Every few years the industry adopts a function word and starts buying with it. Right now the word is "multifunctional": one raw material that humectifies, solubilises, boosts preservation and improves skin feel, so that a label never has to mention the preservative column. The logic is sound. The purchasing language that has grown around it is not.
"Pentanediol" is the clearest case. Two commercial products share the molecular formula C5H12O2 and a molecular weight of 104.15: pentane-1,2-diol, sold under the INCI name Pentylene Glycol (CAS 5343-92-0), and pentane-1,5-diol (CAS 111-29-5). They differ only in the position of one hydroxyl group. One of them supports a preservative-free claim. The other cannot, because the property that does the work is geometric, not chemical.
The same confusion runs through three materials that arrive in the same brief: menthanediol, 4-butylresorcinol and the potassium carbonate used to set the pH. This guide separates them by what each can and cannot do, and ends with the four lines that belong on the purchase order.
Five materials, five different jobs
| Substance | CAS No. | Formula | MW | Form at 25 °C | The job it can do |
|---|---|---|---|---|---|
| 1,2-Pentanediol (Pentylene Glycol) | 5343-92-0 | C5H12O2 | 104.15 | Liquid | Humectant, solvent, preservative booster |
| 1,5-Pentanediol | 111-29-5 | C5H12O2 | 104.15 | Liquid | Humectant, plasticiser, polymer building block |
| Menthanediol (p-menthane-3,8-diol) | 42822-86-6 | C10H20O2 | 172.27 | Solid, mp 82–83 °C | Repellent active, cooling agent |
| 4-Butylresorcinol | 18979-61-8 | C10H14O2 | 166.22 | Powder, mp 51–55 °C | Tyrosinase and TRP-1 inhibitor |
| Potassium carbonate | 584-08-7 | K2CO3 | 138.21 | Powder or granule | pH adjuster and buffer |
The table is short, and everything that follows is already in it: two entries share a formula, one is a solid rather than a liquid, one is not a glycol at all, and one is bought for its equivalents rather than its weight.
Two pentanediols, one formula, two different products
Antimicrobial activity in the alkanediol family comes from shape, not from the number of hydroxyl groups. A 1,2-alkanediol with a C4–C8 chain is amphiphilic: the two adjacent hydroxyls hold the water-facing end, the hydrocarbon tail goes into the lipid bilayer, and the membrane leaks. Move the second hydroxyl to the far end of the chain and the molecule becomes symmetric. There is no head and no tail, nothing to insert, and the membrane disruption that makes pentylene glycol useful in a preservation system simply is not there.
The literature is consistent on this geometry. Reviews of preservative mechanism describe the distance between the two hydroxyl groups as a primary variable: the closer they sit, and the longer the chain, the more amphiphilic the molecule and the stronger the effect on the membrane. That is why caprylyl glycol outperforms pentylene glycol, and why 1,5-pentanediol is never discussed in that context. It is a humectant, a plasticiser and a polyester/polyurethane building block, and a good one, but it is not a preservation tool.
This is where the claim lives. Pentylene Glycol is not listed as a preservative in Annex V of the EU cosmetics regulation. It is a multifunctional humectant and solvent that happens to be antimicrobial, which is why a formula carried by 3–5% pentylene glycol plus a chelator can be presented as preservative-free. The claim is regulatory, and it attaches to the substance, which is to say to CAS 5343-92-0. A drum of 1,5-pentanediol delivers the humectancy and none of the claim.
Use levels follow the same logic. Pentylene glycol is normally dosed at 1–5%; published hurdle-approach work reports 0.5% ethylhexylglycerin with 3% 1,2-pentanediol as a working combination, and at none of those levels is the pentanediol a stand-alone preservative. It still needs a challenge test and, almost always, a partner. 1,5-Pentanediol is dosed for moisture and plasticising, often in the same few-percent region, and brings no antimicrobial credit with it.
