Emulsion failures are usually blamed on the emulsifier being wrong. Far more often the emulsifier class is wrong for the base. An oil phase carrying 15% zinc oxide, 20% ethanol, 2% sodium chloride or a working pH of 3.5 sits outside the operating window of the standard ethoxylated nonionic: as ionic strength rises, the polyethylene oxide chain dehydrates, the cloud point falls into the process range, and the emulsion creams or breaks. Phosphate esters are the class formulators reach for at that point, because the terminal phosphate group stays ionised and hydrated under exactly the conditions that defeat an ethoxylate.
That one property explains why the same chemistry appears in a 50+ SPF sunscreen, a long-wear foundation, an AHA serum and a drum of alkaline industrial cleaner. It also explains why "phosphate ester" is not a product but a family - spanning a waxy solid that melts at 42-49 °C to a mobile liquid, with HLB values from about 9.6 upward. This guide covers the seven grades in our range and the numbers that decide between them.
What actually makes a phosphate ester different
A phosphate ester is an alkyl or alkyl-ethoxylate chain terminated by a phosphoric acid head group. Three structural variables, not the INCI name, govern how it behaves:
- Mono-ester to di-ester ratio. The mono-ester carries two free acid groups and is the more hydrophilic, more water-soluble and more strongly emulsifying species. The di-ester is more lipophilic and then acts as the co-emulsifier and oil-phase compatibiliser. The ratio is fixed by the manufacturing route - a polyphosphoric acid route gives a mono-ester-dominant product, while the more common phosphorus pentoxide route gives a di-ester-dominant one - which is why two grades sharing an INCI name can still behave differently in the same formula.
- Acid form versus neutralised salt. The free acid is oil-soluble, reads roughly pH 1.5-2.5 as a 1% aqueous dispersion, and must be neutralised inside the formula. The potassium, sodium, TEA or ammonium salt is water-soluble, reads roughly pH 6-8 as supplied, and can be dosed directly.
- Degree of ethoxylation. Ethoxylating the hydrophobic chain before phosphation adds steric stabilisation on top of the electrostatic stabilisation from the phosphate group, widens electrolyte tolerance further, and shifts the physical form from waxy solid toward liquid.
The consequences are consistent across the class: lower foaming than sulfates and sulfonates, good compatibility with inorganic builders, useful hydrotropic action that solubilises nonionics into high-electrolyte systems, mildness associated with a phospholipid-like head group, and stability from strongly acidic through strongly alkaline conditions - including resistance to hydrolysis in hot alkali - where several other anionic emulsifiers simply decompose.
The seven grades side by side
| Grade | CAS No. | Form and character | Use level |
|---|---|---|---|
| Cetyl Phosphate | 3539-43-3 | C16 acid form, waxy solid; must be neutralised in the formula | Typically 1-3% as primary O/W emulsifier |
| Potassium Cetyl Phosphate | 19035-79-1 | Pre-neutralised C16 salt, powder; HLB 9.6; PEG-free | 1-3% primary, 0.5-1% secondary |
| Triceteareth-4 Phosphate | 119415-05-3 | C16-18 chain with 4 EO, triester; waxy solid melting 42-49 °C; HLB 10-11 | 1-3%; as low as 0.01-1.5% for low-viscosity systems |
| Trilaureth-4 Phosphate | 31800-90-5 | C12 chain with 4 EO, triester; mobile liquid; quick-breaking | 1-4% as primary; also a co-emulsifier in cleansing oils |
| C20-22 Alkyl Phosphate (and) Stearyl Phosphate (and) Behenyl Alcohol | 84962-18-5; 2958-09-0; 661-19-8 | White granule blend of long-chain phosphate esters with a fatty alcohol; water-resistant and thickening | 1-5%, typically 1-3%; neutralise with TEA or NaOH |
| Glycereth-26 Phosphate | 31694-55-0; 12040-65-2 | Polyoxyethylene glycerin phosphate, 26 mol EO; dual chelating and emulsifying function | Set on the base - secondary emulsifier and metal-ion control |
| Castor Oil Phosphate | 447453-01-2 | Ricinoleic-acid-based phosphate ester; pigment dispersant and emulsifier | Set on the base - dispersant level in the pigment paste |
Two numbers in that table do most of the work when a brief arrives. HLB decides which oil phases a grade can carry: at 9.6, potassium cetyl phosphate pairs with medium-polarity esters and triglycerides, while triceteareth-4 phosphate at 10-11 covers the polar organic filters and inorganic pigments that destabilise lower-HLB systems. Physical form decides the process: a liquid can be cold- or hot-processed and pours at 25 °C, whereas a waxy solid at 42-49 °C forces a hot process and contributes body to the finished texture.
