No other raw-material family in the personal care warehouse spans such a wide working range as the fatty acids. Caprylic acid (CAS 124-07-2) pours like a light oil at room temperature; behenic acid (CAS 112-85-6) is a hard, waxy solid that melts 60 °C higher. Both are sold in 25 kg bags, both are quoted per tonne, and both are described by the same few laboratory numbers — yet choosing the wrong chain length, or reading only the melting point on the datasheet, sits behind a large share of failed batches. This guide sets out how to read a fatty acid certificate of analysis, how to match a chain length to a job, and how to turn a free fatty acid into a working emulsifier inside your own vessel.
The chain-length ladder: 16 °C to 80 °C on one shelf
Two structural facts decide everything about a fatty acid: how many carbons the chain carries (C8 to C22 in commercial supply) and whether it carries a double bond. Chain length and saturation set the melting point, and the melting point sets the physical form, the skin feel and the process temperature you must reach before the material becomes usable.
| Product (CAS) | Chain | Melting point | Titer | Acid value from MW | Typical AV spec | Iodine value | Form at 25 °C |
|---|---|---|---|---|---|---|---|
| Caprylic Acid (124-07-2) | C8:0 | 16.5–16.7 °C | — | 389 | — | ≤0.5 | Clear liquid |
| Capric Acid (334-48-5) | C10:0 | 31.0–31.6 °C | — | 326 | — | ≤0.5 | Low-melting solid |
| Caprylic/Capric Acid (68937-75-7) | C8/C10 blend | Liquid range | — | 326–389 | — | ≤0.5 | Clear liquid |
| Lauric Acid (143-07-7) | C12:0 | 44.2–44.8 °C | 43–44 °C | 280 | 278–282 | ≤0.5 | Flakes, prills |
| Myristic Acid (544-63-8) | C14:0 | 53.9–54.4 °C | 52–55 °C | 246 | 243–249 | ≤0.3 | Flakes |
| Palmitic Acid (57-10-3) | C16:0 | 62.9–63.1 °C | 59–63 °C | 219 | 213–221 | ≤1.0 | Beads, flakes |
| Stearic Acid (57-11-4) | C18:0 | 69.6–70.1 °C | 67–68 °C | 197 | 196–204 (high-stearic) | ≤1.0 | Flakes, powder |
| Oleic Acid (112-80-1) | C18:1 | 13–16 °C | ≤10 °C | 199 | 195–205 | 85–95 | Clear liquid |
| Behenic Acid (112-85-6) | C22:0 | 80.0–81.5 °C | — | 165 | — | ≤1 | Hard solid |
| Erucic Acid (112-86-7) | C22:1 | 33.5–33.8 °C | — | 167 | 160–170 | 70–78 | Flakes, solid |
| Coconut Acid (61788-47-4) | C8–C18 blend | 21–26 °C | 22–26 °C | — | 258–275 | 6–13 | Soft opaque solid |
Note where titer sits relative to melting point. Titer is the solidification point measured on cooling, and for these acids it runs 2–4 °C below the melting point, because the melt supercools and impurity traces show up on the way down. The melting point is the chemistry number; the titer is the process number — the temperature at which your storage tank sets up and your transfer line blocks.
The four numbers on a certificate of analysis
- Acid value (AV, mg KOH/g). The mass of potassium hydroxide needed to neutralise the free fatty acid in one gram. For a pure single-chain fatty acid the acid value is fixed by molecular weight: roughly 56,106 divided by the molecular weight. Read it backwards and it becomes the most useful line on the sheet — divide 56,106 by the stated acid value and you get the average molecular weight, which tells you the average chain length. An AV of 280 is a C12, 246 is a C14, 219 is a C16, 197 is a C18. A product sold as "stearic acid" with an acid value around 205 is not stearic acid; it is a stearic/palmitic blend, and that is entirely normal — commercial triple-pressed stearic typically runs from about 45/55 down to 70/30 stearic to palmitic, depending on origin and pressing route.
- Saponification value (SV, mg KOH/g). Acid value plus ester value. In a properly distilled free fatty acid, the saponification value should sit within about three units of the acid value. A wider gap means unhydrolyzed glyceride or ester is still carried through from the splitting stage. That costs you lather, adds glycerin load to your soap and darkens the finished bar.
- Iodine value (IV, g I2/100 g). A direct measure of double bonds, and therefore your oxidation-risk meter. Fully saturated acids are specified below 1 (lauric ≤0.5, myristic ≤0.3, palmitic and stearic ≤1.0). Coconut acid carries a genuine 6–13 because of its oleic fraction. Oleic acid at 85–95 and erucic acid at 70–78 each hold exactly one double bond. A lot that drifts above its IV window will yellow and turn rancid earlier in storage, however clean the rest of the certificate looks.
