications.
Clathrate hydrates have been known since 1810 when British scientist Sir Humphry Davy led chlorine gas under high pressure into water and discovered the formation of solid structures. Clathrate hydrates are another form of ice, i.e. water in the solid state where water molecules are primarily held together by hydrogen bonds. However, in this case the molecules form a cage-like structure (clathrate from Latin = bars/lattice) with a cavity inside each cage. These cavities contain gas molecules, for example methane, carbon dioxide or hydrogen sulphide as a “guest species”. In this way, large amounts of gas can be stored in a small volume at ambient pressure and temperature: 1 m3 of clathrate hydrate contains about 164 m3 of trapped gas.
Clathrate hydrates regained attention in the 20th century when they were identified as a source of blockage in natural gas pipelines. In cold areas, methane hydrates may also form, very slowly, from natural gas in the atmosphere and atmospheric humidity. These naturally occurring methane hydrates are of potential interest as alternative fuel sources. If clathrate hydrates could be made at the industrial scale from water, they could offer a safe and affordable alternative to pressurised gas bottles as gas storage containers.
Until now, fast formation of clathrate hydrate required high pressure and low temperature, along with a high concentration of gas dissolved in water. This made the detailed study of their formation process a difficult task. Thanks to the new experimental approach, the clathrate hydrates formation can now be studied and optimised in view of industrial applications, at ambient pressure and temperature.
The new experimental approach uses an acoustic levitator to suspend microscopic volumes of dichloromethane in air. Clathrates spontaneously form in these suspended droplets when water is mixed in, because the surface of the droplets efficiently cools as the dichloromethane evaporates. The time scale of the hydrate formation is in the order of seconds.
This made it possible to study the formation of the clathrate hydrates at the atomic scale, using X-ray diffraction, with a time resolution of less than one second. The gas clathrates could also be stabilized for an adjustable period of time by continuously supplying further solvent using a piezo syringe.
“With this elegant method, we have the unique opportunity to obtain novel insights into how clathrate hydrates form at ambient conditions. In a nutshell: a small droplet can help to understand industrial-scale issues.”
Reference: Chem. Commun., 2011, DOI: 10.1039/c1cc13049h
A two-minute video clip in .wmv format has been made from hi-speed camera recordings of a suspended droplet. The clip nicely shows formation of clathrate hydrate crystals and can be downloaded here.