Percolation and diffusion in two-dimensional microporous media: Pillared clays

Abstract

We have investigated the adsorptive and diffusive properties of N2, H2O, and rare gas atoms (Ar and He) in the pillared layered silicate clay systems [Cr(en)3+3]x[Co(en)3+2−(en)]1−x−L, where L is vermiculite (V), fluorohectorite (FHT), or montmorillonite (M), and (en) is an ethylenediamine ligand. In these mixed ion intercalates the intercalated [Cr(en)3+3] cation, where all three en ligands are coordinated to chromium, represents a laterally small pillaring agent, whereas [Co(en)3+2−en] represents a laterally large, ligand-dissociated species. Such systems are excellent models for two-dimensional microporous media. Adsorption measurements were carried out for N2, H2O, and Ar and diffusion studies were performed using simulation methods for both Ar and He. We find that the adsorptive and diffusive properties depend sensitively on the size of the diffusing species and the concentrations x and (1−x) of the intercalants. For Ar adsorption in the FHT system we observe a percolative response when x reaches 0.79. Using simple geometrical models to describe these microporous media, along with computer simulation, we can understand the x=0.79 percolation threshold. In addition, simulation studies of the relative diffusion rates of He and Ar for x=0 and 1, and comparison of these rates with experimental measurements by Zhou and Solin, suggest that He diffusion near narrow constrictions may be strongly suppressed by quantum effects.