As far as I know there are no two gasses that don\'t mix (excluding demixing by

Question

As far as I know there are no two gasses that don't mix (excluding demixing by gravitational effects). For me, as someone working with fluids and surface tensions a lot, this means that the surface tension between the gasses is small or even non-existent.

I saw in this post that mixing will be governed by the Gibbs free energy: ?G=?H?T?S and that the only thing stopping fluids from mixing would be the enthalpy term which arises from repulsion.

Is it correct to think that the enthalpy will always be small in gasses, because of their low density and thus low interaction? And that this is the cause that gasses will always mix?

(just a footnote: I am talking about 'everyday' gasses here, not gasses compressed at thousands of bars with densities close to liquids)

Explanation / Answer

At STP most gases are nearly ideal, that is the interactions between molecules can be ignored and the gas molecules treated as non-interacting points. This means the enthalpy of mixing is negligable and the mixing is dominated by the entropy.

Obviously this is only true far from a phase transition, and small effects such as the Joule Thompson effect can be measured even at modest pressures.

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