Phenyl trimethicone’s CAS number names one 372.75 g/mol molecule, not a polymer

A brand owner wants more shine from the leave-in hair serum you make for them, and sends three questions before signing off the formula: is phenyl trimethicone water soluble, what does it do on skin and hair, and does it have side effects? The purchase request that reaches procurement carries an INCI name, a CAS number (2116-84-9) and little else. Each question has a short answer. A fourth question, what that CAS number names, changes how you should write the specification.

What is phenyl trimethicone?

Phenyl trimethicone (CAS 2116-84-9, also written phenyltrimethicone) is a silicone fluid used in cosmetics as a skin- and hair-conditioning emollient. PubChem records it as one molecule, C15H32O3Si4, with a molecular weight of 372.75 g/mol: a central silicon atom carrying a phenyl ring and three trimethylsiloxy groups. It does not dissolve in water and belongs in the oil phase.

The name maps onto that structure. “Phenyl” is the aromatic ring on the central silicon, and the rest of the molecule is methyl-capped silicone: three trimethylsiloxy groups, each a silicon atom carrying three methyl groups and joined to the central silicon through oxygen. PubChem’s IUPAC name, trimethyl-[phenyl-bis(trimethylsilyloxy)silyl]oxysilane, spells out the same molecule. A supplier’s Safety Data Sheet (SDS) or Certificate of Analysis (CoA) may use a synonym instead, such as phenyltris(trimethylsiloxy)silane or tris(trimethylsiloxy)phenylsilane.

Phenyl trimethicone gets described as a phenyl-modified silicone polymer, a label that fits the wider family of phenyl silicones better than this substance. The registry identity behind CAS 2116-84-9 is one small molecule with four silicon atoms, not a chain of repeating units with a spread of molecular weights. Whether a commercial lot is that molecule alone, or that molecule with close relatives, is for the supplier’s CoA and technical data sheet (TDS) to show. It is the first question to put to a new supplier.

What does phenyl trimethicone do for skin and hair?

The product listing for phenyl trimethicone names it a skin- and hair-conditioning emollient, an antifoam additive in formulations and an antistatic agent in hair-care products. Gloss comes from the structure. An aromatic ring raises the refractive index of a silicone fluid, and a film with a higher index reflects more light from its surface, which reads as shine on hair and lips. Against dimethicone, an all-methyl silicone, that higher index is the property you are specifying.

On skin, phenyl trimethicone works as an emollient in creams, primers, serums and foundations: it spreads into a light silicone film that softens the after-feel and adds sheen. Set the level on your own sensory panel and stability tests, because sheen and residual film weight both rise with concentration.

On hair, a leave-in serum or shine spray gets two of the listed roles, conditioning and the antistatic effect that controls flyaway, plus gloss from the phenyl ring. Leave-in formats are where that combination pays, because the film stays on the fiber.

The antifoam listing deserves a bench test of its own. The listing names the role, not its size: during mixing, phenyl trimethicone may help a batch release entrained air, by an amount that depends on its level and the rest of the formula. In a shampoo or body wash, the same property can cut into the lather the product is sold on, so compare foam height against a control before you add phenyl trimethicone to a cleanser.

Is phenyl trimethicone water soluble?

No, phenyl trimethicone is not water soluble. The molecule has no hydroxyl, ionic or polyether group to hold it in water, and its outer surface of methyl groups and one phenyl ring repels water. Plan it as an oil-phase ingredient, and let the emulsifier system carry it through a water-continuous cream or lotion.

If your regulatory dossier needs a measured water-solubility figure for phenyl trimethicone, ask the supplier for one. A figure measured on another silicone does not transfer to this molecule.

The oil phase is where the phenyl ring earns its place. An aromatic ring widens the molecule’s compatibility with organic esters and plant oils, so phenyl trimethicone mixes into oil phases that combine silicones with esters. One commercial hair-and-skin blend pairs it with cyclopentasiloxane, dimethiconol, C12-15 alkyl benzoate (an ester) and dimethicone crosspolymer.

Volatility is a separate property. Cyclopentasiloxane, a ring of five dimethylsiloxane units in that blend, is listed as a volatile carrier that evaporates from the film; phenyl trimethicone is listed as a conditioning emollient, not as a volatile. If dry-down time or residue matters to your brief, ask for evaporation data on the phenyl trimethicone grade you are buying.

What about phenyl trimethicone side effects?

The public hazard record for phenyl trimethicone does not give one answer. PubChem’s compound record gathers the hazard classifications that companies have notified for the neat substance, and those classifications differ between notifiers. The current SDS for the grade you buy governs plant handling, and side effects in a cream or serum are assessed on that finished product.

For people handling the neat fluid, read the hazard-identification and toxicology sections of the supplier’s current SDS for your lot before the first drum is opened, and do not infer them from the fact that the material ends up in skin care. Because notified classifications differ, check which ones your supplier’s SDS carries, and read the PubChem record next to the SDS, not in place of it.

For a consumer using a finished cream or serum, side effects belong to that product’s own safety assessment, which reflects the level of each ingredient, the rest of the formula and how the product is used. A raw-material record cannot answer that question in either direction. A brand owner who asks you for a verdict should get the current SDS and a pointer to the finished-product assessment, not a reassurance.

How should you specify phenyl trimethicone?

The same molecule can be made for non-cosmetic uses, so CAS 2116-84-9 alone tells you which molecule to expect, not which grade or which documents came with the lot. Write those into the purchase order:

1. Identity. Quote CAS 2116-84-9 and the INCI name, and check that the chemical name on the CoA and SDS matches PubChem’s IUPAC name or one of its synonyms.

2. Assay. Ask for an identity test and a purity assay on each CoA, so the lot shows how close it is to the single molecule the CAS number names.

3. Grade and documents. Name the cosmetic grade and the documentation your market asks of a cosmetic ingredient; a lot made for a non-cosmetic use can carry the same CAS number without them.

4. Optical check. If gloss is the reason you buy it, ask for refractive index on every CoA and track it lot to lot, since it is the property you are paying for.

5. Physical data. Ask for density, to convert drum volume to invoiced weight, and for evaporation or volatile-content data if residue or dry-down time is part of the brief.

6. Bench validation. Request a sample against your target specification and confirm stability, sensory and, for cleansers, foam before you commit to a supply line.

Methodology: identity data for phenyl trimethicone (CAS 2116-84-9: formula, molecular weight, IUPAC name and synonyms) and the notified hazard classifications are from PubChem CID 62427. The cosmetic roles of phenyl trimethicone and cyclopentasiloxane and the blend composition come from RawSource product and catalog listings.

Editorial note: this article is general technical information for formulation and procurement professionals, not a medical, health, cosmetic-efficacy or safety claim, and RawSource gives no warranty, express or implied, for its use.