Cyclomethicone

• USP–NFCyclomethicone

cyclic dimethylpolysiloxane; cyclopolydimethylsiloxane; cyclopentasiloxane (D5); cyclotetrasiloxane (D4); cyclohexasiloxane (D6); volatile silicone

Fully methylated cyclic siloxanes (mainly octamethylcyclotetrasiloxane to dodecamethylcyclohexasiloxane) · [69430-24-6]

[(CH3)2SiO]n, n = 4–6 · ≈ 297 (D4) to 445 (D6)

Semi-solid base, emollient

Typical concentrations: — · Dosage forms: Semi-solid

Used in topical creams, lotions, gels, sprays and antiperspirants as a volatile carrier and emollient: it spreads easily, gives a light, non-greasy, silky feel and then evaporates without leaving a residue or cooling the skin. Levels vary widely, from a few per cent in emulsions to the major part of anhydrous sticks and sprays. It is also widely used in hair-care products.

A clear, colourless, odourless, volatile liquid of low viscosity. It consists of cyclic polydimethylsiloxanes with four to six repeating units; commercial grades are mainly the pentamer (D5) or blends of D5 and D6.

Boiling point

about 175 °C (D4); about 210 °C (D5)

Viscosity

low, a few mm²/s (about 4 mm²/s for D5) at 25 °C

Density

about 0.95 g/cm³

Surface tension

low, about 18 mN/m

Solubility

insoluble in water; miscible with ethanol, isopropyl alcohol and most hydrocarbons and esters

Chemically stable and inert. Store in an airtight container, because it is volatile, in a cool place away from sources of ignition.

Strong acids and bases can catalyse ring-opening and polymerisation. It is otherwise compatible with most cosmetic and pharmaceutical ingredients.

Hydrolysis of dimethyldichlorosilane gives a mixture of linear and cyclic siloxanes; the cyclic compounds are separated and purified by distillation.

Of low toxicity in topical use, and generally non-irritant and non-sensitising to skin. D4 is classified in the EU as a suspected reproductive toxicant. Because D4, D5 and D6 are persistent and bioaccumulative in the environment, their content in many cosmetic and consumer products is restricted in the EU under REACH.

Observe normal precautions for the quantity handled. The liquid is combustible and spills are slippery; keep away from heat and flames and use in a well-ventilated area.

Included in the FDA Inactive Ingredient Database (topical preparations). Widely used in cosmetics, subject to the EU REACH restrictions on D4, D5 and D6.

Dimethicone; simethicone; silicone elastomers.

Formulators in Europe are moving to alternatives such as low-viscosity linear silicones or volatile hydrocarbons and esters because of the REACH restrictions. Its evaporation leaves the active and non-volatile components concentrated on the skin, which should be considered when assessing local tolerability and dosing.