In a crude approximation you can utilize the barometric formula. In terms of density, rho, the barometric formula states rho(h)=rho(h=0)*Exp(-mgh/(k_BT)) with h being elevation, m the mass of your molecule, and, g=9.8 m/s^2. So clearly, in the presence of gravity you will have a density profile. The heavier molecules are found in higher density at the bottom the lighter at the top. Firefighters know that very well. This is why they crawl on the floor in case of fire and absence of breathing mask. CO molecules (C=12u,O=16u) are lighter  than O2 molecules and thus accumulate in lower concentration near the ground. 

Thanks for all of the answers,

- Now what about the interaction between the molecules (rotation, transformation and vibration)?

- What about the collision of the molecules and the kinetic manner and mixing tendency of the molecules in gaseous phase?

Finally, I think the effect of the temperature is very important because of what mentioned before.

The lower the temperature, the smaller the scale heights of both O2 and N2, and hence the smaller the size of the system required for significant gravitational, or more practically centrifugal, separation of the two gases. Of course such separation would be easier for gases that differ more in molecular weight, say H2 and N2 or H2 and O2, versus O2 and N2 which are quite close in molecular weight.

pdf will guide

Dear Denur,

In my case it is N2&O2.

Thanks dear Dhandapani,

but it is about gas/liquid gravity separation while I want the gas/gas ...

And What about entropy? I think entropy drives a propensity preventing the separation?

whats your opinion?

Dear Dr. Päch,

Because of the following sentence in your valuable reference I thought that it is just about the "2-phase" cases:

"Recall that, besides mole fractions, xi, mass fractions ai and volume fractions αi are often used to determine the state of a two-phase mixture."

As you noticed, I studied the article again and found my case (N2/O2) in the article.

The assay comprises the answer of my question. That is excellent.

Thanks,

Dear Cornwall,

Firstly, the experiment was unbelievable to me but then I studied about and found its fundamentals.

Yes, That is,

Thanks.