nanometer cavity is everywhere, and is crucial to the daily phenomena in geology and biology. However, the properties of nanoscale water may differ greatly from those of bulk water, for example, as indicated by abnormally low dielectric constants of water in nanochannels, near frictionless water flow, or the possible presence of a square ice phase. These properties suggest that nanoconstrained water can be designed for nanofluids, electrolyte materials and technical applications of seawater desalination.
Unfortunately, the challenge of experimental characterization of water at the nanoscale and the high cost of First Principles simulations hinder the molecular level understanding required to control water behavior.
Here, the research team at Cambridge University combines a series of calculation methods to conduct first-principles level research on single layer of water in graphene-like channels. The team found that monolayer water exhibited a surprisingly rich and diverse phase behavior, highly sensitive to temperature and van der Waals pressure acting within nanochannels.
In addition to the non-monotonous change of melting temperature over 400 Kelvin multiple molecular phases, the team also predicted a hexatic phase, which is an intermediate between solid and liquid, and a superionic phase with high conductivity over battery materials. It is worth noting that this suggests that nanolimiting may be a promising pathway to achieve ultraion behavior under easily accessible conditions.
The study was titled "The first-principles phase diagram of monolayer nanoconfined water" and was published in "Nature" on September 14, 2022.
Scientists at the University of Cambridge found that water in a single layer is neither liquid nor solid, and becomes highly conductive at high pressure.
About the way "bulk water" behaves well: it expands when it freezes and 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, researchers have detected several new aqueous phases at the molecular level.
Water trapped between membranes or in tiny nano-scale cavity is common – it is everywhere from the membranes of our bodies to the geological structures. But this nano-limiting water behavior is very different from the water we drink.
Cambridge University researchers combined calculation methods to achieve a first-principle level survey of single-layer water.
researchers found that water confined to a thick layer of single molecules goes through several stages, including the "hexagonal" phase and the "superion" phase. In the hexagonal phase, water is neither solid nor liquid, but is in between. In the superion phase that occurs at high pressure, water becomes highly conductive, quickly pushing protons through the ice in a manner similar to the flow of electrons in the conductor.
Understanding the behavior of nanoscale water is crucial for many new technologies. The success of medical treatment may depend on how the water trapped in our body’s small cavity will react. The development of highly conductive electrolytes for batteries, seawater desalination and frictionless delivery of fluids all rely on predicting the behavior of restricted water.
"For all these fields, understanding the behavior of water is a fundamental problem," said Dr. Venkat Kapil, first author of the paper. "Our method allows for the study of monolayers of water in graphene channels with unprecedented prediction accuracy."
researchers found that single-molecular thick water layers within nanochannels exhibit a rich and diverse phase behavior. Their method predicts several phases, including the hexagonal phase (an intermediate between solid and liquid) and the superionic phase, where water has a high conductivity.
"The hexagonal phase is neither a solid nor a liquid, but an intermediate, which is consistent with previous theory about two-dimensional material ." Kapil said, "Our method also shows that this phase can be observed experimentally by confining water to graphene channels."
"The existence of superion phases under easily accessible conditions is special because this phase is usually present under extreme conditions, such as the core of Uranus and Neptune . One way to visualize this stage of is that oxygen atom forms a solid lattice , and protons flow through the lattice like liquid, just like a child running in a maze."
researchers said that this superion may be important relative to future electrolytes and battery materials because its conductivity is 100 to 1,000 times higher than current battery materials.
results not only help to understand how water works at the nanoscale, but also suggest that "nano-limiting" may be a new way to find the superion behavior of other materials.
paper link: https://www.nature.com/articles/s41586-022-05036-x
related reports: https://phys.org/news/2022-09-phases.html
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