Adsorbate layering in narrow-pore materials

The conditions of layering of adsorbate molecules in porous systems with characteristic sizes of from 1 to 50–100 nm are discussed. The porous systems contain both very narrow pores, in which interaction potentials of pore walls overlap, and comparatively broad pores without overlapping of surface potentials. Three pore size intervals are distinguished. In the first interval, no adsorbate layering occurs, the second interval is characterized by capillary condensation with critical parameters different from their volume values, and, in the third interval, capillary condensation conditions are almost the same as in the volume adsorbtive phase. Criteria of the characteristic pore sizes of different geometries are formulated; the criteria correspond to small volumes in which first-order phase transitions are absent. The boundary between the first and second pore size regions is observed experimentally as the disappearance/appearance of hysteresis loops in adsorption-desorption isotherms measured under strictly equilibrium conditions as the size of pores decreases/increases. A nonuniform distribution of the surface potential is shown to be responsible for the multiplicity of local regions in porous media with their own vapor-liquid coexisting phases. The spinodal transitions in adsorption-desorption in pores can occur between various local regions. An analysis is performed in terms of the lattice gas model with short-range Lennard-Jones interaction of adsorbate molecules with each other and adsorbent walls.