Most materials get compacted or fall apart under pressure, but scientists working in an international collaboration between the US Department of Energy’s Brookhaven National Laboratory and the School of Chemical Sciences at England’s University of Birmingham have discovered some that expand.
These unusual materials may have applications as ‘molecular sponges’ for soaking up chemical pollutants or even radioactive waste.
The idea, according to physicist Thomas Vogt, the lead Brookhaven scientist on the study, is that you squeeze a fluid into tiny pores in the material, thereby increasing its volume. This extra volume can also allow slightly larger molecules or atoms, such as pollutants, to enter the expanded pores. ‘When the pressure is released and the material contracts, the pollutant would be trapped inside,’ he said.
The materials are all zeolites, which are solids containing aluminium, silicon, and oxygen with a three-dimensional structure containing regularly spaced pores within the molecular framework. These nanopores are said to make zeolites very useful for soaking up small molecules, ions, or gases.
The pores are normally filled with positively charged ions, such as calcium or sodium, and water molecules, so zeolites are said to be hydrated. Many are currently used as water softeners and in detergents. Previous studies elsewhere suggested that some zeolites have unusual properties under pressure. The collaborative team was investigating these properties when they discovered superhydrated zeolite, which could soak up twice the normal amount of water.
Using a technique called ‘powder diffraction’ at Brookhaven’s National Synchrotron Light Source (NSLS), the team has deciphered the zeolite’s molecular structure, which for the first time explains this unusual property and shows where the extra water goes.
The experiments were done by subjecting the material to increasing pressure (from normal atmospheric pressure up to 50,000 times that pressure) in a diamond anvil cell. The sample is squeezed between two diamonds in a tiny chamber filled with water or another liquid to transmit the pressure evenly to all sides. The scientists then bombard the sample with an intense beam of x-rays and analyse how this beam is diffracted as it bounces off the sample. Vogt’s colleague Yongjae Lee then translated the diffraction pattern into a three-dimensional molecular structure.
As the pressure increased, the material at first appeared to compress. But as the pressure climbed between 0.8 and 1.5 gigapascals (8,000 to 15,000 times atmospheric pressure), the material expanded along two of its three dimensions.
‘This is not supposed to happen,’ said Vogt. ‘Normally, when you squeeze something, it’s supposed to get smaller. This stuff gets bigger.’ When the pressure increased beyond 1.5 gigapascals, the material compressed once again. Analysis of the molecular structure revealed that, during the expansion, extra water molecules were squeezing into the zeolite’s pores.
The scientists have suggested several ways to exploit the unusual property they call pressure-induced expansion, found in only certain zeolites. One would be to set up a ‘trap door’ mechanism for locking up chemical or radioactive pollutants.
‘When you increase the pressure and the material gets bigger, the pores get bigger, too,’ said fellow researcher and Birmingham University chemist Joseph Hriljac. ‘So we can try to get bigger ions or molecules in there, such as hydrocarbons, mercury, lead, or even radioactive strontium. Then, when you release the pressure, the pore would get smaller and trap the pollutants inside.’
The scientists plan to continue their studies on the expanding zeolites to see if this approach will work.