Dual-lye bar soap trial

[ResolvableOwl]

NaOH:KOH mass ratio	100:0	97:3	89:11	100:0 + sorbitol
Na:K molar ratio	100:0	98:2	92:8	100:0 + sorbitol
Lye concentration XOH/(NaOH+KOH+H₂O)	38.3% | 0%	37.5% | 1.1%	35.2% | 4.3%	38.3% | 0% | 3.1% sorbitol

All made from the same oil blend (30% HO sunflower & RBO, 15% macadamia, plus just enough hard oils to make them harden up in a reasonable time). It has plenty MUFA (52% oleic acid, 3% palmitoleic acid), so it should be somewhat prone to oleic slime. No lauric oils added, to not distort bubbliness properties by coconut & friends (still 5% castor though).
Additives: ROE + 0.8%TOW citric acid. The rightmost bar has another 1.1%TOW sorbitol added, so that I can compare the behaviour of the dual-lye bars with the one of a known bubble booster.
Mild CPOP.

With dual-lye, I have to choose one fixed concentration parameter. I went for the lye molarity of 9.58 mol hydroxide per kg lye (that's why the XOH mass concentration increases with KOH content, because KOH is heavier per mole than NaOH). No idea if that makes sense from a fundamental point of view, but it made sense from my masterbatch blending technique.

Objectives:

• Does a few % of KOH make overly soft bar soap, to a degree where it impedes unmoulding? Apparently not! Following up @KiwiMoose reports of an “emergency dual-lye” batch that went just fine, I was surprised and wanted to know better. The recipe is rather soft, so I gave it two days, and the hardness of all three Na/K soaps was essentially the same. Only the one with sorbitol (right) has a different texture (a bit of a chalky/velvety feel) to it. Not sure if the partially speckled look (stearic spots? false trace? separation? You'll never know with microbatches…) has to do something with the lye composition; the sample size is too small to decide (and has other error sources, like varying temperature of oils and lye between the batches). The bar with sorbitol had much worse soda ash issues than the other three, but once again it's too early to draw conclusions from this observation.
• Three data points for the oleic slime reducing properties of a few % of KOH. (Part of the reason why I have formulated the recipe as high-oleic)
• Bubbliness? The logic: KOH → increased solubility → less slime: is suggestive as well to increase the willingness of the soap to give off lather. Some folks around here have made good experiences with the lather of dual-lye, and this trial can add some grounding to it. (Part of the exclusion of lauric oils, to interfere less with subtle changes in lathering ability.)
• Easier solubility sounds like worse longevity? Let's find out!

 

[Peachy Clean Soap]

View attachment 64399

NaOH:KOH mass ratio	100:0	97:3	89:11	100:0 + sorbitol
Na:K molar ratio	100:0	98:2	92:8	100:0 + sorbitol
Lye concentration XOH/(NaOH+KOH+H₂O)	38.3% | 0%	37.5% | 1.1%	35.2% | 4.3%	38.3% | 0% | 3.1% sorbitol

All made from the same oil blend (30% HO sunflower & RBO, 15% macadamia, plus just enough hard oils to make them harden up in a reasonable time). It has plenty MUFA (52% oleic acid, 3% palmitoleic acid), so it should be somewhat prone to oleic slime. No lauric oils added, to not distort bubbliness properties by coconut & friends (still 5% castor though).
Additives: ROE + 0.8%TOW citric acid. The rightmost bar has another 1.1%TOW sorbitol added, so that I can compare the behaviour of the dual-lye bars with the one of a known bubble booster.
Mild CPOP.

With dual-lye, I have to choose one fixed concentration parameter. I went for the lye molarity of 9.58 mol hydroxide per kg lye (that's why the XOH mass concentration increases with KOH content, because KOH is heavier per mole than NaOH). No idea if that makes sense from a fundamental point of view, but it made sense from my masterbatch blending technique.

Objectives:

• Does a few % of KOH make overly soft bar soap, to a degree where it impedes unmoulding? Apparently not! Following up @KiwiMoose reports of an “emergency dual-lye” batch that went just fine, I was surprised and wanted to know better. The recipe is rather soft, so I gave it two days, and the hardness of all three Na/K soaps was essentially the same. Only the one with sorbitol (right) has a different texture (a bit of a chalky/velvety feel) to it. Not sure if the partially speckled look (stearic spots? false trace? separation? You'll never know with microbatches…) has to do something with the lye composition; the sample size is too small to decide (and has other error sources, like varying temperature of oils and lye between the batches). The bar with sorbitol had much worse soda ash issues than the other three, but once again it's too early to draw conclusions from this observation.
• Three data points for the oleic slime reducing properties of a few % of KOH. (Part of the reason why I have formulated the recipe as high-oleic)
• Bubbliness? The logic: KOH → increased solubility → less slime: is suggestive as well to increase the willingness of the soap to give off lather. Some folks around here have made good experiences with the lather of dual-lye, and this trial can add some grounding to it. (Part of the exclusion of lauric oils, to interfere less with subtle changes in lathering ability.)
• Easier solubility sounds like worse longevity? Let's find out!

