One Branched Acid, Six Backbones: Buying Isostearate Esters by Identity Number

Isostearic acid and stearic acid have the same molecular formula, the same molecular weight of 284.5, and more than 60 °C of melting behaviour between them. The difference is a methyl branch. Every ester made from isostearic acid inherits that difference, and it is the reason a lipstick can be glossy without being brittle and a sunscreen can wet a pigment without turning grainy.

What the branch also does is make purchasing harder. A saturated isostearate and the corresponding stearate have identical formulas, identical molecular weights and identical saponification values. On a certificate of analysis they are the same material. This guide sets out what actually separates them, how the six commercial backbones differ, and what to write on the order so that the drum you receive is the drum you specified.

The acid decides the melting behaviour

A straight C18 chain packs into a crystal lattice, and that lattice melts at 69 °C. A branched C18 cannot pack. The methyl group sits somewhere along the chain - 16-methylheptadecanoic acid is the commonest isomer, and commercial material is a mixture of several - and each branch props the neighbouring chains apart. The result is a liquid that stays liquid to around -20 °C, at the same molecular weight.

Two consequences follow, and both are worth money. The first is oxidative stability. Isostearic acid is made by isomerising oleic acid, and the cosmetic grade is saturated down to an iodine value of 8 or below, so almost no allylic positions are left to oxidise. Esters made from it do not develop the rancid note that oleates do, which matters in a product with a two-year shelf life. The second is pigment wetting. The branched chain disrupts packing at the pigment surface as well, so isostearates disperse treated pigments and mineral filters more evenly than the straight-chain equivalent at the same dose.

Read the acid value as a fingerprint rather than a purity test. A pure C18 monoacid at 284.5 should titrate at about 197 mg KOH/g. The cosmetic grades are specified at 175-195, which corresponds to an apparent molecular weight of 288 to 320. The gap is real: the material carries a small heavy fraction, and how large that fraction is tells you which process made it. Two suppliers quoting "acid value 175-195" are not quoting the same molecule.

Six backbones, six numbers

The saponification column is calculated from the formula, not copied from a data sheet: SV = n x 56,106 / M, where n is the number of ester groups. It is there so you can check a supplier's figure in ten seconds. Octyldodecyl stearoyl stearate at 847.5 with two ester bonds must titrate near 132; a data sheet claiming 180 for that INCI is describing a different molecule.

The table also shows what the acid does not decide. The same branched C18 produces a 326 molecular weight wetting agent and an 891 molecular weight lipstick body. The acid is constant; the backbone is the variable. Glyceryl isostearate is the only one of the six that is an emulsifier rather than an emollient. A single ester on glycerol leaves two free hydroxyls, which pulls the HLB down to 3-4 and makes it a water-in-oil co-emulsifier at 1-3% alongside a primary W/O surfactant. The other five are oil-phase materials and do nothing at an interface on their own.

The COA cannot tell isostearate from stearate

This is the most expensive confusion in the family. Isopropyl isostearate and isopropyl stearate are both C21H42O2 with a molecular weight of 326.5. Both titrate at 172 mg KOH/g. Both have an acid value below 1 and a hydroxyl value near zero. Acid value, saponification value, hydroxyl value, density and even refractive index overlap far enough that a standard COA cannot separate them.

The properties, however, are opposite. Isopropyl stearate is a soft solid at room temperature; isopropyl isostearate is a mobile liquid. Put the wrong one into an anhydrous gel and the product sets in the drum. Put the wrong one into a lipstick and the bullet sweats and bends.

What does separate them is measurable, and belongs on the specification:

• Cloud point or pour point. The branched ester stays clear well below 0 °C; the straight-chain one clouds and sets above 20 °C. This single number catches most substitutions.
• Gas chromatography, reported as branched-isomer content. A cosmetic isostearate should show a defined isomer distribution, not one peak.
• Refractive index at 25 °C. Isostearyl isostearate runs 1.458-1.463; the straight-chain analogue sits measurably higher.
• Iodine value. Residual unsaturation from the oleic feedstock is the fingerprint of the process; a value of zero is suspicious rather than reassuring.

Where each backbone earns its place

• Lip and stick products. Isostearyl isostearate, at 45 mPa.s and a refractive index of 1.458 at 20 °C, gives gloss with a rich rather than slippery feel, and it wets pigment well enough to replace part of the castor oil in a bullet. Octyldodecyl stearoyl stearate is the opposite tool: 83 cP at 25 °C and a spreading value of 205 mm2/10 min, against 1,650 for diisopropyl adipate. It barely spreads, which is what keeps a lipstick from migrating into fine lines.
• Water-in-oil sunscreens. Glyceryl isostearate at 1-3% with a low-HLB primary emulsifier, plus triisostearin for body. Triisostearin's three ester bonds and 891 molecular weight give the film a resistance to wash-off that a monoester cannot.
• Pressed powders and eye shadow. Isostearyl isostearate is used as a binder because it disperses pigment and stays where it is placed. Isocetyl stearoyl stearate, with two ester bonds and a branched C16 alcohol, is the harder binder for the same job.
• Emollient oils and serums. Isopropyl isostearate is the light end: a wetting agent that reduces the drag of silicone and mineral oil without adding shine.

What to put on the purchase order

• INCI and CAS together, on every line. "Isostearate" alone is not a specification; the same trade name covers six different backbones and several acid qualities.
• Saponification value with a calculated check. Ask what the theory number is for the formula you specified, and reject a COA that misses it by more than a few percent.
• Cloud point or pour point, plus isomer distribution by GC. Without one of these you cannot prove the acid was branched.
• Colour and odour. These materials sell into colour cosmetics; APHA or Gardner colour and an odour note decide whether the batch is usable in a white cream.
• The acid's own constants if you are buying an ester to make something else: acid value 175-195, iodine value 8 max, cloud point -20 to +10 °C.

Supply

Shanghai Better Chemical supplies the family from China, with COA, SDS and TDS issued against the end use you declare:

• Glyceryl Isostearate (CAS 66085-00-5) - W/O co-emulsifier and bodying agent, HLB 3-4, 1-10%.
• Isopropyl Isostearate (CAS 31478-84-9) - light, spreading emollient and pigment wetting agent.
• Triisostearin (CAS 26942-95-0) - triglyceride for gloss, body and wash-off resistance.
• Isostearyl Isostearate (CAS 41669-30-1) - rich binder and dispersant for lip and powder.
• Isocetyl Stearoyl Stearate (CAS 97338-28-8) - firm binder for lipstick and balm.
• Octyldodecyl Stearoyl Stearate (CAS 90052-75-8) - cushion and low-spread structure for sticks and sun care.

For the heavy end of the same family, our guide to branched ester emollients and pentaerythrityl tetraisostearate covers the polyol-core route, and the light emollient ester guide covers the straight-chain esters these are usually compared against. Tell us the backbone, the acid quality and the market, and we will confirm grade and documentation before you place the order.