I'm sure Mr Google knows this, but no amount of insisting in different ways
will get him to tell me.
Along the way it seems like that effusion is what I really want and that
some mix of Knudsen and Poiseuille flow is involved and ... - but he STILL
won't tell me :-).

I'm trying to understand an aspect of the simultaneous diffusion
(effusion?*)  of several gases through a permeable membrane between two
spaces.
The classic example is O2 and CO2 passing through the lung wall.  Direction
is opposite (you hope). Rate of transfer is driven by a constant related to
the permittivity (or whatever) of the material for the gas concerned and the
difference in partial pressures of the chosen gas on either side of the
membrane. CO2 passes through the membrane about 100 times as easily as O2
for a given partial pressure.

As above, partial pressures of the gas in question is what's relevant. The
pp of the other gas or gases present is irrelevant. Or, as least to a first
order and at least as far as the text books and mr G tell you.

So far so good. BUT.
1.  How does a given gas "know" the pp of it's fellow on the other side of
the membrane to the exclusion of all other gases at other pp's which may be
present? ie what makes Oxygen "see" Oxygen and not Nitrogen or CO2. What
makes CO2 not "see" Nitrogen etc.
2. Is it possible to "spoof" a gas into reacting to another gas so that it
alters its diffusion rate based on both the pp of the same gas on the other
side AND also wholly or partially due to the presence of this other gas? If
so how, and what are the criteria?

This has implications for all sorts of interesting things including osmosis,
reverse osmosis, gas separation etc.

Any enlightenment most welcome.

_______________

An aside.
If source side has O2 at say 20% (about atmospheric and target has O2 at 7%
then if total pressures on each side are the same (eg atmospheric) then flow
will occur due to the 14%/7% partial pressure differential. To prevent flow
you need to double the target total pressure so you now have 7% x 2P
compared with 14% at 1 P so pps are matched. this implies that to reduce
flow to near zero when there is little target gas present you need to
increase absolute pressure to 'very high'.


          Russell


* effusion occurs when pore size is significantly smaller than the mean free
path.
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