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This answer to the question Pre-mixing cryogenic fuels and using only one fuel tank written by a non-chemist (me) begins with:

At STP:

This does not a priori prove that a solution of the two can not exist. However it does mean that they can not be handled as liquids at the same temperature, making mixing the two more difficult.

We know that liquid air exists which shows that LOX and LN2 can mix together. But methane is an organic molecules and we know that heavier $\text{C}_n \text{H}_{2n+2}$ hydrocarbons include oils and waxes don't like to dissolve in non-organic solvents.

A stoichiometric mixture of oxygen and methane would be 2:1 molar:

$$\ce{ 2O2 + CH4 -> CO2 + 2H2O }$$

Though the two can not be conveniently maintained as liquids at the same temperature, can a stoichiometric mixture of the two exist as a liquid at some (low) temperature and standard pressure?

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    $\begingroup$ I am not sure if both the solutions and mixtures tags apply here. $\endgroup$
    – uhoh
    Commented Mar 22, 2019 at 1:18
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    $\begingroup$ With an appropriate heat of mixing, the solution might well be possible. Not sure if there is literature on this mixture, but if I have time I’ll give it a whirl... $\endgroup$
    – Jon Custer
    Commented Mar 22, 2019 at 1:38
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    $\begingroup$ @JonCuster I hope "give it a whirl" doesn't mean you're going to try to mix the two! $\endgroup$
    – uhoh
    Commented Mar 22, 2019 at 1:40
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    $\begingroup$ just trying to whip it into a nice froth for my coffee in the morning... $\endgroup$
    – Jon Custer
    Commented Mar 22, 2019 at 1:41
  • $\begingroup$ Is methane "heavier" (more dense) than $O_2$ etc at same low T? Why do I doubt this? This is not a primary source but suggests the opposite? Methane is certainly more polarizable but Mw is lower. My guess is the component with the higher melting point (methane) is perfectly soluble in the liquid. The entropy of mixing is probably enough to drive miscibility. As the answer suggests, they form ideal solutions at sufficiently low T. $\endgroup$
    – Buck Thorn ♦
    Commented Mar 22, 2019 at 13:05

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There's a NASA report that looks into this: "ON THE SOLUBILITIES AND RATES OF SOLUTION OF GASES IN LIQUID METHANE", Hibbard and Evans, 1968 and concludes that such mixtures are possible.

Starting on page 8:

Figure 5(a) presents the curves for oxygen, argon, carbon monoxide, and nitrogen. Also shown are the two experimental values for nitrogen. Agreement is excellent at 99.83K and good at 110.9K. The curves for these gases show that solubility should decrease with increasing temperature and the nitrogen data confirm this. This figure shows the mole fraction solubility of oxygen to be 1.0 at 90K. This means that oxygen, which has a normal boiling temperature of 90.1K would continuously condense in, and be miscible in all proportions, with liquid methane at 90K. This is confirmed by reference 11 where, in a study of the solubility of methane in liquid oxygen, it was concluded that these formed a near-ideal solution at -297 F (90K)

(emphasis added) Reference 11 mentioned in there is "Hydrocarbon-Oxygen Systems Solubility", McKinley and Wang, 1960 (unfortunately paywalled) which also has interesting discussion of the stability (i.e. presence or absence of a tendency to explode) of various mixtures.

Figure 5 is reproduced below. Note how the solubility of oxygen rises rapidly as temperature drops.

enter image description here

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  • $\begingroup$ Bingo! What a great find, thank you! I think you can (should) also post an answer at the linked question as well. $\endgroup$
    – uhoh
    Commented Mar 22, 2019 at 4:52
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    $\begingroup$ Pre-mixing fuel and oxygen looks like a Very Bad Idea. Any spark will ignite it, as opposed to fuel stored separately. How stable is such a mixture? How likely are external events which would ignite it, like sparks? Could it be managed technically, in a rocket; e.g. can one ensure that the combustion does not back-propagate through the pumps etc.? (Should I ask a question, perhaps in the space exploration department?) $\endgroup$
    – Peter - Reinstate Monica
    Commented Mar 22, 2019 at 7:10
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    $\begingroup$ @PeterA.Schneider have a look at all the answers to the question linked in the first sentence of the question above. $\endgroup$
    – uhoh
    Commented Mar 22, 2019 at 7:28
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    $\begingroup$ On the other side of the phase diagram is sciencedirect.com/science/article/pii/S0011227567800432/… (Hodges & Burch Cryogenics 7 112-3 (1967) where the looked at the distribution of methane between liquid and vapor phases of oxygen. A quote: "the very high solubility of methane in liquid oxygen precluded the possibility of methane precipitating from liquid oxygen". So, the would seem to mix quite nicely. Whether that is a good idea is left to the imagination (and hopefully left right there). $\endgroup$
    – Jon Custer
    Commented Mar 22, 2019 at 13:38
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To add to the Bob's excellent answer (and expand a bit on my comment there), I've found two other potentially interesting papers to peruse.

The first is R.J. Hodges and R.J. Burch, Cryogenics 7 112-113 (1967), titled "The equilibrium distribution of methane between the liquid and vapour phases of oxygen". They note a "very high solubility of methane in liquid oxygen", with a equilibrium distribution coefficient at 93.15K (1.348 atm of O2) of 3.356 (so about 3x more methane in the liquid than in the gas phase). However, they kept the concentration of methane below the lower explosive limit so that it would not, well, explode.

Of perhaps more interest to the OP is R.L. Every and J.O. Thieme, Journal of Spacecraft and Rockets 2(5) 787-789 (1965) titled "Liquid oxygen and liquid methane mixtures as rocket monopropellants". In the introduction the authors note that previous work shows that the liquids are miscible in all proportions above 90K. Their tests show a specific impulse of almost 300 sec, and an exhaust velocity near 6000 fps.

In addition, since "shock sensitivities were reported" in the earlier work, they did some, well, vaguely disturbing experiments (hey, it was the 60's). "Tests were conducted to determine whether violent stirring or agitation, as found in an impeller-type pump, would detonate the mixture", although they didn't have any explosions there. They then went on to quantify the impact sensitivity, dropping a weight onto a stainless steel beaker of different mixtures from increasing heights until an explosion occurred. Impact sensitivities of 20-60 ft-lb were discovered. They also looked at adiabatic compression as a cause of explosion and found the liquid oxygen/methane mixtures were safer than some other things like nitromethane (which really isn't that comforting).

So, it would appear that the idea of using the liquid mixture as a monopropellant isn't totally off the wall, but please do it somewhere far from me...

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  • $\begingroup$ Those were the days. ;-) This is really interesting info! You may want to also write an answer to Pre-mixing cryogenic fuels and using only one fuel tank while you're at it. $\endgroup$
    – uhoh
    Commented Mar 22, 2019 at 14:06
  • $\begingroup$ Just because it's off the wall doesn't mean the early rocket pioneers wouldn't do it. Somewhere out there is a paper describing experimental tests of the feasibility of nitroglycerine as a monopropellant, and there was a general consensus that chlorine trifluoride would make an excellent oxidizer. $\endgroup$
    – Mark
    Commented Mar 22, 2019 at 20:56
  • $\begingroup$ Shock sensitivities? As in your rocket detonates when your ullage motors push the fuel to the bottom of the tank? $\endgroup$
    – Loren Pechtel
    Commented Mar 23, 2019 at 4:01

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