- Titer (°C). Covered above — insist on the method, and use it for tank and pipeline design rather than as a reactivity figure.
- Plus three lines buyers often skip: colour on the Lovibond 5¼-inch scale (lauric acid typically ≤3.0 Y / ≤0.3 R); after-heat colour, measured after 2 hours at 200 °C, which proves the lot will survive an 80–90 °C soap kettle or a hot esterification without browning; and unsaponifiable matter, the non-acid residue that follows the acid into every downstream reaction.
Liquid, soft or hard: matching the acid to the job
- Caprylic and capric acid — the liquid fraction (124-07-2 / 334-48-5, blend 68937-75-7). C8 melts at 16.5 °C and C10 at 31.6 °C, so the commercial C8/C10 blend stays a clear liquid in the drum and in the line: no melting tank, no jacketed piping, no winter logistics problem. Buy this fraction when the process needs a liquid acid — medium-chain triglyceride and ester feedstock, light emollient esters, lubricant and metalworking esters, and low-colour grades for sensitive applications. It is also the fraction behind the pungent coconut-derived odour, so where smell matters, specify distilled and low in C8.
- Coconut acid — the cost-effective workhorse (61788-47-4). A full C8–C18 spectrum with lauric acid at 46–56 % and myristic at 15–21 %, titer 22–26 °C, acid value 258–275. Because it is a blend, its melting point means very little; buy it on titer, acid value, iodine value and GC composition instead. It is the classic feedstock for bar soap and soap noodles and the cheapest route to lauric-type lather without paying for distilled lauric acid. Ask for after-heat colour: a lot that browns at 200 °C will brown in your kettle at 85 °C too.
- Lauric and myristic acid — the lather pair (143-07-7 / 544-63-8). Sodium laurate is the most soluble soap of the saturated series, which is why coconut and palm-kernel oils dominate soap making: lauric acid delivers the fast, abundant, easy-rinsing lather, and myristic acid adds creaminess and bar hardness on top of it. A typical distilled myristic grade carries acid value 243–249, iodine value ≤0.3, titer 52–55 °C, C14 ≥99 % and colour better than 2.0 Y. If a bar feels slimy or lathers weakly, the lauric-to-myristic ratio is the first thing to check.
- Palmitic and stearic acid — the structurants (57-10-3 / 57-11-4). Hard, waxy solids that build body, opacity and a dry, non-greasy afterfeel in creams and sticks, and that harden a soap bar. Palmitic melts around 63 °C and stearic around 70 °C, so a high-palmitic grade sets up softer and yields a whiter, more opaque cream, while a genuine high-stearic grade gives a harder bar and a drier film. This is exactly why the stearic-to-palmitic ratio belongs on the purchase order: a routine stearic purchase can be quoted anywhere from 30 % to 70 % stearic, and the difference shows up as a visible change in the finished product.
- Behenic acid — the melt-point lifter (112-85-6). At 80–81.5 °C it is the highest-melting commodity fatty acid, with an acid value near 165. Use it in small amounts (0.5–3 %) to raise the softening point of a stick, to add pearl and body to a cream, or as the industrial precursor to behenyl alcohol and behenyl esters for hair care. It is practically insoluble, so it must be fully melted into the oil phase. Add it late to a cool oil phase and you will find it again as specks in the finished product.
Making your own emulsifier: in-situ soap from a free fatty acid
The most economical emulsifier in a classical cream is not bought at all — it is made in the vessel. Adding a measured amount of alkali to stearic, palmitic or oleic acid forms the corresponding soap, and that soap is the anionic emulsifier that carries the oil phase. The technique is old, cheap and, because the soap is a salt of a weak acid, unforgiving of pH drift.
Step one is stoichiometry, and the acid value hands it to you directly. Fully neutralising 100 kg of fatty acid — any chain length — takes exactly acid value ÷ 10 kg of 100 % KOH. The other alkalis follow by molecular-weight ratio:
| Alkali (molecular weight) | Factor on acid value | 100 kg stearic acid (AV 197) | 100 kg coconut acid (AV 265) |
|---|---|---|---|
| 100 % KOH (56.11) | AV ÷ 10 | 19.7 kg | 26.5 kg |
| 100 % NaOH (40.00) | AV × 0.0713 | 14.1 kg | 18.9 kg |
| Triethanolamine, 99 % (149.19) | AV × 0.2659 | 52.4 kg | 70.5 kg |
Full neutralisation, however, is almost never what a good formula wants. Two ratios cover most practical work:
- 8–12 % of the stoichiometric alkali — roughly 1 part triethanolamine to 10–12 parts stearic acid by weight. Only a fraction of the acid becomes soap; the unreacted stearic acid crystallises into a fine lamellar network. This is the classic vanishing cream: 15–18 % stearic acid with 1.2–1.5 % TEA produces the pearlescent, bodied, dry-touch cream that absorbs without greasy residue, at a fraction of the cost of a purchased emulsifier.
