Fatty acids are one of the few raw-material families where the price list is almost a periodic table: the same carboxylic acid head, repeated with one extra CH2 per step, from the C8 liquids of coconut oil to the C22 amides of rapeseed wax. Because every property that matters tracks that carbon count, buying by chain length beats buying by trade name. This guide walks the ladder, shows how acid value and iodine value encode chain length and saturation, and maps each grade to its real jobs.

The Chain Ladder Is the Product

Walk the saturated series and watch the melting point climb. Caprylic acid (CAS 124-07-2, C8H16O2, MW 144.21) melts at 16.5 degrees and boils at 239 degrees, so it pours without heating. The odd-carbon neighbor, pelargonic acid (CAS 112-05-0, C9H18O2, MW 158.24) supplied at 99.0 percent minimum, melts lower than either even-carbon neighbor, which keeps it liquid for herbicide and ester uses. One more carbon gives the decanoic acid (CAS 334-48-5, C10H20O2, MW 172.26) datasheet phrase worth learning: "white crystalline solid or clear oily liquid above melting point" around 31 degrees. At C14, myristic acid (CAS 544-63-8, MW 228.37) is a white powder melting at 52 to 54 degrees, and palmitic acid (CAS 57-10-3, C16H32O2, MW 256.42) ships as flakes melting at 61 to 62.5 degrees. Two consequences: water solubility falls by roughly an order of magnitude per CH2 while the pKa stays near 4.8, so what changes across the ladder is phase behavior, not acidity. And storage economics follow: a C8 drum sits at ambient temperature, while C16 and C18 flakes need heated lines above 60 degrees.

Acid Value Is a Chain-Length Fingerprint

Every fatty acid carries exactly one acid group, so acid value measures molecular weight directly: one mole of potassium hydroxide per mole of acid makes acid value equal 56,100 divided by molecular weight. A pure C8 acid therefore reads about 389 mg KOH per gram, a C10 acid about 325, a C16 acid about 219. An acid value alone exposes mislabeled blends: a "C8 grade" quoting 220 is C16-rich. Crude tall oil (CAS 8002-09-3) from kraft pulping of pine shows an acid number of 100 to 160 mg KOH per gram, an average molecular weight far above C18, consistent with rosin at 10 to 35 percent, fatty acids at 30 to 60 percent and unsaponifiables at 5 to 20 percent. The other trap is the name: commercial "stearic acid" is a stearic and palmitic blend, historically purified by pressing, so the honest specification lines are the titer test, the acid value and the iodine value, not the word stearic.

Same Carbon Skeleton, Three Different Products: Acid, Salt and Ester

Turning the acid into a salt or an ester keeps the carbon skeleton and changes everything else. The free C8 acid is insoluble in water and does its preservation work in the oil phase; its sodium salt, sodium caprylate (CAS 1984-06-1, C8H15NaO2, MW 166.19), is a white crystalline powder soluble in water and ethanol with a 1 percent solution pH of 6.0 to 8.0, so pharma and food buyers take the salt for the aqueous phase. At the other end, sodium stearate (CAS 822-16-2, C18H35NaO2, MW 306.46) is the classic soap: slightly soluble cold, fully soluble hot, pH of a 1 percent solution near 9.0, the backbone of transparent bars. Glyceryl caprylate (CAS 26402-26-6, C11H22O4, MW 218.29) melts around 35 degrees and serves as a preservative booster and co-emulsifier in cosmetics; the mixed glyceryl caprylate/caprate (CAS 73398-61-5) quotes an acid value of at most 2.0 mg KOH per gram with a saponification value of 300 to 330. On the feed side, glyceryl laurate (CAS 142-18-7, C15H30O4, MW about 274), the monolaurin ester of lauric acid melting near 60 degrees, is bought for its antimicrobial activity against Gram-positive bacteria in gut-health programs, a specification question about free glycerol and monoester content.

Iodine Value: What a cis Double Bond Buys You

Saturation is the second axis, and iodine value is its certificate. A cis double bond kinks the chain, so it softens the product: the oleic-based glyceryl monooleate (CAS 25496-72-4, C21H40O4) melts at 35 to 40 degrees where its saturated counterpart sits far higher, and its datasheet pairs an iodine value of 60 to 90 with an acid value of at most 6.0 mg KOH per gram and an HLB of about 3.8, the profile of a water-in-oil emulsifier. The same unsaturated C18 skeleton drives the amine chemistry: oleylamine (CAS 112-90-3, C18H37N, MW 267.49) has a density near 0.9 and a flash point of 210 degrees, and it is the standard capping ligand in quantum dot synthesis and a corrosion inhibitor. Push to C22 and you reach erucamide (CAS 112-84-5, cis-13-docosenamide, C22H43NO, MW 337.58), the white flake slip agent for plastic films: its long cis chain and amide head migrate slowly to the film surface, lowering the friction of polyethylene bags. The trade-off of every double bond is oxidation, so for unsaturated grades insist on a peroxide value line alongside iodine value, because rancidity is a storage problem you can buy.

Where the Chains Come From, and What Mixtures Hide

Natural fats set the chain-length menu. Coconut and palm kernel oils deliver the C8 to C14 cut, which is why caprylic, capric and lauric derivatives track coconut harvests; palm and tallow feed the C16 to C18 fraction; high-erucic rapeseed is the C22 source behind erucamide; kraft pine yields tall oil as the industrial wild card. Because these are mixtures, ask for the chain-length distribution by gas chromatography, not just the acid value. The triglyceride route shows why: decanoyl and octanoyl glycerides (CAS 65381-09-1) quote a saponification value of 325 to 360 mg KOH per gram with an acid value of at most 0.5, and that saponification window is the fingerprint of the C8 and C10 balance; a C16-rich batch would never reach it. The same logic governs the alcohol siblings behind fatty alcohol polyoxyethylene ethers: carbon count and ethylene oxide moles together are the real product identity.

A Buying Checklist Before You Compare Prices

Close the comparison with eight lines. One, chain distribution by GC in area percent, not a single "assay". Two, acid value, cross-checked against 56,100 divided by the claimed molecular weight. Three, titer or melting point against the ladder above. Four, iodine value for unsaturated grades. Five, peroxide value wherever iodine value is nonzero. Six, saponification value for esters and glycerides. Seven, unsaponifiables and color, the quiet markers of feedstock quality. Eight, physical form and logistics: molten tanker versus 25 kilogram bags, and whether your plant can re-melt C16 to C18 flakes. Suppliers who answer those lines in one e-mail are selling chain lengths; suppliers who only send a price are selling names.