Is Cyclopentasiloxane Water Soluble? Properties, Uses, and Sourcing Specs

By the RawSource Sourcing Desk, Commercial & Sourcing Desk, RawSource

You blend cyclopentasiloxane into an aqueous serum and it beads up, then floats off as a film on the surface. The technical data sheet says it all in one line: solubility in water, 0.017 mg/L. That single number decides which phase the ingredient lives in, how a lot ships and stores, and which regulatory file now follows the molecule across two continents.

The question “is cyclopentasiloxane water soluble” looks like a chemistry trivia search. For a formulator or a sourcing manager, it is the first fork in a procurement decision. Get the phase behavior wrong and you reformulate. Get the regulatory read wrong and you buy inventory you cannot ship into a key market. This guide answers the solubility question directly, then works through the properties, the comparisons, and the specification language that turn the answer into a clean RFQ.

Is cyclopentasiloxane water soluble?

No. Cyclopentasiloxane, the cyclic volatile methylsiloxane known as D5 (CAS 541-02-6, PubChem CID 10913), is practically insoluble in water. The measured value is about 0.017 mg/L at 25 °C, reported as 1.7 x 10⁻² mg/L on its PubChem record. For context, that is roughly seventeen micrograms in a full liter of water. In a beaker, in a reactor, or in a finished emulsion, D5 stays in the oil phase.

So D5 is a lipophilic, oil-loving fluid. It mixes readily with other silicones, with oils, and with light alcohols, and it sits apart from water and water-soluble humectants like glycerin. Every downstream choice follows from that fact: emulsifier selection, the order of addition, viscosity targets, and the storage and shipping classification.

The molecule is a five-unit ring of alternating silicon and oxygen atoms, with two methyl groups on each silicon. Its formula is C10H30O5Si5, confirmed on PubChem. The silicon-oxygen backbone is flexible and the methyl shell is nonpolar, so the surface that water would have to wet is a dense layer of methyl groups. Water has nothing to grab.

Why is cyclopentasiloxane insoluble in water?

Insolubility comes from the methyl shell, not the ring oxygen. Water dissolves a compound when it can form hydrogen bonds or strong dipole interactions with it. D5 presents an outer surface of methyl groups that are nonpolar and offer no hydrogen-bond donors. The oxygen atoms that could accept a hydrogen bond are buried inside the siloxane ring, shielded by the methyls around them.

The result is a fluid that reads as oil to water and as oil to most polar solvents. The same shielding makes D5 chemically stable and low in surface tension, which is why it spreads so easily on skin and hair and then leaves a smooth, non-greasy feel as it evaporates.

This behavior is shared across the cyclic siloxane family and grows stronger as the ring gets larger. More silicon-methyl units mean a larger nonpolar surface and even lower water affinity, which is the trend the comparison below makes concrete.

What solvents does cyclopentasiloxane dissolve in?

D5 dissolves in oils and silicones, plus light alcohols, never in water. For a formulator building the oil phase, that is the working rule. The fluid is miscible with most linear and cyclic silicone fluids, including dimethicone, and it carries silicone gums and dimethiconol into a spreadable blend. It mixes with mineral oil, many ester oils, and a range of plant-derived oils.

Among polar solvents, D5 handles short-chain alcohols. It is miscible with ethanol and isopropanol, which is why volatile-silicone and alcohol systems appear in antiperspirant and quick-dry formats. It does not dissolve in water, propylene glycol, or glycerin, so those stay in the separate water phase.

For a procurement or QC team, this maps to two practical checks. First, any compatibility trial belongs in the oil phase or in an alcohol carrier, never in a water pre-mix. Second, if a supplier sample looks hazy in an oil blend, suspect water contamination or an off-spec residual instead of a true solubility problem. A Karl Fischer water test settles it fast.

How does D5 compare to D4 and D6 on water solubility and volatility?

All three cyclic siloxanes are practically insoluble in water, and solubility falls as the ring grows. D4 (octamethylcyclotetrasiloxane), D5, and D6 (dodecamethylcyclohexasiloxane) share the same chemistry but differ in ring size, and that difference shifts every property a buyer cares about. The table below uses values from each compound’s PubChem record.

