SLES: properties, uses, INCI functions, safety, alternatives

SLES (Sodium Laureth sulfate), is a chemical compound and belongs to a group of salts of sulfated ethoxylated alcohols. It occurs in liquid form or clear transparent slightly yellow gel or white fine powder.

The name describes the structure of the molecule:

• Sodium is the sodium ion (Na+), an alkaline metal.
• Laureth is a term used in the cosmetics industry for a mixture of polyethylene glycol (PEG) and lauryl alcohol. The term 'laureth' is a contraction of 'lauryl ether', where 'lauryl' refers to the 12-carbon lauryl group derived from lauric acid, a medium-chain fatty acid found in coconut oil and palm kernel or palm oil.
• sulphate is a sulphate group (SO4-2-), a sulphur atom bonded to four oxygen atoms and having a negative charge.

The synthesis process takes place in different steps:

• Extraction.The raw materials are lauryl alcohol and ethylene oxide. Lauryl alcohol is typically derived from coconut or palm oil, while ethylene oxide is a petrochemical product.
• Ethoxylation. Lauryl alcohol is reacted with ethylene oxide in a process called ethoxylation. This reaction is catalysed by a base, usually a strong alkali such as potassium hydroxide. The term 'eth' in 'laureth' indicates that ethylene oxide has been added.
• Sulphonation. Ethoxylated alcohol is reacted with sulphur trioxide (SO3) to produce a sulphate.
• Neutralisation. The sulphate is neutralised with sodium hydroxide (NaOH) to produce sodium sulphate.
• Purification. The resulting product is purified through distillation and filtration to remove any unreacted materials and by-products.
• Quality control. The final product is tested to ensure that it meets quality standards.

SLES (Sodium laureth sulfate) must not be confused with SLS  because, although both are similar and have sulphuric acid and lauryl alcohol as their formula, differ in chemical properties. In SLES, which is less aggressive than SLS but is ethoxylated (obtained from ethylene oxide), it is not uncommon to find in SLES ethylene oxide and 1,4-dioxane residues, chemical compounds that are considered carcinogenic.

The term 'eth' refers to the ethoxylation reaction with ethylene oxide after which residues of ethylene oxide and 1,4-dioxane, chemical compounds considered carcinogenic, may remain. The degree of safety therefore depends on the degree of purity of the compound obtained. No manufacturer appears to provide this information on the label, at least as of the date of this review.

A preliminary remark must be made about synthetic surfactants, which can be divided into four groups:

• Anionic (SLS, ALS, SLES), which solubilises well with lipid monolayers but not with lipid bilayers model of liposomes or cytotoxicity tests towards cultured skin cells
• Nonionic amphoteric (CAPB), which solubilises well with both mono- and bilayers
• Biotensioactive (SAP), which penetrates monolayers at 1% dry mass without solubilisation, and probably penetrates bilayers, increasing their size (without solubilisation).

What it is for and where

Chemical intermediate, anionic surfactant, densifying and foaming agent, with good solvency, wide compatibility, strong resistance to hard water, high biodegradation and relatively low skin and eye irritation.

Sodium Laureth sulfate is used in cosmetics, in liquid detergents, such as hair and bath shampoos, dishwashing detergents, toothpastes, bubble bath and hand washing, soap etc.. In chromatography as a reagent, it has excellent properties as a solvent. In printing and dyeing industry, oil and leather, textile,,, it can be used as lubricant, dyeing agent, cleaner, foaming agent and degreasing agent.

Cosmetics

Cleansing agent. Ingredient that cleanses skin without exploiting the surface-active properties that produce a lowering of the surface tension of the stratum corneum. 

Foaming.  Its function is to introduce gas bubbles into the water for a purely aesthetic factor, which does not affect the cleaning process, but only satisfies the commercial aspect of the detergent by helping to spread the detergent. This helps in the commercial success of a cleansing formulation. Since sebum has an inhibiting action on the bubble, more foam is produced in the second shampoo. In practice, it creates many small bubbles of air or other gases within a small volume of liquid, changing the surface tension of the liquid.

Surfactant - Cleansing agent. Cosmetic products used to cleanse the skin utilise the surface-active action that produces a lowering of the surface tension of the stratum corneum, facilitating the removal of dirt and impurities. 

