  Water plays a crucial role in geological, biological, and technological
processes. Nanoscale water confinement occurs in many of these settings,
including sedimentary rocks, water channel proteins, and applications like
desalination and water purification membranes. The structure and properties of
water in nanoconfinement can differ significantly from bulk water, exhibiting,
for instance, modified hydrogen bonds, dielectric constant, and phase
transitions. Despite the importance of strongly nanoconfined water,
experimentally elucidating the nanoconfinement effect on water, such as its
orientation and hydrogen bond (H-bond) network, has remained challenging. Here,
we study two-dimensionally nanoconfined aqueous electrolyte solutions with
tunable confinement from nanoscale to angstrom-scale sandwiched between a
graphene sheet and CaF2. We employ heterodyne-detection sum-frequency
generation (HD-SFG) spectroscopy, a surface-specific vibrational spectroscopy
capable of directly and selective probing water orientation and H-bond
environment at interfaces and under confinement. Remarkably, the vibrational
spectra of the nanoscale confined water can be described quantitatively by the
sum of the individual water surface signals from the CaF2/water and
water/graphene interfaces until the confinement reduces to angstrom-scale (< ~8
{\AA}). Ab initio molecular dynamics simulations confirm our experimental
observation. These results manifest that interfacial, rather than
nanoconfinement effects, dominate the water structure until angstrom-level
confinement.
