In physics and engineering, permeation is the penetration of a permeate through a solid.
It is directly related to the concentration gradient of the permeate, a material's intrinsic
permeability, and the materials' Mass diffusivity. Permeation is modeled by equations such as
Fick's laws of diffusion, and can be measured using tools such as a minipermeameter.
Description The process of permeation involves the diffusion
of molecules, called the permeant, through a membrane or interface. Permeation works
through diffusion; the permeant will move from high concentration to low concentration
across the interface. A material can be semipermeable, with the presence of a semipermeable membrane.
Only molecules or ions with certain properties will be able to diffuse across such a membrane.
This is a very important mechanism in biology where fluids inside a blood vessel need to
be regulated and controlled. Permeation can occur through most materials including metals,
ceramics and polymers. However, the permeability of metals is much lower than that of ceramics
and polymers due to their crystal structure and porosity.
Permeation is something that must be regarded highly in various polymer applications, due
to their high permeability. Permeability depends on the temperature of the interaction as well
as the characteristics of both the polymer and the permeant component. Through the process
of sorption, molecules of the permeant can be either absorbed or adsorbed at the interface.
The permeation of a material can be measured through numerous methods that quantify the
permeability of a substance through a specific material.
Permeability is measured in units of area, commonly meters squared. It can be measured
in a darcy or millidarcy popularized by Henry Darcy. They are not official SI units, however
they are widely used. The permeability coefficient can be calculated in Barrers. Standard SI
units for temperature, time, distance and pressure are used for the gasses and diffusion
coefficients used to calculate permeability. Related terms
Permeant: The substance or species, ion, molecules permeating through the solid.
Semipermeability: Property of a material to be permeable only for some substances and
not for others. Permeation measurement: Method for the quantification
of the permeability of a material for a specific substance.
History Abbé Jean-Antoine Nollet
Nollet tried to seal wine containers with a pig's bladder and stored them under water.
After a while the bladder bulged outwards. He noticed the high pressure that discharged
after he pierced the bladder. Curious, he did 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.
Thomas Graham Graham experimentally proved the dependency
of gas diffusion on molecular weight, which is now known as Graham's law.
Richard Barrer Barrer developed the modern Barrer measurement
technique, and first used scientific methods for measuring permeation rates.
Permeation in everyday life Packaging: The permeability of the package
needs to be matched with the sensitivity of the package contents and the specified shelf
life. Some packages must have nearly hermetic seals while others can be selectively permeable.
Knowledge about the exact permeation rates is therefore essential.
Tires: Air pressure in tires should decrease as slowly as possible. A good tire is one
that allows the least amount of gas to escape. Permeation will occur over time with the tires,
so it is best to know the permeability of the material that will make up the tire with
the desired gas to make the most efficient tires.
Insulating material: Water vapour permeation of insulating material is important as well
as for submarine cables to protect the conductor from corrosion.
Fuel cells: Automobiles are equipped with Polymer Electrolyte Membrane fuel cells to
convert hydrogen fuel and oxygen found in the atmosphere to produce electricity. However,
these cells only produce around 1.16 volts of electricity. In order to power a vehicle,
multiple cells are arranged into a stack. A stack's power output depends on both the
number and the size of the individual fuel cells.
Thermoplastic and Thermosetting Piping: Pipes intended to transport water under high pressure
can be considered as failed when there is a detectable permeation of water through the
pipe wall to the outer surface of the pipe. Medical Uses: Permeation can also be seen
in the medical field in drug delivery. Drug patches made up of polymer material contain
a chemical reservoir that is loaded beyond its solubility, and then transferred to the
body through contact. In order for the chemical to release itself into the body, it must permeate
and diffuse through the polymer membrane, according to the concentration gradient. Due
to the over solubility of the reservoir, transport of the drug follows the burst and lag mechanism.
