Permeation
Under permeation ( latin permeare , penetrate, pass through, walk through ') is the process in which a substance (permeate) having a solids penetrates or migrates. The driving force is a gradient of the chemical potential of the permeate and is regularly replaced in simplified models by measurable concentration or pressure gradients . The permeability is usually given as a measure of permeation in the unit μg cm −2 min −1 .
Description of the process
Without external influences, the permeate always moves in the direction of the lower concentration or the lower partial pressure . The permeation takes place in three sub-steps:
- Sorption at the interface : gases , vapors or chemicals dissolved in liquidsor suspended substances are absorbed on the surface of the solid.
- Diffusion (through the solid): The permeate penetrates (diffuses) the solid material through pores or molecular spaces.
- Desorption : The adsorbate escapes as a gas on the other side of the solid.
Measures to reduce permeation
In order to reduce penetration and diffusion , the selected substance should have the smallest possible free volume (free space between the molecular chains). This can also be influenced by additives or specific processing. For example, the higher the crystallinity or filler content of a thermoplastic , the lower the degree of crosslinking of a thermoset or elastomer . Another measure is to coat the material with materials that have a lower permeability.
Related terms
- Permeate : substance that penetrates ( permeates ) through a solid
- Permeability : Degree of permeability of a solid, a measure of the permeation through the substance in question. This property is influenced by the type of permeate as well as by pressure, temperature, thickness of the solid and the area size.
- Semi-permeability : property of a substance to be permeable to some permeates and impermeable to others.
- Permeation measurement: measuring method for quantifying the permeability of a substance (for a specific permeate )
Historical
Abbé Jean-Antoine Nollet (physicist, 1700–1770)
Jean-Antoine Nollet wanted to seal wine airtight, so he closed a container with a pig's bladder and also immersed it in a water bath. Over time, he noticed that the bladder was bulging more and more outward. When he finally pricked her up, enormous pressure was released. Out of curiosity, he turned the experimental setup upside down, filled the container with water and dipped it in wine. As expected, he now observed a similar bulging of the pig's bladder into the interior of the container. These test results are the first scientific mentions of a type of permeation (this would later be called semi- permeability ).
Thomas Graham (chemist, 1805-1869)
Thomas Graham experimentally demonstrated the dependence of gas diffusion on molar mass . He is the founder of Graham's Law , which is directly related to it.
Richard Barrer (1910-1996)
Richard Barrer laid the foundations for today's measurement technology. He first used modern scientific methods to study permeation. In doing so, he laid the foundation for the permeation measurements that are essential in process chemistry today. The Barrer is named after him, a unit for the gas permeability of thin materials.
Permeation in everyday life
There are numerous situations in everyday life in which permeation plays an essential role:
- Packaging : Packaging should either be completely airtight (e.g. for chocolate or beer ) or selectively permeable (e.g. for oxygen in strawberries ). Therefore, knowledge of the permeation or even the exact rate of permeation is important.
- Car tires : The pressure in the car tire should decrease as slowly as possible. It is therefore interesting to know which gas permeates the slowest through which type of rubber .
- Insulating materials : For thermal insulation, z. B. the water vapor permeability of the materials used is important. The insulation of submarine cables must alsoprotectthe conductor from corrosion .
- Fuel system : The fulfillment of legal requirements in the automotive sector, such as B. CARB (California Air Resource Board) regulations on the Low Emission Vehicle LEV require impermeable barrier materials for fuel lines and tanks .
- Seepage of water on agricultural areas, macadam paths, mastic asphalt pavements
- Liquid water and water vapor from rain and body sweat through clothing for rain, sports and against the cold
- Balloons made of latex can be made thicker-walled and equipped with sealant; those made of polyester film are usually metallized, in particular to allow the easily escaping helium to escape more slowly.
Permeation in membrane technology
In the membrane technology is permeate a central concept. In the case of liquid filtration, it denotes the effect caused by the filtration of z. B. bacteria, hardness builders or heavy metals freed fluid or, in the case of gas filtration, the purified or depleted gas. The substances retained by the membrane during filtration are called retentate (from the Latin retentare = to hold back, to hold on) or concentrate. Membrane filtration systems are used, for example, in the food industry and waste water treatment or gas separation .
Permeation measurement
Films and membranes can be checked for their permeability with any gases as well as with liquids of all kinds.
With gases
The measurement technologies for gases all contain a central module that is shared by the membrane to be tested: on the “feed side”, the test gas flows over the measuring cell , the remaining retentate is diverted. The amount of gas (permeate) that has arrived on the other side is led by the flushing gas to a detector , where the concentration is measured. The graphic on the right shows this schematically using the example of a measuring cell for foils. The upper and lower parts of the cell enclose the centered membrane. An O-ring that rests on the sample seals the interface. This type of cell is usually made of metal such as B. made of stainless steel.
If the permeation is to be measured on pipes , the cell must have a different geometry, e.g. B. in the photo on the right, similar to a Liebig cooler . Here the inner white tube is filled with a gas or a liquid and the permeate collects in the space between the test sample and the glass wall, where it is fed to an analysis unit by a carrier gas that flows upwards through this space. The amount of the permeated substance is then determined there. The gas permeability is expressed in the unit Barrer .
With liquids
Similar to the permeation measurement for gases, the so-called water value is determined in membrane technology . It is used to characterize the performance of a liquid filtration unit and is given in l / (m² · h · bar).
In hydrogeology
This laboratory test is used to calculate the permeability coefficient and is helpful for assessing the rate of groundwater recharge and the spring discharge . It is based on Henry Darcy and follows the law named after him :
With:
- Q - amount of water in m³ / s
- A - flowed through area in m²
- k f - coefficient of permeability in m / s
- i - hydraulic gradient (also hydraulic gradient or potential gradient) in m / m
Individual evidence
- ^ Fritz Röthemeyer, Franz Sommer: Kautschuktechnologie , Carl Hanser Verlag Munich Vienna, 2nd edition, 2006, pp. 525-526, ISBN 978-3-446-40480-9 .