Two Modes of Diffusion in Liquids

We propose that in the case of diffusion in liquids two modes of diffusion are present. The first mode, a flow mechanism, resulting from the movement of the solvent molecules, is the only one available to large diffusants having molecular weights at least an order of magnitude larger than that of the solvent. Smaller diffusants can use this mode but also move by activated diffusion within the lattice of the solvent matrix. The form of the variation of the activation energy for diffusion with size of the diffusant is discussed. Semiempirical calculations of diffusion coefficients and activation energies for several (small to medium‐sized) diffusants in water are performed and the results are compared with available data.