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Supercritical CO2 has been suggested as a potential alternative bio-solvent, replacing water, at high pressures and modestly elevated temperatures.

But what about supercritical N2? ScCO2 is an industrially-useful solvent for organic chemistry, so a good bit of research has been done relevant to its suitability as a biosolvent, but finding relevant info on nitrogen is seems to be significantly more difficult. Apparently, it is useful as a solvent for some drying and cleaning processes--which is what sparked my curiosity--but there seems to be a dearth of information on precisely how it behaves with various complex organic molecules (like, e.g., proteins, nucleic acids, lipids, and so forth).

Nitrogen's critical point occurs at significantly lower pressure than CO2, and cryogenic temperatures, and so would open up a quite different range of interesting alien homeworlds to play with.

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Water is a good solvent because of it's dipole moment

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Water is a good solvent because it has alternating positive and negative charges on either sides of its molecular structure. The highly electronegative oxygen atom pulls electron from the hydrogen towards it, causing a more negative charge on the side of the oxygen atom. This is called a hydrogen bond. Because of the angle between the hydrogen atoms when bonded, this also create a direction where there is a more positive charge due to the two hydrogen atoms. This is called a dipole moment.

Nitrogen is more symmetrical

Diatomic nitrogen (N$_2$), on the other hand, has a strong, covalent triple bond. The bond is 'short' in the sense that the atoms are tightly bound. Due to the nature of a covalent bond, especially one so strong and short as this triple bond, the electrons are shared instead of pulled from one molecule to the other. Thus, there is no dipole moment. Infact, this makes the normally very reactive element nitrogen into an almost inert gas in its N$_2$ form.

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Note how, due to the shared electrons in the middle, there are an equal number of electrons, and equivalent electric charge, from both sides of the molecule. Thus, N$_2$ is not going to be a good solvent.

Carbon dioxide is already a solvent

Carbon Dioxide can be used as a solvent in its liquid form, as well. In general, Carbon Dioxide is a specfic kind of solvent called a lipophile. This is basically the class of substances which dissolve in oil, while hydrophiles are the substances that dissolve in water. Just as some things dissolve in alcohol but not water, those same things will generally disssolve in carbon dioxide, too.

Carbon dioxide does not have to be supercritical to be a solvent, but making it supercritical gives it some additional useful properties. On the other hand, Nitrogen is so inert that it is not a solvent as a liquid, so there is little reason to believe it would be a solvent as a supercritical liquid.

There is a more interesting form of nitrogen

Of course, perhaps the most interesting nitrogen compound, from a biochemistry point of view, is ammonia, which is itself a solvent. Ammonium, its ionic form is a lipophilic solvent like carbon dioxide. Its supercritical form can be used as a solvent as well. So perhaps supercritical ammonia or ammonium is a better place to start.

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  • $\begingroup$ Water is a good solvent for polar and ionic substances because it is polar. And liquid CO2 is a good solvent for non-polar stuff. But supercritical CO2 is weirdly much better at dissolving things like polar proteins... while supercritical water becomes much more lipophilic than liquid water! So given that I already know supercritical N2 can be used as an industrial solvent (while liquid N2 is not a very good solvent at all), I don't think you can conclude much just from the fact that water's solvent properties derive from it's dipole moment, which N2 lacks. $\endgroup$ – Logan R. Kearsley Dec 10 '17 at 4:08
  • $\begingroup$ Supercritical ammonia is an interesting thing to look at, though. It exists at much lower pressures, but much higher temperatures, than either ScN2 or ScCO2. which opens up yet another novel range of potential alien homeworlds.... $\endgroup$ – Logan R. Kearsley Dec 10 '17 at 4:09
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    $\begingroup$ @LoganR.Kearsley I did not find any reference to supercritical N$_2$ as as a solvent. In fact, this very questions pops up on the first page of a google search! You may have seen papers (here, here) about its use in dehydrogenation. I'm not really sure this counts as a solvent. $\endgroup$ – kingledion Dec 10 '17 at 6:21
  • $\begingroup$ Quite by chance, I happened to run across "Many Chemistries Could Be Used to Build Living Systems" by William Bains, (Astrobiology Vol. 2 No. 4, 2004); apparently, liquid N2 is not a particularly good solvent for most things, mainly because it's just too cold (and cryosolvents tend not to be very good in general)... except that silanols dissolve in it really well. And silanes/silanols are super-reactive and unstable at STP, but not at LN2 temperatures. Seems like silicon-based biochemistry and LN2 solvent were made for each other! $\endgroup$ – Logan R. Kearsley Dec 12 '17 at 23:15
  • $\begingroup$ Now, that doesn't necessarily imply anything particularly useful about ScN2, but since near-critical fluids do have a general tendency to dissolve more stuff than the corresponding liquids (in particular, being miscible with gasses), that does suggest that ScN2 might also be suitable. The implications from NAPs Limits of Organic Life in Planetary Systems, which reference's Bain's work, seem to be that there is insufficient experimental evidence to rule out LN2 or ScN2 as a biosolvent, so it's good enough for sci-fi.... $\endgroup$ – Logan R. Kearsley Dec 12 '17 at 23:19

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