There is a high transfer rate of the drug when the patch makes contact with the skin,
but as time increases a concentration gradient is established, meaning delivery of the drug
settles to a constant rate. This is crucial in drug delivery and is used in cases such
as the Ocusert System. Permeation measurement
The permeation of films and membranes can be measured with any gas or liquid. One method
uses 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 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 to an analysing device. Permeation can also be measured through intermittent
contact. This method involves taking a sample of the test chemical and placing it on the
surface of the material whose permeability is being observed while adding or removing
specific amounts of the test chemical. After a known amount of time, the material is analyzed
to find the concentration of the test chemical present throughout its structure. Along with
the amount of time the chemical was on the material and the analysis of the test material,
one can determine the cumulative permeation of the test chemical.
The following table gives examples of the calculated permeability coefficient of certain
gases through a silicone membrane. *1 Barrer = 10-10 cm3· cm /cm2 · s · cm-Hg
Unless otherwise noted, permeabilities are measured and reported at 25C and not From:
THIN SILICONE MEMBRANES-THEIR PERMEATION PROPERTIES AND SOME APPLICATIONS Annals of the New York
Academy of Sciences, vol. 146, issue 1 Materials in, pp. 119–137 W. L. Robb
Approximation using Fick's First law The flux or flow of mass of the permeate through
the solid can be modeled by Fick's first law.
This equation can be modified to a very simple forumula that can be used in basic problems
to approximate permeation through a membrane.
where is the "diffusion flux"
is the diffusion coefficient or mass diffusivity is the concentration of the permeate
is the thickness of the membrane We can introduce into this equation, which
represents the sorption equilibrium parameter, which is the constant of proportionality between
pressure and . This relationship can be represented as .
The diffusion coefficient can be combined with the sorption equilibrium parameter to
get the final form of the equation, where is the permeability of the membrane. The relationship
being
Solubility of a gas in a metal In practical applications when looking at
gases permeating metals, there is a way to relate gas pressure to concentration. Many
gases exist as diatomic molecules when in the gaseous phase, but when permeating metals
they exist in their singular ionic form. Sieverts' law states that the solubility of a gas, in
the form of a diatomic molecule, in metal is proportional to the square root of the
partial pressure of the gas. The flux can be approximated in this case
by the equation:
We can introduce into this equation, which represents the reaction equilibrium constant.
From the relationship .
The diffusion coefficient can be combined with the reaction equilibrium constant to
get the final form of the equation, where is the permeability of the membrane. The relationship
being
See also
Moisture vapor transmission rate Oxygen transmission rate
Carbon dioxide transmission rate Hermetic seal
Milk permeate External links
Permeation in Polymers & Composites Further reading
Yam, K. L., Encyclopedia of Packaging Technology, John Wiley & Sons, 2009, ISBN 978-0-470-08704-6
Massey, L K, Permeability Properties of Plastics and Elastomers, 2003, Andrew Publishing, ISBN
978-1-884207-97-6 ASTM F1249 Standard Test Method for Water
Vapor Transmission Rate Through Plastic Film and Sheeting Using a Modulated Infrared Sensor
ASTM E398 Standard Test Method for Water Vapor Transmission Rate of Sheet Materials Using
Dynamic Relative Humidity Measurement ASTM F2298 Standard Test Methods for Water
Vapor Diffusion Resistance and Air Flow Resistance of Clothing Materials Using the Dynamic Moisture
Permeation Cell F2622 Standard Test Method for Oxygen Gas
Transmission Rate Through Plastic Film and Sheeting Using Various Sensors
F1383: Standard Test Method for Permeation of Liquids and Gases through Protective Clothing
Materials under Conditions of Intermittent Contact.
THIN SILICONE MEMBRANES-THEIR PERMEATION PROPERTIES AND SOME APPLICATIONS Annals of the New York
Academy of Sciences, vol. 146, issue 1 Materials in, pp. 119–137 W. L. Robb
Pharmaceutical Systems for Drug Delivery, David Jones; Chien YW. 2nd ed. New York: Marcel
Dekker, Inc; 1992. Novel drug delivery systems. O.V. Malykh, A.Yu. Golub, V.V. Teplyakov,
"Polymeric membrane materials: New aspects of empirical approaches to prediction of gas
permeability parameters in relation to permanent gases, linear lower hydrocarbons and some
toxic gases", Advances in Colloid and Interface Science, Volume 164, Issues 1–2, 11 May
2011, Pages 89–99, ISSN 0001-8686, 10.1016/j.cis.2010.10.004.