Following this video, @Linda Linsefors and I tried to make a phase change material with a freezing point around 18°C.
Report:
For our first batch (which is all we’ve made so far), we started with 2 l water / 400 ml sodium sulphate / 100 ml table salt. There was a lot of salt left over after several minutes of stirring boiling/near-boiling water, so we added another 500 ml water. There was still a lot of salt but we stopped there.
We added 30 ml xanthan gum. (By the recipe it should have been more like 40 because of the extra water. The goop we ended up with is pretty good goop consistency, but I do think I’d prefer slightly thicker.) The first teaspoon clumped up a lot in the water, so later ones we ran through a sieve which helped a lot.
We boiled and stirred that for about 30 minutes and left it to cool.
Note that sodium sulphate comes in “anhydrous” and “decahydrate” forms. The latter already has 10 water molecules attached, so less of it dissolves and you get less actual salt. We used anhydrous. I kind of assume the guy in the video used that too (it would be weird if he got better dissolving than us with decahydrate) but idk if it’s clarified anywhere. It’s presumably possible to use decahydrate if you adjust quantities appropriately.
We got something that kinda worked, but I was suspicious that it wasn’t behaving like it should, so I got a temperature logging probe (note: needs initial setup with software that’s available for windows or mac, not linux; I used wireshark to capture the USB traffic so maybe I can make linux software but it’s not a priority). Here’s the graph when I put some of it (at a guess about 50 ml) in the freezer and then thawed it:
So yeah. The thing around 3-5° going down could be either freezing or “freezer is temporarily off (as I interpret the rises later on) but ambient temperature is low enough to keep cooling”. Claude thinks there wasn’t nearly enough sodium sulphate, and it’s basically just salty water, which freezes at that temperature; and which apparently has a smooth melting curve. Conversation (I mistakenly said 60 ml xanthan gum there). I’m not very convinced by its recommendations.
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We separately wanted to see what happens when you just mix sodium sulphate and water, because something that freezes at 32° sounded cool. We started with 200 ml water, tried to keep it around 40°, and added sodium sulphate until it stopped dissolving. That ended up being about 80 ml. It should support about 100 g. Google gives density of the powder as 1.4-1.6 g/ml (a crystal is about 2.7 g/ml) so we’d be at about 120 g, there are enough errors in the process that this doesn’t seem off to me.
What we ended up with was something that freezes in the fridge, but not at room temperature (which was about 28° when we tried). I’m currently thawing it so I can put the probe in, and then I’ll freeze-and-thaw it again and see what happens.
I think it took a few freeze-thaw cycles before it seemed like it would fully freeze? First time it looked more like “clear ice with some water not yet frozen”, now it looks like white ice.
e: okay, it hasn’t thawed at all after several hours. I’m pretty sure it just actually freezes above room temperature, and what I saw before was probably just some combination of supercooling and “not fully saturated” and idk what else.
By weight, the Na2SO4:NaCl ratios range from 2 to 5, so maybe it’s not very sensitive to that? (Nighthawk’s is 5, and Claude’s suggested recipe has 5.5.)
But Nighthawk is a wild outlier for H20:Na2SO4. He gives 3.3x by weight, every other recipe is 1.2 to 1.3.
But if Linda and I had too much water, why didn’t it all dissolve? Claude’s “probably because of the NaCl” sounds weird to me because Nighthawk has less NaCl-to-Na2SO4 than any other recipe. Hypothesis: we just didn’t give it long enough / stir well enough.
I’m going to try making 2E-3SPA from this paper, which (by weight) is 51% water, 39% Na2SO4, 10% NaCl. (They use sodium polyacrylate as a thickener and borax as a nucleating agent, which I’m substituting with xanthan gum and “nothing at first, I’ll try diatomaceous earth if I need one because borax is hard to get in the UK. 2E-3SPA has higher latent heat than the other one they make, and similar melting point.
So recipe I plan to try, mixing at ~40°C:
500 ml water
390 g Na2SO4
95 g NaCl
5 ml xanthan gum
Except that I’m going to add the salts gradually, instead of all at once, and keep going at least until they stop dissolving. So in practice I’ll do… it’s going to be convenient to use measuring spoons, not scales, so probably 15 ml Na2SO4 and 5 ml NaCl at a time, which should be 24 g and 6 g. That gives me a bit less NaCl than the recipe, but this recipe was on the high side for NaCl.
(But I should check the density of my salts.)
If they seem to stop dissolving long before I think I’ve put enough in, then, uh, I dunno.
Update:
First complication is that my scales and my measuring jug don’t agree with each other. I trust the jug more than the scales (which are cheap “rotary dial and spring” ones that I’ve had over 10 years), though I didn’t specifically check the jug versus my measuring spoons. 200 ml water (according to the jug) was about 175 g, and 400 ml was about 350 g.
Then 45 ml of Na2SO4 (according to my measuring spoons) was about 60 g on the scales. If we scale that up by 8⁄7 we get 69, which is close enough to the 72 I was expecting.
