Sodium laureth sulfate is in just about every shampoo and foam bath, as avid readers of ingredient lists can confirm. The name tells you pretty much what it is, if you know about these things. It is the sodium salt of sulfated lauryl alcohol, which has been ethoxylated. If you had hadn’t ethoxylated it, it would be sodium lauryl sulfate. If you hadn’t added the sulfate group it would have been sodium laurate. And sodium laurate is a form of soap. So in a sense it is souped up soap. Soap that has an increased ability to foam at the cost of reducing its ability to cleanse.
It is possible to derive the starting material, the lauryl alcohol, from petrochemicals or from animal fat, and both of these have been used in the past. But the overwhelming majority today comes from vegetable oils. You get a better foam if you use lauryl alcohol sourced from vegetable sources, of which the main ones are coconut oil and palm oil. Typically the choice of source is determined by price and availability and it is quite feasible to use a blend of both as a starting point. You can choose to use just coconut oil and call the resulting chemical sodium coco sulfate. You’d need to work pretty hard to distinguish sodium lauryl and sodium coco sulfate in the lab. You’d certainly not see any difference in their behaviour.
The ‘eth’ bit laureth comes from the fact that the lauryl alcohol has been extended by adding ether groups to it. These make the lauryl alcohol a little bit more soluble in water and a bit more bubbly. They are the reason you get a richer foam from a shampoo than you get from a soap, and you get it much more easily. In fact what we call sodium laureth sulfate is actually a mixture of molecules that have different amounts of ether, and some have none at all. So all sodium laureth sulfate contains some sodium lauryl sulfate as well.
Both these materials are present in the vast majority of hair shampoos and foam baths that you can buy on the market at the moment. In fact, in 2000 when some German researchers wanted to investigate this material they found that 97% of their prospective subjects already had it on their skin.
The reason it is popular is that it is pretty much the ideal material to create bubbles. There is a lot of money in bubbles. People will almost always pick a product that gives a “rich creamy lather” as my marketing colleagues would call it. I once gave a talk about shampoos to an audience of chemists and blew a large bubble to demonstrate how good at forming bubbles sodium laureth sulfate was. It got a round of applause. Everyone loves bubbles.
It has also been known for a long time that sodium lauryl sulfate irritates the skin. This is so well known that when skin irritancy is being studied sodium lauryl sulfate is the generally agreed test material used to do it. A lot of the skill of formulating shampoos and cleansers involves coming up with ways of maximising the bubbles while minimising the irritation. This can be done pretty well in fact. Despite its intrinsic irritancy, most people will find that for them the shampoos formulated with sulfates will be a lot milder than sulfate free ones.
So most people will find that sodium laureth sulfate is a material that they will have no problem with. If you have particularly sensitive skin you’ll probably find that you experience less short term irritation with standard formulations.
But there may well be a more subtle long term problem.
It has recently started to become clear why sodium lauryl sulfate has this harmful effect. If you consider the structure of the skin, the outer layer is the stratum corneum. This is made up of dead cells that mover up from the basal layer becoming flatter and harder as they go. These cells are held together by small links, called desmosomes. These are made of protein and the rate at which these links break controls the rate at which the dead cells on the surface of the skin are sloughed off. This is controlled by an enzyme called stratum corneum chymotrypsin enzyme (SCCE) – which is the body’s natural exfoliating agent. It turns out that sodium lauryl sulfate interferes with this process altering the skins ability to lose dead cells.
So what do we make of all this? It turns out that sodium lauryl sulfate might have an adverse effect on your scalp, but the skill of formulation chemists prevents this from being particularly noticeable.
It is hard to say if this matters or not. Certainly most people use shampoo containing sodium laureth sulphate regularly. Many people use it daily. There doesn’t seem to be any obvious harmful effects even after decades of use. But maybe our scalps are just a touch drier than they would be if we used less? If anyone has studied this in detail I haven’t seen the research. What would be most interesting would be to follow a group of people who have given up using shampoo and see what happens to the state of their scalps.
I am unimpressed by the claims made for sulfate free products. All detergents have similar structures and there is simply no evidence that it is the sulphate bit of sodium laureth sulfate that is what makes it irritating. In fact if I had to guess I’d focus on the sodium part. There is some evidence that replacing sodium with alternatives like triethanolamine makes shampoo formulations milder. The sulfate free idea seems to me to be simply a marketing ploy.
Skin irritancy is not as well understood as we would like. I’d love to know the answer, but all I can do is scratch my head. In fact we might all be scratching our heads a little more often than we need to.
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