The stochastic dynamics of a hydrocarbon molecule moving in a solid body were examined, and a generalized Langevin equation which described the evolution of the coordinates of its center of mass, were derived for the case where the molecule behaved as a rigid linear entity. An explicit expression was obtained, for the friction tensor, which accounted for both molecule/solid interactions and for thermal vibrations of the solid’s lattice. The diffusion coefficient of the hydrocarbon molecule was calculated as a function of the number of C atoms which it contained. The diffusion coefficient exhibited a periodic dependence upon the number of C atoms; with maxima appearing whenever the length of the molecule was an integer multiple of the length of the repeating unit of the lattice, and with minima appearing when it was a half-integer multiple. This phenomenon was called resonant diffusion. The results were compared with experimental data on the diffusion of n-paraffins through zeolite-T, and a qualitative agreement was observed.

R.Tsekov, E.Ruckenstein: Journal of Chemical Physics, 1994, 100[5], 3808-12