The laws of physics say that when a material is squee-zed, it gets smaller. Now US scientists have come up with a material that does the opposite – it expands.
The scientists from the US Department of Energy’s Argonne National Laboratory (AGL) applied high pressures (0.9-1.8GPa) to zinc cyanide at the lab’s Advanced Photon Source. By using different fluids around the material as it was squeezed, they were able to create five new phases of porous material: four crystalline and one amorphous (Journal of the American Chemical Society, 2013, 135(20): 7621; doi:10.1021/ja4012707).
‘It’s like squeezing a stone and forming a giant sponge,’ says AGL chemist Karena Chapman. ‘The pressure-treated material has half the density of the original state. This is counterintuitive to the laws of physics.’
The materials almost double in volume, the researchers report. The resulting new phases contain large fluid-filled pores, such that the combined solid plus fluid volume is reduced, she explains. ‘The bonds in the material completely rearrange.’
The team claims that this is the first time that hydrostatic pressure exerted by a fluid has produced novel porous materials from dense materials with interlaced atomic frameworks.
The type of fluid determined the shape of the pores. Large molecule fluids such as isopropanol or ethanol created a distorted, orthorhombic polymorph, while fluids such as water or methanol produced polymorphs with different topologies: ‘diamondoid’, ‘londaleite’ and ‘pyrite-like’.
Porous or framework materials can be used to trap, store and filter materials. By tailoring release rates, scientists can adapt these frameworks to deliver drugs and initiate chemical reactions for the production of everything from plastics to foods. ‘This discovery will likely double the amount of available porous framework materials, which will greatly expand their use in pharmaceutical delivery, sequestration, material separation and catalysis,’ Chapman says.
These findings have been shown before in porous materials where a liquid enters and fills the pores, increasing the volume, just like when a balloon is inflated, says Andrew Goodwin of the University of Oxford. ‘But what is new here is that the material reorganises completely. Bonds break and new ones form, and that is completely novel as far as I’m aware. No-one would think of using pressure to make a new material but that is what the researchers are suggesting: a novel synthetic approach.’