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Very low temps with Isopropanol/Ice mix?

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LAN_403

Backyard Skunkworks

Sun Apr 06 2008, 05:43AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

After reading about a brine solution and how it doesnt freeze until around 0F (-18C) I thought that if the salty water were replaced with a solvent that has very little hydrogen bonding and low freeze point (such as isopropanol) that the temp would go very low.

Heres my theory:
Given that a lot of heat is lost in order for ice to form, an equal amount of heat must be gained in order for the ice to melt, when you add isopropanol almost none of the water gets a chance to re-freeze and as a result the iso cools down. Shouldn't this continue to happen until it reachs it's freezing point of -89C (-128F)?

Given how easy it is to recover the isopropanol (with salting out) wouldn't this be a pretty good method to get within 61C (116F) of cryogenic temps?

Or am I totally off base with this?

I'll try some experiments soon...

Bored Chemist

Sun Apr 06 2008, 09:05AM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

Ice and methanol baths are fairly commonly used to cool things in labs. I don't know how well IPA would work but, as you say, it's easy to recover most of it so it's worth a try.
There's snow on the ground here in the UK so perhaps I might try it later.

quicksilver

Sun Apr 06 2008, 03:00PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

I once used acetone and dry-ice. THAT was as cold a bath as I could piece together. Nice thing was that the acetone was not corrupted by the CO2 and could be re-used. ..An all together great way to achieve sub zero.

Backyard Skunkworks

Tue Apr 08 2008, 06:12AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

I'll be trying this out in a few days, the theory behind this seems pretty sound. The idea seems to be that single molecules of the ice will be randomly melting (at even very low temps) due to molecular chaos, each one will require some energy in the form of heat to undergo phase transition into liquid and as a result heat is gained from the IPA, causing the temp to drop.

Anyone have any ideas on things I might try dissolving in the IPA to further depress the freezing point?

Dr. Shark

Tue Apr 08 2008, 08:22AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Sorry to sound negative, but maybe I'll save you disappointment down the road. I am not quite sure I understand what your theory is, but these things can actually be calculated, and that is done using thermodynamic quantities like entropy and enthalpy rather than vague ideas about hydrogen bonds.
From what I remember from a list of cooling mixtures, the best you can do is with ammonium nitrate, which may get you to the range of -40C.
This is actually the mix used in disposable cold packs sold for sports injuries, so if you can find some of those, I suggest giving it a try.

Bored Chemist

Tue Apr 08 2008, 04:58PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

Unless you happen to know the enthalpy and entropy changes for ice dissolving in aqueous IPA at various temperatures, the thermodynamics isn't as useful as some ice, some IPA and a thermometer.
To the extent that anyone knows those quantities they were measured by studies of melting ice in IPA. It's practically impossible to calculate them from first principles.

Backyard Skunkworks

Sat Apr 12 2008, 03:26AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

I did a quick mini-experiment where I took ice at -13C from my freezer, crushed it and added 20ml of >99% IPA. The resulting mixture reached a temp of... -13C! Hmm... looks like a failure? I'll try with a slightly larger quantity.

Odd thing is the ice didn't melt at all, over a few hours. Am I wrong that ice will still melt in tiny amounts even below normal freezing temp? Am I wrong that melting ice is endothermic?

...

Sat Apr 12 2008, 05:19AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I would agree that you are incorrect in your assumption that ice will melt below its freezing point.

Imagine taking a block of aluminum (ie, frozen form of liquid aluminum) and putting it in a tub of IPA, I wouldn't expect the aluminum block to start melting....

Bored Chemist

Thu Apr 17 2008, 05:35PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

Well, it snowed again today so I decided to try this.
I got lots of snow and put it in a bowl. I then put the (sealed) tin of IPA in it.
After a while the tin of IPA was cooled down to near 0C.
I took 100g of the (rather wet) snow and put it in a thermos flask. Then I added 100g of IPA.
I sealed the flask and shook it for a while. Then I measured the temperature- it fell to about -17C.
About as good as a domestic freezer.

What@id like to find is some data about eutectic points for mixtures of alcohols and ice.

Backyard Skunkworks

Fri Apr 18 2008, 12:40AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

Great to hear that it does indeed work to some degree, I'll try that large scale experiment soon also and see if I can get it to go past -20ish.

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