Figure 4. Mass transport and diffusion: towards a free volume theory.

(a) Comparison of the self (D_s) and collective (D₀) diffusion. Inset: Scaling of the permeance K with the self-diffusion. (b) Rescaled diffusion coefficient D_s × n, with the alkane length n, versus the free volume fraction . The latter is calculated independently for a given adsorbed amount Γ, see Supplementary Information. The dashed line is the prediction of the free volume theory, , see text. Inset: bare data for the self-diffusion coefficient for the various alkanes. The colour code is the same as Fig. 1: methane (black), propane (blue), hexane (green), nonane (yellow) and dodecane (red).