Layering at Free Liquid Surfaces

Abstract

We show that simple liquids, with appropriate choices of the isotropic pair interaction, may exhibit surface layering above the melting temperature. Results for the liquid surface have been obtained by Monte Carlo simulations in slab geometry. Surface layering appears at temperatures below approximately one-third of the critical temperature for very different choices of pair interaction. The high melting temperature of the Lennard-Jones crystal preempts the observation of the oscillatory density at the free liquid interface, while model pair interaction potentials, built to reproduce some properties of mercury and the alkali metals, have low melting temperature, uncovering the region of surface layering.

Intrinsic profiles and the structure of liquid surfaces

“…The strongly structured density profiles obtained for cold liquids made possible the use of a gaussian deconvolution to get shaper intrinsic density profiles [13]. Similar intrinsic profiles were extracted from the experimental X-ray reflectivity data for Hg [10] and Ga [11].…”

“…The first unequivocal experimental evidence appeared in 1995, from the analysis of the X-ray reflectivity of Hg and Ga [10,11]. Earlier DF results had shown weak layering effects for some liquid models [12], but the development of several cold-liquid models, with pairwise isotropic interactions but very low ratios, T t /T c ≈ 0.1, between the temperatures of the triple and the critical points, showed that the strong layering in ρ(z; A o ) was not specific of liquid metals [13]. The effects of CW fluctuations on oscillatory profiles had already been analyzed by Evans [14].…”

“…The effects of CW fluctuations on oscillatory profiles had already been analyzed by Evans [14]. The amplitude of the oscillations in ρ(z; A o ) is damped by a factor L −η x , with an exponent η = kT /(πγσ 2 ) that is usually larger than 4, but it may be as low as η ≈ 1 for cold liquids at T ≈ 0.1T c ; so that the oscillations are preserved in ρ(z; A o ) even for large A o [13].…”

Local structure of liquid–vapour interfaces

“…The first is a truncated and shifted Lennard-Jones (LJ) liquid [28,29] is the critical temperature. To observe a stronger effect of the liquid-gas interface, we also consider a model potential which approximates Sodium (Na), whose triple point lies at a rather lower temperature relative to criticality of T N a tr /T N a c = 0.22 [8,10]. This model is known to exhibit strong surface layering around the triple point, which is also seen in ab initio simulations [11].…”

“…However, further simulations [6] and density functional theory [2,7] showed that little surface layering is expected for materials based on van der Waals interactions, and that the interfacial profile is dominated by a smooth change in density. Relative to the critical temperature T c , metals typically remain liquid at rather lower temperatures than Lennard-Jones like materials, and here pronounced surface layering is indeed found [7][8][9][10][11]. An important related problem to the free interface is the confinement introduced by a hard wall, crystal or similar external field, which may be tackled with density functional theory [7,12], simulation [13] and experiment [14,15].…”