The local structure factor near an interface; Beyond extended Capillary-Wave models

Abstract

We investigate the local structure factor $S(z;q)$ at a free liquid-gas interface in systems with short-ranged intermolecular forces and determine the corrections to the leading-order, capillary-wave-like, Goldstone mode divergence of $S(z;q)$ known to occur for parallel wavevectors $q\to 0$. We show from explicit solution of the inhomogeneous Ornstein-Zernike equation that for distances $z$ far from the interface, where the profile decays exponentially, $S(z;q)$ splits unambiguously into bulk and interfacial contributions. On each side of the interface, the interfacial contributions can be characterised by distinct liquid and gas wavevector dependent surface tensions, ${\sigma }_l(q)$ and ${\sigma }_g(q)$, which are determined solely by the $bulk$ two-body and three-body direct correlation functions. At high temperatures, the wavevector dependence simplifies and is determined almost entirely by the appropriate bulk structure factor, leading to positive rigidity coefficients. Our predictions are confirmed by explicit calculation of $S(z;q)$ within square-gradient theory and the Sullivan model. The results for the latter predict a striking temperature dependence for ${\sigma }_l(q)$ and ${\sigma }_g(q)$, and have implications for fluctuation effects. Our results account quantitatively for the findings of a recent very extensive simulation study by Höfling and Dietrich of the total structure factor in the interfacial region, in a system with a cut-off Lennard-Jones potential, in sharp contrast to extended Capillary-Wave models which failed completely to describe the simulation results.

20 pages, 9 figures (submitted to J. Phys: Condens. Matter)

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• Drying and Wetting Transitions of a Lennard-Jones Fluid: Simulations and Density Functional Theory
• Why Noether's Theorem applies to Statistical Mechanics
• Capillary Wave Theory of Adsorbed Liquid Films and the Structure of the Liquid-Vapor Interface
• Finite-size corrections for the static structure factor of a liquid slab with open boundaries
• Structure of liquid--vapor interfaces: perspectives from liquid state theory, large-scale simulations, and potential grazing-incidence X-ray diffraction
• Microscopic determination of correlations in the fluid interfacial region in the presence of liquid-gas asymmetry
• Correlation function structure in square-gradient models of the liquid-gas interface: Exact results and reliable approximations