The synonym trap is where the wrong drum gets shipped
Both molecules are traded as "pentanediol", and the synonym lists make it worse. Pentane-1,5-diol is sold as pentamethylene glycol, 1,5-pentylene glycol and, on more than one supplier page, simply "pentylene glycol" — the exact INCI name that belongs to the 1,2 isomer. A purchase order that says "pentylene glycol" and a supplier who quotes by synonym can meet in the middle on the wrong product, and the two are not priced alike.
Assay will not save you, because both are specified at 98–99% and both pass. Three constants on the certificate will: refractive index (1.436–1.440 for the 1,2 isomer against about 1.450 for the 1,5), density (0.95–0.975 against 0.994 g/cm³ at 25 °C) and boiling point (206–215 °C against 239–242 °C). If a supplier's COA shows a refractive index near 1.450 next to the name Pentylene Glycol, the drum is the wrong isomer, and the conversation should stop there.
Menthanediol is a solid, and that changes the dosing line
p-Menthane-3,8-diol (CAS 42822-86-6, C10H20O2, MW 172.27) gets grouped with the diols, but it behaves like an active. It melts at 82–83 °C, so it arrives as a waxy solid rather than a pourable liquid and has to be melted or pre-dissolved before it reaches the water phase. Its two commercial jobs are cooling and insect repellence; as a repellent it is reported to perform in the range of DEET at comparable application rates, with the usual guidance that it is not intended for children under three.
The specification that decides performance is stereochemistry. PMD has three stereocentres and eight possible isomers, and the trade material is a mixture: refined oil of lemon eucalyptus runs at roughly 64–70% PMD as a cis/trans blend, while synthetic material from citronellal can be supplied at 95% or higher. Published work on Aedes albopictus repellency found the (1R)-cis isomer clearly the most active of the diastereomers tested. Two drums both labelled "PMD 95%" can therefore deliver different protection if the isomer ratio is not on the COA.
There is a regulatory split as well. As a repellent, PMD-rich oil of lemon eucalyptus sits on the biocide track, notified under the EU Biocidal Products Regulation and registered with Canada's PMRA; as a cooling agent in cosmetics it is assessed as a cosmetic ingredient, and in China it is listed among cosmetic raw materials and is used as a flavouring (FEMA 4053, JECFA 1416). One CAS, two dossiers. If the end use carries a repellent claim, the biocide documentation is what an importer will ask for, and a cosmetic-grade COA will not answer the question.
4-Butylresorcinol is not a diol, and it does not belong in the preservation argument
4-Butylresorcinol (CAS 18979-61-8, C10H14O2, MW 166.22) is a resorcinol, not a glycol: two hydroxyls on an aromatic ring, plus a butyl chain that makes it oil-soluble. Its job is pigment control through inhibition of tyrosinase and TRP-1, at 0.1–0.5%, and it is bought as a white to off-white crystalline powder melting at 51–55 °C. It has no role in preservation, and a formulator who adds it expecting a hurdle effect is paying active prices for the wrong mechanism.
Two purchasing details matter more than the marketing. First, the pH window is narrow and the published guidance is not uniform: suppliers quote 3.0–6.0 and, just as often, 5.0–6.5 for the finished formula. Ask which one was actually validated, because the answer decides whether your buffer is compatible with the active you are paying for. Second, resorcinols oxidise, and oxidation shows up as colour drift in a white emulsion. The practical response is to specify assay ≥99% with heavy metals under 10 ppm, to pre-dissolve in a glycol or ethanol phase instead of dispersing the powder, and to keep the finished product in opaque packaging with as little headspace as the fill allows.
