Compare the cetyl alcohol molecule to the stearic acid molecule. It has 16 carbons (stearic has 18) and it has an OH (oxygen-hydrogen or hydroxy group) whereas stearic has a carboxyl group at the head (OOH). Cetyl alcohol is a short chain fatty alcohol; stearic acid is a long chain fatty acid. So what does this all mean to us?
Cetyl alcohol can be used like stearic acid to thicken our lotions for a creamier feel. You can use it up to 5% in lotions and creams, but I like to use it up to 3% because I don't want things too thick! You can use it as the emollient - oil or butter - in a lotion for "oil free" moisturizing.
Because it is saturated, it is considered resistant to rancidity, so the shelf life is considered to be very long. (A lot of manufacturers say it is resistant to rancidity forever - nothing lasts forever, but it is a very long time!) And it has a melting point of 49˚C.
It is more emollient than stearic acid, and works in conjunction with a cationic quaternary compound like BTMS to increase the conditioning agent's substantivity (it clings more to your hair). Where stearic can feel waxy and draggy, cetyl feels glidier and greasier (in a good way).
Stearic acid and cetyl alcohol can be used interchangeably, for the most part. I liken a cream with cetyl alcohol to Cool Whip - it's glidier and silkier - whereas I think of a cream with stearic acid as whipped butter - still glidy, but definitely not silky and a lot heavier.
Join me tomorrow to compare and contrast (I always hated that phase in university!) cetyl alcohol and stearic acid.