Permeation Totally Explained

Everything about Permeation totally explained

Permeation, in physics and engineering, is the penetration of a permeate (such as a liquid, gas, or vapor) through a solid, and is related to a materials intrinsic permeability. Permeability is tested by permeation measurement.

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.

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.

History

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'll be called semipermeability).

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's associated directly with that.

Richard Barrer (1910 - 1996)

Barrer built up the modern measurement techniques, "Barrer" and first used scientific methods for measuring permeation rates.

Permeation in everyday life

Packaging: The permeability of the package (materials, seals, closures, etc) needs to be matched with the sensitivity of the package contents and the specified shelf life. Some packages must have nearly hermetic seals while other can (and sometimes must) be selectively permeable. Knowledge about the exact permeation rates is therefore essential.

Tires: Air pressure in tyres should decrease as slowly as possible. Therefore it's 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, for example CARB (California Air Resource Board) for Low Emission Vehicles, it's essential to use barrier materials for fuel hoses and tanks.

Permeation measurement

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.

Further Information

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