Vitamin C (L-Ascorbic Acid)

The most abundant antioxidant in human skin, essential for collagen cross-linking, but notoriously difficult to formulate.

The Pinnell Parameters

Dr. Sheldon Pinnell established the parameters necessary for aqueous L-ascorbic acid to penetrate the stratum corneum effectively:

1. Form: Must be pure L-ascorbic acid (not a derivative).
2. Concentration: Optimal absorption occurs between 15% and 20%. Concentrations above 20% do not increase absorption and significantly increase irritation.
3. pH: Must be formulated at a pH of < 3.5 to keep the molecule protonated (uncharged), allowing it to pass through the hydrophobic lipid bilayer.

Synergy with Ferulic Acid and Vitamin E

Aqueous L-ascorbic acid oxidizes rapidly into dehydroascorbic acid and eventually erythrulose (which can cause a fake-tan effect). The addition of 1% Alpha-Tocopherol (Vitamin E) and 0.5% Ferulic Acid:

• Doubles the photoprotection of the formulation.
• Significantly stabilizes the L-ascorbic acid molecule.
• Provides network antioxidant effects (Vitamin C regenerates oxidized Vitamin E).

Derivatives

Due to L-AA's instability, numerous derivatives exist. While more stable and less irritating, they require enzymatic conversion in the skin to pure L-ascorbic acid, and clinical data on efficacy is weaker.

• Sodium Ascorbyl Phosphate (SAP) - Shows promise for acne.
• Magnesium Ascorbyl Phosphate (MAP)
• Tetrahexyldecyl Ascorbate (THD) - Lipid soluble, deep penetration.