Science-y thought/question

[DeeAnna]

"....when combining sodium citrate with lye should not react..."

Yes, that is correct.

"...but then how do you explain the soap getting softer when adding sodium citrate to the mix? ..."

Well, why not? Just cuz it doesn't react with lye, doesn't mean it can't get into trouble otherwise.

Sodium citrate is a humectant, meaning it wants to draw water to it and form a "complex". To a chemist, a complex is not a disturbing psychological problem; it is a loose association of molecules linked by weak chemical bonds. It's like a bunch of kids who hang out together -- if you find one, you'll find them all. If you add enough sodium citrate to a bar soap, you may see some softening as the sodium citrate molecule pulls water to itself to form a large lump of molecules. This will alter the crystalline structure of the soap framework. A rough analogy might be for my hypothetical bunch of kids to go to a school dance. The bunch of kids is so large, it alters the arrangement of the dancers -- they have to change how they dance so they can move around the bunch of kids.

A sodium ion, on the other hand, is very small and very strong. It wants to pull any and all suitable molecules and ions as close to it as possible. The result is the structure becomes more compact and stronger, rather than larger and weaker. And, yes, Morpheus, this is exactly like the effect of sodium on clay soil.

 

[SoapBro]

"....when combining sodium citrate with lye should not react..."

Yes, that is correct.

"...but then how do you explain the soap getting softer when adding sodium citrate to the mix? ..."

Well, why not? Just cuz it doesn't react with lye, doesn't mean it can't get into trouble otherwise.

Sodium citrate is a humectant, meaning it wants to draw water to it and form a "complex". To a chemist, a complex is not a disturbing psychological problem; it is a loose association of molecules linked by weak chemical bonds. It's like a bunch of kids who hang out together -- if you find one, you'll find them all. If you add enough sodium citrate to a bar soap, you may see some softening as the sodium citrate molecule pulls water to itself to form a large lump of molecules. This will alter the crystalline structure of the soap framework. A rough analogy might be for my hypothetical bunch of kids to go to a school dance. The bunch of kids is so large, it alters the arrangement of the dancers -- they have to change how they dance so they can move around the bunch of kids.

A sodium ion, on the other hand, is very small and very strong. It wants to pull any and all suitable molecules and ions as close to it as possible. The result is the structure becomes more compact and stronger, rather than larger and weaker. And, yes, Morpheus, this is exactly like the effect of sodium on clay soil.

Very interesting, so you're saying that if i add a large amount of sodium citrate to my soap and it becomes very soft, its not that im neutralizing the lye, the soap would be created as usual but it would just be softer because the citrate is affecting its structure?

so if i let the soap stand and dry out in the mold for like 4 weeks, it should eventually harden back up after it drys and i'll be left with a bar of soap that has a very high amount of sodium citrate crystals?

 

[HoneyLady]

Thank you all very much!! :clap:

I'm following (and working hard to understand and keep up with) this thread. I'm keeping notes. There might be a quiz later . . .

I am not a science whiz, by any means. I have to view and review material several times to fully grasp it. (I can write a paper about it with no trouble, but you wouldn't want me as a hands-on lab partner! "I add the water TO the lye, right?" )

Thank you for asking and debating the question. And THANKS (again!) Dee Anna for being such a pal about explaining this to us. You have a knack for communicating the complicated clearly and easily.

Realizing that a complex is not a problem to a scientist, helps me understand several of the geeks I've loved in my lifetime. :-D

Carry on!
~HoneyLady~

 

[reinbeau]

FTLooks to me like y'all are doing fine without me! You have a point about sodium ions -- Na+ -- and hardness. Like most things in life, the idea has some limits, but it's useful to know.

Talking about bar type soaps, there are two components to soap ... the solid soap framework and the liquid film within the solid framework. The liquid film contains water, ions including Na+, other dissolved materials such as sugars and such, and dissolved soap molecules.

Soap, believe it or not, is fairly insoluble when sodium ions are present. The more sodium ions in the liquid fDRilm, the less soap will be dissolved in the liquid. Instead, the soap becomes part of the solid soap framework and this acts to harden bar soap.

There are limits to this in that the short chain, saturated soaps from coconut and PKO are more soluble in the presence of Na+ ions than longer chain saturated soaps (palm, tallow, lard) and unsaturated soaps (olive and the other liquid fats).

So even though a salt soap made with CO gets really hard-like-a-rock, it lathers well. An olive soap would be a dismal failure as a salt bar because it would be hard AND be so insoluble it would not lather well. This is also partly the reason why olive liquid soaps can be thickened by adding table salt, but liquid soaps made with CO do not thicken well with table salt.

