SORPTION HYSTERESIS AND NETWORK STRUCTURE OF PORES IN POROUS SUBSTANCES .1. SIMULATION OF CAPILLARY CONDENSATION IN ADSORPTION PROCESS

Pore size distribution in porous substances has so far been analyzed on the conventional assumption that each pore is isolated from other pores. This paper proposes a three-dimensional network structure (TNS) model for a more realistic analysis of the distribution taking account of pore structure. The TNS pore system comprises cylindrical unit pores, which are of various sizes and are randomly distributed on each site of an appropriate lattice model (Table 1). It was confirmed experimentally that variation in the thickness of sorbed layers could be neglected in a first approximation (Figs. 1~4). This finding allows us to describe the state of a sorbate in terms of the sorbed layer on the pore wall and the condensate on the layer. The condensation of the sorbate in unit pores in the TNS system can proceed in three routes (reversible route I, irreversible I, and intermediate III). The route and the vapor pressure for condensation in a unit pore depend not only on the pore size but also on the environment, i. e., whether the adjacent unit pores have already been filled with liquid or not. Based on the TNS model, the process of capillary condensation with increasing vapor pressure was simulated by a digital computer (Figs. 5~7). The results revealed that more than half of unit pores, particularly, most of larger ones were filled via route I (Figs. 8, 9, and Table 2). Accordingly, it was concluded that for a practical analysis of pore size distribution, not a cylindrical meniscus but a spherical one should be assumed with respect to the adsorption branch of isotherms regardless of the mode of network structure. © 1979, The Chemical Society of Japan. All rights reserved.