Read the trend across the row. Larger rings are less water soluble, less volatile, more viscous, and higher in flash point. D4 evaporates fastest and flashes lowest, so it is the most aggressive solvent and the most demanding to store.

D6 evaporates slowest and behaves more like a light emollient oil than a quick carrier. D5 sits in the middle, which is why it became the default volatile silicone for personal care: fast enough to leave a dry finish, with a flash point that is manageable in a standard cosmetic plant. Full records sit on PubChem for D4 and D6.

The volatility difference also drives substitution decisions. Where a formula needs the dry, fast finish that built D5’s reputation, D6 evaporates too slowly to match it; where a formula wants longer slip and a touch of residual emolliency, D6 or a non-volatile silicone reads better than D5. That trade between speed of evaporation and staying power, not water solubility, is usually the real question behind a switch between rings.

One sourcing consequence matters here. D4 is the most heavily scrutinized member of the family, so its presence as a residual impurity in D5 is a specification issue, not a footnote. More on that in the buying section.

What does insolubility mean for how cyclopentasiloxane is used?

Insolubility plus volatility is the formula behind D5’s signature feel. Because it stays in the oil phase and then flashes off near body temperature over time, D5 acts as a carrier that delivers other actives and silicones, spreads them thinly, and vanishes without a greasy film. That profile underpins its main applications:

1. Antiperspirants and deodorants, where D5 carries the active and dries to a non-tacky finish.
2. Hair care, where D5 and dimethiconol blends add slip and shine, then evaporate so the hair is not weighed down. The trade-offs for hair are covered in Is Cyclopentasiloxane Good or Bad for Hair?
3. Skin care and color cosmetics, where D5 thins heavy oils, improves spreadability, and gives a silky after-feel.

In an emulsion, D5 is part of the internal oil droplet, stabilized by an emulsifier, not dissolved in water. That is the central design point: you do not solubilize D5, you emulsify it or carry it in a compatible oil or alcohol. For background on where volatile and non-volatile silicones fit, see What Is Silicone Oil? Types, Viscosity Grades, and Bulk Sourcing and the broader Beauty & Personal Care hub.

Emulsion design follows from the same logic. Because D5 is volatile, it should join the oil phase late and at a controlled temperature, since holding it hot for long under an open vessel loses material to evaporation and shifts the final ratio. Silicone-compatible emulsifiers, often other silicones such as the PEG/PPG dimethicone copolyols, carry D5 into a stable internal phase more reliably than conventional fatty-alcohol systems.

For a sourcing manager, that links the D5 spec to the emulsifier spec. A change in either can move the viscosity and stability of the finished product, so qualify them together, not one at a time.

Non-volatile silicones play the opposite role. Where D5 is meant to leave, dimethicone is meant to stay and form a protective film, as described in Discover the Unique Benefits and Applications of Dimethicone. Many formulas pair the two: D5 for delivery and feel, dimethicone for lasting conditioning.

How does water insolubility change handling and wastewater?

Insolubility shapes the environmental and handling profile as much as the formulation. Because D5 does not dissolve in water, it does not stay suspended in the water column when it reaches a treatment plant. It partitions onto solids and into sludge, and its low water affinity is part of why regulators treat it as persistent and bioaccumulative. Insolubility and degradation resistance travel together for this molecule.

Handling is driven by volatility, not water risk. With a closed-cup flash point of 73 °C, D5 is a combustible liquid, milder to store than D4 at 55 °C but still a fluid that wants sealed containers, good ventilation, and ignition-source control. Confirm the consignment’s classification and packing details against the current SDS for each grade before booking freight.

Storage practice tracks the flash points in the comparison table. D5 at 73 °C and D6 at 91 °C sit well above ambient, so they store as combustible liquids, while D4 at 55 °C runs closer to the flammable boundary and warrants tighter controls. Keep all three in closed, grounded containers away from heat and oxidizers, and confirm the storage class on the current SDS for the specific grade, since blends and stabilized products can carry a different classification than the neat fluid.