I'm not sure I'm really addressing the question directly ... I'm a bit muzzy headed right now, but I hope this is helpful.

DeeAnna, I follow, but should that red soap be salt? If not, I'll crawl back into my corner..... :???:

 

[DeeAnna]

"...so if i let the soap stand and dry out in the mold for like 4 weeks, it should eventually harden back up after it drys and i'll be left with a bar of soap that has a very high amount of sodium citrate crystals? ..."

I don't know the answer to that. Water can be bound to salts fairly tightly or rather loosely. If loosely, yes, that might happen. If tightly, probably not. You'll have to try it.

 

[DeeAnna]

"...should that red soap be salt?..."

"...The more sodium ions in the liquid film, the less soap will be dissolved in the liquid. Instead, the soap becomes part of the solid soap framework and this acts to harden bar soap. ..."

Thanks for asking -- I really did mean soap, but my sentence structure is not the clearest. Soap molecules in a bar soap can be in one of two places. Most are part of the solid crystalline framework of the soap bar. Some are dissolved in the liquid that lies within the solid soap framework. Those soap molecules in the liquid are what I'm talking about.

Did you ever make sugar candy as a kid -- where you put a string into a saturated solution of table sugar and water? Sugar crystals will form on the string over time. If you have, you'll know the sugar can be in two forms -- solid crystals and dissolved sugar. That's kind of how a bar of soap is. More soap is solid and less soap is liquid, but the concept of solid crystals developing from a liquid solution is more or less the same for sugar and for soap.

And this "sugar candy" model, incidentally, is a good way to think of what happens to soap during cure. Some of the hardness and longevity of the soap that comes during cure is due to the simple loss of water. But some of the hardness comes from the gradual development of the solid soap framework. Anything that interferes with the development of the framework during cure will tend to keep the soap softer. Some types of additives as well as polyunsaturated fats (fats high in linoleic or linolenic acids) interfere with the development of the soap framework.

 

[reinbeau]

Awesome, thank you so much, DeeAnna. Your explanations are far clearer and easier to absorb than some of the other explanations I've read out there.

 

[MorpheusPA]

Very interesting, so you're saying that if i add a large amount of sodium citrate to my soap and it becomes very soft, its not that im neutralizing the lye, the soap would be created as usual but it would just be softer because the citrate is affecting its structure?

so if i let the soap stand and dry out in the mold for like 4 weeks, it should eventually harden back up after it drys and i'll be left with a bar of soap that has a very high amount of sodium citrate crystals?

Not necessarily. Like DeeAnna mentioned, the citrate can still soften the soap if it's interfering with the formation of soap crystals (and if the citrate itself is softer than the soap itself).

One of the shortcuts we're all taking is assuming that the stuff is all balanced in the proper amount. Stuff in enough of anything to break up the soap crystals and you're likely to have a problem. Add the proper amount and they'll help hold the crystals together.

So if you add too much sodium lactate, you'd end up with with a really soft soap. Sodium lactate is a liquid at room temperature, and most of us don't account for it in our recipes by discounting water or altering lye amounts.

 

[MorpheusPA]

A sodium ion, on the other hand, is very small and very strong. It wants to pull any and all suitable molecules and ions as close to it as possible. The result is the structure becomes more compact and stronger, rather than larger and weaker. And, yes, Morpheus, this is exactly like the effect of sodium on clay soil.

Yes for soil chemistry, although deflocculation of soil bits from the (single bond) sodium ion also comes into play. We have the same problem with magnesium (a double bond) just because of the ion radius.

Interestingly, if you were to make soap from calcium hydroxide, I'd expect a rock hard (literally rock-like) soap due to the double bonding. Plus feature very low water solubility and horrible, thin lather.

It wouldn't be a nice soap at all, but we shoot for 75% Ca saturation in soils due to the double bond aspect. It tends to pull soils into bits (flocculate it) that allow water and air in around it.

Soap is a very different animal than soil, and we're working at the molecular level, not the sub-mm particle level as we do with soils. But it's sure fun how the same ions and chemistry come into play with both!

 

[SoapBro]

"...so if i let the soap stand and dry out in the mold for like 4 weeks, it should eventually harden back up after it drys and i'll be left with a bar of soap that has a very high amount of sodium citrate crystals? ..."

I don't know the answer to that. Water can be bound to salts fairly tightly or rather loosely. If loosely, yes, that might happen. If tightly, probably not. You'll have to try it.

I'll give it a shot with a small batch of 100% CO with 100% sodium citrate added at trace, i'll report results.