[artistinneedofhelp]

Hello chemistry people of the internet!

I am in need of some advice. I am an artist working on a sculpture installation where as part of the work, I am using the chemical sodium acetate trihydrate C2H9NaO5 (SAT from here on), commonly known as "hot ice". Please forgive that I am not very well versed in chemistry, so I might use the wrong terms for things, but I am eager to learn and would really appreciate any help.

I am making large PVC bags of SAT, and I need it to stay supercooled in liquid form for several weeks. In my smaller scale (about 2 litres) tests I have been able to make this work no problem, but moving up to larger scale (about 20 litres), I have had trouble with getting the SAT to stay in clear liquid form. Even when I did get it to stay as clear liquid after melting down the SAT, filling the bag with it and cooling it down, after about three days it started forming white crystal formations inside the liquid. It still hasn't reacted and turned solid after more than a week, and actually seems surprisingly stable in this state (hasn't reacted even when the bag was moved), but still what I am aiming for is a clear bag of liquid.

So there are a few things I have suspected of having an impact on this, but I'd really appreciate your input if you have any more ideas or can expand on mine:

- Adding more water (as long as there is enough) does not solve the problem – it doesn't make it any less prone to crystallising, it just makes the SAT be softer when it is in its solid form.

- However, how the bag of SAT is cooled down has a clear effect on it staying liquid or not. As the several instructions for making "hot ice" I've read suggest, it should be cooled down faster than in just room temperature. I have filled the bags with the SAT when it still has been hot after melting it down, since it is virtually impossible to fill the bags with room-temperature SAT without it reacting and turning hot and solid. After filling the bag with hot SAT and sealing the bag shut, I have either put the bag in a cold water bath or taken it outside to cool in the nighttime when it is colder. Now if I, for example leave it outside to cool for too long, it seems that the corners of the bag cool down too fast compared to the centre mass of the bag, and it starts forming white crystals at the corners.  If, however I just leave it in room temperature to cool, it just turns solid as it cools slowly. So it's clear I do need to cool it down faster, but somehow I should manage to make it cool down in a uniform manner. Any tips on this? Is it better the faster I can get it to cool, or is there a sweet spot where it happens fast enough but not too fast?

Here's the material I am using: http://www.ebay.co.uk/itm/161509704151?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
I am also filtering it when melting it down, as I noticed it does have some visible impurities in it.

- Now another thing I have considered could be the issue is the type of plastic. I made most of the smaller scale tests using large "Minigrip" ziploc bags, and with those it worked well. Those are made of PE polyethylene plastic. However, the problem with PE plastic for my use is that in the thickness I've been able to buy it in my area, it is too weak – especially at the seams when I heat seal them. In general it tends to not take well to heat, and I do need to fill the bags when the SAT is hot, and in large quantities/weight, the test bag I made with PE plastic wouldn't hold it, but broke at the seams. So I moved to PVC plastic, with which I have been able to make very strong seams, and which can sustain high temperatures without blistering or breaking. But so, I have been able to fill PVC bags of SAT and have it stay clear liquid for several days, but in this large quantity of 20 litres, so far it always starts to crystallize at some point, even if it doesn't actually turn solid.

Any ideas on what I could still try, or how I could improve my process? I am not working in laboratory conditions, but have tried to make my working process and environment as clean as possible. If you have any questions, please ask.

I would very much appreciate any notes or suggestions. The exhibition is coming up soon and I'm getting a bit anxious thinking if I should somehow change my plans.

[Arkcon]

This is a cool project, but you may be facing some obstacles that are beyond your reach.  Furthermore, I don't see the point ... you want to make a supersaturated sodium acetate solution, you want it to stay clear ... why not bags of water?  OK, I assume eventually you intend to allow it to crystallize eventually ...

The whole trick is that sodium acetate is prone to supersaturation https://en.wikipedia.org/wiki/Sodium_acetate#Heating_pad  The trick will only work if there is too much sodium acetate to stay in solution, that is in solution.  So adding more water will ruin that effect.  The recipe you're following, dissolving sodium acetate in its own water, is a pretty cool trick. 

There is also a need to avoid nucleation sites.  These are tiny crystals of sodium acetate or even common dust.  You will have to prevent these from being there, and the larger volume you have, the more likely something will slip by.  You can try filtering, but that will become problematical to do to a large volume of hot liquid.

There's also the matter of the container.  Rough or easily scratched materials can make a nucleation site as well.  So you've got to shop around to look for a container that's the best.

[artistinneedofhelp]

Thanks for the quick reply Arkcon, I really appreciate it! 

"The trick will only work if there is too much sodium acetate to say in solution, that is in solution." Sorry but umm, what does this mean? To say in solution that is in solution?