Oops I didn’t want to post in your link:

Thank Ü. interesting & informative
Update: the Bar on far right looks the best. Hopefully you will post images of a ( bubble test ) .

 

[ResolvableOwl]

Update: the Bar on far right looks the best.

It is for sure the smoothest, i. e. it has no irregular stearic spots here and there. However, I have the suspicion that it has caught a mild form of the cracking&growing disease. It is larger than the other three (hard to see in this photo, but clearly visible IRL, particularly now after some drying time). That means that it is somewhat chalky to the touch, and doesn't have the silky gloss of the others. IME this can also dramatically impair bubbling ability, which would be a shame, because it'd render it useless as a comparison for the others.

Hopefully you will post images of a ( bubble test ) .

We'll see!

 

[KiwiMoose]

I'm reporting back on my dual-lye soap. It's lovely and creamy, but lasted only 1 week in the shower, as opposed to the two weeks I usually get out of a bar of soap.

 

[LisaBoBisa]

So this isn't dual-lye wet shave soap... it's for bathing?

 

[Ugeauxgirl]

Interesting experiment. I'm reluctant to try dual lye for some reason. Extra lather is a definite plus, reduced longevity isn't. Hmmm

 

[DeeAnna]

So this isn't dual-lye wet shave soap... it's for bathing?

Yes, it's for bathing. What they're talking about here is bath soap made with mostly NaOH with only a small amount of KOH. I normally use 5% KOH to 95% NaOH. This percentage is a chemical proportion, not a weight proportion, by the way.

I'm surprised your soap has such a short life in the bath, @KiwiMoose. I routinely use KOH in my bath soap and honestly don't see a great difference in the longevity versus all NaOH.

edit: A KOH-NaOH mixture will be the most useful for a soap that's high in oleic acid (soap high in olive oil soap or other high oleic oil) or a soap that is high in palmitic and stearic acids (soap high in lard or palm). A small % of KOH helps an oleic soap to go into solution easier so it tends to make less of that stringy oleic slime. And a small % of KOH helps a soap high in palmitic and stearic to be a bit more water soluble so it dissolves and lathers easier.

Many soap makers like soap that lathers well. To get that, they tend to use more coconut oil (CO) because CO creates a soap that produces more lather without a lot of effort. The downside of using more CO is the soap can be more harsh and drying to the skin. A little KOH can be used as an alternative to using more coconut oil -- you can get a similar level of lather without adding harshness.

 

[cmzaha]

I use the 5/95% dual in all my soaps and never noticed a difference in longevity nor did my longtime customers, and believe me when I say they would let me know.

ETA: my soap recipes are high tallow/lard, high palm/shea, or high palm/lard formulas. I also soap with at least 50% vinegar as water replacement.

 

[AliOop]

@KiwiMoose what was your KOH/NaOH ratio, if I may ask? I use the same 95/5 ratio as @DeeAnna, with no reduced longevity that was noticeable to me.

EDIT: Looks like I posted at the same time as @cmzaha with same experience, too.

 

[DeeAnna]

Here's an article of mine that explains more about dual-lye soap -- Dual lye recipe | Soapy Stuff

 

[LisaBoBisa]

I use the 5/95% dual in all my soaps and never noticed a difference in longevity nor did my longtime customers, and believe me when I say they would let me know.

ETA: my soap recipes are high tallow/lard, high palm/shea, or high palm/lard formulas. I also soap with at least 50% vinegar as water replacement.

Does the vinegar react with the lye?
Edit: ignore this question! I just read the KiwiMoose thread on vinegar

 

[KiwiMoose]

@KiwiMoose what was your KOH/NaOH ratio, if I may ask? I use the same 95/5 ratio as @DeeAnna, with no reduced longevity that was noticeable to me.

EDIT: Looks like I posted at the same time as @cmzaha with same experience, too.

95/5
But also used a coconut cream recipe, and also the soap was only 5 weeks old. All contributing factors I'm sure.

 

[LisaBoBisa]

Here's an article of mine that explains more about dual-lye soap -- Dual lye recipe | Soapy Stuff

My world has just expanded....

If my KOH says it's "90-95% pure", I could probably pick any number in that range and be safe, since it's only a small fraction of my lye and my superfat will save me...

 

[DeeAnna]

If my KOH says it's "90-95% pure", I could probably pick any number in that range and be safe, since it's only a small fraction of my lye and my superfat will save me...

Yep, that's pretty much right. Some people would use the highest purity (95%), some the lowest (90%), and others go with the middle.

