It sounds like a contradiction in terms, but the world's first liquid with permanent holes has been announced. Such a strange material could be used to trap gasses for many industrial applications, such as capturing methane or carbon dioxide to reduce their contribution to global warming.
Liquids can't have holes. It stands to reason that by virtue of being liquids, they will just flow into the hole and fill it up. Or as a paper in Nature puts it: “The structural rigidity and robustness of solids allows them to contain permanent, uniform cavities of precise size and shape. By contrast, liquids have fluid structures, and any ‘porosity’ is limited to poorly defined and transient intermolecular cavities, most of which are smaller than typical molecules.”
Nevertheless, the same paper announces the creation of a liquid that contains so many holes, it has been dubbed porous. “Solid porous adsorbents offer major benefits," the paper notes, but gasses have a harder time diffusing through them than through liquids. The ideal method for capturing a gas would be a liquid filled wth holes, but, the paper notes, "permanent porosity is not associated with conventional liquids.”
To resolve this problem, the team designed what they call “cage molecules” that contain stable pore spaces about half a nanometer across and with slightly smaller entrances called “access windows.” These cages are then dissolved in 15-crown-5, a clear solvent whose molecules are too large for the pores; solvent molecules have a ring shape so that “no part of any solvent molecule can fit into the cage pores,” according to the authors.
Smaller molecules of gas, however, can disperse through the liquid and be captured by the cages. Although particle-trapping cavities have been produced in liquids before, previous versions had one five-hundredth the concentration of empty spaces, putting the latest work on a different plane.
As a result, the authors claim they have produced “a marked change in bulk properties, such as an eightfold increase in the solubility of methane gas” relative to the same solvent without the cages. Even with one cage for every 12 solvent molecules, the resulting liquid flows at room temperature and would look to the naked eye like an ordinary liquid.
As one of the most damaging greenhouse gasses, and also a valuable fuel, the capture of methane is important in many circumstances. To the authors, however, this is just the beginning.
“Our results provide the basis for development of a new class of functional porous materials for chemical processes,” they write. The cage specifications would be adapted to match the gas to be trapped. “The unifying design principle for these materials is the avoidance of functional groups that can penetrate into the molecular cage cavities.”
Senior author Professor Stuart James of Queens University, Belfast, said in a statement: “A few more years' research will be needed, but if we can find applications for these porous liquids they could result in new or improved chemical processes. At the very least, we have managed to demonstrate a very new principle – that by creating holes in liquids we can dramatically increase the amount of gas they can dissolve.”
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