Polyquaternium-51

What-it-does: moisturizer/humectant

A biocompatible copolymer (a big molecule that consists of more than one type of building blocks) that has the same structure as important cell membrane ingredient, phospholipid. It's a good skin moisturizer, leaves a silky, smooth feel on the skin and can help to reduce irritation caused by some not-so-nice ingredients (like surfactants).