But 45 ml of NaCl was also about 60 g on the scales. Maybe more like 55 g.
I think the scales are just not good enough to confirm or disconfirm anything. I was expecting an actual weight of 54 g which would be 47 if the scales have a 7⁄8 multiplier.
What’s probably going on is that the scales just aren’t very consistent, especially near 0, and also not very precise, and I can’t use them to confirm or disconfirm much of anything.
Anyway, I took 500 ml water and added 15 ml Na2SO4 + 5 ml NaCl at a time, except 3x those quantities for the first load. I’d expect about 16 loads (16 * 15 * 1.6 = 384 g Na2SO4). After 7 loads it seemed like there was a small-but-clear amount not dissolving any more, and after 8 there was a lot not dissolving even though I gave it at least 10 minutes with plenty of stirring. So, uh, wtf.
At that point I started adding xanthan gum. 10 ml, not 5, because I noticed 5 was a miscalculation. I’m letting it cool and set now. I’m not optimistic it’ll work.
(It probably could have used more time in the freezer, but it’s clearly not freezing in the fridge.)
One hypothesis is that my water is too hard (269 ppm calcium carbonate) or otherwise impure. If so, possible avenues for fixing it are buying distilled water, buying regular bottled water, and using a water filter (quick google: apparently that doesn’t help with hardness).
Another hypothesis is that something about my salts is fucked. Impure sodium sulphate? The table salt comes with anti-caking agents, are those causing a problem? The table salt has also been just hanging out in my kitchen in this kind of thing, possibly with some dried beans in there to stop it clumping; maybe I should try some fresh salt?
I’ve done three experiments so far, and very roughly:
3:1 Na2SO4:NaCl by volume (most recent): I got 210 ml Na2SO4 in 1 l water (7x15 ml, in 500 ml)
4:1 Na2SO4:NaCl by volume (Nighthawk’s recipe): I got 160 ml Na2SO4 in 1 l water (400 ml, in 2.5 l)
No NaCl: I got 400 ml Na2SO4 in 1 l water (80 ml in 200 ml)
So there’s not even a clear trend for “more NaCl means less Na2SO4 dissolved”. Maybe because I did nighthawk’s at close to boiling? And/or because it went via the kettle?
Still, regardless of what my numbers show, more NaCl must mean less Na2SO4 dissolves, right? So maybe the next thing to try is, like, 10:1?
Following this video, @Linda Linsefors and I tried to make a phase change material with a freezing point around 18°C.
Report:
For our first batch (which is all we’ve made so far), we started with 2 l water / 400 ml sodium sulphate / 100 ml table salt. There was a lot of salt left over after several minutes of stirring boiling/near-boiling water, so we added another 500 ml water. There was still a lot of salt but we stopped there.
We added 30 ml xanthan gum. (By the recipe it should have been more like 40 because of the extra water. The goop we ended up with is pretty good goop consistency, but I do think I’d prefer slightly thicker.) The first teaspoon clumped up a lot in the water, so later ones we ran through a sieve which helped a lot.
We boiled and stirred that for about 30 minutes and left it to cool.
Note that sodium sulphate comes in “anhydrous” and “decahydrate” forms. The latter already has 10 water molecules attached, so less of it dissolves and you get less actual salt. We used anhydrous. I kind of assume the guy in the video used that too (it would be weird if he got better dissolving than us with decahydrate) but idk if it’s clarified anywhere. It’s presumably possible to use decahydrate if you adjust quantities appropriately.
We got something that kinda worked, but I was suspicious that it wasn’t behaving like it should, so I got a temperature logging probe (note: needs initial setup with software that’s available for windows or mac, not linux; I used wireshark to capture the USB traffic so maybe I can make linux software but it’s not a priority). Here’s the graph when I put some of it (at a guess about 50 ml) in the freezer and then thawed it:
So yeah. The thing around 3-5° going down could be either freezing or “freezer is temporarily off (as I interpret the rises later on) but ambient temperature is low enough to keep cooling”. Claude thinks there wasn’t nearly enough sodium sulphate, and it’s basically just salty water, which freezes at that temperature; and which apparently has a smooth melting curve. Conversation (I mistakenly said 60 ml xanthan gum there). I’m not very convinced by its recommendations.
---
We separately wanted to see what happens when you just mix sodium sulphate and water, because something that freezes at 32° sounded cool. We started with 200 ml water, tried to keep it around 40°, and added sodium sulphate until it stopped dissolving. That ended up being about 80 ml. It should support about 100 g. Google gives density of the powder as 1.4-1.6 g/ml (a crystal is about 2.7 g/ml) so we’d be at about 120 g, there are enough errors in the process that this doesn’t seem off to me.
What we ended up with was something that freezes in the fridge, but not at room temperature (which was about 28° when we tried). I’m currently thawing it so I can put the probe in, and then I’ll freeze-and-thaw it again and see what happens.
