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:

$$D_i^*=D_i\frac{\partial\ln c_i}{\partial\ln a_i}.$$

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.