Cyclodextrin

How it works

Cyclodextrins are made by setting an enzyme to work on starch, which links the glucose units into a closed ring instead of a chain. The result is a shallow cone covered on the outside in hydroxyl groups and thoroughly at home in water, while the inside of the cone is not. That gives it a small greasy pocket in the middle of a water-soluble molecule.

A poorly water-soluble guest — a fragrance material, an oil-loving vitamin, a constituent of an essential oil — can tuck into that pocket. The pair is called an inclusion complex, and the useful part is that the guest now goes wherever the water-loving host goes. Something that would have needed a solvent, or would have separated out of the bottle, stays put in the water phase.

The evidence

Cyclodextrin chemistry is old, thoroughly characterised and used well beyond cosmetics. What it does in a formula is not in dispute: it raises apparent solubility, it shields a guest from oxygen and light, it slows the escape of something volatile, and it dulls a smell by keeping fewer of the odorous molecules free in the air at any moment.

What a finished product may claim is a narrower question, because every one of those effects depends on which guest, which cyclodextrin, and what else in the jar is competing for the same pocket. A complex is an equilibrium — the guest goes in and comes out again, and how far it sits toward “in” is a property of the pair rather than of the ingredient. A box that says it traps odour is describing that equilibrium. The pocket is not a lock, and it is not endless.

Suitability

• Not an active; no skin type needs more of it and none needs to avoid it
• Used to make a formula milder by keeping less of an irritating molecule free at any moment — a real mechanism, and one that moderates how much of it works as well
• Turns up in deodorising, low-fragrance and long-lasting-scent products, and wherever an unstable oil-soluble ingredient has to survive
• Its presence tells you something about the formula, not about the result

Concentration

No figure. The level is set by how much guest there is to complex, roughly in proportion to it, rather than by anything to do with skin. More of it than the guest requires simply sits there.

Conflicts & combinations

Nothing conflicts with it in the ordinary sense, but competition is real. Two oil-loving ingredients in the same formula will both want the pocket, and adding an oil or a solvent can pull a guest back out of it. Which is why this is a decision made with the specific complex in hand, and not one that can be read off an ingredient list.