I think it took a few freeze-thaw cycles before it seemed like it would fully freeze? First time it looked more like “clear ice with some water not yet frozen”, now it looks like white ice.
e: okay, it hasn’t thawed at all after several hours. I’m pretty sure it just actually freezes above room temperature, and what I saw before was probably just some combination of supercooling and “not fully saturated” and idk what else.
I found a bunch of recipes online and compared them: https://docs.google.com/spreadsheets/d/1uroXRGgT6fNKtQ9iAyBuVrW8TJlRNA5d6sPmRir-4kY/edit?usp=sharing
By weight, the Na2SO4:NaCl ratios range from 2 to 5, so maybe it’s not very sensitive to that? (Nighthawk’s is 5, and Claude’s suggested recipe has 5.5.)
But Nighthawk is a wild outlier for H20:Na2SO4. He gives 3.3x by weight, every other recipe is 1.2 to 1.3.
But if Linda and I had too much water, why didn’t it all dissolve? Claude’s “probably because of the NaCl” sounds weird to me because Nighthawk has less NaCl-to-Na2SO4 than any other recipe. Hypothesis: we just didn’t give it long enough / stir well enough.
I’m going to try making 2E-3SPA from this paper, which (by weight) is 51% water, 39% Na2SO4, 10% NaCl. (They use sodium polyacrylate as a thickener and borax as a nucleating agent, which I’m substituting with xanthan gum and “nothing at first, I’ll try diatomaceous earth if I need one because borax is hard to get in the UK. 2E-3SPA has higher latent heat than the other one they make, and similar melting point.
So recipe I plan to try, mixing at ~40°C:
500 ml water
390 g Na2SO4
95 g NaCl
5 ml xanthan gum
Except that I’m going to add the salts gradually, instead of all at once, and keep going at least until they stop dissolving. So in practice I’ll do… it’s going to be convenient to use measuring spoons, not scales, so probably 15 ml Na2SO4 and 5 ml NaCl at a time, which should be 24 g and 6 g. That gives me a bit less NaCl than the recipe, but this recipe was on the high side for NaCl.
(But I should check the density of my salts.)
If they seem to stop dissolving long before I think I’ve put enough in, then, uh, I dunno.
Update:
First complication is that my scales and my measuring jug don’t agree with each other. I trust the jug more than the scales (which are cheap “rotary dial and spring” ones that I’ve had over 10 years), though I didn’t specifically check the jug versus my measuring spoons. 200 ml water (according to the jug) was about 175 g, and 400 ml was about 350 g.
Then 45 ml of Na2SO4 (according to my measuring spoons) was about 60 g on the scales. If we scale that up by 8⁄7 we get 69, which is close enough to the 72 I was expecting.
But 45 ml of NaCl was also about 60 g on the scales. Maybe more like 55 g.
I think the scales are just not good enough to confirm or disconfirm anything. I was expecting an actual weight of 54 g which would be 47 if the scales have a 7⁄8 multiplier.
What’s probably going on is that the scales just aren’t very consistent, especially near 0, and also not very precise, and I can’t use them to confirm or disconfirm much of anything.
Anyway, I took 500 ml water and added 15 ml Na2SO4 + 5 ml NaCl at a time, except 3x those quantities for the first load. I’d expect about 16 loads (16 * 15 * 1.6 = 384 g Na2SO4). After 7 loads it seemed like there was a small-but-clear amount not dissolving any more, and after 8 there was a lot not dissolving even though I gave it at least 10 minutes with plenty of stirring. So, uh, wtf.
At that point I started adding xanthan gum. 10 ml, not 5, because I noticed 5 was a miscalculation. I’m letting it cool and set now. I’m not optimistic it’ll work.
Indeed, no luck:
(It probably could have used more time in the freezer, but it’s clearly not freezing in the fridge.)
One hypothesis is that my water is too hard (269 ppm calcium carbonate) or otherwise impure. If so, possible avenues for fixing it are buying distilled water, buying regular bottled water, and using a water filter (quick google: apparently that doesn’t help with hardness).
Another hypothesis is that something about my salts is fucked. Impure sodium sulphate? The table salt comes with anti-caking agents, are those causing a problem? The table salt has also been just hanging out in my kitchen in this kind of thing, possibly with some dried beans in there to stop it clumping; maybe I should try some fresh salt?
I’ve done three experiments so far, and very roughly:
3:1 Na2SO4:NaCl by volume (most recent): I got 210 ml Na2SO4 in 1 l water (7x15 ml, in 500 ml)
4:1 Na2SO4:NaCl by volume (Nighthawk’s recipe): I got 160 ml Na2SO4 in 1 l water (400 ml, in 2.5 l)
No NaCl: I got 400 ml Na2SO4 in 1 l water (80 ml in 200 ml)
So there’s not even a clear trend for “more NaCl means less Na2SO4 dissolved”. Maybe because I did nighthawk’s at close to boiling? And/or because it went via the kettle?
Still, regardless of what my numbers show, more NaCl must mean less Na2SO4 dissolves, right? So maybe the next thing to try is, like, 10:1?