In water, molecules dance hand in hand: small rings open up, partners switch places, and groups come together again. Their “hands” are hydrogen bonds, constantly forming and breaking.
Inside a very narrow nanotube, the confined space and its walls change the choreography: many hands are left without a partner to hold on to. So what kind of dance emerges? A New Year’s Eve conga line, with the molecules in single file? A solo freestyle? Or a couples’ dance where partners keep changing?
The question reaches far beyond a tiny tube. Water moves through nanometre-scale spaces in cellular membrane channels, porous materials, and membranes designed to filter it. In these spaces, interactions between molecules and surfaces help shape how water moves. Understanding them helps us interpret biological processes and design materials that can control water transport.
Carbon nanotubes provide a model system for studying these interactions. In a recent paper published by NatureXintong Xu and colleagues measured the vibrations of water inside individual nanotubes, detecting the energy transferred from electrons to molecular vibrations.
In a tube with an inner diameter of 2.3 nanometres, the signal resembles that of bulk water. In a 1.4-nanometre tube, by contrast, vibrations characteristic of O–H groups only weakly involved—or not involved at all—in hydrogen bonds become dominant. In our metaphor, the group dance loses cohesion: many hands let go, though that doesn’t necessarily mean every dancer is dancing alone. The signature is similar to one observed in vapour, but this does not show that the water has become gaseous.
Even when cooled, hydrogen bonds in the narrower tube are only partially restored, and vary from one region to another.
By measuring water inside individual nanotubes, researchers have shown how strongly the available space can alter the bonds between its molecules. The dance changes, but the dancers’ arrangement remains to be clarified. Understanding it will help explain how water behaves in tiny spaces that matter for life and filtration technologies.
