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Irritative potential/mildness of surfactants

⏱️ 4 min read
Irritative potential/mildness of surfactants

Surfactants are commonly classified according to their ionic charge, falling into four main categories:

  • Anionic
  • Cationic
  • Amphoteric
  • Non-ionic

These classes not only differ in their physical and chemical properties but also in their degree of skin mildness. The potential for irritation in prototype cosmetic formulations is often assessed using the Zein test. This method utilizes zein, a corn-derived protein structurally similar to keratin found in the skin and hair. Zein is typically insoluble in water unless denatured.

In the test, a measured amount of zein protein is exposed to a solution containing surfactants from the cosmetic formulation. After the interaction, the denatured protein is separated. The Kjeldahl method is then employed to quantify the amount of nitrogen released, which serves as an indicator of protein denaturation. A higher nitrogen content suggests a greater degree of zein breakdown, and therefore, a higher irritation potential of the surfactant formulation. This test provides a useful in vitro approximation of how harsh a surfactant may be on the skin, helping formulators develop milder and more skin-friendly cosmetic products.

The same surfactant sourced from different manufacturers may exhibit varying Zein values due to slight differences in the percentage of active matter, as well as inconsistencies in testing procedures. Furthermore, mildness comparisons are often limited to only a few surfactants, making it challenging to assess larger sets in a standardized and meaningful way.

Colonial Chemical offers a mildness overview of their products based on a simplified, modified Zein test. While the method has certain limitations, it effectively reveals general trends in the irritation potential of various surfactants. The reported values for each product should not be interpreted as absolute measurements, but rather as comparative references.

In terms of skin irritation caused by pure surfactants, the general trend in decreasing order is: Anionic > Cationic > Amphoteric > Non-ionic. In practice, nearly all anionic surfactants, regardless of their specific chemistry, are too harsh to be used on their own. As a result, they are almost always blended with amphoteric or non-ionic surfactants, which significantly reduces their irritation potential.

Irritation potential Zein test

There is a clever marketing tactic surrounding Sodium Coco Sulfate (SCS), with some even claiming it to be a mild alternative to Sodium Lauryl Sulfate (SLS). As the name suggests, SLS is commonly assumed to be derived entirely from lauryl (C12) alcohol. In contrast, Sodium Coco Sulfate is produced from a mixture of fatty alcohols (C8–C18) obtained from coconut oil. Lauryl (C12) alcohol typically makes up about 50% of this mixture.

After the sulfation process, the resulting surfactant blend consists of roughly 50% SLS, with the remainder composed of sulfates derived from other alcohols (C8, C10, C14, C16, and C18). It is important to note that sulfates made from shorter-chain alcohols such as C8 and C10 are actually harsher than SLS, while longer-chain alcohol sulfates (C14, C16, C18) tend to reduce irritation potential. Despite this, Sodium Coco Sulfate still contains approximately 50% SLS, reflecting the lauric acid content of coconut oil.

The presence of SLS in a shampoo or cleanser does not automatically mean the product should be avoided. Many formulations include SLS under the label of SCS. When properly formulated, products containing SLS can be just as mild as some sulfate-free alternatives.

When discussing mildness, it is essential to define a reference surfactant. Claiming that SCS is milder than SLS is largely a marketing statement. SLS is among the harshest surfactants commonly used in the industry, so nearly any alternative could be described as milder—some only marginally so, others significantly. Ultimately, what matters is the mildness of the finished formulation, not the surfactants in isolation.

Do you prefer to avoid sulfates such as SLS, SLES, or SCS in personal care products? If so, look for items that clearly state “sulfate-free” on the label. If this claim isn’t listed, there’s a good chance the product contains them.

Curious about how mild your shampoo really is? During a shower, allow a small amount of your shampoo to enter your eye. You’ll quickly learn its level of mildness. While this method is fast and revealing, it should, of course, be attempted at your own risk.

 

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