# Do gasses always mix because of their Gibbs free energy?

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: $\Delta G = \Delta H - T\Delta S$ 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)

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I agree with John Rennie but I would phrase it differently: gases are defined as those fluids that have negligible interactions between the individual molecules. So as long as the thing deserves to be called a gas, the molecules fly pretty much freely through space and may be combined simply by adding the partial pressures. There's nothing that stops one gas compound from spreading to the whole vessel even if the vessel is filled by other gases. When the obstructions are significant, the stuff is a liquid. – Luboš Motl Feb 27 '13 at 7:20
The division of fluids to liquids and gases is gradual and is governed exactly by criteria like yours: when the surface tension and similar things is negligible, it's gases. When it's not and when the interactions between the molecules become significant, it's a liquid. – Luboš Motl Feb 27 '13 at 7:21
Surface tension is exactly zero at the critical point. Above the critical point one could define a negative surface tension. If one has seen the critical opalescense (due to immense density fluctuations) in experiment, one knows that for the rest of life :=) – Georg Feb 27 '13 at 19:07