Scientists explore the unique structure and behaviour of water molecules confined in nanopores using nuclear magnetic resonance spectroscopy. The study revealed a phase observed in water called the premelting phase. Credit: Wikimedia Commons via Creative Commons Search Repository Image link: https://openverse.org/image/4cc513d0-1bca-4739-a91c-603d3cf14391?q=water+globules&p=16. CC BY.
Researchers have used nuclear magnetic resonance spectroscopy to analyse a paradoxical phase that occurs when water is confined to crystal nanopores.
Despite being one of the most familiar substances on Earth, water holds many secrets that scientists are still working to understand.
When confined to extremely small spaces – such as within certain proteins, minerals, or artificial nanomaterials – water behaves in ways that are drastically different from its bulk liquid form.
These confinement effects are critical for many natural and technological processes, including regulating the flow of ions through cell membranes and the properties of nanofluidic systems.
One intriguing yet poorly understood state of confined water is called the ‘premelting state.’
In this unique phase, water behaves as if it were on the cusp of freezing and melting at the same time, thus defying simple liquid or solid classifications.
However, it has proven difficult to study the premelting state and other confined water dynamics in detail.
While techniques such as diffraction methods (example: X-ray analysis) are useful for pinpointing the positions of atoms other than hydrogen, they are not sensitive enough to capture the picosecond-scale rotational motion of hydrogen and the motion of individual water molecules.
The left image shows a crystal structure of dynamic water molecule cluster like liquid at −20°C in the supercooled water state revealed previously by X-ray structural analysis. The right image shows the crystal structure of water/ice molecule clusters at –90˚C in the premelting state. Red spheres indicate water regularly hydrogen-bonded to the oxygen atoms on the pore wall surface. Credit: Professor Makoto Tadokoro from Tokyo University of Science, Japan Image link: https://pubs.acs.org/doi/10.1021/jacs.5c04573. CC BY.
Mysteries
In a recent study, a research team led by Professor Makoto Tadokoro alongside Lecturer Fumiya Kobayashi and first-year PhD student Mr Tomoya Namiki, from the Department of Chemistry, Tokyo University of Science, Japan, shed new light on the mysteries of confined water.
Their paper reports how they used static solid-state deuterium nuclear magnetic resonance (NMR) spectroscopy to observe the hierarchical dynamics of water confined within the hydrophilic nanopores of a molecular crystal and characterised the premelting state, which is a new phase observed in water.
To perform their experiments, the team produced hexagonal rod-like crystals, with quasi-one-dimensional channels containing a nanopore approximately 1.6nm in diameter and filled them with heavy water (D2O).
By measuring the NMR spectra of a single crystal of {[Co(D2bim)3](TMA).20D2O}n at room temperature, the researchers were able to confirm the existence of a hierarchical, three-layered structure in the contained water molecules.
The unique peaks observed in the spectra corresponded to every layered structure with distinct movements and hydrogen-bonding interactions with one another of the confined water, providing clear evidence of multi-layered organisation.
Furthermore, water confined in the nanopores freezes in a different structure from bulk ice and first melts through a distorted hydrogen-bonded structure, leading to the formation of a premelting state.
Distinct changes
To gain insights into the premelting state, the researchers heated the crystal gradually from low temperature to get the water from a frozen state to a liquid state.
They observed distinct changes in the NMR spectra that confirmed a phase transition into the premelting state, and their measurements revealed the presence of two seemingly contradictory states.
Tadokoro said: “The premelting state involves the melting of incompletely hydrogen-bonded H2O before the completely frozen ice structure starts melting during the heating process. It essentially constitutes a novel phase of water in which frozen H2O layers and slowly moving H2O coexist.”
The researchers measured the spin-lattice relaxation time to quantify the rotational mobility of the heavy water molecules in this new phase.
While the activation energy for the premelting state was far from that of bulk ice, the correlation time was remarkably close to that of bulk liquid water.
Simply put, this means that while the water molecules’ positions were relatively fixed as one would expect of a solid, their rotational motions were extremely fast and liquid-like.
Clarifications
Taken together, these findings build toward a more comprehensive understanding of how water behaves in extreme confinement.
They clarify crucial structural and dynamic aspects, which are important for understanding how water and ions permeate through biological proteins and membranes.
Looking ahead, these insights could also lead to practical innovations.
Tadokoro said: “By creating new ice network structures, it may be possible to store energetic gases such as hydrogen and methane and develop water-based materials such as artificial gas hydrates.”
Controlling the freezing properties of water based on the structure of ice could lead to the creation of new, inexpensive, and safe hydrosphere materials.
Overall, this study ultimately demonstrates that even a substance as common as water still holds fundamental secrets waiting to be unlocked.
The study is published in the Journal of the American Chemical Society.
