Self-diffusion explained

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 medium is the 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]

$$D\_i^*\; =\; D\_i\backslash frac$$

Here,

is the activity of the medium (e.g. water) in the system (e.g. solution) and is the concentration of medium . Due to challenges observing it directly it is commonly assumed to be equal to the diffusion of an isotopically different molecule of the medium in the medium of interest e.g. a molecule of deuterated water in water.^[3] However modern simulations are able to estimate it directly without the need for isotope labeling.^[4]

See also

• Brownian motion
• Diffusion
• Molecular diffusion

Notes and References

1. Web site: IUPAC Compendium of Chemical Terminology 'The Gold Book'. 2. 1997. Blackwell Scientific Publications. S. J. Chalk. 10.1351/goldbook.S05582 . A. D. McNaught, A. Wilkinson.
2. Everett . D. H. . 1972-01-01 . Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry . Pure and Applied Chemistry . en . 31 . 4 . 577–638 . 10.1351/pac197231040577 . 1365-3075. subscription .
3. Mills . R. . March 1973 . Self-diffusion in normal and heavy water in the range 1-45.deg. . The Journal of Physical Chemistry . en . 77 . 5 . 685–688 . 10.1021/j100624a025 . 0022-3654. subscription .
4. Bizzarri . Anna Rita . 1996 . Molecular dynamics simulation evidence of anomalous diffusion of protein hydration water . Physical Review E . 53 . 4 . R3040–R3043 . 10.1103/PhysRevE.53.R3040. 9964761 . 1996PhRvE..53.3040B . subscription .