Diffusion

The process by which sorbate molecules move in the host material, due to random molecular motion. Usually quantified by a diffusion coefficient to express how mobile the sorbed phase is. The diffusion coefficient is a function of temperature and concentration, and can be measured from sorption versus time curves in defined conditions during isothermal gravimetric experiments.

The chemical diffusion coefficient, DC, which is the magnitude of the flux of sorbed phase from one part of the material to another, is in the simplest model described by Fick’s Law. The mobility of the sorbed phase at the microscopic level is described by the self-diffusion coefficient, DS.

N.B. The nomenclature for diffusivity varies between different application and science disciplines.

Diffusion coefficients are not the same as permeation rates. Permeation is a function of both diffusivity and solubility.