  Fluids under extreme confinement exhibit distinctly new properties compared
to their bulk analogs. Understanding the structure and intermolecular bonding
of confined water lays the foundation for creating and improving applications
at the water-energy nexus. However, probing confined water experimentally at
the length scale of intermolecular and surface forces has remained a challenge.
Here, we report a combined experiment/theory framework to reveal changes in
H-bonding environment and the underlying molecular structure of confined water
inside individual carbon nanotubes. H-bonding is directly probed through the
O-H stretch frequency with vibrational electron energy-loss spectroscopy and
compared to spectra from molecular-dynamics simulations based on
density-functional-theory. Experimental spectra show that water in larger
carbon nanotubes exhibit the bonded O-H vibrations of bulk water, but at
smaller diameters, the frequency blueshifts to near the 'free' O-H stretch
found in water vapor and hydrophobic surfaces. The matching simulations reveal
that, in addition to steric confinement, the tube's vibrations play a key role
in breaking up the H-bond network, resulting in an orientationally-dispersed,
non-H-bonded phase. Furthermore, the temperature-dependence of the vibrations
is investigated, providing insights into phase transitions and the
confined-water density. This research demonstrates the potential of the
experiment/theory framework to explore unprecedented aspects of structure and
bonding in confined fluids.
