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What substance that is liquid at STP has the lowest freezing/melting point? I think it's either 2-methylpentane or 3-methylpentane, but I get different numbers from different sources.

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    $\begingroup$ Shouldn't you be looking at very volatile liquids then (i.e. substances which boil barely above room temperature)? For example, n-pentane is still a liquid at STP and it freezes at lower temperatures than either of your suggestions. Methylbutane is even better. $\endgroup$ – Nicolau Saker Neto Mar 7 '15 at 16:42
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    $\begingroup$ A completely different but interesting possibility is very low melting point ionic liquids. Also, if you relax the requirement that the liquid must be a pure substance, you can look into deep eutetic solvents. $\endgroup$ – Nicolau Saker Neto Mar 7 '15 at 18:01
  • $\begingroup$ -196°C (!!) That matches the figures for the most recent copy I could find online of the CRC Handbook of Chemistry and Physics. And no, it wouldn't have to be a pure substance so I'll check out your suggestions. Thanks so much!! :) $\endgroup$ – M.Hutson Mar 8 '15 at 19:00
  • $\begingroup$ Slightly disappointed to find upon further reading (chemicalbook.com/ChemicalProductProperty_EN_CB9465749.htm) that this chemical is so reactive that its applications would be sharply limited. $\endgroup$ – M.Hutson Mar 8 '15 at 19:35
  • $\begingroup$ I wouldn't classify N,N-diethylmethylamine as "extremely reactive", especially not at cryogenic temperatures. It's very similar to triethylamine, which is commonly used in the lab as a mildly strong base which is stable to decomposition in a wide range of conditions. It is also mildly toxic, however, in much the same way ammonia is toxic; it can be safely handled, but care is required to avoid excessive exposure. What exactly is the use you had planned for these cryogenic fluids? $\endgroup$ – Nicolau Saker Neto Mar 8 '15 at 19:43
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After a bit of random searching through small molecules, I found this very large dataset with 28,645 experimentally measured melting points. Sorting them by increasing melting point and checking the first few entries individually, it seems that N,N-diethylmethylamine holds the record, with a melting point of -196°C (!) and a boiling point of 63-65°C.

I also mentioned another possibility for very low melting liquid substances at room temperature, namely ionic liquids. Even though the species present are much larger and have ionic interactions, presumably the ions pack so extremely poorly in the solid that freezing is very unfavourable. Aldrich already offers multigram quantities of ionic liquids melting at -88°C, and ongoing research offers even lower melting alternatives, such as the ones in this article which purportedly do not crystallize until -150°C. This limit may well be improved upon in the next few years.

If mixtures are allowed as well as pure substances, then a combination of small organic molecules or ionic liquids will very likely reach even lower temperatures than the examples above without freezing.

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  • $\begingroup$ Great answer. Quick unrelated side question: why is "N,N-diethylmethylamine" not called "N-ethyl-N-methyl-ethylamine"? (And you are right that e.g. Sigma-Aldrich uses the same name as you)...is that a colloquialism or the bona fide correct name? $\endgroup$ – Curt F. Mar 8 '15 at 5:08
  • $\begingroup$ @CurtF. I'm not sure, I think the second is formally more correct (IUPAC actually would recommend N-ethyl-N-methylethan-1-amine) but the first is just more concise and easier to pronounce. $\endgroup$ – Nicolau Saker Neto Mar 8 '15 at 11:09

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