Self-diffusion describes the diffusive motions of molecules within themselves e.g. the movement of a water molecule in water.
According to the IUPAC definition,[1] the self-diffusion coefficient
of a chemical species in said medium when the concentration of this species is extrapolated to zero concentration.
It can be described by the equation[2]:
Due to challenges observing it directly it is commonly assumed to be equal to the diffusion of an isotope in the medium of interest.
[3] However modern simulations are able to estimate it directly without the need for isotope labeling.
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