Sodium Alpha-Olefin Sulfonate (AOS, CAS 68439-57-6): Properties, Production and Applications

Sodium alpha-olefin sulfonate (AOS) is an anionic surfactant made by sulfonating an alpha-olefin with sulfur trioxide. It is a workhorse in personal care and household cleaning because it foams well, tolerates hard water and is milder than the older sulfonates – and it also plays a technical role in enhanced oil recovery, emulsion polymerisation and agrochemical formulation. This article covers what AOS is, its chemistry, how it is made, its specifications, where it is used, and how it compares with the other anionic surfactants.

1. What AOS is

AOS is the sodium salt of an alpha-olefin sulfonate. Unlike a single pure compound, AOS is a mixture of two related sulfonate species formed in the same reaction – an alkene sulfonate and a hydroxyalkane sulfonate. Both carry a sulfonate (-SO₃Na) head group and a long hydrocarbon tail, so both are anionic surfactants; the mixture is what gives AOS its characteristic performance.

Property	Value
CAS number	68439-57-6
Chemical name	Sodium alpha-olefin sulfonate (sodium C14-16 olefin sulfonate)
Type	Anionic surfactant
Typical hydrophobic chain	C14 – C16 (also C12 – C18 depending on the olefin cut)
Appearance	Light-yellow powder (92%) or light-yellow liquid (34-36%)

2. Chemistry: the two structures

AOS is defined by two structures, which coexist in the product:

Species	Structure	Share of product
Alkene sulfonate	RCH=CH(CH₂)_n-SO₃Na	The majority species
Hydroxyalkane sulfonate	RCH(OH)(CH₂)_n-SO₃Na	The balance

Because the sulfonate group is attached through a carbon-sulfur bond, AOS is stable across a wide pH range – unlike sulfate esters (for example SLES), which hydrolyse in acid or at high temperature. That stability is one reason AOS is used in hard-water, saline and high-temperature systems.

3. How AOS is made

Sulfonation. An alpha-olefin (typically C14-C16) reacts with sulfur trioxide (SO₃) in a falling-film reactor.
Neutralisation. The acidic sulfonation product is neutralised with sodium hydroxide.
Hydrolysis. Controlled heating converts the intermediate into the alkene and hydroxyalkane sulfonates that make up AOS.
Finishing. The product is bleached and adjusted to give the liquid grade (34-36% active), or spray-dried to give the powder grade (90-94% active).

The choice of olefin cut sets the hydrophobic chain length and therefore the foaming and wetting balance; the process controls set the colour (Klett) and the unsulfated matter.

4. Specifications

Property	AOS 92% (powder)	AOS 34-36% (liquid)
Appearance (25 deg C)	Light-yellow powder	Light-yellow liquid
Odour	No strange odours	No strange odours
Active matter (%)	90 – 94	34 – 36
Unsulfated matter (%)	Max 3.0	Max 1.75
Inorganic salt (%, as Na₂SO₄)	Max 5.0	Max 1.5
Free alkali (%, as NaOH)	Max 1.0	Max 0.3
Colour (Klett, 5% aqueous solution)	Max 90	Max 80
Water (%)	Max 3.0	–

Active matter is what you pay for and what sets the formulation; unsulfated matter (unreacted olefin) affects foaming and odour; inorganic salt and free alkali affect viscosity and pH in the finished product; colour (Klett) matters for clear and light-coloured formulations. Confirm all of them on the batch COA.

5. Properties that matter in use

Property	Why it matters
Hard-water tolerance	Calcium and magnesium salts stay soluble (unlike LAS), so performance holds in hard water
Rich, stable foam	Good lather in shampoos, hand soaps and dishwash
Mildness	Lower skin and eye irritation than SLS or LAS, so it suits leave-on and frequent-use products
Sulfonate stability	Stable across a wide pH range and at elevated temperature (no sulfate-ester hydrolysis)
Electrolyte tolerance	Tolerates salt and brine – important in oilfield and some cleaning formulations
Biodegradability	Readily biodegradable, with a good environmental profile
No ethylene oxide	Made without ethoxylation, so there is no 1,4-dioxane issue of the kind associated with some ethoxylated surfactants

6. Applications

Personal care

AOS is a primary or co-surfactant in shampoos, body washes, facial cleansers, hand soaps and oral care. Its combination of rich foam, mildness and hard-water tolerance makes it a good base for gentle, high-lather products.

Household and industrial cleaning

In liquid and powder detergents, dishwash and hard-surface cleaners, AOS supplies foaming and detergency and keeps working in hard water. It is also used in industrial cleaning and textile wetting and scouring.

