Discover the fascinating world of nanoporous ices - water's most counterintuitive state that could transform energy storage, biomedical applications, and materials science.
Imagine a block of ice filled with so many tiny tunnels and cavities that it resembles microscopic Swiss cheese. This isn't ordinary ice—it's nanoporous ice, one of water's most fascinating and counterintuitive states. Unlike the solid ice cubes in your freezer, these ices contain a complex network of nano-cavities and channels built directly into their water molecule framework, creating structures so full of empty space that they could theoretically float on liquid water 1 4 .
For centuries, scientists believed they understood water's basic solid forms. But the discovery of these empty frameworks has overturned conventional wisdom, revealing a hidden world of porous ice structures that exist at the intersection of materials science, chemistry, and physics. These remarkable materials aren't just laboratory curiosities—they represent an entirely new class in the water/ice family with potential applications ranging from clean energy storage to biomedical delivery systems 1 4 .
Complex network of nano-cavities and channels built from water molecules
Nanoporous ices contain permanent nanocavities and channels, making them fundamentally different from conventional ice structures.
To understand why nanoporous ices are so remarkable, we need to place them in context. Water is deceptively complex—despite its simple molecular formula (H₂O), it can form at least 20 different three-dimensional crystalline structures under varying temperature and pressure conditions, designated ice I through ice XX 4 . Most of these are dense, packed-tight configurations we would recognize as solid ice.
Nanoporous ices represent something entirely different. They belong to a special category of low-density porous materials called WOFs (water oxygen-vertex frameworks), named by analogy to other porous materials like MOFs (metal-organic frameworks) and COFs (covalent organic frameworks) 1 4 . What makes WOFs unique is that their frameworks are built entirely from hydrogen-bonded water molecules arranged to create permanent nanocavities and channels.
Dense, packed-tight water molecule arrangement with no permanent pores.
Complex network of nano-cavities and channels with significant empty space.
| Ice Type | Structure Description | Cavity Characteristics | Discovery Year |
|---|---|---|---|
| Ice XVI | Guest-free sII clathrate structure | Two types of polyhedral nano-cavities (512 and 51264) with volumes of 160ų and 307ų respectively | 2014 4 |
| Ice XVII | Framework with spiraling nanochannels | Channel diameter of 6.10Å, capable of reversibly adsorbing gas molecules | 2016 4 |
The story of nanoporous ices begins not with ice itself, but with its filled counterparts—clathrate hydrates. These fascinating structures are host-guest compounds where water molecules form cage-like structures that trap "guest" molecules like methane, neon, or hydrogen 4 . For decades, scientists had theorized that if you could remove these guest molecules without collapsing the water framework, you'd be left with empty cages—a true nanoporous ice.
In 2014, a team of researchers led by Andrzej Falenty achieved what was once thought impossible: they created the first experimentally confirmed nanoporous ice 4 . Their approach was ingenious—instead of trying to build empty cages from scratch, they started with a neon clathrate hydrate (Ne atoms trapped in water cages) and carefully pumped out all the neon guests. The resulting structure, named ice XVI, remained stable below 145K and exhibited the curious property of negative thermal expansion (shrinking when heated) at very low temperatures 4 .
Following the same strategy, scientists soon created a second nanoporous ice by removing hydrogen molecules from a hydrogen-filled ice hydrate 4 . The result was ice XVII, which features an even more intriguing structure of spiraling nanochannels rather than isolated cages. These channels, approximately 6.10Å in diameter, demonstrate reversible gas adsorption—they can repeatedly absorb and release gas molecules without collapsing, making ice XVII a promising candidate for hydrogen storage applications 4 .
Scientists create nanoporous ices by carefully removing "guest" molecules from clathrate hydrates without collapsing the water framework.
The resulting empty water frameworks remain stable at cryogenic temperatures, creating permanent nanopores.
Recent research has revealed that water confined to nanoscale spaces behaves in ways that defy conventional classification. In a groundbreaking 2025 study, researchers from Tokyo University of Science discovered that when water is squeezed into nanosized channels approximately 1.6nm in diameter, it enters a bizarre "premelting state" that exhibits characteristics of both solids and liquids simultaneously 8 .
Using advanced solid-state deuterium NMR spectroscopy, the team observed that in this peculiar state, water molecules maintain relatively fixed positions (as in a solid) while undergoing extremely fast, liquid-like rotational motions 8 . Professor Makoto Tadokoro, who led the research, explains: "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" 8 .
