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The search for empty cages

Irish scientists claim that they have created holes in liquid, like holes in Swiss cheese. By shooting antimatter at it, researchers from Kiel have confirmed the discovery of the world‘s first perma­nently porous liquid.

Liquids do not actually have large stable holes or pores in them. As all the molecules in liquids are moving around, pores constantly appear and disappear again. By contrast, porous solids like zeolites and metal organic frameworks (MOF) have been used in industry for chemical processes such as catalysis and gas separation for some time now. These fixed structures have permanent pores of equal size that can store waste products such as methane. Problems arise time and again, however, if they are to be inserted into existing chemical equipment. Such obstacles would be overcome by porous liquids that act as filters: for example, liquids could easily be pumped through pipes to absorb gas and then be pumped out again.

The researchers are now very close to using their new class of material for this purpose. The material is made up of cage molecules that are dissolved in a crown ether solution. In order to make the cages soluble, the scientists built six crown ether molecule groups into each corner of the cages. In this way, despite the high concentration of cages, they produced a liquid substance at room temperature.

As clear and runny as honey, the unique chemical product looks quite nondescript. To find out whether the cages in the liquid really are empty was the task of a team of experts from Kiel University led by Professor Klaus Rätzke at the research group for Multi-component Materials under Professor Franz Faupel. Using what is known as positron annihilation lifetime spectroscopy – a method mastered by only a handful of research groups worldwide – they also revealed the size of the holes through their experiments.

The Kiel-based researchers have spent ten years de­veloping the technique and normally use it to cha­racterise metals and polymers. For them, empty spaces, what are known as defects in the regular crystalline structure, are crucial for certain charact­eristics of the material. “We look at where atoms are missing in a metal and how large the holes are”, said Professor Klaus Rätzke. For instance, the researchers have also studied two-component adhesives in order to characterise their storage and adhesion qualities.

To find defects in new liquid material, PhD student Tönjes Koschine shot positrons, i.e. antimatter, at a sample of the porous liquid. Positrons annihilate immediately on contact with electrons. “If there are holes in the liquid, this is where there are no electrons, and so the positrons ‘live’ longer there. That is what we measured”, explained Koschine.

The researchers are able to draw conclusions about the size of the pores based on the length of the positrons’ lifetime. “Positrons live around 10 times longer in the holes than if they come into direct contact with electrons, that is a total of two nanoseconds”, said doctoral degree supervisor Rätzke. A nanosecond is equivalent to a billionth of a second. The empty spaces in the cages are therefore around half a nanometre in size, which is the size of two to three atoms. The Kiel-based scientists have thus confirmed the results of simulations carried out within this international research partnership and made an important contribution towards the development and characterisation of new materials. The results were published in the renowned journal Nature.

Denis Schimmelpfennig

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