Hydrogen bonding in water under extreme confinement.

Hydrogen bonding in water shifts from bulk-like to non-H-bonded states when confined inside carbon nanotubes smaller than 2 nm, with vibrational frequencies approaching those of water vapor.

• Why it matters: Understanding water's behavior under extreme confinement is crucial for fields like nanofluidics, material science, and biological systems, yet direct molecular-level insights have been limited.
• What they did: Using in situ vibrational electron energy loss spectroscopy (vEELS) with nanoscale resolution, the study examined water in individual CNTs of varying diameters, analyzing vibrational frequency shifts and structural transitions.
• The result: Findings demonstrate that as CNT diameter decreases, water's hydrogen bonds weaken, approaching a non-H-bonded state, revealing the quantum and dynamic nature of confined water and informing future nanoconfined fluid research.