This research bridges the gap between studies of nanoporous ices and the behavior of water in biological systems, helping explain how water and ions permeate through protein channels in cell membranes 8 .
Comparison of molecular behavior in different water states
Creating and analyzing these exotic ice forms requires sophisticated techniques and carefully controlled conditions. Researchers in this field employ a diverse array of methods spanning temperature control, gas manipulation, and structural analysis.
| Tool/Method | Function in Research | Specific Application Examples |
|---|---|---|
| Cryogenic Systems | Maintain extremely low temperatures | Keeping ice XVI stable below 145K 4 |
| Gas Manipulation | Introduce or remove guest molecules | Pumping neon from clathrates to create ice XVI 4 |
| Neutron Diffraction | Determine atomic structure | Mapping spiraling channels in ice XVII 4 |
| Solid-State NMR | Study molecular motion and environment | Analyzing premelting state in confined water 8 |
| Multimodal Analysis | Combine multiple measurement types | Simultaneous X-ray scattering and infrared spectroscopy 3 |
Begin with clathrate hydrate containing guest molecules as structural templates.
Extract guest molecules through controlled pressure and temperature conditions.
Use multiple analytical techniques to verify structure and properties.
The experimental approach to creating nanoporous ices typically follows a multi-step process involving template selection, guest removal, structure stabilization, and characterization using advanced analytical techniques.
The potential applications of nanoporous ices extend far beyond fundamental scientific interest. Their unique properties—high surface area, tunable pore sizes, and excellent biocompatibility—make them promising candidates for numerous technological applications 1 4 .
Perhaps the most immediate application lies in gas storage and separation. Ice XVII's ability to reversibly adsorb hydrogen molecules suggests potential for clean energy storage 4 . Similarly, the nanocavities in these ices could be tailored to selectively capture specific gas molecules, enabling more efficient gas purification systems for industrial processes 1 .
Professor Tadokoro speculates that "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" 8 . This could lead to inexpensive and safe materials for storing hydrogen as a clean fuel alternative.
The excellent biocompatibility of water-based frameworks positions nanoporous ices as potential vehicles for drug delivery and other medical applications 1 . Unlike synthetic nanoparticles that may trigger immune responses or toxicity, ice-based delivery systems would comprise the body's most natural solvent—water—as their primary structural component.
Potential applications include targeted drug delivery, controlled release systems, and even tissue engineering scaffolds where the ice structures could serve as templates for biological growth.
In materials science, nanoporous ices could serve as templates for creating other porous materials or as model systems for studying confinement effects. Their ability to influence freezing properties based on ice structure could lead to new hydrosphere materials with tailored characteristics 8 .
Additional applications might include environmental remediation (capturing pollutants), catalysis (providing confined reaction environments), and even information storage through controlled phase transitions.
| Aspect | Theoretically Predicted | Experimentally Confirmed |
|---|---|---|
| Number of Structures | Multiple stable phases predicted at negative pressures | Two confirmed structures (ice XVI and XVII) 4 |
| Stability Range | Guest-free sII and sH clathrates predicted to be stable at negative pressures | Ice XVI stable below 145K 4 |
| Formation Method | Direct nucleation from liquid water predicted unlikely | Successful formation via guest removal from clathrate templates 4 |
| Future Research | More reliable structures and phase diagrams predicted with machine-learning force fields 1 | Many predicted structures await experimental confirmation 1 |
The discovery and development of nanoporous ices reminds us that even the most familiar substances still hold profound secrets waiting to be unlocked. As researcher Xiao Cheng Zeng notes, "The field of porous ices is still emerging" 1 , with many computationally predicted structures awaiting experimental confirmation.
What makes this field particularly exciting is its interdisciplinary nature—combining elements of materials science, chemistry, physics, and engineering to explore water's hidden capabilities. As machine-learning methods advance and force fields become more accurate, researchers anticipate discovering even more members of this unique ice family 1 .
The story of nanoporous ices exemplifies how fundamental scientific exploration—driven by curiosity about nature's building blocks—can lead to unexpected technological revolutions. From clean energy solutions to biomedical breakthroughs, these fascinating structures demonstrate that sometimes, the most revolutionary materials are hiding in plain sight, in the most ordinary of substances: water.
As we continue to unravel the mysteries of these remarkable materials, one thing becomes increasingly clear: when it comes to water, we've only begun to scratch the surface of its hidden potential.
Key milestones in nanoporous ice research