## self-diffusion coefficient

https://doi.org/10.1351/goldbook.S05582
The diffusion coefficient Di* of species i in the absence of a chemical potential gradient. It is related to the diffusion coefficient Di by $D_{i}^{*}=D_{i}\ \frac{\partial (\ln c_{i})}{\partial \ln a_{i}}$ where ai is the activity of i in the solution, and ci is the concentration of i. If an isotopically labelled species (i*) is used to study diffusion, the tracer diffusion coefficient, Di*, is practically identical to the self-diffusion coefficient provided that the isotope effect is sufficiently small.
Source:
PAC, 1972, 31, 577. 'Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry' on page 617 (https://doi.org/10.1351/pac197231040577)