Ask three suppliers for "a light, dry emollient ester" and you get three different materials with the same viscosity on the certificate. That is the trap in this part of the oil phase. Viscosity describes how a liquid pours; it says nothing about how the film behaves once it is on skin. Two esters that both read 6 mPa.s can differ by nearly half again in spreading area, and the difference a consumer notices is the spreading, not the number in the spec sheet.

This guide works through eleven light esters we stock - the isopropyl esters, the octyl and cetyl esters, the C12-15 and cetearyl grades, and the glycerol triesters - against the three properties that actually decide performance: viscosity, spreading rate and polarity. It closes with the two commercial questions that come up in almost every enquiry: what the famous rabbit-ear comedogenicity score really means, and how to use the saponification value to catch the isomer traps that no certificate of analysis can resolve.

Three numbers, three different jobs

Viscosity, spreading rate and polarity are independent measurements. Only the first one is printed on every technical data sheet, which is why it gets used as a proxy for the other two - and why substitution projects fail.

  • Viscosity (mPa.s) is a bulk property. It predicts how the raw material pours, how it thickens the oil phase and how it behaves in a pump or a stick. It does not predict feel.
  • Spreading rate (mm²/10 min) is the area a fixed 20 µL drop wets on standardised filter paper in ten minutes at 25 °C - the method introduced by U. Zeidler in 1985 and still the industry reference for the slip-and-vanish feel of an oil. It is the number to buy on when the brief says light, dry or fast-absorbing.
  • Polarity is not one number at all, but for procurement purposes a ranking - low, medium or high - is enough. It predicts what the ester can dissolve (organic UV filters, crystalline actives, pigments, fragrance) and how well it tolerates silicones, hydrocarbons and electrolytes.

Zeidler's bands are still quoted: below 300 mm²/10 min is low spreading, 300 to 1000 medium, above 1000 high. Notice how coarse the middle band is - it spans a factor of three. Almost every ester in this article sits inside it, which is precisely why "medium spreading" on a data sheet is not a specification you can buy against.

Viscosity is a method-dependent number, and the spread is wide

Before comparing two quotes, check the conditions. Published viscosities for the same INCI routinely differ by a factor of two, because temperature, instrument and shear rate are not standardised between suppliers.

  • Isopropyl Palmitate is listed at 6.2 mPa.s (25 °C, rotational) in one source and 8 mPa.s (20 °C) in another.
  • Octyl Stearate appears as 12-15 mPa.s in one TDS and 29 mPa.s in a published oil table - the same INCI, twice the number.
  • Cetyl Ethylhexanoate is quoted at 10 mPa.s (25 °C), 14 mPa.s (20 °C) and 7-14 mPa.s (20 °C) by three different suppliers.

None of them is necessarily wrong. The consequences are two. First, an ester specification should state the test temperature and the method, and a purchase order should not accept "viscosity: approx." without them. Second, spreading rate is the more portable number: the drop size, the substrate and the timing are fixed by the method, so figures from different laboratories sit far closer together than viscosity figures do. When you have to substitute one ester for another without a sensory panel, match on spreading rate first.

The eleven esters, side by side

ProductCASFormulaMWViscosity, mPa.sSpreading, mm²/10 minPolarityUse level
Isopropyl Laurate (IPL)10233-13-3C15H30O2242.4--Medium-high2-10%
Isopropyl Myristate (IPM)110-27-0C17H34O2270.54.6-6 (25 C)707Medium-high1-10%; to 20% as UV-filter solvent
Isopropyl Palmitate (IPP)142-91-6C19H38O2298.56.2-8625Medium2-15%
Octyl Palmitate (2EHP)29806-73-3C24H48O2368.713 (20 C)-Medium3-10%
Cetyl Ethylhexanoate59130-69-7C24H48O2368.77-14 (20 C)-Medium3-30% (3-9% typical)
Octyl Stearate (2EHS)22047-49-0C26H52O2396.712-29 (25 C)697Low-medium1-10%
C12-15 Alkyl Ethylhexanoate90411-66-8Blend---Low-mediumto 22% (CIR, leave-on)
Cetearyl Isononanoate111937-03-2C25/C27 blend383-41114.9-22 (20 C)700-780Medium3-20%
Butylene Glycol Dicaprylate/Dicaprate211107-84-5C20H38O4 (di-C8)342.59.6813Low-
Triethylhexanoin7360-38-5C27H50O6470.725-35 (25 C)-Low0.5-20%
Triisooctanoin7360-38-5C27H50O6470.725-35 (25 C)-Low0.5-20%

Spreading values are from the standardised 20 µL filter-paper method at 25 °C; viscosities are supplier data at the temperature shown. Where sources disagree the figure is given as a range - the ranking is reliable, the absolute value is indicative. Polarity follows the common supplier ranking and is a relative, not an absolute, scale. A dash means we have no published figure we are prepared to quote; ask us and we will test the batch.

