Mass diffusivity

Diffusivity, mass diffusivity or diffusion coefficient is a proportionality constant between the molar flux due to molecular diffusion and the gradient in the concentration of the species (or the driving force for diffusion). Diffusivity is encountered in Fick's law and numerous other equations of physical chemistry.

The diffusivity is generally prescribed for a given pair of species and pairwise for a multi-species system. The higher the diffusivity (of one substance with respect to another), the faster they diffuse into each other. Typically, a compound's diffusion coefficient is ~10,000× as great in air as in water. Carbon dioxide in air has a diffusion coefficient of 16 mm²/s, and in water its diffusion coefficient is 0.0016 mm²/s.Diffusion
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Diffusivity has dimensions of length² / time, or m²/s in SI units and cm²/s in CGS units.

Temperature dependence of the diffusion coefficient

Solids

The diffusion coefficient in solids at different temperatures is generally found to be well predicted by the Arrhenius equation:

$D = D_0 \exp\left(-\frac\right)$

where
*''D'' is the diffusion coefficient (in m²/s),
*''D''₀ is the maximal diffusion coefficient (at infinite temperature; in m²/s),
*''E''_A is the activation energy for diffusion (in J/mol),
*''T'' is the absolute temperature (in K),
*''R'' ≈ 8.31446J/(mol⋅K) is the universal gas constant.

Liquids

An approximate dependence of the diffusion coefficient on temperature in liquids can often be found using Stokes–Einstein equation, which predicts that

$\frac = \frac \frac ,$

where
* ''D'' is the diffusion coefficient,
* ''T''₁ and ''T''₂ are the corresponding absolute temperatures,
* ''μ'' is the dynamic viscosity of the solvent.

Gases

The dependence of the diffusion coefficient on temperature for gases can be expressed using Chapman–Enskog theory (predictions accurate on average to about 8%):

$D = \frac\sqrt,$

where
* ''D'' is the diffusion coefficient (cm²/s),

* ''A'' is an empirical coefficient equal to $1.859 \times 10^ \mathrm$
* 1 and 2 index the two kinds of molecules present in the gaseous mixture,
* ''T'' is the absolute temperature (K),
* ''M'' is the molar mass (g/mol),
* ''p'' is the pressure (atm),
* $\sigma_ = \frac(\sigma_1 + \sigma_2)$ is the average collision diameter (the values are tabulated page 545) (Å),
* Ω is a temperature-dependent collision integral (the values are tabulated but usually of order 1) (dimensionless).

Pressure dependence of the diffusion coefficient

For self-diffusion in gases at two different pressures (but the same temperature), the following empirical equation has been suggested:
$\frac = \frac ,$
where
* ''D'' is the diffusion coefficient,
* ''ρ'' is the gas mass density,
* ''P''₁ and ''P''₂ are the corresponding pressures.

Population dynamics: dependence of the diffusion coefficient on fitness

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'':
$D = D_0 e ^,$

where $D_0$ 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).

Effective diffusivity in porous media

The effective diffusion coefficient describes diffusion through the pore space of porous media. It is macroscopic in nature, because it is not individual pores but the entire pore space that needs to be considered. The effective diffusion coefficient for transport through the pores, ''D''_e, is estimated as follows:
$D_\text = \frac,$
where
*''D'' is the diffusion coefficient in gas or liquid filling the pores,
*''ε_t'' is the porosity available for the transport (dimensionless),
*''δ'' is the constrictivity (dimensionless),
*''τ'' is the tortuosity (dimensionless).

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.

Example values

Gases at 1 atm., solutes in liquid at infinite dilution. Legend: (s) – solid; (l) – liquid; (g) – gas; (dis) – dissolved.

See also

* Atomic diffusion
*Effective diffusion coefficient
*Lattice diffusion coefficient
*Knudsen diffusion

References

{{DEFAULTSORT:Mass Diffusivity
Transport phenomena
Diffusion