- 33–50 % of stoichiometric — roughly 1 part TEA to 2–3 parts stearic acid. Enough soap to emulsify a real oil load of mineral oil, esters and silicones into a stable lotion. The finished system settles at pH 7.5–8.5, typically around 8.
Process discipline matters more than the exact ratio:
- Put the alkali in the water phase and the fatty acid in the oil phase, and bring both to 70–75 °C. Combine oil into water under high shear for 5–10 minutes, then continue slow stirring through the cooldown so the soap crystallises while the emulsion is still fluid.
- Run the alkali as the limiting reagent. Free triethanolamine left in the finished product adds odour and irritation potential, and with nitrosating preservatives it raises nitrosamine concern — which is why TEA should sit at or just below what the pH target requires, never in excess.
- Remember that every other acidic ingredient competes for the same alkali. AHAs, ascorbic acid, salicylic acid and acidic preservatives will all consume TEA and thin or break the emulsion. Add them after the soap has formed, after cooldown, and re-check pH afterwards.
- Respect the pH ceiling. Below roughly pH 7 the soap reverts to free fatty acid and the emulsion collapses. If the product must live at pH 5–5.5 — an AHA serum, a low-pH cleanser — the stearic/TEA route is simply not available, and the answer is a nonionic or synthetic anionic emulsifier: glyceryl stearate, a PEG ester or a polyglyceryl ester will all hold an emulsion at low pH without a soap.
Sodium, potassium or magnesium stearate: three soaps, three jobs
Once the acid has been neutralised, the choice of cation is a choice of solubility, and solubility decides the physical form of the finished product.
| Soap (CAS) | Cation | Water behaviour | Physical character | Datasheet melting point | Where it wins |
|---|---|---|---|---|---|
| Sodium Stearate (822-16-2) | Na+ | Less soluble; dissolves on heating | Dense, stable lather; rigid crystalline structure | about 245 °C | Bar soap, syndet bars, solid sticks, industrial soaps, 1–3 % as a cream and toothpaste thickener |
| Potassium Stearate (593-29-3) | K+ | Noticeably more soluble and dispersible | Softer, looser, more voluminous lather | about 215 °C | Liquid and cream soaps, shaving cream, emulsions, textile auxiliaries, latex processing |
| Magnesium Stearate (557-04-0) | Mg2+ | Practically insoluble, hydrophobic | No lather; slippery, water-repellent powder | about 88 °C | Tablet and capsule lubricant, pressed powders, anti-caking and release agent |
The sodium-versus-potassium decision is a solubility decision: sodium builds structure, potassium builds fluidity. It is the reason the same stearic acid sits at the bottom of a bar soap and inside a pump bottle of liquid soap. The practical consequence is that the two are not freely interchangeable in an existing formula — swapping one for the other shifts the phase behaviour enough to require a fresh stability run.
Magnesium stearate belongs to a different world, the pharmaceutical and powder one, and its specification is written around hydrophobicity and surface area rather than lather. A pharmacopoeial grade is accepted on magnesium content 4.0–5.0 %, stearic acid not less than 40 % of total fatty acids, stearic plus palmitic not less than 90 %, loss on drying not more than 6.0 %, residue on ignition 10.0–16.0 %, lead not more than 10 ppm, and specific surface area 5–20 m2/g. That last line is the one that separates a well-behaved lubricant from a batch that over-lubricates: at 0.25–2.0 % in tablets the stearate lamellae shear off and coat the granules, and a high surface area combined with too long a blend reduces tablet hardness and slows dissolution. For capsule fills the usual band is 0.5–1.5 %.
One limitation binds every soap in the table: hard water. Calcium and magnesium ions in the wash water precipitate the soap as insoluble calcium and magnesium stearate — the chemistry of bathtub ring, and of lather that dies on contact. Soap-based cleansers aimed at hard-water markets need a chelator in the formula, whether EDTA, a tetrasodium salt or a biodegradable alternative. And it is worth remembering that concentrated soap solutions are difficult to prepare at all, because these soaps are only marginally soluble in water; that single fact is the historical reason synthetic surfactants took over the cleansing market.
The two unsaturated acids: oleic and erucic
- Oleic acid (112-80-1) — the winter-pourable liquid. Melting point 13–16 °C, titer below 10 °C, acid value 195–205, iodine value 85–95, and colour typically 10 Y / 1 R or better on the Lovibond 5¼-inch scale. Buy it on the combination of acid value, iodine value and GC composition, because a grade with 80–90 % C18:1 behaves quite differently from a "red oil" carrying more saturated and polyunsaturated material — the latter hazes at low temperature, oxidises faster and smells. Keep peroxide value below 10, store away from light and heat, and consider nitrogen blanketing: the double bond is the oxidation site, and rancid odour and yellowing both start there.
