Density functional study of layering at liquid surfaces.
We analyze the density profiles for liquid-vapor interfaces within two density functional (DF) approximations, applied to simple fluid models which have low ratios between their triple and critical temperatures. The observation of layering structures at low T is discussed in relation with the Fisher-Widom line for each model. Although we find no apparent correlation between the amplitude of the oscillatory density decay mode and the approach to T(FW) , that temperature sets a threshold for the generation of nonmonotonic structures within a fixed distance of the interface. The rapid decay of the oscillatory mode amplitude with T may be interpreted as a result of the capillary wave (CW) damping of strongly structured intrinsic density profiles. The layering in the presence of gravitylike external fields indicate that the effective transverse size which might be built in the DF approximations is around 10+/-2 molecular diameters; however, that interpretation has to allow for an effectively reduced damping exponent, i.e., an effective surface tension for the CW Hamiltonian which is larger than the value obtained directly from the DF grand-potential minimization.
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Departamento de Física Teórica de la Materia Condensada (C-V), Universidad Autónoma de Madrid, E-28049 Madrid, Spain.
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