Abstract

It has been recently suggested that many structures in nature have fractal geometries: polymers¹, diffusion limited aggregates², sandstones³, Vycor glasses⁴ and various irregular surfaces⁵. Avnir et al.^5,6 have demonstrated that for porous surfaces the concept of surface area is not meaningful. The area as measured by molecular adsorption changes with the size of the adsorbate molecules (the yardsticks). It has been proposed^5,6 that many porous surfaces are characterized by their fractal dimension d̅ in a way which relates the number of adsorbed molecules to their cross sectional areas (yardstick sizes), σ, There have been only a few works^7-9 which study the dynamics of molecules adsorbed in Vycor glasses and on porous silica gels. However no attempt has been made to relate the observations to a geometrical parameter. Interestingly enough both porous Vycor and porous silica gel have been proposed as fractal systems.^4,5

© 1985 Optical Society of America