The Comparison, in One Table
| L-ascorbic acid | THD ascorbate | |
|---|---|---|
| What it is | Pure vitamin C | An oil-soluble vitamin C ester |
| Also listed as | Ascorbic acid | Tetrahexyldecyl ascorbate, ascorbyl tetraisopalmitate |
| Published evidence | The most studied topical form | Much less clinical data |
| Stability | Oxidizes; turns orange then brown | Stable |
| pH needed | Low, around 3.5 or below | Works in neutral formulas |
| Irritation | Can sting, especially sensitive skin | Gentle |
| Typical format | Water-based serum | Oil or anhydrous serum |
The Trade: Evidence Versus Stability
L-ascorbic acid is the form most of the published research on topical vitamin C uses. To penetrate skin it needs a low pH, and absorption work found it peaks at around 20% concentration. It is also unstable: exposed to air and light it oxidizes, turning orange and then brown as it loses potency. Well-formulated versions add vitamin E and ferulic acid, which the research found improve its stability and effect. The broader picture is on the vitamin C monograph.
THD ascorbate attaches fatty chains to vitamin C to make it oil-soluble and stable. It does not need a low pH, rarely stings and keeps for far longer. The catch is that it must be converted to ascorbic acid in the skin to work, and there is far less published clinical evidence on how well that happens or what results to expect.
