As soon as l-ascorbic acid is mixed with water it starts oxidizing. This makes the l-ascorbic acid ineffective and also dangerous, because oxidized L-ascorbic acid is a pro-oxidant which may increase the formation of free radicals.

So, the challenge has been to make stable L-ascorbic acid products, or to make l-ascorbic acid itself stable.

Currently there are three ways this has been done:The Skinceuticals Vitamin C Plus Ferulic Acid + Vitamin E formulation, Anhydrous C: and 

Vitamin C derivatives: 

To improve the stability of vitamin C in skin care, scientists have been looking for alternative solutions. One of these has been to create Vitamin C derivatives. Usually these are made by combining ascorbic acid with other molecules. Examples of these are the water soluble Magnesium Ascorbyl Phosphate and the oil soluble Tetrahexyldecyl Ascorbate (which is also known as Ascorbyl Tetraisopalmitate)

Tetrahexyldecyl Ascorbate ( Ascorbyl Tetraisopalmitate)

Tetrahexyldecyl Ascorbate is the latest development in Vitamin C derivatives.

Tetrahexyldecyl Ascorbate is oil soluble, just like your skin, which means that it easily penetrates into the skin.

It is not acidic and it does not require a low pH to get it to penetrate the skin, so it does not sting. It is also stable in air and water for up to 18 months, which means it will work as an anti-oxidant long after you start using it.

Studies have shown that Tetrahexyldecyl Ascorbate is an effective skin care product. It effectively suppresses UVB-induced skin pigmentation, possibly through its anti-oxidative activity.

Tetrahexyldecyl Ascorbate also prevented UVA induced oxidative stress, and it boosted collagen synthesis.

Magnesium Ascorbyl Phosphate is a water-soluble vitamin C derivative. Magnesium Ascorbyl Phosphate is not acidic so it does not irritate the skin and it is more stable than vitamin C.

Studies have shown that Magnesium Ascorbyl Phosphate is JUST as effective at boosting skin collagen synthesis as L-ascorbic acid.

Magnesium Ascorbyl Phosphate has proven to be an effective skin lightener, reducing hyperpigemnetation in some test subject.

Finally, studies have shown that Magnesium Ascorbyl Phosphate protects against UVB induced skin damage, such as inflammation and free radical formation, both duuring and after exposure to sunlight.