What liquids last the longest in space?

I am trying to find something that is somewhat stable as a liquid in space. What happens to various liquid elements and compounds in a vacuum? Is there a list or table of the effects of local space on different liquids at different temperatures?

https://astronomy.stackexchange.com/questions/28531/what-liquids-can-be-found-in-the-void-space

• All liquids do have a vapor pressure depending on temperature. If ambient pressure is well below vapor pressure, the liquid can not exist for along time, it will vaporize completly. A lower temperature will decrease vapor pressure, but with a much lower temperature the liquid will turn to a solid. But such solids in vacuum may sublimate, direct transition from solid to gas.
– Uwe
Nov 25, 2018 at 21:07
• I guess the question is what liquid has the lowest vapour pressure at its melting point. With the exception of helium, it should be easy enough to keep the liquid at that temperature by either shading it from the Sun or focusing sunlight on it. I can't find any tables of such things, but I wonder is something like a low-melting salt might be good candidate. The ionic liquid is fairly stable, but the vapour would more or less have to be plasma. Nov 25, 2018 at 21:17
• Found webbook.nist.gov/cgi/… from which you can read off vapour pressures. Eg sodium chloride just above its melting point is about 50Pa. Nov 25, 2018 at 21:23
• There was an experiment with water long ago, over 50 years, see Project high water. 86 metric tons of water was released above 100 km and turned into ice clouds. The ice then sublimed to water vapor.
– Uwe
Nov 26, 2018 at 15:02
• I feel like Apiezon is probably a good option. Feb 26, 2021 at 23:51

While a table may exists somewhere called "handy space liquids" it may not be reliable. I think you many need to boot-strap your way there by making your own table first.

The three primary considerations here as a function of temperature and pressure:

• will it boil?
• will it freeze?
• will it evaporate too fast?

(sounds like the beginning of a song somehow)

Pressure:

We can ignore several complicated subtleties by assuming that the total and partial pressures outside the liquid are close enough to zero to call them zero.

Temperature:

In this answer I explain the equation for an estimate of the equilibrium temperature of a blackbody heated by visible light, and radiating in infrared light.

$$T \sim \left( \frac{(1-a_{vis})}{e_{ir}} \frac{I_{Sun}}{4 \sigma} \right)^{1/4}$$

where $$a_{vis}$$ is the visible light albedo, $$e_{ir}$$ is the infrared emissivity (both should really be weighted averages over the appropriate wavelength ranges), $$\sigma$$ is the Stefan–Boltzmann constant (about 5.67E-08 W m^-2 K^-4), and I is the intensity of sunlight, and for 1AU is the solar constant and about 1360 W/m^2.

You will have to treat your liquid Earth satellite as if it were a

Spherical Cow in orbit:

Most non-metal things have high infrared emissivity, but metals including eutectics can be very low as well. Materials also vary widely in visible light albedo. For ranges of 0.1 to 0.9, the equilibrium temperature at 1 AU from the sun ranges from about -100 C to +200 C:

emissivity (IR)  albedo (vis)  Equilib. T (K) / (C)
---------------  -----------   ---------------------
0.1            0.1                    482 /  208
0.1            0.9                    278 /   +5
0.9            0.1                    278 /   +5
0.9            0.9                    160 / -112


So you will need to carefully consider the optical properties of the material, and then factor in any impurities as well.

Freezing & Boiling:

After you've considered the equilibrium temperature, you'll need to look at the freezing and boiling points of each material you consider to make sure it doesn't freeze (therefore unsuitable for your question) or boil away quickly.

Evaporation:

Once you have a candidate, then look up the rate of evaporation. Liquids with a high evaporation rate will disappear more quickly than those that evaporate slowly. Your problem is that evaporation is caused both by thermal motion in the liquid and by incident photon flux, and you have a lot of that! The same photons of sunlight that keeps your liquid from freezing also can stimulate evaporation, even if the liquid is fairly transparent like water. Water evaporation rate is strongly affected by visible light photon flux beyond it's impact on the water's temperature.

You can get some idea about the evaporation rate in the dark from looking up the vapor pressure

The equilibrium vapor pressure is an indication of a liquid's evaporation rate

but it is not a replacement for knowing the evaporation rate in the dark, and adding to that the photon-induced evaporation rate.

Good luck!

Try it for pure water, and for gallium or some eutectic, and see what happens!

