Making Hard Soap with Potassium Hydroxide (KOH)

[DeeAnna]

..There are many people who make bar soap from wood ash... They add salt but claim to make a bar that is almost as hard as NaOH soap....

Yes, this is a reasonably correct statement. If you use the same blend of fats, just saponify the soap with KOH or NaOH, you will find the all-potassium soap is softer and the all-sodium version will be harder.

The piece of the puzzle that you're missing is that it's not just the alkalis (KOH or NaOH) that affect hardness. The salts also present in the soap will affect the hardness of a soap.

If you add salt (sodium chloride, table salt) to a potassium-based soap at the point when the soap still contains a generous amount of water, some of the sodium from the salt will replace some of the potassium contained in the soap molecules.

That ion replacement will create a mixed sodium and potassium soap. This soap is likely to be somewhat firmer than an all-potassium soap, depending on how much of the potassium has been replaced by sodium.

But you won't be able to get a 100% sodium-based soap out of this treatment. The soap is not going to be as hard as an all-sodium soap made with the same blend of fats.

 

[AliOop]

However, in my opinion, the softness or hardness of any soap bar always depends on the oils or fats used.

That's only partially true. As @DeeAnna explained above, whether the soap is hard or soft also depends on which type of lye/alkali is used for saponification. Sodium alkalis will produce a hard soap, whereas potassium alkalis will produce a soft soap, usually more of a paste.

 

[Albercook]

That's only partially true. As @DeeAnna explained above, whether the soap is hard or soft also depends on which type of lye/alkali is used for saponification. Sodium alkalis will produce a hard soap, whereas potassium alkalis will produce a soft soap, usually more of a paste.

I'm starting to get it. All things being equal Sodium alkalis are harder. But why? It seems that the length of the fatty acids and the amount of branching would make a difference.
Sodium is more electronegative but Potassium Hydroxide is more exothermic. So there is a bigger difference between the about of energy required to separate K+ and OH- and the energy released when the become solvated by water than the corresponding process with NaOH. Hence the first is more exothermic but Na+ is more electronegativity than K+ so it is attracted more strongly to the carboxylate anions.

So the answer is electronegativity?

 

[Albercook]

I have not made wood ash soap….yet!
However, in my opinion, the softness or hardness of any soap bar always depends on the oils or fats used. I’ve probably seen most of the same videos you have and now you have me curious. I’ll probably go back and watch a couple of the more successful wood ash soap videos and pay attention to the fat/oils used. I would imagine that the best fat to use would be tallow, or considering my recent experience with 60% cocoa butter, it might be worth trying.

I think it is starting to make sense to me. Assume that the stoichiometry is correct and the reactions have all gone to completion that the amount of leftover water and the left over oils are the same.

I propose that
Longer fatty straighter fatty acids make fats that are solid at room temperature and harder soaps. Shorter fatty acids with more bends (unsaturated, double bonds) the thinner the oil and the more liquid the soap. (There might be something funky about olive oil. I hear it does not follow the rules)

On the other hand the more electronegative the cation the stronger it will be attracted the carboxylate anion on the end of the hydrocarbon chain.

Has anyone ever tried Lithium Hydroxide? If I'm right it should make soap so hard it might be brittle.

Let me know if this makes sense.

I still don't understand many things like pH and salt addition just to name two.

Thanks for reading all this.

 

[AliOop]

I think it is starting to make sense to me. Assume that the stoichiometry is correct and the reactions have all gone to completion that the amount of leftover water and the left over oils are the same.

I propose that
Longer fatty straighter fatty acids make fats that are solid at room temperature and harder soaps. Shorter fatty acids with more bends (unsaturated, double bonds) the thinner the oil and the more liquid the soap. (There might be something funky about olive oil. I hear it does not follow the rules)

On the other hand the more electronegative the cation the stronger it will be attracted the carboxylate anion on the end of the hydrocarbon chain.

Has anyone ever tried Lithium Hydroxide? If I'm right it should make soap so hard it might be brittle.

Let me know if this makes sense.

I still don't understand many things like pH and salt addition just to name two.

Thanks for reading all this.

You are correct that the different fatty acids create different soap properties, including harder v. softer, more or less bubbly, etc. However, the alkali used makes the difference between soap that will become solid, and soap that becomes a soft, dilutable paste that will remain diluted. You can try to dilute bar soap, but it won't stay diluted for more than a few days or so - it will always congeal and become "snotty."

All I know is that the difference has something to do with how the soap molecules are arranged after saponification - sorry I don't have more specific scientific knowledge than that. For more info, read this website which was written by DeeAnna, who has a PhD in chemistry.

 

[Albercook]

You are correct that the different fatty acids create different soap properties, including harder v. softer, more or less bubbly, etc. However, the alkali used makes the difference between soap that will become solid, and soap that becomes a soft, dilutable paste that will remain diluted. You can try to dilute bar soap, but it won't stay diluted for more than a few days or so - it will always congeal and become "snotty."

All I know is that the difference has something to do with how the soap molecules are arranged after saponification - sorry I don't have more specific scientific knowledge than that. For more info, read this website which was written by DeeAnna, who has a PhD in chemistry.

Thanks