Possible to make sodium stearate from stearic acid?

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myriad

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I need (pure) sodium stearate for one of my recipes - a recipe which does not go through saponification.

Unfortunately sodium stearate is hard to find in Europe, I'm only seeing it sold at great expense from large chemical companies.
However, stearic acid is easy to find here, and when combined with our friend NaOH, apparently becomes sodium stearate.

Searching online, I haven't seen any instructions online on how to do this. Would I enter the stearic acid into soapcalc, melt it down with some water, add my lye water (zero superfat), let the reaction occur, and end up with a pretty pure form of sodium stearate? My fear is that stearic acid can react suuuper fast / get chunky in oil recipes, I can foresee it reacting way too quickly alone and not getting properly mixed.
 
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Yes, yes, and yes.
Stearic acid = free fatty acid, and FFAs react with NaOH instantly to form the respective salts. Your idea is correct, I wouldn't do it otherwise either.
BUT there are a few caveats, some of which you have already foreseen.
First, stearic acid is a powder, melting point 69°C, and next to insoluble in water. To make things complicated, sodium stearate (the resulting salt) isn't particularly well soluble in water either. On the contrary, it is used in soap formulations to limit the solubility. The trick to here is to use heat and dilution. Make a dilute* NaOH solution (lye), bring it to a simmer, and slowly sprinkle in the stearic acid flakes under heavy stirring (but don't whip up too much foam). The more dilute, the easier to dissolve, but should you need sodium stearate as a solid (as water-free as humanly possible) in the end, it also means longer drying time.
(Your instincts about feeling uncomfortable to just stir heaps of stearic acid into concentrated lye are fully appropriate – people are highly unhappy in soaps with just 3% stearic acid in an oil blend.)

*Unfortunately, I don't know exactly what “dilute” means here. IIRC, around 5% solutions of sodium stearate are clear at the boiling point of water, and turn into a jelly upon cooling – but don't quote me on these numbers.

Once concentrated enough solutions will jellify/become solid upon cooling and evaporation. At some point it's somewhat similar to normal soap. But since there is no glycerol in there, it can dry out completely (over a few weeks…), to form a hard and brittle solid, similar to chalk or styrofoam. You can then crush it into a powder and (if needed) further dry it (exsiccator, vacuum, …).

One word of caution towards commercially available stearic acid: Trading understands two slightly different things under “stearic acid”. First the stearic acid per se, i. e. the C18 carboxylic acid, obtained from hydrogenated vegetable oils or slaughterhouse waste. But at least as common (e. g. in candles) is that this term refers to a blend of stearic and palmitic acid as gained from hydrogenated and/or fractionated palm oil (“palm stearin”). It is highly unfortunate that this difference is often neglected. This is because for technical purposes, palmitic acid (C16 carboxylic acid) behaves similar enough to stearic for this to not be an issue.
BUT what replacing some C18 by C16 does (in your case) is that it alters the SAP, so you cannot longer rely on a soap calculator to predict how much NaOH you have to use to neutralise the acid. A good manufacturer datasheet might help (with SAP tabulated; many vendors do), otherwise you'll have to resort to titration.
If you specifically need the stearate salt (without palmitate), then you have to obtain your stearic acid from a source that promises very low palmitic acid contamination.
 

DeeAnna

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What you propose will work fine. Yes, it's going to react very quickly and be difficult to work with.

Use a hot process method -- let it react with the lye solution while stirring as best you can. It's going to seize (get very firm very quickly), but don't panic. Just wait -- the heat of reaction will cause the soap to get warm enough to gel and become softer and somewhat easier to work with.

Unless you're using pure stearic acid, you're not going to get pure stearic soap. Typical commercial "stearic acid" is roughly half stearic acid and half palmitic acid.

Also, remember NaOH is not 100% pure. Unless you compensate for the actual NaOH purity in your calculations, there will be a small amount of stearic acid in the resulting soap. Not sure if this is a problem for what you want to do. Just something I thought I'd mention.
 

Johnez

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DeeAnna and Resolvable Owl have covered the important points.

I'll go a different direction though and ask why you need straight sodium stearate? Could it be a recipe for a soap you're trying to duplicate from ingredients list? If that is the case note that you don't need straight sodium stearate because as you've found you can make it as a part of a reaction with lye-just like sodium tallowate (beef tallow+NaOH) or any other "-ates" you find on labels.

What I'm getting at is a recipe formed out of these ingredients:

Beef tallow, coconut oil, olive oil, water, sodium hydroxide, glycerin.

....will result in a label that reads like this:

Sodium Tallowate, sodium cocoate, sodium olivate, glycerine, water.
 

DeeAnna

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If the ingredients list in a soap specifically states sodium stearate, then stearic acid needs to be one of the starting ingredients. An accurate ingredients list would not include "sodium stearate" as an ingredient if the stearic acid in the soap comes from a fat such as tallow. As you noted, that would be named sodium tallowate, not sodium stearate.

To stick up for the OP's quest to make just sodium stearate --

There are good reasons why someone might want to make sodium stearate all on its own for use as a raw ingredient. Sodium stearate is used in many products, including deodorant and lotions, to thicken and stabilize the texture of the product.
 
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Johnez

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If the ingredients list in a soap specifically states sodium stearate, then stearic acid needs to be one of the starting ingredients. An accurate ingredients list would not include "sodium stearate" as an ingredient the stearic acid in the soap comes from a fat such as tallow. As you noted, it would be sodium tallowate, not sodium stearate.

I wasn't implying the sodium stearate can be listed were if to come from tallow, but that's a good point to make clear. My point was that while one *can* make sodium stearate first to add to soap, one does not *need* to make sodium stearate first to make the soap. One can have the same outcome (an arguably easier and more feasible route IMO) by starting with stearic acid and saponifying it with everything else.

From the "superfat" and "lye" terms, and the fact that this question is posted in the lye based soap forum I immediately assumed he means to make soap. I could be wrong, we don't have much info to go on here. I didn't know sodium stearate could be used for deodorant though, that is definitely relevant to my interests. 🤔
 
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I'm with @Johnez: As long as we don't have more input from @myriad about the final use, and tolerable side products (like moisture, glycerol, sodium palmitate, excess NaOH or FFA), any further effort is based on speculations and assumptions, and leads to nothing.

I didn't know sodium stearate could be used for deodorant though, that is definitely relevant to my interests. 🤔
Yes. Many twist-tube deodorant sticks are essentially M&P soaps, and the main ingredients read like: sodium stearate, glycerol, ethanol (in some order).
In this case (regardless of what OP is aiming for), glycerol as a by-product would be absolutely appropriate, and stearin triglycerides (like from palm/soy “wax”) would be much easier to work with than the free FFA. The glycerol discount calculation is no excuse: in a complete saponification, each g of NaOH liberates 0.768 g of glycerol from triglycerides.
 

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