Sulfate-Free Cosmetics Are Not a Panacea

In the collective unconscious, sulfur is associated with the Gates of Hell. Or, for some, with the oracles provided by Sybilla, the Roman prophetess whose manic proselytization may have been produced by inhaling ethylene gas or a mixture of ethylene and ethane coming from a naturally-occurring vent. Even today, the adjective “sulfurous” is often used to label actions or ideas charged with having an infernal, hellish or, at least, sinful intent.

No wonder, then, that the word “sulfate” has come a long journey from alchemy to chemistry to modern skin care, and has turned into the most feared, despised, scary, hatred, abhorred, repugnant concept in cosmetics! And no wonder that “sulfate-free” is the consensual, immaculate label pointing to the only path that has to be walked to reach true salvation.

Let’s review the chemistry.


Sulfur Salts

A formally-trained teacher of theoretical chemistry will tell us that salts are what one obtains by mixing an acid with a base. Fine.

Salts are generally more or less soluble in water. Some salts, such as sodium chloride, play crucial roles in supporting life. Sulfur salts or sulfur-containing molecules are used in a variety of practical applications. Calcium sulfate is used as a fertilizer. Copper sulfate is used to fight pests and also as a fungicide. Magnesium sulfate is known as epsom salt. Taurine’s chemical name is 2-aminoethanesulfonic acid. Traces of sulfur dioxide are added to fermenting wines to scavenge oxygen, thus hindering the growth of those aerobic microorganisms that would turn ethanol into acetic acid. The human body contains about 140 grams of sulfur per 150 pounds of body weight. Penicillin and cephalosporin contain sulfur, and elemental sulfur is used in the vulcanization of rubber. Last but not least, colloidal sulfur is used against acne, rosacea and seborrheic dermatitis.

So, why is there such a psychologically violent, quasi-allergic visceral reaction against sulfates?


Sulfates in Skin Care

Dodecanol [CH3(CH2)11OH] reacts with sulfur trioxide (SO3) to produce the acid hydrogen lauryl sulfate [CH3(CH2)11SO4H]. Upon neutralization with bases such as sodium hydroxide, one obtains the salt sodium dodecyl sulfate (SDS), aka sodium lauryl sulfate (SLS), a molecule having a Molecular Weight MW=288.

The addition of the sulfate moiety to the dodecanol chain makes it very hydrophilic. As a matter of fact, the water solubility of dodecanol is about 4mg per liter whereas the water solubility of SDS is 200 grams per liter. At 25°C, SDS has a critical micelle concentration of about 10mM (~0.28%).

It so happens that SDS is a great detergent; its surfactant properties make it appropriate for industrial cleansing and, at lower concentration, it is used in shampoos, soaps, toothpastes, shaving creams and bubble baths. It is also an ideal ingredient to prepare stable emulsions and topical skin care products. It is not cancerogenic and it is generally recognized as safe (GRAS).

Unfortunately, because of its chemical properties, it is very good at denaturing proteins and at solubilizing lipids. This is of interest for biochemical and histological analyses but is not so good for skin care. Indeed, after topical application, it has been shown that 5% SLS causes significant lipid depletion in vivo even at a 1-minute contact. SLS treatment does indeed lead to the selective removal of cholesterol, cholesterol ester, free fatty acids and sphingolipids from the stratum corneum.

An analysis by electron microscopy shows the absence of intercellular lamellae after SLS treatment.¹ It has also been shown that topically applied SDS provokes a remarkable erythema and a dramatic increase of transepidermal water loss (TEWL), whereas cetyltrimethylammonium bromide only provokes a mild erythema and a slight increase in TEWL. At variance, Triton X-100 does not provoke erythema and does not promote the increase of the TEWL.² In human skin exposed to 1% SDS in patches for 24 hours, the majority of the components of the Natural Moisturizing Factor (NMF) decrease for a few days and only return to normal levels at day 10 after treatment.³

This is to say that, when it is used in cleansers or in leave-on topical creams, SDS disrupts the lipid lamellae, denatures epidermal proteins, extracts lipids, proteins and NMF, favors the penetration of foreign bodies across the stratum corneum and turns out to be incredibly irritating. This is why the skin care industry discouraged its use and has generated the claim “sulfate-free” to advertise products that do not contain SDS.


Conclusion

Now, is this information sufficient to cause goose bumps when hearing the word “sulfate?” After all, it can be surmised that the properties of disrupting the lipid layers and denaturing proteins are more the consequence of the dodecane aliphatic tail than of the sulfate anionic head, aren’t they? Dodecanol itself is considered to be an irritant, and so are expected to be its Aluminum and Boron salts.

So, let us be clear. SDS is an irritant, its mechanism of action has been elucidated. We all agree that its use must be avoided in personal care products—no doubt about that. Should, therefore, every sulfate, intended as “every salt containing the SO42- group,” be automatically struck from the list of cosmetically-acceptable ingredients? Not automatically, of course, and only if the thorough analysis of its toxicological properties compels us to it. 

References

1. Imokawa et al Importance of intercellular lipids in water retention properties of the stratum corneum: induction and recovery study of surfactant dry skin. Arch Dermatol Res 1989; 281:45-51
2. Ruissen et al (1998) Differential effects of detergents…J.Invest. Dermatol 110 : 358-363
3. Hoffman DR, Kroll LM, Basehoar A, Reece B, Cunningham CT, Koenig DW. Immediate and extended effects of sodium lauryl sulphate exposure on stratum corneum natural moisturizing factor. Int J Cosmet Sci. 2014 Feb;36(1):93-101. doi: 10.1111/ics.12101. Epub 2013 Nov 20. PMID: 24152302

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Paolo Giacomoni, PhD
Insight Analysis Consulting
paologiac@gmail.com
516-769-6904
 
Paolo Giacomoni acts as an independent consultant to the skin care industry. He served as executive director of research at Estée Lauder and was head of the department of biology with L’Oréal. He has built a record of achievements through research on DNA damage and metabolic impairment induced by UV radiation as well as on the positive effects of vitamins and antioxidants. He has authored more than 100 peer-reviewed publications and has more than 20 patents.