Surfactant - Emulsifying agent. Emulsions are thermodynamically unstable and are used to soothe or soften the skin and emulsify, so they need a specific, stabilising ingredient. This ingredient forms a film, lowers the surface tension and makes two immiscible liquids miscible. A very important factor affecting the stability of the emulsion is the amount of the emulsifying agent. Emulsifiers have the property of reducing the oil/water or water/oil interfacial tension, improving the stability of the emulsion and also directly influencing the stability, sensory properties and surface tension of sunscreens by modulating the filmometric performance.

Safety

It is used in mild soaps and shampoos. However, it can cause eye irritation if used in large quantities. Since it is not mandatory to indicate the percentage or quantity of the chemical on labels, it is still difficult to know how much Sodium Laureth sulfate is in the product. The scientific literature that has dealt with this chemical compound for decades has concluded in favour of a recognition of the irritant properties of Sodium lauryl ether sulphate. Since it is not uncommon for ethylene oxide (1) and 1,4-Dioxane, a synthetic cyclic ether traditionally used as a stabiliser (2), to be found in this chemical compound during the production process, the IARC (International Agency for Research on Cancer ) warns that ethylene oxide is carcinogenic to humans (3) and 1,4-Dioxane is potentially carcinogenic to humans (4). The real problem is that no manufacturer declares SLES as free of these two compounds on the label. So we cannot know if and how much ethylene oxide and 1,4-dioxane are present in the product we have purchased. In addition, 1,4-dioxane does not degrade easily and is therefore considered to be a water pollutant that must be removed using special techniques (5).

From the above, a rather negative picture emerges for SLES, and my opinion is to be cautious and not to buy products containing SLES unless the absence of ethylene oxide and 1,4-dioxane is clearly indicated.

 It is also important not to mistake the acronyms SLES (Sodium lauryl polyoxyethylene ether sulfate Sodium Laureth Sulfate) with SLS (Sodium lauryl sulfate), also a surfactant but much less aggressive.

Alternatives

The most relevant studies on this chemical compound have been selected with a summary of their contents:

Sodium Laureth sulfate studies

Typical characteristics of the commercial product Sodium lauryl polyoxyethylene ether sulfate Sodium Laureth Sulfate (SLES)

PSA	84.04000
LogP	4.48150
Safety

• Molecular Formula: C₁₄H₂₉NaO₅S
• Molecular Weight: 332.431 g/mol
• CAS: 15826-16-1
• EC Number: 239-925-1 
• UNII 410Q7WN1BX
• DSSTox Substance ID: DTXSID2029298    DTXSID70274019
• MDL number  
• PubChem Substance ID 
• InChI=1S/C14H30O5S.Na/c1-2-3-4-5-6-7-8-9-10-11-12-18-13-14-19-20(15,16)17;/h2-14H2,1H3,(H,15,16,17);/q;+1/p-1  
• InChl Key      ASEFUFIKYOCPIJ-UHFFFAOYSA-M
• SMILES    CCCCCCCCCCCCOCCOS(=O)(=O)[O-].[Na+]
• IUPAC   sodium;2-dodecoxyethyl sulfate
• RTECS    KK7890000

Synonyms:

• SLES
• Sodium dodecylpoly(oxyethylene) sulfate
• Sodium Laureth sulfate

 References______________________________________________________________________

(1) Vleugels LF, Pollet J, Tuinier R. Polycation-sodium lauryl ether sulfate-type surfactant complexes: influence of ethylene oxide length. J Phys Chem B. 2015 May 21;119(20):6338-47. doi: 10.1021/acs.jpcb.5b02043.

(2) Black RE, Hurley FJ, Havery DC. Occurrence of 1,4-dioxane in cosmetic raw materials and finished cosmetic products. J AOAC Int. 2001 May-Jun;84(3):666-70.

(3) Ethylene oxide. IARC Monogr Eval Carcinog Risks Hum. 1994;60:73-159. 

(4) 1,4-Dioxane. IARC Monogr Eval Carcinog Risks Hum. 1999;71 Pt 2(PT 2):589-602.

Wilbur S, Jones D, Risher JF, Crawford J, Tencza B, Llados F, Diamond GL, Citra M, Osier MR, Lockwood LO. Toxicological Profile for 1,4-Dioxane. Atlanta (GA): Agency for Toxic Substances and Disease Registry (US); 2012 Apr.

(5) Scaratti G, De Noni Júnior A, José HJ, de Fatima Peralta Muniz Moreira R. 1,4-Dioxane removal from water and membrane fouling elimination using CuO-coated ceramic membrane coupled with ozone. Environ Sci Pollut Res Int. 2020 Jun;27(18):22144-22154. doi: 10.1007/s11356-019-07497-6.