Structure and adsorption at gas–solid interfaces: Layering transitions from a continuum theory

Packing effects in liquid-like films, adsorbed from bulk gas at the interface with an attractive solid substrate, may cause the fluid to adopt layer-like structure near the substrate. Such structure is inherent in the lattice gas model of adsorption where it is known to give rise to a sequence of layering transitions at low temperatures, provided the substrate (wall) is sufficiently attractive. We have used a nonlocal density functional theory to describe a continuum fluid near strongly attractive structureless walls. The theory yields oscillatory density profiles arising from packing effects. At low temperatures T (typically 0.5≲T/Tc ≲0.6, where Tc is the bulk critical temperature) the adsorption may increase with pressure in a series of discontinuous jumps, each corresponding to the appearance of a new dense layer in the adsorbed film. The transitions are first order, with very small metastable regions. As in the lattice gas model the transitions exhibit critical points; above the critical temperature of a given layer the adsorption increases smoothly with increasing pressure. However, we anticipate certain qualitative differences between the transitions we observe in the continuum theory and those of the lattice gas model, since the latter overemphasizes the role of hard-core repulsion. Layering transitions also occur for fluids confined in narrow pores. We find that in cylindrical pores the transitions are shifted to lower pressures. Moreover, capillary condensation (the first order transition to a ‘‘liquid’’ that fills the pore) competes with layering resulting in a rapid truncation of the sequence of layering transitions. The implications of our results for the interpretation of adsorption measurements performed near a bulk triple point are discussed and possible surface phase diagrams are suggested.