[jr2]

Sep 24, 2004
Law-breaking liquid defies the rules

Physicists in France have discovered a liquid that "freezes" when it is heated. Marie Plazanet and colleagues at the Université Joseph Fourier and the Institut Laue-Langevin, both in Grenoble, found that a simple solution composed of two organic compounds becomes a solid when it is heated to temperatures between 45 and 75°C, and becomes a liquid when cooled again. The team says that hydrogen bonds are responsible for this novel behaviour (M Plazanet et al. 2004 J. Chem. Phys 121 5031).

Solids usually melt when they are heated, and liquids turn into gas, although exceptions do exist when heating leads to chemical changes that cannot be reversed, such as polymerisation. However, a reversible transition in which a liquid becomes a solid when heated has never been observed until now.

Plazanet and colleagues prepared a liquid solution containing α-cyclodextrine (αCD), water and 4-methylpyridine (4MP). Cyclodextrines are cyclic structures containing hydroxyl end groups that can form hydrogen bonds with either the 4MP or water molecules.

At room temperature, up to 300 grams of αCD can be dissolved in a litre of 4MP. The resulting solution is homogenous and transparent, but it becomes a milky-white solid when heated. The temperature at which it becomes a solid falls as the concentration of αCD increases.

Neutron-scattering studies revealed that the solid phase is a "sol-gel" system in which the formation of hydrogen bonds between the αCD and the 4MP leads to an ordered, rigid structure. At lower temperatures, however, the hydrogen bonds tend to break and reform within the αCD, which results in the solution becoming a liquid again.

Molecular dynamics simulations by Plazanet and co-workers confirmed that the cyclodextrine ring becomes distorted as it is heated up to close to the solidification temperature. The hydrogen bonds within the αCD break and the hydroxyl groups rotate towards the outside, which allows a network of bonds to form between the different molecules. The team has found a number of cyclodextrine/pyridine systems that also become solid when heated, and is now looking more closely at the structure of the sol-gel system to understand the solidification mechanism in more detail.
About the author

Belle Dumé is Science Writer at PhysicsWeb

[Hellstryker]

what the hell will come next, a replacement for water? O_o

[jr2]

jr2, Bobboau and 0 Guests are viewing this topic.

*waits for wisdom from above* 

[karajorma]

As soon as I read the line about hydrogen bonding I had a pretty good handle on what was going on there. It's one of those things that had you asked me if it could happen I'd have said probably but obviously I wouldn't be able to name any examples. To be honest I'm kinda surprised that this is the first time it has been seen.

The mechanism surprised me a bit though. I'd have expected it to work by ring-opening rather than distortion of the ring. Then again I only ever worked with monosaccharides and that was years ago.

[Nuke]

what would happen if you kept pushing up the temperature? would it become harder/melt/catch on fire/explode? i only ask because of potential applications for high temperature materials, you know things such as rocket engines and fusion reactors

[karajorma]

I'd imagine that it would caramelise if you pushed the temperatures too much higher.

We're basically talking about something not too dissimilar to starch in terms of chemical properties. By the way, in case that sounds tasty I should warn you that the other chemical  4-methylpyridine probably has a putrid fishy smell.


Hydrogen bonding isn't that strong in the scheme of things. It's strong enough to stop water being a gas at room temperature but it's nowhere near as strong as metalic bonds (let alone ionic or covalent bonds like you find in rubies or diamonds).

[Mika]

I don't have access to that journal, but the thing I was wondering is actually the circumstances when the discovery was made. Was it accidental or something which was simulated beforehand?

Mika

[G0atmaster]

another cool chemistry oddity is the so-called "Water Bridge," formed by subjecting pure, deionized water to extremely high voltage:

http://www.physorg.com/news110191847.html

[Hellstryker]

Interesting.. depending on how fast you could get the water to flow and stay within such a confined space you could have quite an interesting weapon there..

[FUBAR-BDHR]

Hmm I was thinking more in the line of shields.   Some kind of array of these bridges that could be instantly frozen when a projectile is detected heading toward an area.

[Hellstryker]

I was thinking somthing more along the lines of a defense grid for military buildings

[FUBAR-BDHR]

Defense grid, shield array what's the diff?  Still does the same thing.

[jr2]

another cool chemistry oddity is the so-called "Water Bridge," formed by subjecting pure, deionized water to extremely high voltage:

http://www.physorg.com/news110191847.html

[G0atmaster]

God makes truly amazing toys 

[karajorma]

Don't troll.

I don't have access to that journal, but the thing I was wondering is actually the circumstances when the discovery was made. Was it accidental or something which was simulated beforehand?

My bet is that it was an accident. I've not used it but 4-methylpyridine seems like a reasonable choice of solvent for an experiment. I can easily imagine a PhD student or post doc wanting to warm the solution up before the experiment and finding it turned solid when he tried.

[G0atmaster]

Were you referring to me, kara?  I didn't know that was considered trolling, sorry sir.

[Mefustae]

Were you referring to me, kara?  I didn't know that was considered trolling, sorry sir.

Oh, don't be coy.

[G0atmaster]

more vocabulary!! what the heck is a coy?!

[redsniper]

www.dictionary.com !!!
DO YOU USE IT!!!

[G0atmaster]

Thank you for that.  No I don't regularly.

So now you WANT me to troll?

Kara says don't troll, Mefustae says not to be coy, whose synonym is shy.  You're sending me mixed signals!