Editing Molecular diffusion (section)

== Tracer, self- and chemical diffusion ==
[[File:Diffusion self1.svg|thumb|340px|Self diffusion, exemplified with an isotopic tracer of radioactive isotope <sup>22</sup>Na]]
[[File:Diffusion4.svg|thumb|340px|Example of chemical (classical, Fick's, or Fickian) diffusion of sodium chloride in water]]
<!-- "Wasser" should be "Water" in second figure. -->Fundamentally, two types of diffusion are distinguished:
* ''Tracer diffusion'' and ''Self-diffusion'', which is a spontaneous mixing of molecules taking place in the absence of concentration (or chemical potential) gradient. This type of diffusion can be followed using [[Isotopic labeling|isotopic tracers]], hence the name. The tracer diffusion is usually assumed to be identical to [[self-diffusion]] (assuming no significant [[Kinetic isotope effect|isotopic effect]]). This diffusion can take place under equilibrium. An excellent method for the measurement of [[self-diffusion]] coefficients is [[pulsed field gradient]] (PFG) [[NMR]], where no isotopic tracers are needed. In a so-called NMR [[spin echo]] experiment this technique uses the nuclear spin precession phase, allowing to distinguish chemically and physically completely identical species e.g. in the liquid phase, as for example water molecules within liquid water. The self-diffusion coefficient of water has been experimentally determined with high accuracy and thus serves often as a reference value for measurements on other liquids. The self-diffusion coefficient of neat water is: 2.299·10<sup>−9</sup>&nbsp;m<sup>2</sup>·s<sup>−1</sup> at 25&nbsp;°C and 1.261·10<sup>−9</sup>&nbsp;m<sup>2</sup>·s<sup>−1</sup> at 4&nbsp;°C.<ref name =wasser>{{cite journal | last1=Holz | first1=Manfred | last2=Heil | first2=Stefan R. | last3=Sacco | first3=Antonio | title=Temperature-dependent self-diffusion coefficients of water and six selected molecular liquids for calibration in accurate 1H NMR PFG measurements | journal=Physical Chemistry Chemical Physics | publisher=Royal Society of Chemistry (RSC) | volume=2 | issue=20 | year=2000 | issn=1463-9076 | doi=10.1039/b005319h | pages=4740–4742| bibcode=2000PCCP....2.4740H }}</ref>
* ''Chemical diffusion'' occurs in a presence of concentration (or chemical potential) gradient and it results in net transport of mass. This is the process described by the diffusion equation. This diffusion is always a non-equilibrium process, increases the system entropy, and brings the system closer to equilibrium.

The [[Mass diffusivity|diffusion coefficient]]s for these two types of diffusion are generally different because the diffusion coefficient for chemical diffusion is binary and it includes the effects due to the correlation of the movement of the different diffusing species.