Tetrahexyldecyl ascorbate is a nonirritating, stable derivative of vitam C .Typical concentrations are 1-2%. Mixes well with most creams or lotions.
First, it is stable (if pH < 5), easy to formulate and a joy to work with for a cosmetic chemist.
Second, because it's oil-soluble, its skin penetration abilities seem to be great. So great in fact, that it surpasses the penetration of pure vitamin C threefold at the same concentration and it penetrates successfully into the deeper layers of the skin (that is usually important to do some anti-aging work). There is also in-vitro data showing that it converts to AA in the skin.
Third, ATIP seems to have all three magic abilities of pure vitamin C: it gives antioxidant protection from both UVB and UVA rays, it increases collagen synthesis (even more than AA) and it has a skin brightening effect by reducing melanogenesis by more than 80% in human melanoma cell cultures.