  Two different theories are used to understand the liquid-vapor interfaces:
the Van der Waals theory and the capillary waves theory. But comparing both
come up a problem of interpretation of the interface density profiles obtained,
for example, with the Density Functional Theory (DFT). As a consequence emerge
the question of how the surface fluctuations are included on traditional
density profiles (usually named equilibrium density profiles). Last years, new
insights on the role of capillary waves were possible by analyzing X-ray
reflectivity experiments and performing computer simulations of liquids with
low melting temperature. In particular, the density profile exhibits a layering
structure which is considered a key property to elucidate a new interpretation
of those profiles as intrinsic density profiles. This dissertation aims to
investigate these questions within the DFT using simple fluids with a pairwise
interactions that reproduce important phase-diagram properties of liquid
metals. Two generics questions were explored: the relevance of Fisher-Widom
line and the role of capillary waves on the nature of interface obtained with
approximations WDA and FMT. It hypothesized the existence of a density profile
with strong layering properties whose structure is reduced by capillary waves.
Then the effect of surface fluctuations is described by introducing an
effective transversal size which imposes a limitation of the spectrum of
surface fluctuations incorporated on the DFT. However, an explicit methodology
to unfreeze the capillary waves over a postulated intrinsic profile exempt of
surface fluctuations was proven still a challenge. As a consequence it
suggested that other previous results describing the liquid surface using the
equilibrium DFT may conduct to unphysical properties.

Statistical Mechanics