A drop of water inside a "sandwich" of two layers of graphene will freeze into a new form of ice, with its molecules in a square grid pattern never found in nature, researchers have discovered.
An atomically thin layer of water between the layers of graphene — flat, single-atom-thick sheets of carbon — will freeze into the unique form of ice at room temperature, they say.
Researchers at the University of Manchester in England put water on a graphene sheet, then placed another layer on top. As the water evaporated and the sheets were pressed together until they were less than a nanometer apart, pockets of water molecules were trapped in the sandwich.
Using electron microscopy, the researchers discovered these pockets contained square ice "with symmetry completely alien to water molecules," they reported in the journal Nature. One particular benefit of using one atom thick graphene is that it does not block high-magnification electron microscopy that shows the pattern of molecules, allowing researchers to spot individual water molecules.
Previous theoretical work had suggested the possibility of such room-temperature square ice, although lead researcher Andre Geim said he doubted whether that was true.
"To our own surprise, we found exactly what theory predicted: an ice which is only one atom thick," says Geim, who shared the Nobel Prize in Physics in 2010 for groundbreaking work to isolate the first molecules of graphene.
Although water is all around us, lots of it exists in locations that, to us, appear to be bone dry, he points out.
"You go to the driest desert, there will be water absorbed everywhere," Geim says. "You crack any rock, there will be capillaries inside filled with water."
Up to now, little was known concerning the behavior of water in such tiny, confined spaces, he says.
"Until recently, we didn't even know that the structure of water would be so different from the structure of conventional ice, when it goes to the nanoscale."
In conventional ice, the oxygen-hydrogen bonds are organized in a three-dimensional, tetrahedral or pyramid shape.
In the square ice, however, all the atoms lie in a flat plane, with each oxygen-hydrogen bond at a right angle.
Although just a laboratory finding at the moment, the discovery of this counterintuitive behavior of water at the molecular scale could eventually lead to more efficient technologies including filtration, distillation and desalination, Geim says.
"Finding out how the water behaves in a capillary is a big part of what we need to do to make a good filter," he says. "This is a very important step."
