Scientists detected new phases of water acting like neither a liquid nor a solid
Researchers from the University of Cambridge have found that water behaves neither like a liquid nor a solid in a single molecule layer and that under extreme pressures, it becomes electrically conductive.
Water normally expands when it freezes, and it has a high boiling point. However, the new research demonstrates that when water is compressed to the nanoscale, its properties change dramatically.
The results were published in Nature on September 14.
A thorough understanding of the behavior of water has not yet been possible due to the difficulties in experimentally characterizing its nanoscale phases. However, the Cambridge-led team describes how they were able to anticipate the phase diagram of a one-molecule thick layer of water with extraordinary accuracy, according to the statement.
Hexatic and Superionic phases
Scientists discovered that water that is contained in a layer that is one molecule thick travels through several phases, including a "hexatic" phase and a "superionic" phase.
The water behaves in the hexatic phase as something in-between, neither a solid nor a liquid. At greater pressures, during the superionic phase, the water turns highly conductive, and protons are propelled swiftly through ice in a manner akin to the movement of electrons in a conductor.
"The hexatic phase is neither a solid nor a liquid, but an intermediate, which agrees with previous theories about two-dimensional materials," said Dr. Venkat Kapil from Cambridge's Yusuf Hamied Department of Chemistry, the paper's first author.
"Our approach suggests that this phase can be seen experimentally by confining water in a graphene channel."
"The existence of the superionic phase at easily accessible conditions is peculiar, as this phase is generally found in extreme conditions like the core of Uranus and Neptune. One way to visualize this phase is that the oxygen atoms form a solid lattice, and protons flow like a liquid through the lattice, like kids running through a maze," he also added.
The findings not only point to a new way to discover superionic behavior in other materials but also to a new way to understand how water behaves at the nanoscale.
Water in nanoscale cavities is ubiquitous and of central importance to everyday phenomena in geology and biology. However, the properties of nanoscale water can be substantially different from those of bulk water, as shown, for example, by the anomalously low dielectric constant of water in nanochannels, near frictionless water flow, or the possible existence of a square ice phase. Such properties suggest that nanoconfined water could be engineered for technological applications in nanofluidics, electrolyte materials, and water desalination. Unfortunately, challenges in experimentally characterizing water at the nanoscale and the high cost of first-principles simulations have prevented the molecular-level understanding required to control the behavior of water. Here we combine a range of computational approaches to enable the first-principles-level investigation of a single layer of water within a graphene-like channel. We find that monolayer water exhibits surprisingly rich and diverse phase behavior that is highly sensitive to temperature and the van der Waals pressure acting within the nanochannel. In addition to multiple molecular phases with melting temperatures varying non-monotonically by more than 400 kelvins with pressure, we predict a hexatic phase, which is an intermediate between a solid and a liquid, and a superionic phase with a high electrical conductivity exceeding that of battery materials. Notably, this suggests that nanoconfinement could be a promising route towards superionic behavior under easily accessible conditions.
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