Acid form or pre-neutralised: the first decision
With cetyl phosphate the same molecule is available as the free acid (CAS 3539-43-3) and as the potassium salt (CAS 19035-79-1). Choose on process capability rather than price alone.
- Free acid gives the formulator control over the counter-ion, ships less water and costs less per unit of active - but the neutralisation must actually be carried out. Add the base (KOH, NaOH or triethanolamine) to the water phase, then emulsify. Under-neutralising leaves free acid in the finished product: pH drifts low, the emulsion is coarse and it separates on storage. Over-neutralising pushes pH past the useful window and can thin the system rather than build it.
- Pre-neutralised salt removes that step and its variability. It is the practical choice where the plant has no reliable way to hold the neutralisation point, or where the buyer does not want the counter-ion fixed by the supplier's choice.
One behaviour of the pre-neutralised powder catches people out: potassium cetyl phosphate dissolves fully in neither water nor oil, and undissolved particles remain in the dispersion. That is normal. Do not filter them out.
Electrolyte, hard water and pH: where nonionics give up
The difference between a phosphate ester and an alcohol ethoxylate is not a matter of degree. A nonionic ethoxylate is held in solution by hydrogen bonding between water and its EO chain; add salt or hard-water calcium and magnesium, and those ions bridge and dehydrate the chain, solubility collapses and the system separates at its cloud point. The phosphate head group carries a permanent negative charge, so it maintains hydration and inter-droplet repulsion through high ionic strength and across a wide pH range instead.
In practice that translates into published tolerances rather than adjectives:
| Property | Typical phosphate ester | Ethoxylated nonionic |
|---|---|---|
| pH stability window | Roughly 3-9 for triceteareth-4 phosphate; about 4-9 for potassium cetyl phosphate, optimal near skin pH 5.5-5.8 | Cloud point falls sharply as pH moves away from neutral |
| Electrolyte load | Triceteareth-4 phosphate tolerates up to about 2% NaCl; class described as resistant to hard water and inorganic salts | Phase separation and haze as salt rises; the "salt curve" turns over |
| High-pigment and high-ethanol loads | Explicitly documented as stabilisable | Commonly the trigger for instability |
| Foam profile | Between low-foaming nonionics and high-foaming ether sulfates | Low, but collapses in electrolyte |
Two secondary properties come free with the same chemistry and are worth knowing when the brief is not purely cosmetic: phosphate esters inhibit corrosion and behave as hydrotropes, solubilising nonionic surfactants into high-electrolyte systems where they would otherwise separate out.
Sun care: water resistance and filter compatibility
Sun care is the application that made this class mainstream, because it combines three destabilisers at once - high oil load, high pigment or filter load, and often ethanol or salt. Potassium cetyl phosphate at 1-3% is used as the primary emulsifier in high-SPF systems, where it contributes to SPF and to water resistance. It should be combined with a lipophilic co-emulsifier, and fatty alcohols are the most compatible choice; the pair builds a liquid crystalline gel network in the oil phase and at the droplet interface within the first 24 hours.
Where a single system must carry both organic filters and inorganic ZnO or TiO2, triceteareth-4 phosphate at HLB 10-11 is the more forgiving choice, and its ethoxylated chain adds steric buffering that the non-ethoxylated grades do not have. For a genuinely water-resistant, high-SPF or high-thickening brief, the C20-22 alkyl phosphate blend at 1-3% is built specifically for that job: the long C20-22 chains and the behenyl alcohol give a structured interface and a higher yield value, which is what holds a film on skin through sweat and swimming.
