What Is Isostearyl Isostearate?
Isostearyl isostearate (INCI: Isostearyl Isostearate, CAS 41669-30-1) is a branched, fully saturated liquid ester formed from a branched C18 fatty alcohol and a branched C18 fatty acid. Its molecular formula is C36H72O2 and its molecular weight is about 537 g/mol. The material is a clear to slightly hazy, low-odour liquid at room temperature and is supplied as a vegetable-derived or synthetic cosmetic emollient ester.
The branching in the molecule is the reason for its behaviour. Branched chains stay liquid and spread evenly instead of crystallising, and a saturated ester has no reactive double bonds, so it resists oxidation and rancidity far better than many natural oils. That combination gives formulators a light skin feel with the stability of a synthetic ester.
Functional Roles in a Formulation
Emollient: softens and conditions skin and hair, reduces moisture loss and improves the condition of a compromised barrier.
Solubiliser: helps disperse crystalline actives, UV filters and pigments, improving formulation homogeneity and stability.
Spreading agent: lowers the drag of heavier oils and improves the evenness of coverage in creams, lotions and colour cosmetics.
After-feel modifier: leaves a silky, non-greasy touch even though it is an oil-phase ingredient, without the heavy residue of some traditional emollients.
Because it performs several of these functions at once, it is often used as a partial replacement for mineral oil or for low-viscosity esters rather than as an additive at trace level.
Head-to-Head Comparison with Common Emollients
| Emollient | Skin feel | Spreadability | Oxidative stability | Notes for formulators |
|---|---|---|---|---|
| Isostearyl isostearate | Silky, light, non-greasy | Good, even film | High; saturated branched ester | Wide compatibility, useful pigment wetting |
| Mineral oil | Heavier, more occlusive | Moderate | Very high | Low cost but can feel greasy and is not always preferred in premium textures |
| Isopropyl myristate | Very light, fast absorbing | High | Good | Lower molecular weight, more penetration and higher irritation or comedogenicity potential in some users |
| Caprylic and capric triglyceride | Light, dry | High | High | Good general carrier, less conditioning and less solubilising power |
| Natural plant oils | Rich, variable | Variable | Lower; prone to rancidity | Marketing appeal, but batch-to-batch variation and odour must be managed |
The practical conclusion is that isostearyl isostearate sits between the light, penetrating esters and the heavy occlusive oils. It gives a more elegant texture than mineral oil, greater stability than most natural oils, and a lower irritation profile than short-chain esters such as isopropyl myristate.
Technical and Quality Data
| Parameter | Typical value | Why it matters |
|---|---|---|
| INCI name | Isostearyl Isostearate | Required for labelling and formulation documents |
| CAS number | 41669-30-1 | Identity and regulatory reference |
| Molecular formula and weight | C36H72O2, about 537 g/mol | Sets the spread and skin feel profile |
| Appearance | Clear to slightly hazy liquid | Visual screen for quality |
| Assay | 98% minimum | Consistency of performance between batches |
| Acid value | 1.0 mg KOH/g maximum | Indicates hydrolysis and residual free acid |
| Iodine value | Low, typically 5 g I2/100 g maximum | Confirms the saturated, oxidation-resistant structure |
| Water content | 0.1% maximum | Prevents emulsion and preservative problems |
| Specific gravity at 20 °C | about 0.86 | Used in formulation calculations |
Application Areas and Typical Use Levels
Isostearyl isostearate is used wherever a light, stable emollient is needed in an oil phase. Typical applications include facial creams and moisturisers, BB and CC creams, sunscreens and after-sun products, body lotions, and lip care and colour cosmetics. Use levels are usually set by the texture target rather than by a regulatory ceiling: skin care emulsions commonly carry 2 to 15% of the oil phase, and lip and colour products may use considerably higher levels when a rich, glossy film with good pigment wetting is wanted.
In hair care it is used to add slip and conditioning without a heavy coating. In anhydrous and stick formats it helps control structure and pay-off. The ester is generally compatible with common oil-phase ingredients, silicones, esters and hydrocarbon carriers, and it is stable in the presence of typical actives, which is why it is often selected as the emollient when a formula also carries crystalline UV filters or pigments that need to be dispersed evenly.
Sourcing Considerations for Bulk Emollient Esters
Confirm identity against the INCI name and CAS number, and ask for the origin route, since both vegetable-derived and synthetic material are traded under the same chemical identity.
Review the certificate of analysis for assay, acid value, iodine value, colour, water content and heavy metals, and keep a retained sample per batch.
Ask whether the supplier documents natural origin content against ISO 16128 if the product will carry a natural sourcing claim, and check cosmetic regulation compliance for the destination market.
Confirm packaging format, shelf life, storage temperature and recommended retest interval; keep containers closed because cosmetic esters can absorb odour and moisture in storage.
Texture claims sell products, but the specification behind them is what keeps a formula reproducible. Comparing two emollient esters on skin feel alone, without matching assay, acid value and stability data, is one of the most common causes of a formulation drifting after a supplier change.
Frequently Asked Questions
Q: What is isostearyl isostearate used for?
It is used as a light, stable cosmetic emollient ester in skin care, sun care, colour cosmetics and hair care, where it also acts as a solubiliser, spreading agent and after-feel modifier.
Q: How does isostearyl isostearate compare with mineral oil?
It gives a silkier, lighter and less greasy after-feel than mineral oil and is easier to use in premium textures, although it is a higher-cost oil-phase ingredient.
Q: Is isostearyl isostearate better than isopropyl myristate?
It has a higher molecular weight and a lower irritation and comedogenicity potential, and it provides more conditioning, while isopropyl myristate spreads faster and feels lighter on the skin.
Q: Does isostearyl isostearate go rancid?
It is a saturated branched ester with a low iodine value, so it resists oxidation far better than unsaturated natural oils and does not develop the rancid odour those oils can produce.
Q: What is the CAS number of isostearyl isostearate?
CAS 41669-30-1, with the INCI name Isostearyl Isostearate, corresponding to the ester of isostearyl alcohol and isostearic acid.
Q: What should a buyer check when sourcing bulk isostearyl isostearate?
Confirm INCI name and CAS number, assay, acid value, iodine value, colour, water content and heavy metals on the certificate of analysis, plus natural origin documentation if a claim is planned.
