Decamethylcyclopentasiloxane
[CAS 541-02-6]

Identification

• Classification: Organic raw materials >> Hydrocarbon compounds and their derivatives >> Cyclic hydrocarbon
• Name: Decamethylcyclopentasiloxane
• Synonyms: 2,2,4,4,6,6,8,8,10,10-Decamethyl-1,3,5,7,9,2,4,6,8,10-pentoxapentasilecane
• Molecular Formula: C₁₀H₃₀O₅Si₅
• Molecular Weight: 370.77
• CAS Registry Number: 541-02-6
• EC Number: 208-764-9
• SMILES: C[Si]1(O[Si](O[Si](O[Si](O[Si](O1)(C)C)(C)C)(C)C)(C)C)C

Properties

• Density: 1.0±0.1 g/cm³ Calc.^*, 0.959 g/mL (Expl.)
• Melting point: -44 °C (Expl.)
• Boiling point: 210.0 °C 760 mmHg (Calc.)^*, 237.9 °C (Expl.)
• Flash point: 95.6±23.0 °C (Calc.)^*, 76 °C (Expl.)
• Index of refraction: 1.427 (Calc.)^*, 1.396 (Expl.)

^* Calculated using Advanced Chemistry Development (ACD/Labs) Software.

Safety Data

• Hazard Symbols: GHS07 Warning
• Risk Statements: H227-H315-H319-H413
• Safety Statements: P273-P210-P264-P280-P370+P378-P337+P313-P305+P351+P338-P332+P313-P403+P235-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Chronic hazardous to the aquatic environment	Aquatic Chronic	4	H413
Skin irritation	Skin Irrit.	2	H315
Eye irritation	Eye Irrit.	2	H319
Acute toxicity	Acute Tox.	3	H331
Specific target organ toxicity - single exposure	STOT SE	3	H335

chemBlink Chemical Story

Decamethylcyclopentasiloxane, CAS 541-02-6, is usually called D5 or cyclopentasiloxane. It is a cyclic volatile methylsiloxane consisting of a five-membered Si-O ring in which each silicon bears two methyl groups. The formula C10H30O5Si5 captures the unusual architecture: an inorganic-like siloxane backbone surrounded by organic methyl groups.

D5 became important in personal-care formulation because it combines low surface tension, easy spreading and volatility. It can carry other ingredients across skin or hair, provide a smooth lubricious feel, and then evaporate rather than leaving a heavy oily residue. The European Commission's Scientific Committee on Consumer Safety lists functions including antistatic, emollient, solvent, viscosity control and hair conditioning. D5 has consequently appeared in deodorants, hair products, skin creams and other cosmetics, and it has also been used in cleaning and industrial solvent applications.

The Si-O-Si backbone behaves differently from an ordinary carbon chain. Flexible bond angles and weak intermolecular interactions contribute to the fluidity and low surface tension characteristic of volatile silicones. D5 can also serve as a feedstock or cyclic monomer in silicone chemistry, where ring-opening polymerization can generate polysiloxane chains.

Volatility, however, also means environmental release. D5 has been extensively evaluated for environmental persistence and bioaccumulation, and regulatory treatment differs by jurisdiction and use pattern. In Europe, restrictions have been adopted for certain cyclic siloxanes in cosmetic and other applications, so modern discussion of D5 must include both its formulation advantages and its environmental fate.

D5 therefore represents a broader story in materials design. Its cyclic siloxane structure was valued because it delivered a sensory combination difficult to obtain from conventional oils: rapid spreading, slip and evaporation. The same volatility and chemical stability that enabled those benefits also created questions about where the molecule goes after use.

References:
1. European Commission SCCS. Opinion on Decamethylcyclopentasiloxane (Cyclopentasiloxane, D5) in Cosmetic Products, SCCS/1549/15.
2. Chemical Insights. Decamethylcyclopentasiloxane: A Common Air Pollutant.
3. Sigma-Aldrich, Decamethylcyclopentasiloxane, CAS 541-02-6.

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