- Oleic acid is also the right acid for liquid soaps. Triethanolamine oleate is markedly more soluble and more fluid than triethanolamine stearate, so a pumpable liquid soap or a clear cleansing gel is built on oleic rather than stearic acid. Identical stoichiometry, identical rules — only the physical result differs.
- Erucic acid (112-86-7) — the C22:1 intermediate. Derived from high-erucic rapeseed oil: melting point 33.5–33.8 °C, acid value 160–170, iodine value 70–78, supplied as white to pale-yellow flakes at 90 % or higher assay. Its value is as an industrial intermediate rather than an emollient. It is the feedstock for erucamide, the dominant slip and anti-block agent in polyethylene and polypropylene film; for behenyl alcohol by hydrogenation, the C22 alcohol used in premium hair care; and for brassylic acid, the monomer behind nylon 13,13. It also brings high-temperature lubricity and oxidative stability to greases and metalworking fluids, which is why high-erucic rapeseed oil remains a strategic industrial crop even though it left the food chain decades ago.
- Erucic acid carries a compliance flag worth stating plainly. Edible-oil law caps it: under EU Regulation (EU) 2023/915, edible oils must contain less than 2 % erucic acid by weight. That limit is the reason low-erucic canola replaced high-erucic rapeseed oil in edible applications, and the reason the erucic acid trade is now overwhelmingly industrial. If a buyer's channel is food, confirm the erucic limit for the destination market before quoting; for cosmetics, lubricants and plastics, the industrial grade is the correct specification.
The sourcing checklist: six lines for the purchase order
Almost every dispute over a fatty acid shipment comes down to one of six lines. Put all six on the order, and name the test method for each.
| Line | What to specify | Why it matters |
|---|---|---|
| Acid value (mg KOH/g) | A tight band, not a maximum — for example 278–282 lauric, 243–249 myristic, 196–204 high-stearic | It fixes the stoichiometry of every downstream soap and ester reaction, and it is your check on average chain length |
| Saponification value | Within about 3 units of the acid value for a distilled free fatty acid | A wider gap exposes unhydrolyzed glyceride or ester carried through from splitting |
| Iodine value | ≤0.5 lauric, ≤0.3 myristic, ≤1.0 palmitic and stearic, 6–13 coconut, 85–95 oleic, 70–78 erucic | Oxidation stability and shelf life; a drifted IV predicts yellowing and odour |
| Titer (°C), with method | The actual solidification point | It decides storage temperature, tank heating and whether a line blocks in winter |
| Colour, initial and after heat | Lovibond 5¼-inch cell (for example ≤3.0 Y / ≤0.3 R), plus 2 h at 200 °C | Proves the lot will not darken in your kettle or your esterification reactor |
| Composition by GC | Chain-length distribution with explicit cut-offs | The only way to tell a genuine C18 stearic from a C16/C18 blend, or a coconut acid from a palm-kernel acid |
Two packaging questions are also worth settling before you commit a container, because they affect plant cost more than the unit price does: is the material supplied as flakes, prills or a liquid grade, and is it packed in 25 kg multiply paper bags, 180 kg drums or a flexitank? The form decides whether your plant needs a melting tank; the pack decides your floor space and your labour.
Supply and grades
Shanghai Better Chemical supplies the full fatty acid and soap range, with COA, TDS and MSDS on request:
- Saturated fatty acids: Caprylic Acid (CAS 124-07-2), Capric Acid (CAS 334-48-5), Caprylic/Capric Acid blend (CAS 68937-75-7), Lauric Acid (CAS 143-07-7), Myristic Acid (CAS 544-63-8), Palmitic Acid (CAS 57-10-3), Stearic Acid (CAS 57-11-4), Behenic Acid (CAS 112-85-6).
- Unsaturated fatty acids: Oleic Acid (CAS 112-80-1), Erucic Acid (CAS 112-86-7).
- Blends and soaps: Coconut Acid (CAS 61788-47-4), Sodium Stearate (CAS 822-16-2), Potassium Stearate (CAS 593-29-3), Magnesium Stearate (CAS 557-04-0).
Related reading: our guides to fatty alcohol chain lengths, metallic stearates and the glyceryl stearate family cover the adjacent raw materials in the same soap and emulsion systems.
Send us the acid value band, the chain length and the physical form you need — and, if a soap system is involved, the pH your finished product has to hold — and we will quote the grade and packing that fits.