New water phases detected

Scientists from the University of Cambridge have discovered that water acts neither as a liquid nor as a solid in a one-molecule layer and that at high pressures it becomes very conductive.

Much is known about how “bulk water” behaves: it expands when it freezes, and it has a high boiling point. But when water is compressed to the nanoscale, its properties change dramatically.

By developing a new method to predict this unusual behavior with unprecedented accuracy, the researchers have discovered several new water phases at the molecular level.

Water trapped between membranes or in tiny nanoscale cavities is widespread — it can be found in everything from membranes in our bodies to geological formations. But this nano-confined water behaves very differently than the water we drink.

So far, the challenges of experimentally characterizing the nanoscale water phases have prevented a full understanding of its behavior. But in an article published in the magazine Naturethe Cambridge-led team describes how they used advances in computational approaches to predict the phase diagram of a layer of water one molecule thick with unprecedented accuracy.

They used a combination of computational approaches to enable the study of a single layer of water at the first principles level.

The researchers found that water trapped in a layer one molecule thick goes through several phases, including a ‘hexatic’ phase and a ‘superionic’ phase. In the hexatic phase, the water acts as neither a solid nor a liquid, but something in between. In the superionic phase, which occurs at higher pressures, water becomes highly conductive and propels protons rapidly through ice, similar to the flow of electrons in a conductor.

Understanding the behavior of water at the nanoscale is crucial for many emerging technologies. The success of medical treatments can depend on how water trapped in small cavities in our bodies reacts. The development of highly conductive electrolytes for batteries, water desalination and the smooth transport of liquids all depend on how confined water will behave.

“For all of these areas, understanding the behavior of water is the fundamental question,” said Dr. Venkat Kapil from the Yusuf Hamied Department of Chemistry in Cambridge, the first author of the paper. “Our approach enables the study of a single layer of water in a graph-like channel with unprecedented prediction accuracy.”

The researchers found that the one-molecule-thick layer of water inside the nanochannel showed rich and diverse phase behavior. Their approach predicts multiple phases including the hexatic phase – an intermediate between a solid and a liquid – and also a superionic phase where the water has high electrical conductivity.

“The hexatic phase is neither a solid nor a liquid, but an intermediate, consistent with previous theories about two-dimensional materials,” Kapil said. “Our approach also suggests that this phase can be observed experimentally by confining water in a graphene channel.

“The existence of the superionic phase under easily accessible conditions is peculiar, since this phase generally occurs under extreme conditions such as the cores of Uranus and Neptune. One way to visualize this phase is that the oxygen atoms form a fixed lattice and protons flow through the lattice like a liquid, like children running through a maze.”

The researchers say this superionic phase could be important for future electrolyte and battery materials because it has electrical conductivity 100 to 1,000 times higher than current battery materials.

The results will not only help understand how water works at the nanoscale, but also suggest that “nano-encapsulation” could be a new way to find the superionic behavior of other materials.