Effusion

As effusion (from Latin Effusio, outpouring ) such physical processes are known in which substances, mostly gases, but also liquids, leaving a closed container by passing through the molecular lattice migrate through, and they do this atom by atom or molecule. For this purpose they must be sufficiently small, but they also have a suitable spatial geometry and charge distribution (see also Van der Waals forces). The diameter of the apertures should be small molecular weight compared to the mean free path ( See also Brownian motion ) of a effundierenden molecule. Otherwise, an ordinary leak would be present, through which gases and liquids spread diffusively mix or what is going according to distinctly different physical laws than the effusion. In contrast to diffusion, and the transfusion through a semipermeable membrane, a particle stream runs in the effusion only in one direction and the effusion ends after a sufficiently long time, usually with a complete emptying of the container.

The effusion rate of a gas depends only on the temperature - from and by the molecular weight - that can be controlled under laboratory conditions. Therefore, one can use this technique to determine this weight with unknown substances and then find out by combustion analysis, which has the chemical formula substance. Today we used instead of the much more accurate effusion mass spectrometry, which moreover requires only minimal quantities.

Well-known examples

From the air navigation is common that many hydrogen- filled buoyancy lose their gas filling gradually. For helium gas, there is even no effusionssicheren and so dense vessels in normal terrestrial conditions.
Some aromatic hydrocarbons from fuels such as gasoline, diesel, kerosene, effusion with walls made from certain plastics from tanks or canisters into the air, which has already led to accidents by " befuddled " motorists who their intoxierenden effect were exposed for too long, and also the risk of explosion increased dangerously near unsuitable vessels.
In the ultra-high vacuum (UHV ) held molecular beam epitaxy (MBE ) effusion cells are used to enable the flow of material. They contain a melt which can be heated to above 1000 degrees ( for example, gallium in GaAs MBE). The partial pressure, and thus the material flow are a function of temperature. The crucibles must be made (e.g., boron nitride) of a refractory material are electrically heated and an outlet opening.
During the enrichment of uranium for nuclear fission gaseous uranium hexafluoride ( UF6) can be driven by membranes. Since the lighter UF6 - which contains the fissile 235U and 238U not - effundiert slightly faster, the uranium can accumulate. This technique was applied during the Manhattan Project in Oak Ridge; the required system (codename K -25) occupied an area of 17 hectares.

Laws

The still found by Thomas Graham without knowledge of atomic or molecular structures and contexts, and in 1833 published Graham's law says that the discharge rates of different gases are inversely proportional to the square roots of their densities at the same pressure. It is quite accurately and for the effusion of liquids and gases, because the density is a function of molecular weight and this is a good approximation to the ( average ) molecular diameter obeying also in the phase of the passage through the wall itself non- gases and substances lowest viscosity of the gas laws, and as long as they move into this area as isolated atoms or molecules that are essentially no neighbors.

According to Graham 's law, a simple relationship between the two substances Effusionsgeschwindigkeiten results under otherwise identical conditions:

Rate1: effusion of gas 1
Rate2: effusion of gas 2
M1: molar mass of gas 1
M2: molar mass of gas 2

The effusion of a gas is calculated as:

Rate: effusion rate of the gas
P: pressure
A0: surface of the hole, which is held by the effusion
NA: Avogadro constant
M: molar mass
R: universal gas constant
T: temperature

Near the absolute zero of temperature the specific circumstances that facilitate the effusion significantly inter alia by the absence of thermal molecular motions.

Not correct, later versions, however, the ' very thick walls ' and' - as described by Evangelista Torricelli 1644 hydrodynamic variable, Torricelli's theorem or the Torricelli theorem called, describes the original version of the effusion - to which it did not go to him small outlet openings ' into account, etc., you get close relative.