A few thoughts for a lazy Saturday, Fuchia asks: A bit off topic but I'm curious about making a Vitamin C cream. I have a face cream recipe I make now and love (it does not contain Vitamin C but uses Optiphen Plus as a preservative) and keep reading online that you can just add a small amount of L Asorbic acid to it and it will keep for a few weeks. Why can't it keep for longer then that (ie 6 months to a year) and is there another step involved in making Vitamin C cream or is it that easy?
(From this post on Vitamin C.) It isn't easy to add Vitamin C to a product. Because it's really unstable in water and it doesn't easily penetrate our skin. Plus pH 3 is really acidic and that's not a great pH for our lotions or serums to be. And it degrades easily when exposed to oxygen.
So let's say you want to use Vitamin C in your creations. Is it possible? It is. The ideal product would be a non-ionic anhydrous product or emulsion in an air tight container (so a lotion or serum not including any cationic ingredients - like BTMS or cationic polymers - or anionic ingredients - like our bubbly surfactants - is right out). You can use it with silicones or oils as an anhydrous creation. But Vitamin C is water soluble, so how the heck would we get it into a creation with little to no water?
ascorbyl palmitate in a serum or lotion as your source of Vitamin C, for instance. You can use the water soluble Vitamin C in an emulsion, but you will see some degradation of the ingredient, so don't choose a pump bottle but something like a malibu/tottle or disc cap to keep it less exposed to the air. (As I note, I found tetrahexyldecyl ascorbate at Lotioncrafter's site. This is another ester with Vitamin C and very stable. I also found VitaC Stable at the Herbarie, which is Vitamin C with a phosphate added for stability. Or MAP at the Formulator Sample Shop.)
Or you could dissolve it in water and create an emulsion, although this will be less effective than using the ester. And it can oxidize quickly, making your products an orange/brown colour (that's when you know it's oxidized!), which isn't great. So it's looking like using one of the esters is the best choice.