The Italians have a colorful expression – to make a hole in water – to describe an effort with no hope of succeeding. Researchers at Queen's University Belfast (QUB), however, have seemingly managed the impossible, creating a class of liquids that feature permanent holes at the molecular level. The properties of the new materials are still largely unknown, but what has been gleaned so far suggests they could be used for more convenient carbon capturing or as a molecular sieve to quickly separate different gases.
Porous materials are a jack-of-all-trades of the engineering world. Their larger surface area, lighter weight and filtering abilities are used to create high-performance batteries and supercapacitors, build lighweight supermaterials, or filter out CO2 before it leaves factory smokestacks.
When it comes to carbon sequestration in particular, scientists have already come up with plenty of readily available materials – including clay and coffee grounds – to do the job. But while effective and inexpensive, such solid-state materials are not easily retrofitted to existing plants.
Researchers Stuart James and team have now demonstrated a class of liquids that is permanently hollow at the molecular level and could be employed for more convenient carbon capturing or to manipulate gases in new and more effective ways.
To create a porous liquid, the scientists simply designed hollow cage molecules to place in a solvent. The solvent is chosen so its molecules are too big to enter the cages, leaving those spaces available for an external gas to fill. The resulting concentration of empty cages is about 500 times greater than in similar solutions.
The solvent picked for the study was the crown ether 15-crown-5, and the cages were designed to fit the molecules of carbon dioxide, methane, nitrogen and xenon. After testing, the scientists reported that their porous liquid was able to store eight times the amount of methane gas as bare crown ether.
Such a figure is remarkably high for a liquid, and opens the possibility to employ these materials for carbon sequestration. Porous solids are often more effective at collecting carbon in absolute terms, but a system based on liquids would likely be easier to retrofit.
Porous liquids could also be used as an effective gas separator. Even when gas molecules saturate the liquid, they can be quickly displaced by other organic molecules whose size is a better fit for the cages. For instance, even as xenon gas is saturating the solution, a small amount of chloroform will suddenly cause the gas to be released.
James and team are now at work to further study these liquids and find how their properties can be used for practical applications. A paper describing the advance was published in the latest issue of the journal Nature.