# Does hydrazine crystallize when freezing?

When discussing problems of keeping hydrazine from freezing on spacecraft we stumbled upon a question: can pure hydrazine separate through fraction-freezing from the stabilizing substances like UDMH? That would largely depend on what form it takes. If it crystallizes, it's a risk - it's quite unstable when pure. If it turns amorphous, embedding the stabilizer within the structure, it's moot. Publicly available resources are quite skimpy on descriptions of freezing hydrazine (or I just suck at searching). So, -

Does hydrazine crystallize when freezing? Can it trigger fraction-freezing separation if it's the first freezing substance?

• Pretty much everything crystallizes when freezing. The exceptions are relatively rare. – Ivan Neretin Dec 6 '18 at 10:06
• @IvanNeretin: non-polar substances tend to just grow more dense gradually turning into amorphous solids. – SF. Dec 6 '18 at 10:28
• Even if this were mostly true (which it isn't), it doesn't apply to hydrazine, which is polar. – Ivan Neretin Dec 6 '18 at 10:30
• @SF. - There are a hundred-odd stable(ish) elements that all seem to have crystalline solid phases... – Jon Custer Dec 6 '18 at 14:58
• @JonCuster: And there are millions of complex hydrocarbon compounds with lengthy chains that don't.... – SF. Dec 6 '18 at 15:17

I would take a different position on answering the question, and look at the water-hydrazine binary phase diagram. Looking at J. A. McMillan and S.C. Los, J. Chemical Physics 42(1) 160-161 (1965) one finds the experimentally determined phase diagram. With enough water in the mix, one proceeds smoothly from the pure hydrazine melting point down to a slight eutectic near the monohydride, but only 3K lower in temperature.

Further, the same authors have another paper, J. Chem Phys 42(3) 829-834 (1965) that discusses the nonequilibrium phase transformations. The first sentence in the abstract says

Although neither hydrazine nor water supercool to a large extent, their mixtures do. Aqueous solutions of hydrazine may be strongly supercooled and stabilized as rigid glasses.

In the introductory paragraph, one also finds

Since the phase diagram of this system is known, it was tempting to look for a correlation between the irreversible processes of crystallization and the equilibrium phase transformations. The hydrazine-water system is particularly useful for this purpose, since crystallization can indeed be induced in the entire range of concentration. Samples of different hydrazine content were then supercooled until glass stabilization occurred and then warmed up at different rates. The processes of glass transformation and crystallization were studied, and values of the free-energy barriers could be obtained for the latter process.

The rest of this second paper is devoted to measuring and describing thermodynamically the crystallization curves at different water-hydrazine compostions.

So, yes, you can definitely get a glass, if cooled quickly enough, but it just might decide to phase separate on you if it warms up a bit.

For the case of (only) hydrazine-1,1-dimethylhydrazine, the phase diagram is covered in J. A. McMillan, Journal of Chemical and Engineering Data 12(1) 39-40 (1967). This shows a very slow decrease in the liquidus temperature as 1,1-dimethylhydrazine is added, with the eutectic occurring at 94% mole percent 1,1-dimethylhydrazine at 214K, only 61K below the pure hydrazine melting point. There are no compounds. Further, the article states

Unlike the systems of the components with water, the mixture hydrazine-1,1-dimethylhydrazine does not supercool

which indicates that phase separation and crystallization of the two components will readily occur. On the other hand, the samples were sealed in glass under nitrogen, and if kept in the dark were stable over a 6 month period.

• Unfortunately hydrazine for use as rocket fuel is stabilized with UDMH, not water, which would be very bad for performance of the propellant. – SF. Dec 6 '18 at 17:05
• @SF. - noted. additional information added specifically on hydrazine-1,1-dimethylhydrazine. It isn't a particularly good antifreeze... – Jon Custer Dec 6 '18 at 17:57
• It was good enough for the military :) Huge thanks for the great answer! – SF. Dec 6 '18 at 23:24

I found an old research paper which states that hydrazine forms monoclinic crystalline structure in molecule pairs.

Given the ongoing discussions on whether a rocket-fuel mixture separates or not as it cools, I'm adding this paper which says hydrazine monohydrate forms trigonal crystals. I admit to massive ignorance as to the exact compounds used in fuels.

• The usual hydrazine fuel is stabilized with UDMH (Unsymmetric dimetylhydrazine, $NH_2 - N(CH_3)_2$; properties similar to hydrazine but much more stable, −57 °C freezing point and acts both as stabilizer and antifreeze agent on plain hydrazine.) – SF. Dec 6 '18 at 15:23