Self-diffusion
# 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.

