Physicists Squeeze Water Molecules Into an Entirely New State
Beyond solids, liquids, and gases.
Physicists from the Department of Energy's Oak Ridge National Laboratory have discovered a new state of water that cannot be explained as a solid, liquid, or gas. It is a peculiar, and like exceedingly common, "other" state that water molecules are forced to exist in in conditions of extreme confinement. Here, quantum effects begin to take over, leaving behind the rules of classical physics that we're used to: solids, liquids, and gases just stop making sense.
The group's work is published in the current Physical Review Letters.
To start, we need to imagine just a single water molecule: two hydrogen atoms bound to a single oxygen atom. This molecule is placed in a tiny natural channel cleaved through the hexagonal crystals of the mineral beryl. The channel, which is large enough to host just a single water molecule, is only about 5 angstroms across, or roughly one ten-billionth of a meter. According to the physicists, such confinement should be fairly common in the natural world, taking place in certain geological and biological environments such as soils, mineral interfaces, and cell walls.
Atoms themselves are only about 1 angstrom across, so the beryl channels are really more straitjackets than cages. Trapped like this, the water molecules begin to demonstrate tunneling behavior, becoming "delocalized." As we should expect in the quantum world, the molecules and their constituent atoms are able to exist in multiple states at once.
This winds up looking like the ring in the illustration above. The hydrogen atoms of the water molecule begin to inhabit six different symmetric orientations simultaneously, with the oxygen atom stuck in the middle.
Normally, a water molecule is arranged with the oxygen atom in the middle at the two hydrogen atoms off to one side, with the resulting configuration looking like a ">" symbol. In a beryl channel/cavity, this configuration can have one of six different orientations corresponding the six different walls of the hexagonal passageway. This is maintained so long as the hydrogen atom doesn't have enough energy to jump into a new configuration.
That's what "should" happen, anyhow. But the quantum world is much weirder.
Instead, we wind up with a central oxygen atom and a surrounding ring of fuzzy "peaks." The hydrogen atoms are no longer jutting out from the oxygen like antenna and are instead existing in a suspended average of all of the possible orientations, with the effect increasing with increasing temperature. This is the result of the hydrogen atoms "tunneling" between different possible configurations.
The ORNL physicists were able to observe this in action using neutron scattering. Here, neutrons are used to probe subatomic structures rather than the electrons and photons of optical and X-ray imaging methods. Neutrons have the handy property of being electrically neutral, so they don't get knocked around by materials that may lie between the observer and the thing to be observed. Their lack of charge also means that they won't interfere with the charged particles they're tasked with observing.
So, neutron scattering is a crucial imaging technique for understanding material properties at atomic scales, and it also happens to be one that ORNL is very well-equipped for thanks to the lab's Spallation Neutron Source, which provided the physicists with their first views of the tunneling water molecules. A second round of experiments was conducted using high-energy neutrons from the ISIS neutron facility at Rutherford Appleton Laboratory in the UK.
This new state of water winds up having some interesting properties. For one thing, the molecule's center of mass is shifted away from the two outlying hydrogen atoms to its central oxygen atom. (Remember that rather than hanging out to one side, the hydrogens are now spread out around a circle.)
Even more interesting is that a water molecule in this configuration loses its electric dipole moment. Normally, a water molecule is more negatively charged at its oxygen corner (the vertex of the "<" shape) and more positively charged on the side with the hydrogen atoms. With these atoms spread around, there's no longer this asymmetry.
With the molecule's dipole moment lost, it should no longer be interested in boding with other atoms/molecules. The result then is water that can no longer claim to be a "universal solvent," the property that more or less makes the biological world go 'round. I'm not sure if it'd then be more apt to call this form of water super-pure or just "dead."