CAPILLARY CONDENSATION IN STRUCTURED PORES

We investigate capillary condensation in slit-like pores with structur ed walls, described by a simple lattice gas model based on the mean-fi eld approximation. Most theoretical workers did not go beyond very sim ple capillary models with perfect symmetry, thus being unable to map e xperiments in real pores with rather complex structure. In this paper we deal with chemical inhomogeneity, produced by a periodic wall poten tial, that causes fluid attraction and repulsion altering along the wa lls. We observe a new mechanism of capillary condensation, characteriz ed by a splitting of the equilibrium ''gas-liquid'' transition, and we concentrate on the crucial role the typical length for the wall inhom ogeneity, lambda, plays compared with the pore width, H. Our microscop ic results are confirmed by a macroscopic analysis that leads to a mod ified Kelvin equation, giving a quantitative prediction of capillary c ondensation in structured pores. Furthermore our model explains the na ture of the critical isochore as measured in mesopore condensation exp eriments by Thommes and Findenegg. (C) 1996 American Institute of Phys ics.