Abstract
Adsorption on single planar walls and filling of slits with identical planar walls are investigated in the frame of the density functional theory. In this sort of slits the external potential is symmetric with respect to its central plane. Calculations were carried out by applying both the canonical and grand canonical ensembles (CE and GCE, respectively). The behavior is analyzed by varying the strength of the adsorbate-substrate attraction, the temperature T, and the coverage Γℓ. Results obtained for physisorption of Xe on alkaline surfaces are reported in the present work. Prewetting (PW) lines and wetting temperatures, Tw, are determined from the analysis of adsorption on single walls. The filling of slits is analyzed for temperatures T > Tw. It is found that whenever for a given Xe-substrate combination the adsorption on a single wall exhibits a first-order wetting transition then asymmetric profiles that break the left-right symmetry of the external potential appear in the filling of an equivalent slit. These spontaneously symmetry breaking (SSB) solutions occur in a restricted range of Γℓ with a T-dependent width. In the case of closed slits analyzed in the CE scheme, the obtained asymmetric profiles exhibit lower Helmholtz free energies than the symmetric species and, therefore, could be stabilized in this geometry. For open slits, the GCE scheme yields all the symmetric and SSB states in the corresponding convex regimes of the free energy. It is shown that both the CE and the GCE frames yield three coexistent states, two symmetric and one asymmetric twofold degenerate. Both a PW line and the related SSB effect terminate at the same temperature. For rather strongly attractive surfaces reentrant SSB states are found at a fixed value of T.
Highlights
The study of the adsorption of fluids on solid walls is of considerable current interest
This theory is based on the existence of a grand canonical functional (T, μ, [ρ(r)]) of the one-particle density ρ(r) in the system, which depends parametrically on the two thermodynamic variables T and μ19 (T, μ, [ρ(r)]) = FDF(T, [ρ(r)]) − μ N, (2)
Since in the Canonical Ensemble (CE) scheme the calculations are performed at a fixed n, whilst μce is an unknown quantity, Eq (18) is used to evaluate μce and ρ0 is obtained from the integrated density profile n = ρ0
Summary
Sikkenk et al.[4,5] have found by carrying out molecular dynamics calculations that asymmetric density profiles of the fluid may appear in the process of filling a planar slit built up of moderately attractive identical walls This phenomenon is known as spontaneous symmetry breaking (SSB) effect and disappears above a critical temperature Tsb. The authors of Ref. 4 explained the occurrence of structures with different symmetries in terms of the balance of substrate-liquid γ sl, substrate-vapor γ sv, and liquid-vapor γ lv surface tensions. Adsorbate-substrate combination the adsorption on a single planar wall exhibits a first-order wetting transition asymmetric profiles appear by filling an equivalent slit, and both these phenomena PW and SSB terminate at the same temperature.
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