When water is trapped in narrow, nanoscale cavities, it enters an intermediate phase that is neither solid nor liquid, but somewhere in between. This is the finding of an international team of researchers who used statistical physics, quantum mechanics and machine learning to study how the properties of water change when it is confined in such small spaces. By analysing the pressure-temperature phase diagram of this nanoconfined water, as it is known, the team found that it exhibits an intermediate “hexatic” phase and is also highly conducting.
The properties of water on the nanoscale can be very different from those we associate with bulk water. Among other unusual features, nanoscale water has an anomalously low dielectric constant, flows almost without friction and can exist in a square ice phase.
The study of nanoconfined water has important real-world applications. Much of the water in our bodies is confined within narrow cavities such as the spaces inside cells, between membranes and in small capillaries, notes team leader Venkat Kapil, a theoretical chemist and materials scientist at the University of Cambridge, UK. The same is true of water locked inside rocks or trapped in concrete. Understanding the behaviour of this water could therefore be central to biology, engineering and geology. It could also be important for developing future aqueous nanodevices and for applications such as nanofluidics, electrolyte materials and water desalination.
In recent years, researchers have fabricated artificial hydrophobic capillaries with nanoscale dimensions. This has enabled them to measure the properties of water as it passes through channels that are so narrow that water molecules do not have enough space to display their usual hydrogen bonding pattern.
Just one molecule thick
In the latest work, Kapil and colleagues studied water trapped between two graphene-like sheets, such that the water layer was just one molecule thick. Using atomistic simulations, which aim to model the behaviour of all the electrons and nuclei in a system, they calculated the water’s pressure-temperature phase diagram. This diagram, which plots temperature on one axis and pressure on the other, reveals the most stable phase of water at a given pressure-temperature condition.
“These simulations are usually very computationally expensive, so we combined many state-of-the-art approaches based on statistical physics, quantum mechanics and machine learning to reduce this cost,” Kapil tells Physics World. “These computational savings allowed us to rigorously simulate the system at different pressures and temperatures and estimate the most stable phases.”
The researchers found that monolayer water boasts a surprisingly varied phase behaviour that is highly sensitive to temperature and pressure acting within the nanochannel. In certain regimes, it shows a “hexatic” phase, which is intermediate between a solid and liquid as predicted by the so-called KTHNY theory that describes the melting of crystals in 2D confinement. This theory earned its developers the 2016 Nobel Prize for Physics for advancing our understanding of the phase behaviour of 2D solids.
High electrical conductivity
The researchers observed that nanoconfined water becomes highly conducting, with an electrical conductivity 10–1000 times higher than that of battery materials. They also found that it ceases to exist in a molecular phase. “The hydrogen atoms start moving almost like a fluid though a lattice of oxygen, say like children running through a maze,” explains Kapil. “This result is remarkable since such a conventional ‘bulk’ superionic phase is only expected to be stable in extreme conditions like the interiors of giant planets. We have been able to stabilize it under mild conditions.
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“It looks like confining materials in 2D can lead to very interesting properties or properties that their bulk counterparts only exhibit at extreme conditions,” he continues. “We hope our study will help unveil new materials with interesting properties. Our bigger goal, however, is to understand water, especially when it is subject to very complex conditions like inside our bodies.”
The team, which includes researchers from University College London, the Università di Napoli Federico II, Peking University and Tohoku University, Sendai, now hopes to observe the phases they have simulated in real-world experiments. “We are also studying 2D materials other than graphene-like ones since these systems could in principle be synthesized and studied in the laboratory,” Kapil reveals. “A one-to-one comparison with experiments should therefore be possible – fingers crossed.”
The present work is detailed in Nature.