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Diffusion
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Molecular diffusion, often called simply diffusion, is a net transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion. The result of diffusion is a gradual mixing of material. In a phase with uniform temperature, absent external net forces acting on the particles, the diffusion process will eventually result in complete mixing or a state of equilibrium.
Molecular diffusion is typically described mathematically using two Fick's laws.
usion is of fundamental importance in many disciplines of physics, chemistry, and biology.
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Molecular diffusion, often called simply diffusion, is a net transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion. The result of diffusion is a gradual mixing of material. In a phase with uniform temperature, absent external net forces acting on the particles, the diffusion process will eventually result in complete mixing or a state of equilibrium.
Molecular diffusion is typically described mathematically using two Fick's laws.
Applications
Diffusion is of fundamental importance in many disciplines of physics, chemistry, and biology. Some example applications of diffusion:
Significance
Diffusion is part of transport phenomena. Of the mass transport mechanisms, molecular diffusion is known as a slower one. Molecular diffusion is generally superimposed on, and often masked by, other transport phenomena such as convection, which tend to be much faster. However, the slowness of diffusion can be the reason for its importance: diffusion is often encountered in chemistry, physics and biology as a step in a sequence of events, and the velocity of the whole chain of events is that of the slowest step. For example, the rate at which a chemical reaction progresses can be entirely limited by the rate of diffusion of reactants/products to/from the place where the reaction occurs.
The speed of diffusion can be approximately illustrated as follows (at room temperature)
- In gas: 100 mm in one minute;
- In liquid: 0.5 mm in one minute;
- In solid: 0.0001 mm in one minute.
Transport due to diffusion is slower over long length scales: the time it takes for diffusion to transport matter is proportional to the square of the distance. Conversely, diffusion can be quite fast over small length scales; inside a living cell, chemicals are almost entirely transported by diffusion.
The above numbers should be treated only as an illustration of the slowness of diffusion. Great differences exist in the diffusion speed between particular systems, particularly in the solid state. For example:
- Hydrogen gas in solid iron at 10 °C - diffusion coefficient of 1.66x10-13 m2/s;
- Aluminum in solid copper at 20 °C - diffusion coefficient of 1.3x10-34 m2/s.
When diffusion speed is proportional to the square root of the diffusion coefficient, then the hydrogen in iron diffuses over 10 orders of magnitude faster than does aluminum in copper.
In biology
In cell biology, diffusion is a main form of transport for necessary materials such as amino acids within cells. Diffusion of water is classified as 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.
Tracer and chemical diffusion
Fundamentally, two types of diffusion are distinguished:
- Tracer 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 tracers, hence the name. The tracer diffusion is usually assumed to be identical to self-diffusion (assuming no significant isotopic effect). This diffusion can take place under equilibrium.
- 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 diffusion coefficients 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.
Non-equilibrium system
Because chemical diffusion is a net transport process, the system in which it takes place is not an equilibrium system (i.e. it is not at rest yet). Many results in classical thermodynamics are not easily applied to non-equilibrium systems. However, there sometimes occur so-called quasi-steady states, where the diffusion process does not change in time, where classical results may locally apply. As the name suggests, this process is a not a true equilibrium since the system is still evolving.
Chemical diffusion increases the entropy of a system. This is nothing else than saying that diffusion is a spontaneous and irreversible process. Particles can spread out by diffusion, but will not spontaneously re-order themselves (absent changes to the system, assuming no creation of new chemical bonds, and absent external forces acting on the particles).
Other types of diffusion
The spreading of any quantity that can be described by the diffusion equation or a random walk model (e.g. concentration, heat, momentum, ideas, price) can be called diffusion. Some of the most important examples are listed below.
An experiment to demonstrate diffusion
Diffusion is easy to observe, but care must be taken to avoid a mixture of diffusion and other transport phenomena.
It can be demonstrated with a wide glass tube, paper, two corks, some cotton wool soaked in ammonia solution and some red litmus paper. By corking the two ends of the wide glass tube and plugging the wet cotton wool with one of the corks, and litmus paper can be hung with a thread within the tube. It will be observed that the red litmus papers turn blue.
This is because the ammonia molecules travel by diffusion from the higher concentration in the cotton wool to the lower concentration in the rest of the glass tube. As the ammonia solution is alkaline, the red litmus papers turn blue. By changing the concentration of ammonia, the rate of color change of the litmus papers can be changed.
Another, simpler experiment to show diffusion is to drip a drop or two of food colouring into a glass of water. At first the food colouring will be very dark and concentrated at the spot where it hit the water, but after a while it will drift apart and fill the whole glass with a lighter shade of its colour. This is the dye diffusing in the water.
See also
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