If I understand what you mean, I think that I hadn't quite really understood this supersaturation thing, so now I'm thinking that the problem might have been just me adding water! I mean, I didn't add much, but since I had been reading that if you overheat it so that the water in it evaporates away, and it doesn't stay liquid, then add more water, and also that you can make sodium acetate trihydrate from sodium acetate anhydrous by adding water, so all of this added up in my mind as "hey, try adding a bit of water!". I didn't add much, but still. I could at least TRY a batch without adding any now and see if that works.

And yes, indeed I am intending to have it crystallise eventually during the exhibition, so a bag of water is not quite the same.  There is a knife hanging above the bag from an electromagnet and some unsuspecting viewer will (via a sensor) trigger the electromagnet to drop the knife, which has a bit of sodium acetate crystals on it to act as a nucleation site once it punctures the bag. I have already managed to do this twice (with 20 litres) and film it too. So I know it's possible, it's just that now I've had trouble with getting the liquid to stay clear for a longer time.

[Arkcon]

I meant exactly what I said.  You have dilute solutions, strong solutions, saturated solutions and then you have supersaturated solutions.  A supersaturated solution is a solution that has more than can dissolve under those conditions.  At any time, it will dump the excess solute out.  Particularly with the trigger you have in mind.  Like I said, disrupting the solution by vibrating it may be enough, which is how those heating packs I linked to work.  Rough surfaces, or dust are also a problem.  Diluting the solution won't give you the effect you want.

[Dan]

There is a knife hanging above the bag from an electromagnet and some unsuspecting viewer will (via a sensor) trigger the electromagnet to drop the knife, which has a bit of sodium acetate crystals on it to act as a nucleation site once it punctures the bag. I have already managed to do this twice (with 20 litres) and film it too.

Cool.

You say the crystallization problem is in the corners and you are heat sealing. So if I understand correctly, you are getting crystallisation at the seams?

Maybe the seams are too rough and you are getting nucleation sites there. Could you take a single sheet and make a bag by tying it up like a lolly pop wrapper (can't think of a better description, hope it makes sense), so there are no seams?

[artistinneedofhelp]

Great, thank you for the replies, I really appreciate the comments!

I'm open to the possibility of being wrong, but I don't think that the seams are the problem. The seams don't really get rough from heat sealing them, they are actually quite smooth.  To me it seems like the reason the crystallisation has at times started from the corners is because the corners of the pillow-like bag are thinner and thus cool faster than the bulk of the bag. And from my experience, it's not good for the liquid to cool unevenly. In my smaller scale tests I noticed that if one part of the bag was touching something very cold (like a frozen cooling pack), this is a problem and causes crystallisation. Instead, it works better eg. in an ice bath of cold water where the bag is surrounded by cool water in a uniform manner.

I'm going to try making a new large bag this weekend, this time not adding water, or if I absolutely have to, very little and distilled.

I will also be extra careful with the heat, and not overheat the solution. This was suggested to me now too somewhere else. That I don't let it get hotter than it needs to in order to melt down to liquid form.

I'll let you know if this works! If you have any extra tips or suggestions, I'd be happy to hear them!

[DrCMS]

Could you try it by putting the solid sodium acetate trihydrate into a bag, sealing it up, heating it up in a hot water bath to melt, sealing the bag again without any air pockets and finally cooling the bag  down in a cold water bath?

[artistinneedofhelp]

Hi DrCMS! Thank you for the comment!

What you're suggesting does indeed work in smaller scale, but in larger scale like with the 20 litre bags, it's not really a viable option since the bag doesn't fit into any pots that would fit the stove top, to have it in boiling hot water. And when I've tried melting the SAT down in a large tub with hot water, and keep swapping out some with boiling water while covering the thing in a silver blanket that keeps the heat in, it still takes FOREVER for the chemical to melt. And this is in smaller scale, so with a 20 litre bag, I don't think this is really an option for me.

[artistinneedofhelp]

Hi!

If any of you are interested, here you can see a little snippet of what I was working on: https://vimeo.com/145652999

This is a trailer for the actual exhibition that is up in Helsinki, which includes both video and sculptures made of sodium acetate trihydrate.

Thank you for you comments and *delete me* I had to remelt the sodium acetate trihydrate so many times and work really long hours, but by the end of it I managed to make these large 25 litre "bags" of it to stay as a supercooled liquid that I could then set off by dropping a knife into it.  It didn't stay completely clear like water, but instead started forming these crazy beautiful crystals inside the liquid, but somehow it still stayed liquid and didn't affect the actual reaction.

Anyhoo, thanks again!

[Arkcon]

That was pretty cool, I wasn't expecting the crystalization to look like that.  Now, you have to look at the artistic message of conduction this experiment on an even larger scale.  Like this artist did: https://en.wikipedia.org/wiki/Copper%28II%29_sulfate#Art

[Borek]

Thanks for sharing!