  The dielectric constant of interfacial water has been predicted to be smaller
than that of bulk water (= 80) because the rotational freedom of water dipoles
is expected to decrease near surfaces, yet experimental evidence is lacking. We
report local capacitance measurements for water confined between two
atomically-flat walls separated by various distances down to 1 nm. Our
experiments reveal the presence of an interfacial layer with vanishingly small
polarization such that its out-of-plane dielectric constant is only
approximately 2. The electrically dead layer is found to be two to three
molecules thick. These results provide much needed feedback for theories
describing water-mediated surface interactions and behavior of interfacial
water, and show a way to investigate the dielectric properties of other fluids
and solids under extreme confinement.
