Diffusion coefficient: (also called diffusivity or mass diffusivity)
- It is a proportionality constant used in the Fick’s first law of diffusion; the law is about rate of diffusion per unit time.
J = - D.A X [(C1 – C2)/d]
J is flux or rate of diffusion; D ~ Diffusion coefficient, A ~ surface area, (C1 – C2) is concentration difference or concentration gradient, d ~ diffusion distance.
- For spherical molecules, D is given by the ‘Stokes-Einstein equation’:
D = - kT/{6(pie)rn)}
k ~ Boltzmann’s constant
T ~ temperature (degrees kelvin)
r ~ radius of the molecule
n ~ viscosity of the medium.
In summery, the rate of diffusion would be directly proportional to: surface area, concentration gradient, and temperature; it will be inversely proportional to diffusion distance, radius of the molecule, and viscosity of the medium.