Vitamin C ester

Tetrahexyldecyl ascorbate is an oil-soluble vitamin C derivative — ascorbic acid bound to fatty acids. It exists as an answer to two well-known problems with pure vitamin C: it is unstable, and it tends to irritate.

Pure ascorbic acid oxidises quickly in water. A serum that started out clear turns yellow within weeks and then brown — and by then it no longer does what it was bought for. It also needs a very low pH to reach the skin at all, and that acidity is behind the stinging that puts many people off vitamin C for good.

The ester sidesteps both. In oil it is stable, it needs no acidic pH, and it behaves calmly on skin. The price is patience: the molecule must first be split in the skin to release the vitamin C, so results come gradually rather than within a fortnight. For care meant to last years that is the better bargain — the product can be used every evening and it does not spoil.

In the No4. Midnight Mineral Pressed Serum the vitamin C ester joins natural vitamin E and gamma oryzanol in the antioxidant composition, working towards a brighter and more even look. The rich balm texture is exactly the environment it belongs in — the oil phase is its home.

One practical test of quality is colour. A serum with pure ascorbic acid that has turned brown belongs in the bin regardless of the date on the box — oxidation cannot be reversed. With an oil-soluble ester that problem disappears and the product stays usable for its whole shelf life. It also means it need not be treated as fragile goods: ordinary bathroom light and being opened every evening do it no harm.