Self diffusion

According to IUPAC definition, self-diffusion coefficient is the diffusion coefficient D i ∗ of species i when the chemical potential gradient equals zero. It is linked to the diffusion coefficient D i by the equation:

D i ∗ = D i ∂ ln ⁡ c i ∂ ln ⁡ a i .

Here, a i is the activity of the species i in the solution and c i is the concentration of i . This term is commonly assumed to be equal to the tracer diffusion determined by watching the movement of an isotope in the material of interest.