Okay, we’ve familiarized ourselves with the different types of surfactants commonly used in cleansers. But how do they actually affect the skin? Let’s find out.
Now, cleansers are typically in contact with the skin anywhere from 15-60 seconds, or longer if a “specialized” cleanser is used; such as a hydroxy acid-, benzoyl peroxide-, or sodium sulfacetamide-based one. During this time, it is the top layer of skin, the stratum corneum (SC) that is most affected.
The SC has a brick and mortar style structure to it. The bricks are non-nucleated keratinocytes that also lack cytoplasmic organelles; they will henceforth be referred to as corneocytes. While physiologically dead, they still remain active in the sense that they house bundles of keratin and low-weight molecular and water-soluble amino acids and their derivatives. Together they are known as the natural moisturizing factor (NMF), not including the keratin. The water content of the SC, which makes up ~15% of the total weight, resides primarily around this NMF. The mortar is primarily made up of lipids that have been extruded from the lamellar granules–oblong-shaped organelles that are part of the keratinocytes in the layer of skin immediately below the SC, the granular layer (stratum granulosum).
When the SC is exposed to a cleanser, which typically contains anywhere from ~5-10% of surfactants, the lipids of the “mortar” can be damaged, as well as the NMF parts of the “bricks.” Therefore, this article will detail what happens to these structures immediately after the skin is cleansed.
The Effects of Surfactants on Proteins
While proteins are by far the most dominant type of molecule present in the SC at ~70%, it can be said that at the end of the day, the % of water present in the SC is the most impacting. After all, dry skin is the result of having the overall water content of the SC lowered, since water keeps the SC from cracking and remaining pliable. However, because the optimal concentration of water is inherently and directly tied to how well the proteins (and lipids) do their respective jobs, it would be an overgeneralization to only focus on the water content.
As stated before, dry skin (one of the defining characteristics of “over-cleansing”) is a manifestation of having the SC’s water content lowered. Ironically enough, when the skin is first cleansed, the water content skyrockets. But the mechanism by which this occurs, is rather counterintuitive.
The total water content skyrockets because the corneocytes, which are filled with proteins (keratin and NMF), have a natural affinity for water. It’s where the water content of the SC naturally resides after all. Therefore, when you cleanse, most of the water absorbed by the SC will be done so by the corneoctyes, causing them to swell. Surfactants increase the swelling further by altering or even destroying the corneocytes’ ability to regulate water content. This happens because the charged head groups of the surfactants interact with the charged areas of the various proteins such as aquaporins present in the corneocytes, which can cause denaturation–meaning that the proteins are inactivated. Not to mention that the removal of NMF–the primary compounds that water binds to, also increases. But that’s due more to the effect that surfactants have on lipid, rather than proteins. We’ll get into that in the next section.
But does that even matter, now that there’s so much extra water in the SC? Can we say goodbye to dry skin? Unfortunately, we cannot. This increase in water is temporary. Remember, the proteins’ ability to regulate water has been comprised. It’s a double-edged sword. They can’t prevent water from coming into the corneocytes, but they also can’t prevent water from leaving. As you know, water evaporates. There goes our water… This is why it is recommended to apply moisturizer soon after cleansing. In addition, perhaps the necessity of toners is now apparent? (We’ll find out soon).
Keep in mind that the degree of “destruction” depends on the potency of each individual surfactant, the amount present, the duration of contact; among other factors.
The Effects of Surfactants on Lipids
Like water, lipids comprise about ~15% of the SC by weight. Similarly, the function of lipids in the SC is to keep the skin cohesive and crack-free (and provide some antioxidant protection as well).
The main components of the lipid content in the SC, are fatty acids, cholesterol, and ceramides. Most of the studies discuss the effects that surfactants have on the first two molecules, likely because ceramides are present at the lowest concentration, not to mention that they are the least susceptible to solubilization.
Unlike with proteins, there are actually two main mechanisms by which lipids in the SC are compromised by surfactants: solubilization and insertion.
Solubilization is when the surfactants sweep up the lipid content of the SC and form a sort of bubble or micelle around it. This is what allows the lipids to be carried away by water when rinsing. This is how surfactants for example, remove foundation off the skin. But if too much of the lipid content is removed, the corneoctyes become exposed, which then increases water loss. The NMF (remember, this is what water primarily binds to) no longer has its protective layer of lipids to help hold on to that water, which can now more freely leave the corneocytes and be washed away with the surfactants. The stronger the surfactants and the more of them are present, the more and more lipids will be leeched away; thereby exposing deeper and deeper layers of the SC, which leads to greater and greater water loss.
Solubilization occurs primarily at the top of the SC, though it would make sense that depending on the size of the micelles formed, which in-turn are dependent on the surfactant(s) used, micelles themselves may penetrate into the lipid matrix of the SC–something that recent studies have somewhat confirmed.
Sometimes referred to as intercalation, the second method by which surfactants damage the lipid matrix is via insertion. This is when single surfactant molcules (monomers) partition into the lipid matrix of the SC, which then alters both its rigidity and formation. The more the monomers bury their way into the matrix, the further it is destabilized. With the formation of the matrix in disarray thanks to the surfactant monomers, which again reduce the surface tension between oils and water, the lipids can no longer prevent water from coming or going. There goes our water… Again.
This mechanism of insertion tends to occur in the deeper layers in the SC, where the monomers (as opposed to micelles) can penetrate more easily. They are smaller in size, right? Note that this is part of the reason why small-headed surfactants like sodium lauryl sulfate, are more potent than surfactants with larger head groups, such as sodium cocoyl sarcosinate.
How Do These Effects Add Up?
All this crazy talk about protein denaturation and lipid solubilization sounds pretty scary, right? How in the world can the skin survive such onslaughts? Fortunately, the skin is remarkably resilient. But even the best comebacks can be worn down and suppressed by repeated attacks. To find out what negative long-term effects surfactants can have on the skin, keep reading!
Your blog is amazing. I learn tons of things here. Comprehensively explain every details and easy to read! Do you have a list of books to read for improve skincare knowledge? Please recommend
Thanks! I use a variety of dermatology, chemistry, and biology textbooks to improve my understanding of skin care, as well as regularly exploring relevant PubMed articles. 🙂