In Peter T. Pugliese, MD's, article "Physiology of the Skin: Vitamin C in Skin Care," the power of vitamin C is explored. Following is information about the available forms forms of vitamin C for esthetician use.
Only four or five of the major forms of vitamin C are available to the esthetician, although there are many more. To set the record straight, natural and synthetic vitamin C are chemically identical, and there appears to be no clinically significant difference in either the bioavailability or the bioactivity.
Pure vitamin C or crystalline form. Pure vitamin C is L-ascorbic acid. The L designation refers to how the molecule rotates light. It is interesting that although almost all natural sugars, such as glucose, are D forms, ascorbic acid, which is derived from glucose, is an L form. Crystalline ascorbic acid dissolves easily in water and may have a very faint tinge of yellow. If exposed to air for several hours to several days, it will turn yellow in the presence of iron that may be in the water. Most water-soluble preparations of ascorbic acid will oxidize and turn yellow throughout time unless something is added to control oxidation. Ferulic acid, an antioxidant, is used in one form of aqueous vitamin C to help reduce the oxidation. It takes high levels of crystalline ascorbic acid to penetrate the skin, approximately 5–15%. Preparations of more than 15% tend to be irritating to the skin. The pH should be no higher than 3 in these preparations.