One particular aspect of the complex behavior of confined water, that in fact still lacks a deep understanding, is its peculiar premelting state. Confined water—which occurs in nanopores with diameters of about 1.6 nanometers—has special characteristics that differentiate it from bulk water. This study increases the scientific community’s foundational understanding of how water behaves when confined. It creates the foundation for breakthroughs in material science and energy storage applications.
Complex liquids form when there is a weird dance between solid/frozen and liquid-like properties. Water trapped in these nanopores freezes into a crystalline structure that is much different than traditional ice. Instead of adopting a regular crystalline structure, frozen confined water has an all-dominant distorted hydrogen-bonded structure.
Understanding the Premelting State
In the premelting state, water molecules confined in the space have high energy and perform liquid-like rotational motions. Because they are so dense, they rather literally stay put like a solid. This fascinating state can be studied at very high pressures, all the way down to very low temperatures, up to -90 ˚C. The fast-moving picosecond-scale movements, frequent rotations, and possible quantum behavior of these water molecules shake the basic principles of solid-state behavior often seen in liquids.
One such phenomenon that researchers are into. They note that in the premelting state, incompletely hydrogen-bonded H2O begins to melt even before the whole crystal structure completely thaws upon heating. That would be a compelling sign for the existence of a third phase of water where frozen H2O sheets coexist with tertiary H2O to, you know, party.
“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,” – Prof. Tadokoro.
Hierarchical Structure and Motion
Confined water exhibits a hierarchical, three-layered structure. Each layer has different motions and hydrogen-bonding patterns, making it even more challenging to understand this rare state. This is because the very top layer acts more like a liquid than any other layer, and the layers underneath move more like solids.
This layered behaviour is interesting because it marks a challenge to some of the old orthodoxies surrounding the structure of water. Animations of how water interacts at each of these layers reveal that confined water exhibits remarkable properties. These key findings reveal vast new exciting applications in medicine, optics, nanotechnology and beyond.
Previously researchers found it difficult to measure the complex movements of single water molecules because the available methods fall short. Traditional approaches, including diffraction methods such as X-ray crystallography, do not have enough sensitivity to reveal this kind of atomic detail. Nuclear magnetic resonance (NMR) spectroscopy has been shown to be a particularly powerful tool for demonstrating the premelting state. It gives them very tangible details about the behaviors and interactions at play.
Implications for Future Research and Applications
The implications of knowing more about confined water and its premelting state are more than just scientific interest. According to Prof. Tadokoro, since the performance of these structures can be readily controlled, it may be possible to achieve breakthroughs in energy storage technologies.
“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,” – Prof. Tadokoro.
Storing gases more efficiently could help make energy systems cleaner, cheaper, and more reliable. This historic step is particularly significant for tapping renewable energy and climate-friendly materials. The potential development of water-based materials indicates opportunities for innovation across various industries.