A fractal analysis of adsorption processes by pillared swelling clays

We have applied the concept of fractal dimension D of solid surfaces, developed by Pfeifer and Avnir [J. Chem. Phys. 79, 3566 (1983)] to adsorption on pillared clays, namely, on layer lattice silicates in which the layers are propped apart by large organic or inorganic cations. D was determined either from the radius dependence of the saturation coverages of various yardstick molecules on the same pillared clay, or, alternatively, from the saturation coverages of one type of yardstick molecule (nitrogen) on various surfaces covered with pillars of increasing size. Both methods yield D≂2, or slightly below 2, within experimental error, in the range ∼4–16.5 Å. This confirms the common assumption that mica-type surfaces are molecularly smooth, and shows that the distribution of pillars in the pillared clays that we examined is close to homogeneity. The relation between D, microporosity, and molecular sieving is briefly discussed.