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A preservative is not an ingredient you buy, it is a system you build. Two suppliers can ship the same INCI name at the same assay and only one of them will pass a challenge test in your emulsion, because the certificate describes the drum and not the water phase where the organisms actually live. A few of the decisions below are made by chemistry, and a few are made by regulators.
Microbes grow in water, so the value that matters is the concentration of free active in the aqueous phase, not the percentage in the container. Anything that pulls the active out of the water lowers the effective dose. Nonionic surfactants are the usual culprit: Polysorbate 80 (CAS 9005-65-6) is an excellent solubiliser precisely because it forms micelles, and a preservative sitting inside a micelle is not available to act on a cell wall. Oils, fatty alcohols and thickeners do the same thing by partition, which is why a lotion and a clear gel built from the same preservative at the same percentage can need genuinely different dose rates. Ask for the recommendation on your base, not on water.
Benzalkonium chloride is a family, not a compound. Benzalkonium Chloride carries two CAS numbers for that reason — 8001-54-5 for the general alkylbenzyldimethylammonium chloride mixture and 63449-41-2 for the defined commercial grade — and the alkyl chain distribution, typically C8 to C18, decides how it behaves. It is normally supplied as a 50% or 80% aqueous solution, so the number on a quotation is a solution strength, not an active content.
The whole quaternary ammonium group is inactivated by anionic surfactants and by soap, and it adsorbs onto cellulose and some plastics, so packaging and the rest of the formula both matter. Didecyldimethylammonium Chloride (CAS 7173-51-5, C22H48ClN, MW 362.08) is the twin-chain member of the family, more oil-soluble and more at home in hard-surface and fabric hygiene than in an emulsion. Dodecyltrimethylammonium Chloride (CAS 112-00-5) and cetyltrimethylammonium chloride (CAS 112-02-7, C19H42ClN, MW 320) sit lower in the same series; the C16 member is sold mainly as a conditioner, at 25% or 50% solution, and its antimicrobial action is a useful side effect rather than the reason it is bought.
The counter-ion is part of the product identity and changes the CAS. Benzalkonium Bromide (CAS 7281-04-1) is the bromide salt of the same cation, stable in weakly acidic to neutral conditions and favoured in medical disinfection, antiseptics and ophthalmic irrigation, where it is reported to hold up better against some chlorine-resistant organisms. Substituting one salt for the other is a specification change, not a purchasing convenience.
Chloroxylenol, better known as PCMX (CAS 88-04-0, C8H9ClO, MW 156.61), is still permitted as a cosmetic preservative in the EU at up to 0.5%, and in the United States it is one of the actives the FDA left in place for consumer antiseptic washes when it removed triclosan and triclocarban in its 2016 final rule.
Triclocarban (CAS 101-20-2, C13H9Cl3N2O, MW 315.58) is the cautionary tale from the same bench: chemically a near relative, commercially finished as a preservative in consumer wash products in the US after that rule and not permitted as a cosmetic preservative in the EU at all. Zinc Pyrithione (CAS 13463-41-7, C10H8N2O2S2Zn, MW 317.7) has followed a parallel curve, from the default antidandruff active to no longer permitted in cosmetic products in the EU since March 2022, while the US OTC monograph still allows it in rinse-off shampoos at 0.3% to 2% and not in leave-on products. A supplier's technical sheet will not tell you any of this. The annexes will.
DMDM Hydantoin (CAS 6440-58-0, C7H12N2O4, MW 188.18) and Sodium Hydroxymethylglycinate (CAS 70161-44-3) contain no formaldehyde as an ingredient, yet both work by releasing it in the aqueous phase. That mechanism is why each sits on the EU positive list with a maximum that is expressed literally as formaldehyde, and why a “formaldehyde-free” claim made on the strength of the INCI list alone is a claim about the label rather than the reaction. The glycinate is the friendlier of the two to handle, supplied as a water-soluble powder at 98% or better with a 1% solution at pH 6 to 8.
Chlorphenesin (CAS 104-29-0) is a genuine broad-spectrum preservative, mild and low-odour, but it is normally a partner for phenoxyethanol rather than a stand-alone system. Around it sits a group of materials usually described as boosters, and it is worth being clear that they are not interchangeable with a preservative: Caprylohydroxamic Acid (CAS 7377-03-9, C8H17NO2, MW 159.23) works mainly as an iron chelator and starves organisms of a metal they need; Glyceryl Caprylate (CAS 26402-26-6, C11H22O4, MW 218.29) partitions into microbial membranes and disrupts them; and Hinokitiol (CAS 499-44-5, C10H12O2, MW 164.2) is a tropolone from hinoki cypress that binds metal ions and brings a long record in Japanese oral and cosmetic use.
Salicylic Acid (CAS 69-72-7, C7H6O3, MW 138.12) is the clearest case of a molecule with two working lives. It is permitted to 0.5% when it is preserving and to 2% when it is doing something else, and the function you claim changes what has to appear on the pack. Read the annex for the function, not for the acid.
None of this needs a new laboratory. It needs the second column of the technical sheet and the annex behind it, which is where the difference between a preservative that works and one that merely complies actually lives.
