Adsorption of Ne on alkali surfaces studied with a density functional theory

Asymmetric profiles and prewetting lines in the filling of planar slits with Ne

The filling of slits with identical planar walls is investigated in the frame of the density functional theory. For this kind of slit, the confining potential is symmetric with respect to its central plane. Closed and open systems are studied by applying, respectively, the canonical and grand canonical ensembles (CE and GCE). Results obtained for the confinement of fluid Ne by alkaline surfaces are reported. The behavior of these systems is analyzed by varying the strength of the Ne-substrate attraction, the temperature T, and the coverage Γ(l). It is assumed that the one-body density of the fluid, ρ(r), is uniform along the (x, y) planes parallel to the walls, becoming a function of the coordinate z perpendicular to those planes. Two sorts of solutions are found for the density profile: (i) symmetric ones that follow the left-right symmetry of the potential exerted by the walls and (ii) asymmetric ones that break the symmetry of the slit. The pores are wide enough for determining prewetting (PW) lines and wetting and critical PW temperatures, i.e., T(w) and T(cpw), from the analysis of symmetric solutions provided by both the CE and GCE schemes. Asymmetric species are examined in detail for T > T(w). It is shown that for a given Ne-substrate pair at a fixed T both the CE and GCE frames yield only one asymmetric 2-fold degenerate stable profile (formed by a "thin" wetting film at one wall and a "thick" wetting film at the other) coexisting with two symmetric profiles (formed by "thin" or "thick" wetting films at the two walls), while the remaining asymmetric states are at best metastable. This feature occurs along PW lines and disappears at T(cpw).

Confinement of Ar between two identical parallel semi-infinite walls

The confinement of Ar in planar slits of two identical parallel semi-infinite walls of alkali metals, alkaline-earth metal Mg, and CO(2) is investigated within the framework of the density functional theory. It is assumed that (1) the fluid atoms interact via a recently proposed effective attractive pair potential with strength, epsilon(ff), which reproduces the experimental data of the surface tension of the liquid-vapor interface at the bulk coexistence curve, and (2) the adsorption on the walls is described by ab initio potentials characterized by a well depth, W(sf). In this way the systems were studied in the framework of a realistic approach. We found that for small coverages, the slit is always filled by forming two symmetric vapor films, one at each wall. For increasing coverage the behavior depends on the ratio W(sf)/epsilon(ff) and the temperature T. In the case of alkali metals, we found at the triple point, T(t), of the adsorbate a regime of average density rho(av) ( *) in which the ground state exhibits asymmetric density profiles, leading to the so-called spontaneous symmetry breaking (SSB) effect. The SSB appears at an average density rho(sb1) ( *) and disappears at a higher average density rho(sb2) ( *). When T is increased, the range of densities rho(sb1) ( *)<or=rho(av) ( *)<or=rho(sb2) ( *) diminishes and eventually the SSB disappears at a critical temperature, T(sb), which coincides with the critical prewetting temperature T(cpw) observed in the adsorption on a single wall. For T>T(cpw) the slit is filled symmetrically up to the phase transition to capillary condensation. All these features are examined as a function of the strength of the substrate and the width of the slit. Furthermore, no SSB effect was found for Mg and CO(2).

Adsorption of Ar on planar surfaces studied with a density functional theory

The adsorption of Ar on planar structureless substrates of alkali metals, alkaline-earth metal Mg, CO2 , and Au was analyzed by applying a density functional formalism which includes a recently proposed effective attractive pair potential conditioned to Ar. It is shown that this approach reproduces the experimental surface tension of the liquid-vapor interface over the entire bulk coexistence curve for temperatures T spanning from the triple point Tt up to the critical point Tc. The wetting properties were studied over the entire range temperatures Tt<-->Tc. It was found that Ar wets all the investigated surfaces. The adsorption isotherms for alkali metals exhibit first-order phase transitions. Prewetting lines were resolved even for the less attractive surfaces. In the cases of Mg, CO2 , and Au a continuous growth for T> or =Tt was obtained. A comparison with experimental data and other microscopic calculations is reported.