Cyclopentasiloxane (D5) is a common synthetic silicone widely incorporated into personal care items such as hair care products, antiperspirants, and cosmetics. Cyclopentasiloxane is definitively not water-soluble; its chemical composition makes it fundamentally incompatible with water.
How Cyclopentasiloxane Interacts with Water
Cyclopentasiloxane is classified as a cyclic siloxane, a silicone that exhibits strong hydrophobic properties, meaning it repels water. The molecule’s structure is built upon a backbone of alternating silicon and oxygen atoms arranged in a ring, with non-polar methyl groups attached. This lack of polarity prevents the molecule from forming the hydrogen bonds required to dissolve in polar solvents like water. Due to its water-repellent nature, D5 is often used in products where water resistance or a moisture barrier is desired, such as in waterproof makeup and some sunscreens. While insoluble in water, D5 demonstrates good solubility in various organic solvents and alcohols. Formulators must use specialized emulsifiers to successfully blend D5 into water-based products and create a stable mixture.
The Role of Volatility in Product Performance
A defining characteristic of Cyclopentasiloxane is its high volatility, meaning it evaporates rapidly at room temperature after application. This property makes it an effective carrier fluid in cosmetic formulations. The ingredient helps to dissolve and evenly distribute other heavier or active substances, such as fragrances or conditioning agents, across the skin or hair.
As the product is applied, the D5 quickly vaporizes, leaving behind only the intended beneficial ingredients. This rapid evaporation ensures a non-greasy, light, and silky sensory feel on the skin or hair, preventing the tacky residue often associated with heavier oils. This is beneficial in products requiring a quick-drying finish, including spray deodorants, antiperspirants, and lightweight hair serums.
Consumer Concerns and Global Regulation
Regarding human safety, the Cosmetic Ingredient Review (CIR) Expert Panel has evaluated Cyclopentasiloxane and determined it is safe for use in cosmetic products at current concentrations. Studies indicate that the molecule does not penetrate the skin in measurable amounts and evaporates quickly, limiting systemic exposure. The primary concerns surrounding D5 relate to its environmental fate, specifically its potential for persistence and bioaccumulation in aquatic environments.
Due to these environmental factors, regulatory bodies, particularly in the European Union (EU), have implemented restrictions on D5 use. Since 2020, EU regulations have prohibited the placing on the market of wash-off cosmetic products containing Cyclopentasiloxane concentrations above 0.1% by weight. This measure targets products that are immediately rinsed down the drain before the ingredient has a chance to evaporate. The EU has expanded these restrictions, announcing a prohibition on D5 in leave-on cosmetic products at concentrations equal to or above 0.1% by weight, set to take effect in 2027. These regulatory updates reflect a global effort to mitigate the environmental impact of persistent ingredients, even as safety panels affirm the ingredient’s minimal risk to human health.