Potassium carbonate: the pH lever, and the two traps under it
Potassium carbonate (CAS 584-08-7, K2CO3, MW 138.21) is a standard way to raise pH without adding sodium, and it is the historic alkali behind soft soap, which is why it still turns up in bath and soap products. What makes it a purchasing problem rather than a formulation one is that it is bought by weight and works by equivalent.
| Neutraliser | MW | Equivalents per mole | g per equivalent | Equivalents per kg |
|---|---|---|---|---|
| Potassium carbonate (K2CO3) | 138.21 | 2 | 69.1 | 14.5 |
| Potassium hydroxide (KOH) | 56.11 | 1 | 56.1 | 17.8 |
| Sodium hydroxide (NaOH) | 40.00 | 1 | 40.0 | 25.0 |
| Triethanolamine (TEA) | 149.19 | 1 | 149.2 | 6.7 |
Read the last column and the price list stops being comparable. A tonne of potassium carbonate delivers 14.5 kmol of base; a tonne of caustic soda delivers 25. If the K2CO3 quote is only 15% below the NaOH quote per tonne, you are paying more per equivalent, not less. Convert before you compare, and convert again if the quotation is written on a K2O basis: potash is traded that way, and the factor from K2CO3 to K2O is 0.682.
The first trap is strength. A 1% solution sits near pH 11.6 and a 10% solution at 11.8–12.2, and the carbonate/bicarbonate pKa of 10.33 means the buffer holds around pH 10–11, not at the pH 5–6 where a butylresorcinol system wants to live. Potassium carbonate is the material for getting into the alkaline region, not for trimming a formula at pH 5.5. For that, use a bicarbonate or an amine.
The second trap is the acid. Carbonate plus acid gives off carbon dioxide, so any low-pH system — an AHA toner, a citric-acid buffered gel, a benzoic-acid preserved emulsion — will build headspace pressure in a sealed tube and can push a flip-top open in transit. The powder is also strongly hygroscopic: left open, it absorbs moisture and CO2 and converts to potassium bicarbonate, so the assay falls in the bag rather than in the reactor. Specify the assay on a dry basis, watch the loss on ignition, and keep the packaging sealed.
Four lines for the purchase order
- Name, CAS and INCI together. "Pentanediol" on its own is ambiguous: write the CAS and, where one exists, the INCI. For pentylene glycol the pair is 5343-92-0 and Pentylene Glycol, and 111-29-5 is a different product with a different job.
- The constants that separate isomers. For the C5 pair, put refractive index, density at 25 °C and boiling point on the specification, not just assay. Assay cannot tell 1,2 from 1,5.
- The function-specific numbers. For PMD, the PMD content and the cis/trans ratio; for butylresorcinol, assay ≥99%, heavy metals and the validated pH window; for potassium carbonate, assay on a dry basis and loss on ignition.
- The regulatory purpose. Repellent, cosmetic cooling agent, preservative booster and pH adjuster are four different dossiers. State the end use and ask for the documentation package that matches it, rather than the one the supplier happens to have on file.
Supply
Shanghai Better Chemical supplies the full set from China, with COA, SDS and regulatory statements issued against the end use you declare:
- 1,2-Pentanediol, bio-based (Pentylene Glycol, CAS 5343-92-0) — 1–5% humectant, solvent and preservative booster for skin, eye and baby care.
- 1,5-Pentanediol, bio-based (CAS 111-29-5) — humectant, plasticiser and polyester/polyurethane building block.
- Menthanediol (p-menthane-3,8-diol, CAS 42822-86-6) — cooling agent and repellent active for sprays, lotions and lip care.
- 4-Butylresorcinol (CAS 18979-61-8) — oil-soluble brightening active at 0.1–0.5%.
- Potassium carbonate (CAS 584-08-7) — pH adjuster and buffer for cosmetics, soap and bath products.
If you are building a preservative-free system, the 1,2-hexanediol and phenoxyethanol comparison covers the C6 diol that usually sits beside pentylene glycol, and the preservative system selection guide covers the hurdle set for detergents, wipes and personal care. Tell us the application, the claim you intend to make and the market you are selling into, and we will confirm the grade, the isomer and the documentation before you place the order.