Reading the ladder by family

  • The isopropyl esters - IPL, IPM and IPP. One alcohol, three acid chain lengths from C12 to C16, the tightest viscosity ladder in the group and the highest polarity of the eleven. This is the solvent-plus-emollient family: they dissolve lipophilic actives, fragrance and organic UV filters, cut the greasy afterfeel of heavier oils and improve spreading. IPM is the classic UV-filter solvent and is used at 10-20% in sun care for that reason, against 1-10% in general skin care. Spreading falls as the acid chain grows - IPM at 707 mm²/10 min against IPP at 625 - which is the C14-versus-C16 difference showing up on the substrate.
  • The octyl and cetyl esters - 2EHP, 2EHS and cetyl ethylhexanoate. These pair a branched C8 acid with a C16 or C18 alcohol. Molecular weight is close to double that of the isopropyl esters, so they stay in the oil phase rather than penetrating, and they bring a cushion the isopropyl esters do not have. Within the group, octyl palmitate and cetyl ethylhexanoate share the formula C24H48O2 - see the isomer trap below. Note how high the assessed use levels go: cetyl ethylhexanoate is used from 3% to 30% depending on the system, against 3-10% for most emollient esters, because its branched acid refuses to crystallise and keeps the texture stable at load.
  • The C12-15 and cetearyl grades. Both are blends rather than single molecules, and both are sold on electrolyte tolerance and stick behaviour rather than on spreading. C12-15 alkyl ethylhexanoate reduces the oiliness, viscosity and hardness of stick products while staying stable in the presence of antiperspirant salts, and it is assessed safe in leave-on products to 22%. Cetearyl isononanoate sits at the rich end of this article's ladder - 14.9-22 mPa.s and 700-780 mm²/10 min - a medium-spreading, medium-polarity workhorse for creams, sun care and colour.
  • The polyol esters - butylene glycol dicaprylate/dicaprate and the two glycerol triesters. Highest molecular weight, lowest polarity, widest compatibility. Triethylhexanoin is soluble in low-polarity esters, high-polarity esters, vegetable oils, castor oil, white oil, low-viscosity silicone fluid and ethanol, and insoluble only in water; it also stays liquid below -30 °C, which is why it appears in frost-resistant and cold-process systems. Butylene glycol dicaprylate/dicaprate is the lightest of the three - 9.6 mPa.s against 25-35 - and spreads furthest at 813 mm²/10 min.

Polarity decides what the ester can dissolve

Feel is the visible half of the choice. Solvency is the half that decides whether the formula is stable, and it follows polarity.

  • Dissolving solids. If the brief involves a crystalline UV filter, a pigment dispersion or a crystalline active, start at the polar end - the isopropyl esters first, then cetyl ethylhexanoate. For the filter-by-filter solubility picture and the regulatory limits that go with it, see our guide to choosing organic UV filters.
  • Crystalline-active caution. The most effective solvents are also the ones most capable of changing how a crystalline active recrystallises or partitions between phases. Qualify the carrier on the finished formula, not on a solubility table.
  • Silicone compatibility. This is where the branched C8-acid esters earn their price. Cetyl ethylhexanoate is reported soluble at 20%, 50% and 80% in ethanol, cyclopentasiloxane, 10 cSt dimethicone and 100 cSt dimethicone, as well as in olive oil and liquid paraffin - a genuinely two-phase carrier. Cetearyl isononanoate and the glycerol triesters sit at the low-polarity end and further from the silicone trade-off.
  • Electrolytes. Antiperspirant and deodorant sticks are the stress test. C12-15 alkyl ethylhexanoate is recommended specifically for that reason: it reduces the oiliness, viscosity and hardness of the stick while remaining stable in a high-salt matrix.

Cushion, by contrast, is bought with molecular weight and with the acid side of the molecule - the lever worked through in our guides to branched ester emollients and to polyol ester emollients. The esters in this article are the light end of the same map.

Comedogenicity: what the 5-point score actually means

Isopropyl myristate and isopropyl palmitate carry some of the most-quoted warnings in the industry, and nearly all of them trace back to a single assay. In the rabbit-ear test the substance is applied to the inner surface of the rabbit ear for two to four weeks and the follicular keratinisation is graded 0 to 5. In the 1989 survey that is still the source of most published exclusion lists, isopropyl myristate scored 5 out of 5 and isopropyl palmitate 4 out of 5; octyl palmitate is also listed at 4-5.

Three things need saying before those numbers are used to reject a material.

  • The model is over-sensitive by design. Kligman and Mills showed in 1982, using a human model, that substances which are potent comedogens in the rabbit ear can produce little or no comedone formation in human skin. The rabbit follicular unit was chosen because its response is accelerated, not because it matches human skin. The working threshold is that a rabbit score has to exceed 3 before it indicates a meaningful human risk.
  • The finished product is the real test. A 2006 re-evaluation of the comedogenicity concept in the Journal of the American Academy of Dermatology concluded that finished products made with comedogenic ingredients are not necessarily comedogenic. Dose, the rest of the oil phase and the emulsion structure all intervene.
  • The regulatory conclusion is not in doubt. The CIR Expert Panel has assessed isopropyl myristate and isopropyl palmitate repeatedly, most recently in the 2013 amended safety assessment of alkyl esters, which concluded that this class is safe in cosmetics when formulated to be non-irritating. Isopropyl myristate was reported in more than 1,100 cosmetic products, at up to 77% in some hair products. The EU has set no restriction.