For wastewater and spill planning, treat D5 as an oil. Skim and contain instead of diluting, since dilution does not work on a fluid that will not enter solution. Keep it out of drains, and route waste through a licensed chemical handler.

What regulations apply to cyclopentasiloxane?

D5 is regulated as a persistent, bioaccumulative substance, and the rules are tightening. The European Chemicals Agency (ECHA) classifies D4, D5, and D6 as very persistent and very bioaccumulative (vPvB), with D4 also identified as persistent, bioaccumulative, and toxic (PBT).

Under REACH Annex XVII, D4 and D5 have been restricted in wash-off cosmetic products to below 0.1% by weight, and a 2024 amendment extends restrictions to leave-on cosmetics and adds D6, with staggered compliance dates. Confirm the exact effective dates and product scope directly with ECHA before formulating to any threshold, because the transitional periods differ by use.

In the United States, the Environmental Protection Agency is running a TSCA risk evaluation of D4, the most scrutinized member of the family. Details sit on the EPA risk evaluation page for octamethylcyclotetrasiloxane. Because D4 can appear as a residual impurity in commercial D5, the D4 evaluation reaches into D5 procurement through the impurity spec, not only through the use of D4 itself.

The practical takeaway for a buyer is simple. Solubility is not the only reason to read the regulatory file; it is part of the reason the file exists. Track the ECHA restriction status for the markets you sell into, and put a residual-D4 limit on every D5 purchase so a low-spec lot cannot quietly push a finished product over a cosmetic threshold.

What should buyers specify when sourcing cyclopentasiloxane?

Write the specification around the two facts that matter most: D5 is an oil-phase fluid, and its D4 residual is a regulated impurity. A clean RFQ for cosmetic-grade D5 should name the following on the purchase spec and require each value on the certificate of analysis for the lot:

1. Assay (D5 content), with a minimum percentage.
2. Maximum residual D4 content, set to the tightest limit your destination markets require.
3. Water content by Karl Fischer, with a maximum, since trace water shows up as haze in oil blends.
4. Viscosity range near 3.9 cSt at 25 °C, to confirm you received D5 and not a heavier siloxane or a blend.
5. Appearance and color, specified as a clear, colorless liquid.

Grade matters because the residual-D4 level is set by how the D5 was made and purified, not by the D5 molecule itself. Cosmetic-grade material is fractionated to drive residual D4 down, while technical or industrial grades aimed at non-cosmetic uses may carry a looser D4 limit at a lower price.

Buying a technical grade into a cosmetic line to save on unit cost is a common way to inherit a regulatory problem. Match the grade to the end use, and write the D4 limit into the contract instead of assuming it.

Two further moves protect a program. Require origin and lot number on every CoA so a flagged batch is traceable, and ask for the current SDS and any vPvB or REACH restriction statements in the supplier’s documentation pack. For related volatile-silicone chemistries, Cyclotetrasiloxane (D4) and Cyclohexasiloxane (D6) carry their own specs and restriction profiles, and adjacent ingredients like phenyl trimethicone follow similar solubility logic, covered in Phenyl Trimethicone: Uses, Safety, Solubility, and Bulk Sourcing.

To compare cosmetic-grade specifications and request a quote with your residual-D4 and water-content limits attached, start from the Cyclopentasiloxane (D5) product page and submit those targets with the RFQ. Specifying the impurity limits up front keeps the quotes comparable and the regulatory exposure visible before a lot ever ships.

Methodology: physical property values (water solubility, boiling and flash points, viscosity, vapor pressure, and density) are drawn from the PubChem compound records cited above (CIDs 10913, 11169 and 10911), and molecular formulas were confirmed against PubChem. Regulatory status is summarized from ECHA’s REACH Annex XVII restrictions and the EPA TSCA program; verify current effective dates and product scope with those agencies before formulating to a threshold.

RawSource Editorial

Commercial & Sourcing Desk