Poor stearic acid. It's often overlooked for its glidier, silkier cousin cetyl alcohol, but this fatty acid is an inexpensive way to thicken a lotion, anhydrous body butter, lotion bar, and more.
Stearic acid is a saturated, long chain fatty acid with 18 carbon molecules, which is to say it is a chain of carbons and hydrogens with a carboxyl group (that COOH you see at the end of the chain) at the head. It's called an acid because this carboxyl group is called a carboxylic acid. It's found in most of our oils and butters, and can be found in the human body in our muscles.
It isn't an emulsifier, but it can help to stabilize an emulsion and it's a great thickener for our lotions. It is considered as part of the oil phase in the HLB system - its HLB value is 15.5 - and it has a melting point of 69.6?C, which is why we must heat and hold our lotions at 70?C or higher when using stearic acid. (We should do this all the time, but it's especially important when we're using stearic acid or a butter or oil that contains stearic acid!)
Why do we heat & hold our lotions"
Why do we heat & hold anhydrous products"
There is an emulsifier you can make from a combination of stearic acid and triethanolamine (TEA). (As an aside, Lush uses this emulsifier all the time in things like their Dream Cream and Helping Hands lotion.) These combine to make an alkaline soap that emulsifies oil in water. I have never tried this combination before, so I'll refer you to this great article about vanishing creams and how to make this emulsifier!
I know I mentioned this a few paragraphs above, but I think it bears repeating: Stearic acid on its own isn't an emulsifier. When combined with TEA, it creates a type of soap that can emulsify. You can't just use stearic acid as an emulsifier as it will fail.
You're probably quite familiar with how to use stearic acid in lotions if you've read my blog for any length of time. It's suggested to include it in the...