Enhanced oil recovery and oilfield

AOS is one of the surfactants used in surfactant and foam flooding for enhanced oil recovery, because it tolerates the high salinity and hardness of reservoir brine and stays stable at reservoir temperature. It is also used as a foaming agent in drilling and workover fluids.

Emulsion polymerisation

As an anionic emulsifier, AOS stabilises monomer droplets and polymer particles in emulsion polymerisation (for example acrylic and styrene-acrylic latices), contributing to particle-size control and latex stability.

Agrochemicals and other uses

Wetting and dispersing aid in wettable powders, suspension concentrates and other formulations.
Fire-fighting foam concentrates.
Textile wetting, scouring and dyeing assistants.

7. AOS vs LAS, SLES and SLS

Surfactant	Type	Hard water	Mildness	Notes
AOS	Anionic (sulfonate)	Excellent	Good	Stable across pH; no EO; good foam
LAS (linear alkylbenzene sulfonate)	Anionic (sulfonate)	Poorer (can precipitate)	Lower	Very cost-effective; the detergent workhorse
SLES (sodium laureth sulfate)	Anionic (sulfate + EO)	Good	Very good (mild to eyes)	Contains EO; can carry 1,4-dioxane unless controlled
SLS (sodium lauryl sulfate)	Anionic (sulfate)	Moderate	Lower (more irritating)	Strong cleansing; more irritating than AOS or SLES

In short: AOS gives hard-water tolerance and sulfonate stability with good mildness; LAS is the low-cost option for commodity detergents; SLES is the mildest to eyes but brings ethoxylation; SLS cleans hard and irritates more. Formulators often blend two or three to balance cost, foam, mildness and viscosity.

8. Formulation and dosage

Application	Typical active level (starting point)
Shampoo and body wash	About 8 – 15% total active surfactant (AOS often part of the blend)
Liquid detergent / dishwash	About 5 – 20% active
Powder detergent	About 5 – 15% active
Emulsion polymerisation	About 0.5 – 3% on monomer
Enhanced oil recovery	About 0.2 – 0.5 wt% in the flood (project-specific)

Levels are starting points: build the formulation and confirm foam, viscosity, mildness and stability by test. AOS blends well with amphoteric and nonionic surfactants, which are often used to boost mildness and viscosity.

9. Sustainability and safety

Biodegradability – AOS is readily biodegradable, with a good aquatic profile for an anionic surfactant.
No ethoxylation – because AOS is not ethoxylated, there is no 1,4-dioxane concern of the kind that applies to some ethoxylated surfactants.
Mildness – lower irritation than SLS, which is why it suits frequent-use and leave-on applications.
Handling – as with any surfactant, avoid eye contact and prolonged skin contact; use the SDS for handling, storage and first aid.

10. Packaging, storage and supply

Grade	Packaging
Powder (92%)	25 kg PP/PE bags; 1000 kg jumbo bags; palletised for container shipment
Liquid (34-36%)	200 L drums; 1000 L IBCs; ISO tanks; bulk

Keep the powder sealed and dry; protect the liquid from freezing and from prolonged high temperatures. Both grades are supplied with a per-batch COA and SDS, and third-party inspection is available on request. Tell us the grade (powder or liquid), the active matter you need, the tonnage and the delivery port for a quotation.

Frequently Asked Questions

What is AOS used for?

As an anionic surfactant in personal care (shampoos, cleansers, oral care), household and industrial cleaning, enhanced oil recovery, emulsion polymerisation and agrochemical formulations.

What is the difference between the 92% powder and the 34-36% liquid?

They are the same surfactant at different active levels. The powder is spray-dried for powder detergents and for lower freight cost; the liquid is easier to pump and dose and is used in liquid formulations.

Is AOS better than LAS or SLES?

It depends on the job. AOS gives better hard-water tolerance and sulfonate stability than LAS and contains no ethylene oxide (unlike SLES). LAS is cheaper for commodity detergents; SLES is milder to the eyes. Many formulations blend them.

Does AOS contain 1,4-dioxane?

No – AOS is not ethoxylated, so it does not carry the 1,4-dioxane issue associated with some ethoxylated surfactants. Confirm specifications with your supplier.

Why is AOS used in enhanced oil recovery?

Because it tolerates high-salinity, high-hardness reservoir brine and remains stable at reservoir temperature, making it suitable for surfactant and foam flooding.

Is AOS biodegradable?

Yes – AOS is readily biodegradable and has a good environmental profile compared with many anionic surfactants.