Substantivity

Putting a material on skin and having it stay there are two different problems. Substantivity is the second one: how much of what was applied is still in place after water, a towel, a shirt collar and a few hours have each had a turn.

Affinity for the surface decides it, not dose. Skin sits at an acidic surface pH and carries a net negative charge there, so positively charged materials are drawn to it and hold on through a rinse — which is why a conditioning agent survives washing and a plain humectant does not. Size contributes: a large polymer has many points of contact and comes away slowly, while a small water-soluble molecule leaves with the first splash. Solubility settles the rest: what dissolves in water goes where the water goes.

It explains things that otherwise look inconsistent. A cleanser can leave an ingredient behind although the product itself is rinsed away. A sunscreen can be applied properly and be largely gone after a swim or a sleeve. A toner that felt marvellous can be doing nothing an hour later — not because the ingredient was wrong but because nothing held it.

Two cautions. Substantivity describes where a material ends up, not whether it is worth having there; staying put is not itself a benefit. And it cuts both ways: whatever binds well to skin is harder to wash off, which matters when the material is one that disagrees with you.