[juul]

I'm looking for a compound or solution with a melting point at, or close to, -80 C. It should remain liquid up to at least 50 C and should not be flammable or especially toxic.

I'm not really sure how to approach this problem so I'm not so much looking for someone to hand me the answer and more for a bit of guidance in what I need to understand and where I can learn more.

(btw I tried to read the rules first but clicking the link toward the top gives "Access denied for user ''@'localhost' (using password: NO)")

[Mitch]

Try typing freezing points for common solvents into your preferred search engine. Here is a list I found.

https://www.chemistry.mcmaster.ca/mcnulty/Solvent%2520Properties.pdf

Thank you for pointing out the forum rules link is broken, we'll investigate.

[Enthalpy]

Some silicone oils may fit your request. Wacker's AK10 has -80°C pour point, I didn't find its flash point immediately.

Alkanes are less easy, but phytane and farnesane fit. One expensive, farnesane was produced experimentally in kg quantity by
https://amyris.com/
Whether they went to mass production and sales? Or would you produce it, say from nerolidol or citronellal?
https://www.chemicalforums.com/index.php?topic=56069.msg338107#msg338107
https://www.chemicalforums.com/index.php?topic=56069.msg326163#msg326163
JP-10 = exo-tetrahydrodicyclopentadiene, a rocket and jet fuel used among others by cruise missiles, has mp=-79°C or -85°C and fp>+55°C
http://webbook.nist.gov/cgi/cbook.cgi?ID=2825-82-3&Units=SI
My frenzy about alkanes and amines with a wide liquid range, there:
https://www.chemicalforums.com/index.php?topic=56069.0

You might have a look at brake fluids and hydraulic fluids, but -80°C is uncommon for them. Airliners need such hydraulic fluids.

Some fluorocarbons would answer the request nicely but they are in the process of legal banning.

Could you detail the use or the additional desired properties? Good lubricant, insulator, cheap, transparent, long-lasting under sunlight and air, water-repellent, non-corrosive, good coolant...? Make or buy?

[juul]

Could you detail the use or the additional desired properties? Good lubricant, insulator, cheap, transparent, long-lasting under sunlight and air, water-repellent, non-corrosive, good coolant...? Make or buy?

Wow thanks for all that info! I'll look into your suggestions immediately. The use is as a thermal buffer to prevent biomedical samples from rising much above -80 for some time during power loss for areas with unreliable power. Large enthalpy of fusion would be good. It will be sealed from sunlight and air. It doesn't have to be non-corrosive. The safer it is for humans and environment the better. No preference for make vs. buy.

Thanks again!

[juul]

Oh and I should mention that keeping the cost down is important. We'll need at least 1000 ml of this per freezer backup unit.

[juul]

It seems that NASA already has something like this but I haven't been able to get more info on it

[Mitch]

I think what you need are those cold gel packs: https://en.wikipedia.org/wiki/Ice_pack

Making the gel shouldn't be that expensive.

[Enthalpy]

I hadn't found quickly a mixture of water and antifreeze that melts at -80°C. Not propylene glycol, not ethylene glycol, not erythritol, not glycerol, but possibly a mixture of several glycols in water, remembering that ethylene glycol is a bit toxic. Nasa's attempts were possibly in this direction, rather than an alkane, if they sought compatibility with polyethylene.

Besides water and alcohols, alkanes offer a big fusion heat, fluoroalkanes less so. They are not always compatible with polyethyleneS and polypropyleneS, but with other usual plastics they are. Non-flammable criterium suggests the molecule shall be bigger than 2,6-dimethyloctane, and melting point criterium tells it must be branched.

Farnesane is a first idea

bp=+243°C mp<-71°C fp+109°C wow, so it does burn with a wick or as a mist, but matches won't light a leak on concrete. Other sources tell mp around -100°C. Farnesane makes 99,76% of the AMD-200 fuel attempt by Amyris, where many kg were produced. My feeling is that the company meanwhile targeted easier markets than jet fuels.
BEWARE the syntheses I suggest in the linked threads are NOT tried and I'm no chemist, so they can be complete cr*p.

2,4,6-trimethyl-dodecane has phase-change properties similar to farnesane. It seems rare too.


Unless a company like Amyris decide to produce these compounds, they are expensive lab rarities, so synthesising would be cheaper. And if you produce such an alkane, other uses and customers await you.

JP-10 is mass-produced

it melts at -79°C from NIST (Chickos et al is doubtful), and its flash point is very little above +55°C. Its kerosene smell can be misleading.

Grafting an alkane tail to cheap myrcene, then saturating everything, could be a flexible design, as the tail's length adjusts the melting point. Here myrcene


Do not believe melting points in general. Measures are rare, most values are software estimates, but these fail grossly on melting points.

[juul]

Wow, thanks again Enthalpy for all this info! I definitely have some new directions to pursue