Stearic Acid A Beauty Basic Chemical
Oh, stearic acid a beauty basic chemical, how we love you as a co-emulsifier and thickener! Inexpensive and multi-functional, you make a lotion become a decadent cream!
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 carboxylic acid. It has two different part of each end. The polar head of the molecule – the carboxylic or COOH group – is called by hydrophilic (love-water) and the tail (the other end) is called by hydrophobic (hate-water), so it’s an emulsifier, bringing together water and oil!
You might remember a picture of a similar molecule, esters. An ester is a chain containing a carboxyl group where the H of the OH joins up with the OH of alcohol to create an ester and water. If you bring an alcohol to stearic acid and they undergo esterification, you get an ester called glycol stearate or glycol distearate!
Is stearic acid an emulsifier? Actually, no. 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. (This should be done all the time, but it’s especially important when we’re using stearic acid or butter or oil that contains stearic acid!)
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