Cyclopentasiloxane, also known as D5, is part of a class of silicon-based compounds known as silicones. This category of ingredients is massive, ranging from stiff solids (like silicone kitchen instruments, for example) to extremely lightweight liquids. Despite some fear-mongering you may hear about silicones, each silicone has unique properties and they can’t be lumped together. Many silicones are quite beneficial for the skin, helping to lock in moisture and strengthen the moisture barrier; others are crucial to make stable, elegant formulas.
Cyclopentasiloxane is an extremely lightweight silicone that evaporates completely. It’s been shown to be completely safe when used in leave-on products. It’s not recommended for use in wash-off products like cleansers, simply because that won’t give the ingredient time to evaporate. Cyclopentasiloxane is the reason C+C Serum is so lightweight and leaves behind only the active ingredients.