Diffusion

From Wikipedia, the free encyclopedia

This article may require cleanup to meet Wikipedia's quality standards.
Please improve this article if you can. (August 2007)

This article needs additional citations for verification.
Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (August 2007)

This article is about the physical mechanism of diffusion. For alternative meanings, see diffusion (disambiguation).

The term diffusion is derived from the Latin verb diffundere which means "to spread apart" but can also mean "to pour away" or "give vent to" and one encounters two usages which can be associated with Fick's laws. Fick's first law deals with the passage of a gas through a membrane or porous material while Fick's second law is concerned with the dispersion of particles that occurs when there are different concentrations within a container.

In the second sense of diffusion is the spontaneous net movement of particles from an area of high concentration to an area of low concentration. For example, diffusing molecules will move randomly between areas of high and low concentration but because there are more molecules in the high concentration region, more molecules will leave the high concentration region than the low concentration one. Therefore, there will be a net movement of molecules from high to low concentration. Initially, a concentration gradient—a smooth decrease in concentration from high to low—will form between the two regions. As time progresses, the gradient will grow increasingly shallow until the concentrations are equalized.

Diffusion is a spontaneous process (more familiarly known as a "passive" form of transport, rather than "active"); it is simply the statistical outcome of random motion. Diffusion increases entropy, decreasing Gibbs free energy, and therefore is thermodynamically favorable. Diffusion operates within the boundaries of the Second Law of Thermodynamics because it demonstrates nature's tendency to wind down, as evidenced by increasing entropy.^[1]

The diffusion equation provides a mathematical description of diffusion. This equation is derived from Fick's law, which states that the net movement of diffusing substance per unit area of section (the flux) is proportional to the concentration gradient (how steeply the concentration changes in space), and is toward lower concentration. (Thus if the concentration is uniform there will be no net motion.) The constant of proportionality is the diffusion coefficient, which depends on the diffusing species and the material through which diffusion occurs. Fick's law is an assumption that may not hold for a given diffusive system (e.g., the diffusion may depend on concentration in addition to concentration gradient), in which case the motion would not be described by the normal (simple, Fickian) diffusion equation. An analogous statement of Fick's law, for heat instead of concentration, is Fourier's law.

The mechanism of diffusion is "Brownian motion" whereby a molecule makes a random walk about a central location since by kinetic theory the mean velocity of a particle is zero if it is not subject to any external forces. Due to collisions with neighboring molecules the motion of the particle is characterized by a mean free path which tends to confine the particle. But since there is no potential field acting to restore a particle to its original position, it is still free to move about the vessel or liquid in which it is located. The Laplacian in the diffusion equation indicates that the dispersion of the particles is second order effect, i.e., due to changes in the concentration gradient.

Diffusion is often important in systems experiencing an applied force. In a conducting material, the net motion of electrons in an electrical field quickly reaches a terminal velocity (resulting in a steady current described by Ohm's law) because of the thermal (diffusive) motions of atoms. The Einstein relation relates the diffusion coefficient to the mobility of particles.

In cell biology, diffusion is a main form of transport within cells and across cell membranes.

Osmosis

Metabolism and respiration rely in part upon diffusion in addition to bulk or active processes. For example, in the alveoli of mammalian lungs, due to differences in partial pressures across the alveolar-capillary membrane, oxygen diffuses into the blood and carbon dioxide diffuses out. Lungs contain a large surface area to facilitate this gas exchange process.