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True soap precipitate identity
Hello chemists,
I am currently formulating a liquid soap by the following formula ratios:
85% coconut oil
15% olive oil
1-5% excess KOH
Once saponified the paste is diluted with water to desired strength. Citric acid may be used to adjust ph to 10.
The issue I am seeing is that a light solid precipitate is forming days later that looks like settling dust bunnies. The degree of settling seems loosely tied to citric acid use (the more and quicker its added the greater the effect); however it will appear even without adding citric acid. My question is what could this be and how could I control its formation?
The saponification reactions appears complete (perfectly clear product except for settling and tests for excess KOH), so I do not believe its unsaponified product. The amount of settling does not appear to be linked to wt/wt% of soap paste mass or behave like an oil. I have noticed that when extremely diluted the ph of the product has a tendency to drift lower within a few days (ph 10 to ph 8.5 in 2-3 days). As I mentioned as citric acid seems to affect this precipitate formation I thought it may be senstitive to ph change. However, my last test removed citric acid and instead of water used a 0.5M sodium bicarbonate/sodium carbonate [ph10] buffer, and the precipitate still formed/settled over the weekend.
Nothing filters out on filter paper after diluting the paste and the containers have been thoroughly cleaned.
Any ideas?
I appreciate any help,
RDchemist15
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5 Replies
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I would suspect the white lipid layer would be from the unsaponifiables in the olive oil. You could try lowering your percentage of olive or dropping it completely and see what happens. We don’t use an excess of more than 1% KOH (which is later neutralized with citric science) and get crystal clear all-coconut liquid soap.
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Hi @Gene,
I had that thought at one point too but our olive oil has an unsaponifiable content of ~30g/kg which in the final formula would amount to <0.015%. Our coconut oil content is ~15g/kg which would amount to ~0.042%. If it was unsaponifiable content wouldn’t removing the olive oil be futile as the unsaponifiable matter in coconut oil would still be much greater?
Its not really a lipid layer either. It looks like a coating of dust like you’d see gathering in a corner of your home.
I’ll try an olive oil knockout though in the meantime and see if that improves anything.
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The only soaps that are even moderately soluble at room temperature are potassium laurate, myristate, and oleate, the potassium salt of acids from coconut oil, and the sodium oleate. The other sodium and potassium soaps of the saturated fatty acids require elevated temperatures for solution.
You will note that olive oil contains palmitic, stearic, linoleic, linolenic acids. Do you see them in the above list? No, thought not.
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@Belassi Coconut oil contains palmitic and stearic acids and both potassium salts should be soluble. The knockout should confirm this is potassium linoleate and potassium linolenate then. I’ve got samples on stability at 45C that are still exhibiting this precipitate. I don’t have subscription access to your paper, do they list the temperature where these salts are moderately soluble at elevated temperatures? It would be nice to do that quick test in lab to check.
This formula was based off a popular national brand so I guess their solution in manufacturing was to settle the insoluble salts and pump carefully off the top then.
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It has long been known in soaping communities that liquid soap has to be “matured” to allow the insoluble salts to settle out. Then the clear soap is separated. It would make more sense to use the required fatty acids, in my view, rather than the oils.
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