Permeation
From Wikipedia, the free encyclopedia
Permeation, in physics, is the penetration of a substance (permeate) through a solid. Primum mobile is the concentration gradient. Permeability is tested by permeation measurement.
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[edit] Description
The permeate always migrates to the lower concentration in three steps:
- Sorption (at the interface): Gases, vapour or dissolved chemicals or suspended substances are adsorbed at the surface of the solid.
- Diffusion (through the solid): The permeate penetrates the solid material through pores or molecular gaps.
- Desorption: The adsorbate leaves the solid as a gas.
[edit] Related terms
- Permeate: The substance permeating through the solid.
- Permeability: The grade of transmissibility of a solid, meaning how much penetrates in a specific time, dependent on the type of permeate, pressure, temperature, thickness of the solid and the area size.
- Semipermeability: Property of a material to be permeable only for some substances and not for some others.
- Permeation measurement: Method for the quantification of the permeability of a material for a specific substance.
[edit] History
[edit] Abbé Jean-Antoine Nollet (physicist, 1700 - 1770)
Nollet tried to seal wine containers with a pigs bladder and stored them under water. After a while the bladder was bulged outwards. He remarked the high pressure that discharged after he pierced the bladder. Curious he made the experiment the other way round: He filled the container with water and stored it in wine. The result was a bulging inwards of the bladder. His notes about this experiment are the first scientific mention of permeation (later it will be called semipermeability).
[edit] Thomas Graham (chemist, 1805 - 1869)
Graham proved the dependency of the gas diffusion on the molecular weight with experiments. He developed Graham's law that is associated directly with that.
[edit] Richard Barrer (1910 - 1996)
Barrer built up the modern measurement techniques, "Barrer" and first used scientific methods for measuring permeation rates.
[edit] Permeation in everyday life
- Packaging: Packages should be either absolutely airtight (e.g. for chocolate or beer) or selectively permeable (e.g. for oxygen with strawberries). Knowledge about the exact permeation rates is therefore essential.
- Tyres: Air pressure in tyres should decrease as slowly as possible. Therefore it is good to know which gas permeates least through the rubber wall.
- Insulating material: Water vapour permeation of insulating material is important as well as for submarine cables to protect the conductor from corrosion.
- fuel systems: To meet legal regulations, e.g. CARB (California Air Resource Board) for Low Emission Vehicles, it is essential to use barrier materials for fuel hoses and tanks.
[edit] Permeation measurement
The Permeation of films and membranes can be measured with any gas or liquid. The method contains a central module which is separated by the test film: the testing gas is fed on the one side of the cell and the permeated gas is carried to the detector by a sweep gas. The diagram on the right shows a testing cell for films, normally made from metals like stainless steel. The Photo shows a testing cell for pipes made from glass, similar to a Liebig condenser. The testing medium (liquid or gas) is situated in the inner white pipe and the permeate is collected in the space between the pipe and the glass wall. It is transported by a sweep gas (connected to the upper and lower joint) to an analysing device.
[edit] External links
Commercial provider for permation measurements: http://www.mecadi.com/en-literatur.htm The page contains additional measurement methods amongst others.
