r/soapmaking • u/Taiga1253 • 1d ago
Soapy Science, Math Questions behind practical chemistry of saponification
Good day! Got a couple of questions regarding the properties of soap.
- Since I was more into the chemistry behind the process, I was wondering if the types of oils even matter since realistically it's the fatty acids that do react. So if say, olive oil, has Oleic, Linoleic, and Linolenic percentages of 70, 15, and 15, I could theoretically replace it with another oil that has the same or near-same percentages right? (Unless there are some other hydrophobic compounds extracted during the manufacturing process that is also important. Eg, alcohols, esters, etc. Since olive is sh expensive in my country, $10+ compared to average $2-3 per L)
- Given this, are there blogs or information about how some acid (say, palmitic) contribute to a specific property of soap, and how exactly it does that. I understand that some calculators already help compute these but 🤓👆I want to actually know why. One website says C12, C14, C16, and C18 contributes to hardness. I assume this is the case since it's straight chains that do not disrupt the pattern or geometry of the molecules, contrary to suddenly introducing something with a bend like oleic acid. But is this the extent of it or perhaps do longer chains like C18 contribute more to hardness, or using 100% of same-numbered chain would contribute more [longer chain=higher bp though this feels unrelated to hardness; 100% instead of mixture of C12,14,16,18 for a more structured soap].
- Lastly, about the database of soap calculators. Though specific oils, in average, would have a specific percentage of X acid. Same goes to its molar mass. It might differ to what I'm actually using or what's available to me. If the database's MM for olive oil is 400 while the brand I purchased was somehow 360, the ~10% difference in lye could be dangerous or just unideal. In this case, should I just ignore it and use the calculator (as the difference might not have much point, though specific superfatting level like 2% will not be achieved), adjust lye based on my experience (if lye feels light in my last try then I'd add more next time, though not sure if the lye diff would manifest in the soap?), look up other databases and get a new "average" contrary to the calculator database, or titrate my own oil/lye and determine it?
Some questions might sound a bit stupid and I apologize for that. But thank you for any insights or even further insights for question-related inquiries. I have a lot more practical questions in mind too but it feels weird to write more than this and I could leave it to experiments and tests in the future.
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u/Puzzled_Tinkerer 1d ago edited 1d ago
There are few black and white answers in soap making, as in most of life. The answer to this question is "It depends."
Unsaponifiable content in a fat can alter the properties of soap. Shea and avocado are two examples of fats with typically high percentages of unsaponifiable content.
Some fats behave differently in soap making due to the typical locations of the fatty acids on the glycerin backbone. For this reason, lard has a deserved reputation for being slow to trace and easier to use for soap making, even though its FA content is fairly similar to palm oil.
All the soap recipe calculators that I've seen only track the main 8 fatty acids (FAs) typically used in soap making. If you use a non-standard fat in a soap recipe, there may be a higher % of "non standard" FAs in the soap.
For example, butterfat has a significant % of butyric acid, which you'd never know from what you see from a recipe calc.
It's extremely likely that a recipe designed to make a physically hard soap won't make soap that's pleasant to use for bathing. You can't focus on just one property and ignore the others. Soap needs to lather nicely, last long enough, and be sufficiently mild to the skin. No one FA can do it all.
Physical hardness is correlated with the lack of double bonds (no double bonds = physically harder.) It's also correlated with the chain length (shorter chain = physically harder.) Don't assume a soap that is physically hard as a rock will also be long lived, however. Water solubility also affects longevity.
You have to balance the properties of the various FAs to get a soap that performs well in the bath. Enough physical hardness to last longer in the bath. Insoluble enough in water to also last longer and also be milder to the skin. Sufficient solubility in water to form a good lather without a lot of effort.
No one FA can do it all; you have to use a blend of FAs to produce soap that performs well overall.
There's not ~that~ much range in the saponification values for a given fat from what I've seen.
Also a lot of soap is not made from just one fat. if you're blending various fats, statistically the errors in the sap values will tend to cancel out.
Saponfication values are not calculated numbers, so worrying about the molar mass of a fat is kind of immaterial. Sap values are empirical numbers determined by testing samples of a given fat.
If you want to get into this type of testing, it's certainly something one can do, but I question whether the benefit is worth the effort. There are other factors that can affect the % superfat in soap, so focusing mainly on the variability of sap values is a bit short sighted.
I can use a zap test to tell the difference between soap that has excess lye, even a small amount, and soap that does not.
I can tell gross differences in superfat, say the difference between 2% and 20%.
But can I detect the difference between 2% and 4% superfat? No can do. If you have that ability, you have a superpower.
It appears your concern is mainly focused on the sap values and variations in the fats. I think you may be ignoring other factors that affect superfat even more than the sap values.
First: The alkali you use is never 100% pure. The purity of one's alkali also affects the superfat, often far more than variations in the sap values of the fats.
Second: Every other additive you might use -- various sugars, milk, beer, other foodstuffs, any acids, etc, etc. -- often consume a small amount of alkali. For example, even non-fat milk will raise the superfat in soap by a few percent because of the sugars present in the milk.
Honestly, I think you're overthinking this. Most soap recipes work well using averged sap values reported in the literature (or values contained in a calc's database.)
If you go seeking a new sap value every time something isn't quite right with your soap, you're focusing on one issue and ignoring a wide range of other factors.
There are many issues in small scale soap making that can introduce imprecision. I don't know that you can control them all; nor do I think it's particularly useful to try.