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Old May 20th 2019, 02:40 PM   #1
glassgal
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Soap Saponification Question

Hi, I'm trying to understand what chemically happens when soap is made with rancid oils (fatty acids). No one ever seems able to answer it, so I'm hoping someone here can help (I'm not studying chemistry, I make soap).

In a typical batch of soap, you add lye solution (NaOH+H2O) to a fatty acid solution (not a single oil, but multiple oil blends). You mix it all up, add heat, and you get soap + glycerin+ water which evaporates out while drying.

Using rancid oils however also produces soap. The 'word on the street' is that if you use rancid oils to make your soap, your soap will get rancid. It does NOT get rancid immediately, it makes regular soap, not rancid soap.

Where do the rancid aldehydes and ketones go from the smelly oils if it's making regular soap?

For illustrative purposes, here are the formulas for 2 common soaping oils:
Peanut Oil: C30H45N9O5
Coconut Oil: C4H8NNaO2

Lye Solution: NaOH + H2O

Of course, more oils are used than the above, but to keep it as simple as possible, lets say there's a 50/50 of Peanut Oil and Coconut Oil blended. This is mixed with the lye.

If you were to use rancid oils, that would be CHO for the aldehydes, and RC (R being a carbon group) for the ketones....

How does saponification differ after adding the rancidity factor (all fatty acids used are rancid)?

I would be REALLY grateful for an answer, this question has puzzled me a long long time. Thank you.

Last edited by glassgal; May 20th 2019 at 02:42 PM.
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Old Jun 4th 2019, 12:26 AM   #2
oz93666
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It appears consuming rancid oil is not a danger .. I suspect less than 1% of the oil has broken down , but taste buds are very sensitive , so oil has a consume by date ....

I would imagine there would be no effective difference using rancid oil to make soap ..I can't think the soap would later turn rancid if made from rancid oil.

Since it seems not unusual to use rancid oil to make soap , I'm sure it's perfectly OK .
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Old Jun 4th 2019, 04:46 AM   #3
glassgal
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Thank you for the response! Since I posted the question, I've done some experimentation. I happened to have some roughly 15 year rancid oils. They smell unbelievably bad. The types of oils are, Peanut, Shea Butter, Canola, Palm, Coconut (all soaping oils). The most stinky of all is the Palm oil.

The soaps made from the above are pretty darn bad. They are off by color, and do not attract oils or dirt as regular handmade soaps do.

I'm guessing that the rancid O molecules in a recently rancid oil would appear further up the long chain, but as the oil becomes more rancid, start filling in all available spots until the 'sticky' end that is normally 2 hydrophillic ends result in a hydrophobic tail that has ketones stuck to it.

The resulting soap does not seem to dissolve oils, and will not quickly wash away without a ton of rinsing and scrubbing just to remove the soap.

Mildly rancid oil (as you mentioned, the edible kind) seems fine. But as you increase the rancidity level in oils, it produces less and less usable soap.

It's one of the areas that I've seen zero discussion and study of, where saponification is such a hugely important area in Organic Chemistry. I was hoping someone would be interested enough to look into it, because it's kind of interesting, and because I actually have the oils to assist in experimenting. Not many people have 15 year old rancid soaping oils laying around.
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Old Jun 5th 2019, 01:46 AM   #4
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Yes ...15 year old oil must be rare and perhaps my previous comments do not hold ..

Interestingly enough I live on an oil palm and coconut plantation , I'm a UK citizen who moved out to Thailand to retire ... both products are very cheap here , palm oil for cooking sells in the supermarket for $2 US for a gallon
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Old Jun 5th 2019, 05:48 AM   #5
glassgal
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Oh my goodness! That must be wonderful! I've been to Thailand... the people, the culture are amazing. Do you export the oil to the US?
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