Just to propose something specific in an answer:

I would first guess Gallinstan (by mass: 68.5% Ga, 21.5% In, 10.0% Sn), at least if we're talking about near-room-temperature applications: Supposedly, it melts at -19°C, and has about a μPa or less vapor pressure at 500°C. (This is much lower than Hg, but I don't know if it's lower than pure gallium because the lowest temperature number I've found for that is 1.588×10^-6 atm at 1230K, see Vapor Pressures of Inorganic Substances. IX. Gallium.) It's boiling point is supposedly above 1300°C (Ga boils at 2400°C, In at 2072°C, Sn at 2602°C). Some other liquid metals might also be appropriate.

Purely ionic liquids are also a good idea (ones with lower melting points than water exist). Some kind of ionic liquid (or maybe just ionic liquids in general) coated in silver (don't ask me how) has been proposed for use on the surface of the moon, see below. (It's possible they were going to freeze the ionic liquid before they deposited silver on it though, since it seems to me like that might work for their application. I haven't read the paper, though.)

Something else I wonder about is lava. Many asteroids that are too small to hold much atmosphere are believed to have been molten once and large areas of the Earth's Moon are believed to be lava flood plains. I don't know if lava's vapor pressure can be low (depending on exact composition of course) or if it's just that the surface froze protecting the interior and/or that a lot of it did evaporate and then escape to space or deposit elsewhere on the planet but there was so much that a lot was still left. If you consider that evaporation only happens at the surface of a body, then you can see how a square-cube law could come into play, where larger volumes last longer, at least if the volume-increase isn't just a flat plain of larger area and the same thickness. It's also true that the rock may not have had to have been liquid for very long to explain the features we see. Still, something to put through @uhoh's proposed analyses.

I'm not exactly sure how to find vapor pressures of silicate melts, but I know that different elements evaporate out at different rates. Sossi & Fegley 2018 (Thermodynamics of Element Volatility and its Application to Planetary Processes) goes in a lot of detail about exactly how different elements evaporate out of molten rock and mentions compilations of thermodynamic data called JANAF and IVTAN. Lock, et al 2018 (The Origin of the Moon Within a Terrestrial Synestia), which has some graphs that might be informative, used data from JANAF and also the GRAINS code from Petaev 2009 (The GRAINS thermodynamic and kinetic code for modeling nebular condensation). Walter & Carron 1964 (Vapor pressure and vapor fractionation of silicate melts of tektite composition) mentions specific, rather high, experimental vapor pressures (though it sounds like these were mostly dissolved volatiles), as well as "a boiling point curve ... [that] differs from the equilibrium vapor pressure curve due to surface tension effects" in its abstract, but I can't currently even reach the paper (just like I can't reach Petaev 2009).

One thing to notice about all of these proposals is that they have very strong bonds compared to, say, water, (metallic bonds and ionic bonds). These compounds usually have to become electrically neutral atoms or molecules/ion-pairs to evaporate. Only at very high temperatures do they vaporize directly into plasma

This means it takes a lot of energy for them to evaporate. Lava usually contains a lot of ionic bonding too, and also may contain very large polymer-like ions and molecules.* Very large molecules usually have to break up for a substance containing them to vaporize, which also often takes a lot of energy. These sorts of considerations could help you in looking for candidate liquids.

*These chains are also often quite short, particularly if the temperature is high, the pressure is low, or the melt contains a lot of "water". Bjørn Mysen 1983 and 2014 discuss these effects on the degree of polymerization at length: The Structure Of Silicate Melts Water-melt interaction in hydrous magmatic systems at high temperature and pressure. Vacuum is obviously low pressure, though probably about the same as 1 atm in its effects on silicate polymerization, except that it might allow even more of the "water" to escape quickly, leaving an almost completely anhydrous melt. (Geology frequently deals with thousands of atm of pressure, so 1 atm is often approximately 0 atm in geology.)

• +1 Great answer! I adjusted the formatting a bit so we can see the individual links more clearly, and added a link to my answer in Astronomy SE related to ionic liquids with silver on top.
– uhoh
Feb 10, 2021 at 6:35
• I added links to some more papers about silicate melts. It seemed odd to me to use "Water-melt interaction in hydrous magmatic systems at high temperature and pressure" as my only given source for talking about silicate melts that would be at ~0 pressure and probably contain almost no "water". Feb 26, 2021 at 22:46
• ha! :-) my goodness you've added quite a bit, your diligence and stewardship is laudable!
– uhoh
Feb 26, 2021 at 23:04
• It should be noted that these are papers that I've looked at a bit, not papers that I've actually read. I've read a little bit of "The Origin of the Moon Within a Terrestrial Synestia" and have been planning to read the whole thing, and I've skimmed "Water-melt interaction in hydrous magmatic systems at high temperature and pressure" more thoroughly than the others, but the others I've mostly just scrolled up and down through a bit or even just read the abstract. Feb 26, 2021 at 23:22