Shampoo products are formulated with a variety of different surfactants, of which sodium lauryl ether sulfate (SLES) is one of the most widely used. To understand why requires an understanding of its structure, properties and processing conditions used to make shampoo with SLES
What is SLES and how is it made ?
Figure 1: the n associated with the ether group denotes the degree of ethoxylation of SLES, which is usually between 1, 2 or 3 oxyethylene units. However, commercially SLES is referred to by the average number of ethoxylate groups present. The length of the carbon chain is usually 12 carbon atoms.
SLES consists of a negatively charged sulfate head group and an alkyl ether hydrocarbon tail. It is produced through a multistep process. First, dodecyl alcohol which is usually derived from palm or coconut oil is ethoxylated. The ether then reacts with sulfur trioxide to produce an ethoxylate. The ethoxylate is neutralised with sodium hydroxide to give SLES and water. In industry, the production process of SLES is shown in Figure 2.
The properties of SLES can be tuned by varying the degree of ethoxylation, denoted by n and the length of the hydrocarbon chain
Figure 2: In the manufacturing process of SLES, sulfation of the ethoxylate can occur using chlorosulfonic acid or oleum. The alkyl ether sulfate is neutralised with NaOH.
Physical and chemical properties of SLES
Properties:
- Good solubility in water and at increased temperatures
- Compatible with hard water
- Acts as a cleansing agent
- Good foaming ability for both high viscosity and low pH products. However, to create light and airy foams, foam boosters are used to affect the micellar properties of SLES and decrease electrostatic repulsion between the molecules in foam films
- An increase in ethoxylation increases solubility and reduces both precipitation and foam volume in presence of metal ions in hard water
- Salt increases viscosity of shampoo products with SLES
Environmental impacts of SLES
Figure 3: Ecotoxicological and biodegradation tests on SLES
Although SLES is generally reported to be biodegradable in standard tests, with degradation rates between 7 h and 30 days, depending on the initial conditions, data on its biodegradation in environmental studies are quite scarce.
Laboratory ecotoxicological tests pointed out detrimental effects of SLES for aquatic organisms, while data on the terrestrial species are rather poor so far and further studies at the expected environmental concentrations are necessary.
Health concerns about SLES
SLES generally shows lower skin irritation than sodium lauryl sulfate (SLS). However, 12 carbon atom chain SLES are found to be more irritating than any other chain lengths. Increasing ethoxylation reduces the skin irritating properties of SLES.
The levels of SLES used are 10% w/w to as this leads to moderate levels of skin irritation.
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