Knowing what I do about the chemical production industry, I pick the middle. Manufacturing plants that make chemicals don't like to produce product that is at the lower limit because they risk making not-pure-enough product that customers will (rightfully) be unhappy with. But they don't like to produce product that is at the high end either. Overly pure product costs more money to make, although customers won't complain.

So I think it's a safe bet to assume the actual purity is somewhere in the middle. But I can see why others call it differently.

 

[cmzaha]

Does the vinegar react with the lye?
Edit: ignore this question! I just read the KiwiMoose thread on vinegar

This is really a better question for a new thread but to answer your question yes vinegar reacts with the lye to create Sodium Acetate. But when using it to soap with you have to add in extra lye at the rate of 0.0357 x the amount of vinegar you will be using in your batch to get the amount of extra NaOH needed. If you do not factor in the extra lye all you do is up your superfat.

 

[LisaBoBisa]

Yep, that's pretty much right. Some people would use the highest purity (95%), some the lowest (90%), and others go with the middle.

Knowing what I do about the chemical production industry, I pick the middle. Manufacturing plants that make chemicals don't like to produce product that is at the lower limit because they risk making not-pure-enough product that customers will (rightfully) be unhappy with. But they don't like to produce product that is at the high end either. Overly pure product costs more money to make, although customers won't complain.

So I think it's a safe bet to assume the actual purity is somewhere in the middle. But I can see why others call it differently.

Then I'll use 92%, like you do in your example! Thanks a million for all this information

 

[LisaBoBisa]

Q: if I add 2% (of my oil weight) citric acid to my water, and an extra 6.24g NaOH for every 10g citric acid, is the KOH stronger, since K's less electronegative, and will it all react with the citric acid before the NaOH has a chance to? I see that @DeeAnna advises doing it this way on this page, but I'm curious to know why it won't monkey with my 95/5% NaOH/KOH balance. Wouldn't it be better to add extra KOH instead?

I get to take gen chem 2 this semester, and it's been fun seeing soapy stuff line up with the chapters we cover (just finished acids and bases)

 

[DeeAnna]

Q: if I add 2% (of my oil weight) citric acid to my water, and an extra 6.24g NaOH for every 10g citric acid, is the KOH stronger, since K's less electronegative, and will it all react with the citric acid before the NaOH has a chance to? I see that @DeeAnna advises doing it this way on this page, but I'm curious to know why it won't monkey with my 95/5% NaOH/KOH balance. Wouldn't it be better to add extra KOH instead?

I have people calculate the additional alkali requirement as NaOH alone for several reasons.

The main one is it's easier for the average soap maker to do the calculations using just one alkali. I had to pick one, so I picked NaOH since most soap makers are likely to have more NaOH on hand than KOH. You can add additional alkali as all KOH rather than NaOH if you like. Or you can add both alkalis in a 95:5 stoichiometric ratio if you want to do the math. Or a 50:50 stoich ratio if that seems appropriate.

Another reason why I have people calculate the extra alkali as NaOH alone is I don't perceive any difference in the performance of soap whether the extra alkali is NaOH or KOH or some blend of both. But there's value in trying this out and seeing for yourself, so give it a whirl.

The last reason is, as soap makers, we're dealing with a lot of what I call "hidden error" -- actual saponification values of fats are different than the averaged values used by soap recipe calculators, the purity of the alkali(s) we are using varies from manufacturer to manufacturer and also with the way the alkalis are stored, additives we use in our soap can consume alkali that we don't account for, etc. All these hidden errors make soap making less precise than what you might think.

In other words soap making isn't quantitative analytical chemistry where measuring to 0.001 gram is critical. Work as accurately as is reasonable, but don't sweat the small stuff.

 

[Becky1024]

Q: if I add 2% (of my oil weight) citric acid to my water, and an extra 6.24g NaOH for every 10g citric acid, is the KOH stronger, since K's less electronegative, and will it all react with the citric acid before the NaOH has a chance to? I see that @DeeAnna advises doing it this way on this page, but I'm curious to know why it won't monkey with my 95/5% NaOH/KOH balance. Wouldn't it be better to add extra KOH instead?

I get to take gen chem 2 this semester, and it's been fun seeing soapy stuff line up with the chapters we cover (just finished acids and bases)

I teach gen chem and would love to have you in my class! It’s fun when students “connect the dots” to real life!

DeeAnna had a very good explanation. I will add that the electonegativity difference between Na and K is so small that it probably won’t make much difference since they are in solution with water. The ion that is closest to the citrate will grab it.

 

[cmzaha]

Since I am just a soapmaker and Not a chemist I followed what DeeAnna recommended and just figured my extra alkali as NaOH, it is just easier, although I use vinegar, not citric acid I still have to figure out my extra alkaline and an Edta/Sodium Gluconate combination as my chelator.

Soap is very forgiving with cure times even lye heavy soaps, to a point, can cure out safely.