Gas-Gas Miscibility Gaps?

Many binary phase diagrams display a miscibility gap in the solid state (Ni-Au) as well as in the liquid state (check out the monotectic transformations in the Cu-Se system).

What about gas-gas miscibility gaps? Gases behave ideally at low pressures and high temperatures — ideal gas behavior for individual species, and ideal solution behavior for gaseous mixtures); at high pressures, especially, one suspects that deviations from either kind of ideal behavior must be possible. We certainly know about deviations from ideal gas behavior for pure species at high pressures; how about deviations from ‘ideal solution’ behavior that might give rise to miscibility gaps? As the following extract (from Ira N. Levine’s excellent text Physical Chemistry — 5th edition) shows, such gas-gas miscibility gaps do exist:

[On p. 361] Although it is often stated that gases are miscible in all proportions, in fact several cases of gas-gas miscibility gaps are known. Examples include CO2 – H2O, NH3 – CH4, and He – Xe. These gaps occur at temperatures above the critical temperatures of both components, and hence by the conventional terminology … involve two gases. Most such gaps occur at rather high pressures and liquid-like densities. However, n-butane – helium shows a miscibility gap at pressures as low as 40 atm. See R.P. Gordon, J.Chem.Educ., 49, 249 (1972). [Link added]

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