  The density profiles of liquid/vapor interfaces of water-alcohol (methanol,
ethanol and propanol) mixtures were studied by surface sensitive synchrotron
X-ray scattering techniques. X-ray reflectivity and diffuse scattering
measurements, from the pure and mixed liquids, were analyzed in the framework
of capillary-wave theory to address the characteristics length-scales of the
intrinsic roughness and the shortest capillary-wavelength (alternatively, the
upper wave-vector cutoff in capillary wave theory). Our results establish that
the intrinsic roughness is dominated by average interatomic distances. The
extracted effective upper wave-vector cutoff indicates capillary wave theory
breaks-down at distances on order of bulk correlation lengths.
