Scientists at the University of Cambridge discovered that water in a single molecule layer does not act as a liquid or a solid, and that it becomes highly conductive at high pressures.
Much is known about the behavior of “bulk water”: 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 for predicting this unusual behavior with unprecedented accuracy, researchers have discovered several new phases of water at the molecular level.
Water trapped between membranes or in tiny nanocavities is common – it can be found in everything from the membranes in our bodies to geological formations. But this nano-formed water behaves very differently from the water we drink.
To date, challenges of experimental characterization of water phases at the nanoscale have prevented a full understanding of its behavior. But in paper Published in the magazine temper naturethe Cambridge-led team describes how they used advances in computational methods to predict the phase diagram of a thick layer of a single water molecule with unprecedented accuracy.
They used a range of computational methods to enable the first principles level to be achieved for a single layer of water.
The researchers found that water confined to a thick layer of a single molecule goes through several phases, including a “hexagonal” phase and a “supra-ionic” phase. In the hexagonal phase, water acts as neither a solid nor a liquid, but rather as something in between. In the supra-ionic phase, which occurs at higher pressures, water becomes highly conductive, rapidly pushing protons through the ice in a manner similar to the flow of electrons in a conductor.
Understanding the behavior of water at the nanoscale is critical to many new technologies. The success of medical treatments can depend on how the water trapped in small cavities in our bodies interacts. The development of highly conductive electrolytes for batteries, water desalination, and frictionless fluid transportation depends on predicting how confined water will behave.
“For all of these areas, understanding the behavior of water is the key question,” said Dr Venkat Kapil of the University of Cambridge. Youssef Hamid Department of Chemistry, the first author of the paper. “Our approach allows the study of a single layer of water in a graphene-like channel with unprecedented predictive accuracy.”
The researchers found that the thick layer of a single water molecule inside the nanochannel exhibited rich and varied phase behaviour. Their approach predicts several phases including the hexagonal phase – an intermediate phase between a solid and a liquid – and also a super-ionic phase, in which water has a high electrical conductivity.
“The hexagonal phase is neither a solid nor a liquid, but an intermediate, which is consistent with previous theories about two-dimensional materials,” Capel said. Our approach also suggests that this phase can be seen experimentally by confining water in the graphene channel.
“The existence of the supra-ionic phase in accessible conditions is strange, as this phase is generally found in extreme conditions such as the cores of Uranus and Neptune. One way to visualize this phase is that the oxygen atoms form a solid lattice, and the protons flow like a liquid through the lattice, like children who run through a maze.”
The researchers say this super-ionic phase could be important for future electrolyte and battery materials because it exhibits 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 dressing’ could be a new avenue for finding the superlative behavior of other materials.
Dr. Venkat Kapil is a Junior Research Fellow at Churchill College, Cambridge. The research team included Dr. Christoph Schran and Professor Angelos Michaelides from Youssef Hamid Department of Chemistry ICE groupworking with Professor Chris Piccard in the Department of Materials and Mineral Sciences, Dr. Andrea Zinn of the University of Naples Federico II and Dr. Jie Chen of Peking University.
Angelos Michaelides et al. “Schematic diagram of the first principles stage of ultrafine monolayer waterNature (2022). DOI: 10.1038/s41586-022-05036-x