Colour cosmetics and pigment dispersion
Pigment loading is where a formula stops behaving like an emulsion and starts behaving like a filled suspension. Castor oil phosphate is used as a dispersant as much as an emulsifier - phosphated castor oil derivatives are documented for improved water solubility together with excellent pigment-dispersing performance - which makes it useful in colour cosmetics and in preparing pigment pastes. Potassium cetyl phosphate tolerates high pigment loads and gives snow-white emulsions, useful in foundations and mascaras. Where the requirement is a long-wear foundation or a tone-up cream that has to hold powder without turning chalky, the C20-22 alkyl phosphate blend is the grade that adds thickening at the same time as water resistance.
Low-pH and active-rich formulas
Formulas built around AHA, BHA, ascorbic acid or other acid actives run at pH 3-4, which is the range where most emulsifier classes are least comfortable. Triceteareth-4 phosphate is stable from about pH 3.0 to 9.0 and is explicitly positioned for systems containing AHA and BHA, and the class as a whole is stable in acid at elevated temperature. This is also why the same chemistry is used in aqueous industrial cleaners built on strong alkali: the head group survives both ends of the pH scale in a way that a soap or an ethoxylate does not.
Light textures, cleansing oils and quick-breaking systems
Trilaureth-4 phosphate is the odd one out in the table because it is a liquid at ambient temperature and behaves as a "quick-breaking" emulsifier: the emulsion collapses on contact with skin and releases a fresh, aqueous sensation rather than a waxy film. That makes it the grade for fluid lotions, serums, sprays and light sunscreens at 1-4%, and it is widely used as the co-emulsifier in cleansing oils, where it works alongside primary emulsifiers such as Sorbeth-30 Tetraoleate or PEG-20 Glyceryl Triisostearate to convert the oil into a rinse-off milk on contact with water.
Home care, industrial cleaning and metalworking
The same chemistry carries into non-cosmetic applications, and the reasons are identical. In alkaline detergents and industrial cleaners, phosphate esters act as hydrotropes that keep nonionic surfactants in solution at the electrolyte concentrations those formulas require, and they bring corrosion inhibition and antistatic performance with them. Their low foam makes them suitable for spray and CIP cleaning, where foam is a handling problem. In metalworking fluids the hard-water tolerance is the decisive property: calcium and magnesium in the dilution water break a nonionic emulsifier's hold on the lubricating oil, whereas the phosphate ester keeps the oil emulsified. Do not assume a cosmetic grade transfers unchanged to these uses, however - check active content, colour and any specification your process requires.
Running the emulsification
- Temperature. Melt and disperse the phosphate ester in the heated oil phase at roughly 75-90 °C, or add potassium cetyl phosphate to the heated water phase at 80-85 °C. High temperature is required for long-term stability, not merely for dispersion.
- Shear. These grades are not shear-sensitive and do not need aggressive homogenisation; adapt the shear to the equipment rather than to the emulsifier.
- Mixing through cool-down. Keep the batch under agitation while it cools. Undisturbed cooling is one of the most common causes of a coarse emulsion with these grades.
- Below 40 °C. Add the neutraliser and any temperature-sensitive ingredients below 40 °C, then apply a second, shorter homogenisation step.
- Ripening. Allow 24-48 hours before judging viscosity or texture. The liquid crystalline structure that provides the stability continues to build after the batch is filled, so a viscosity read at two hours is not the final number.
- Co-emulsifier. Always pair the amphiphilic phosphate grades with a lipophilic co-emulsifier - fatty alcohols such as Cetearyl Alcohol C16-18 (CAS 67762-27-0) or Cetyl Alcohol C16 (CAS 36653-82-4) are the best documented partners - to avoid insoluble crystal formation and to build the lamellar network.
Specification and procurement checklist
- Acid value (mg KOH/g) and free phosphoric acid content. These two numbers determine how much base the formula needs and how much unreacted phosphoric acid the customer inherits.
- Mono-ester to di-ester ratio, and whether the supplier can hold it or adjust it on request. Ratio drift between lots shows up as viscosity drift in the customer's batch.