The commercially useful reading is this: the rabbit-ear score is a shortlisting signal for one specific application - leave-on facial products aimed at oily and acne-prone skin - and nothing more. For that application, take it seriously and move to the lower-polarity, higher-molecular-weight end of the table: C12-15 alkyl ethylhexanoate, assessed safe in leave-on systems to 22%; cetyl ethylhexanoate; butylene glycol dicaprylate/dicaprate; or triethylhexanoin. For body care, rinse-off, sun care and hair care, IPM and IPP have decades of CIR clearance and are usually the cheaper and better-performing answer. What you should not do is reject an ester on the strength of a number that came off a rabbit ear.

Two isomer traps, and the arithmetic that catches them

Two substitutions look safe on a certificate of analysis and are not. Both fall out of one calculation, because the saponification value of a simple ester is fixed by its molecular weight:

  • Monoester: SV = 56,106 ÷ M
  • Diester: SV = 2 × 56,106 ÷ M
  • Triester: SV = 3 × 56,106 ÷ M

The constant is the molecular weight of KOH scaled to milligrams per gram, so the expression is exact for a pure single-component ester and a few percent out for a blend. Run it across the table above and the first trap appears at once: octyl palmitate (29806-73-3) and cetyl ethylhexanoate (59130-69-7) share the formula C24H48O2, therefore the molecular weight 368.7, therefore the same saponification value of 152. One is a straight-chain C16 acid esterified with a branched C8 alcohol; the other is a branched C8 acid esterified with a straight-chain C16 alcohol. They behave differently - the branched acid is the one that refuses to crystallise - and no saponification value, acid value, refractive index or density will separate them. If your specification says "C24H48O2, MW 368, liquid ester", you have not specified a material.

The second trap is the glycerol triesters. Triethylhexanoin and triisooctanoin carry the same CAS number, 7360-38-5, and the same formula, C27H50O6. That is not a data error: isooctanoic acid is the commercial name that has been applied for decades to 2-ethylhexanoic acid, the branched C8 acid that makes triethylhexanoin, and the two INCI names describe the same molecule. But other suppliers use "isooctanoic acid" for a different branched C8 acid mixture whose representative structure is 6-methylheptanoic acid, with its own separate CAS number. The name resolves two ways; the CAS number does not. Order by CAS together with INCI, and put the supplier's own product code on the purchase order as well.

The triesters also show why it pays to do the arithmetic rather than copy a data sheet. For C27H50O6 the molecular weight is 470.7, so a triester must saponify at 3 × 56,106 ÷ 470.7 = 358 mg KOH/g. Suppliers quote 352-362 and 340-370, which agree; one supplier quotes 252-262, which implies a molecular weight of 655 and cannot describe this molecule. The two sets of figures differ by a single leading digit, so a typographical error is the likely explanation - but a buyer cannot confirm that from outside, and a receiving specification that copies the wrong figure will reject good material.

Using the saponification value at goods-in

Three numbers on the COA catch most ester problems before the drum is opened, and all three are cheap to run.

  • Saponification value - ester content and, through the calculation above, average molecular weight. A value off by more than a few percent means either the wrong ester or a blend with something lighter or heavier.
  • Acid value - free fatty acid left over from incomplete esterification or from hydrolysis in storage. Below 1.0 mg KOH/g is normal for a light emollient ester; a rising acid value across batches points to hydrolysis.
  • Hydroxyl value - free hydroxyl groups. For a fully esterified monoester or triester this should be low, a couple of mg KOH/g at most. A high hydroxyl value on triethylhexanoin means partial glycerides, which change both the texture and the solubility profile.

Add colour (APHA) and refractive index as identity checks and the five together will accept or reject a batch without a full GC. For the same arithmetic applied to the heavy end of the ester range, see our guide to polyol ester emollients, where saponification and hydroxyl values cross-check the polyol core.

Specification checklist

  • INCI name and CAS number, both. Not the trade name, and not the formula. Cetyl ethylhexanoate is not octyl palmitate, and triisooctanoin is not a second product alongside triethylhexanoin.
  • Viscosity with test temperature and method. A number without conditions cannot be compared against another supplier's.
  • Spreading rate if the brief is about feel. Ask for the method as well. A figure without substrate and drop size is not comparable either, but within one laboratory the ranking is stable and useful.
  • Solvency data for the specific actives in your formula. Where a crystalline UV filter or active is involved, ask for the solubility curve, not a yes or no.
  • Saponification, acid and hydroxyl values. Run the first one through the arithmetic before you issue the specification.
  • Comedogenicity statement, scoped to the application. Present the rabbit-ear figure, its source and the CIR conclusion together; a raw score without context will be misread internally.
  • Heavy metals, residual solvent and microbial limits if the ester is going into a cosmetic or a topical pharmaceutical product.

Supply

All eleven esters are in stock and shipped worldwide from our Shanghai facility:

Tell us the application, the feel you are targeting and the actives the ester has to carry, and we will confirm the grade, the spreading range and the goods-in specification before you place the order.