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Using a metal–organic framework, researchers reduced the volume of a water-based mixture by 7%.
Liquids are difficult to compress because there isn’t much space between molecules.Credit: Napaporn Leadprathom/Alamy
Researchers have achieved the notoriously difficult task of squashing a liquid, using a technique that allows water to be compressed by up to 20 times more than is normally possible.
Liquids typically can’t be compressed much because there isn’t any space between molecules. Stuart James at Queen’s University Belfast and his colleagues found a way to compress water by exploiting a technology that they have been working on called porous liquids; counterintuitively, these liquids have empty spaces inside them. The team, which published its work1 in Advanced Materials, says that its approach has resulted in the best compression of a liquid so far.
To make the compressible liquid, James and his team took a porous solid in which the molecules are arranged in a cage configuration known as a metal–organic framework, or MOF. In this material, called ZIF-8, the pores’ insides are hydrophobic, meaning that they repel water.
The team made the MOF into extremely small particles — between 30 and 200 nanometres — and dispersed it in water, creating an opaque white solution. Because the pores in ZIF-8 are hydrophobic, they remain empty when mixed with water at normal pressures. “You’ve got something that looks like milk; it’s a dispersion of these tiny particles. But about 7 or 8% of it is actually empty space,” says James. When the team put the liquid under pressure, the water molecules were forced inside the empty pores, decreasing the liquid’s volume. At 500-bar pressure — 500 times atmospheric pressure — the volume dropped by 7%.
The technique still requires significant pressure to work. James hopes that by tuning the system he might be able to compress a liquid at lower pressures. The team has tried mixing the water with methanol, which made it easier to push the water molecules inside the pores.
Not all researchers agree that the material represents a compressible liquid. Yaroslav Grosu, who studies porous media at the CIC energiGUNE research centre in Vitoria-Gasteiz, Spain, sees the system as a dispersion of nanoscale particles, rather than the homogeneous fluid that James describes. Grosu says that squeezing liquids into and out of pores in these particles is well studied, and that research should extend the applications of this kind of technology.
James says that the definition of homogeneous materials isn’t clear-cut. He thinks that the technique could be useful in shock absorption or acoustics, because the compressibility of liquids, solids and gases determines how fast sound travels through them. Ultimately, James says, he pursued the idea of making a compressible liquid for fun. “I love an oxymoron.”