- pH of a 1% aqueous dispersion, stated with the dilution. Expect roughly 1.5-2.5 for acid grades and 6-8 for potassium-neutralised grades. A salt grade reading acidic means incomplete neutralisation.
- Active content and water content, given as an assayed range with the method - not "typical".
- Colour on the APHA or Gardner scale. Colour is the earliest visible indicator of thermal damage or excess free phosphate, and it matters directly in white emulsions.
- Residuals from ethoxylation. For the ethoxylated grades, ask for declared limits on 1,4-dioxane and diethylene glycol, the two impurities that regulators and brand customers look for first.
- Cation form and EO moles, stated explicitly - potassium versus sodium versus TEA changes solubility and skin feel, and a different EO number is a different product.
- Toxicological and regulatory status. Cetyl phosphate and its salts have been reviewed by the Cosmetic Ingredient Review, and the alkyl phosphates were assessed separately. For China, be aware that reported leave-on use levels in the ingredient directory are around 1.7% for C20-22 alkyl phosphate and around 2.8% for trilaureth-4 phosphate - useful reference points when a customer is building a safety assessment file.
- Packaging and handling. Liquids are typically supplied in drums and IBCs; waxes and granules in 25 kg boxes or bags. Waxy solids need heated or well-controlled storage to remain pourable.
- Documentation. Batch COA including acid value, pH, active content and colour; TDS; SDS in the customer's language; and a sample for a trial in the customer's own base.
Selection quick answers
A high-SPF, water-resistant sunscreen with organic and inorganic filters? Triceteareth-4 phosphate at 1-3%, or potassium cetyl phosphate at 1-3% with a fatty alcohol co-emulsifier. A long-wear foundation or tone-up cream that has to hold powder? The C20-22 alkyl phosphate blend at 1-3%. A foundation that separates on storage? Move the pigment to a castor oil phosphate dispersion first, then emulsify. An AHA or BHA serum at pH 3.5? Triceteareth-4 phosphate - this is the range where ethoxylates fail. A fluid spray or a "water-burst" lotion? Trilaureth-4 phosphate at 1-4%; it is also the co-emulsifier for a cleansing oil. A cleansing oil that must turn into a milk on contact with water? Trilaureth-4 phosphate with a primary emulsifier such as Sorbeth-30 Tetraoleate. A hard-water detergent or metalworking fluid? The phosphate ester class, chosen for low foam. The customer wants a PEG-free emulsifier? Potassium cetyl phosphate at HLB 9.6. The batch is coarse and thins out after a week? Check whether the neutralisation was completed and whether a lipophilic co-emulsifier is present - with these grades the emulsifier is rarely the fault.
Shanghai Better Chemical supplies the phosphate ester emulsifier range - Cetyl Phosphate (CAS 3539-43-3), Potassium Cetyl Phosphate (CAS 19035-79-1), Triceteareth-4 Phosphate (CAS 119415-05-3), Trilaureth-4 Phosphate (CAS 31800-90-5), the C20-22 Alkyl Phosphate (and) Stearyl Phosphate (and) Behenyl Alcohol blend (CAS 84962-18-5; 2958-09-0; 661-19-8), Glycereth-26 Phosphate (CAS 31694-55-0; 12040-65-2) and Castor Oil Phosphate (CAS 447453-01-2) - together with the co-emulsifiers, oil phases and rheology modifiers these systems are built with: Cetearyl Alcohol C16-18 (CAS 67762-27-0), Cetyl Alcohol C16 (CAS 36653-82-4), Glyceryl Stearate (CAS 31566-31-1), Caprylic/Capric Triglyceride (CAS 95912-86-0), Isononyl Isononanoate (CAS 59219-71-5), Polyhydroxystearic Acid (CAS 27924-99-8), Dimethicone 350 (CAS 63148-62-9) and Xanthan Gum (CAS 11138-66-2). Every lot ships with a COA covering acid value, pH, active content, appearance and colour, with TDS, MSDS and samples for line trials. Send us the oil phase, the target pH and the electrolyte load, and we will confirm which grade holds it - along with current pricing and lead time.