More accurately, the diffusion coefficient times the local concentration is the proportionality constant between the negative value of the mole fraction gradient and the molar flux.
This distinction is especially significant in gaseous systems with strong temperature gradients.
Diffusivity derives its definition from Fick's law and plays a role in numerous other equations of physical chemistry.
The diffusivity is generally prescribed for a given pair of species and pairwise for a multi-species system.
Typically, a compound's diffusion coefficient is ~10,000× as great in air as in water.
The diffusion coefficient in solids at different temperatures is generally found to be well predicted by the Arrhenius equation:
where Diffusion in crystalline solids, termed lattice diffusion, is commonly regarded to occur by two distinct mechanisms,[3] interstitial and substitutional or vacancy diffusion.
The former mechanism describes diffusion as the motion of the diffusing atoms between interstitial sites in the lattice of the solid it is diffusing into, the latter describes diffusion through a mechanism more analogue to that in liquids or gases: Any crystal at nonzero temperature will have a certain number of vacancy defects (i.e. empty sites on the lattice) due to the random vibrations of atoms on the lattice, an atom neighbouring a vacancy can spontaneously "jump" into the vacancy, such that the vacancy appears to move.
By this process the atoms in the solid can move, and diffuse into each other.
Of the two mechanisms, interstitial diffusion is typically more rapid.
[3] An approximate dependence of the diffusion coefficient on temperature in liquids can often be found using Stokes–Einstein equation, which predicts that
where The dependence of the diffusion coefficient on temperature for gases can be expressed using Chapman–Enskog theory (predictions accurate on average to about 8%):[4]
is obtained when inserting the ideal gas law into the expression obtained directly from Chapman-Enskog theory,[8] which may be written as
is the molar density (mol / m3) of the gas, and
At moderate densities (i.e. densities at which the gas has a non-negligible co-volume, but is still sufficiently dilute to be considered as gas-like rather than liquid-like) this simple relation no longer holds, and one must resort to Revised Enskog Theory.
[9] Revised Enskog Theory predicts a diffusion coefficient that decreases somewhat more rapidly with density, and which to a first approximation may be written as
is the radial distribution function evaluated at the contact diameter of the particles.
For molecules behaving like hard, elastic spheres, this value can be computed from the Carnahan-Starling Equation, while for more realistic intermolecular potentials such as the Mie potential or Lennard-Jones potential, its computation is more complex, and may involve invoking a thermodynamic perturbation theory, such as SAFT.
For self-diffusion in gases at two different pressures (but the same temperature), the following empirical equation has been suggested:[4]
where In population dynamics, kinesis is the change of the diffusion coefficient in response to the change of conditions.
In models of purposeful kinesis, diffusion coefficient depends on fitness (or reproduction coefficient) r:
is constant and r depends on population densities and abiotic characteristics of the living conditions.
This dependence is a formalisation of the simple rule: Animals stay longer in good conditions and leave quicker bad conditions (the "Let well enough alone" model).
The effective diffusion coefficient describes diffusion through the pore space of porous media.
The effective diffusion coefficient for transport through the pores, De, is estimated as follows:
where The transport-available porosity equals the total porosity less the pores which, due to their size, are not accessible to the diffusing particles, and less dead-end and blind pores (i.e., pores without being connected to the rest of the pore system).
The constrictivity describes the slowing down of diffusion by increasing the viscosity in narrow pores as a result of greater proximity to the average pore wall.
It is a function of pore diameter and the size of the diffusing particles.
Gases at 1 atm., solutes in liquid at infinite dilution.
Legend: (s) – solid; (l) – liquid; (g) – gas; (dis) – dissolved.