When Surfactants Meet Our Stratum Corneum
The stratum corneum is the outermost layer of our epidermis and our skin's principal permeability barrier. This is what we commonly refer to as the skin barrier. It controls water loss from within while restricting the penetration of allergens, irritants, toxins, pollutants, microorganisms and other substances from the environment
The stratum corneum is often described as a brick-and-mortar structure. Keratinocytes (skin cells), as they migrate from the basal layer towards the skin surface, progressively differentiate, eventually becoming flattened, hardened corneocytes (dead skin cells) that have lost their nuclei. These corneocytes form the bricks. They are embedded in a highly organised extracellular lipid matrix, the mortar, composed predominantly of ceramides, cholesterol and free fatty acids.
In chemistry all fats are lipids but not all lipids are fats. Lipids include fats, oils, waxes, phospholipids (major components of cell membranes), ceramides, and sterols such as cholesterol. They are a large diverse group of natural molecules that are insoluble or poorly soluble in water.
Ceramides, cholesterol and free fatty acids, including linoleic acid the essential fatty acid abundant in Vitis V Face TonIQ, are arranged into tightly packed lamellar layers. Their composition, proportion and molecular organisation are fundamental to our skin barrier integrity.
They are not dirt. They are not impurities. They are critical for skin barrier integrity.
They are part of the biological architecture of healthy skin.
And every time we cleanse with a surfactant found in skincare cleansers, body wash and soap, we expose our lipid-based barrier to chemistry specifically designed to interact with lipids.
Cleansers are basically detergents
Most commercial facial and body cleansers rely on surfactants (surface-active agents) a class of chemical compunds to perform their foaming and lipid removal action.
Surfactants have one part of their chemical structure that is attracted to water and another that is attracted to lipids. In water, they group together to form structures called micelles, which surround oily material and allow it to be dispersed in water and washed away.
By the way micellar water contains surfactants.
It is the dual attraction, one to water and one to lipids, that makes surfactants effective cleansers.
It is the same fundamental chemical principle that allows dishwashing liquid to remove grease from a plate.
Surfactants interact with lipids (fats and oils) so those lipids can be removed with water.
The problem is that our skin barrier is lipid based.
Our skin barrier depends on lipids. Surfactants are designed to interact with lipids.
When surfactants meet our stratum corneum
Surfactants do not differentiate between with the substances we intend to remove and our structural lipids of our skin
Surfactants can interact with the skin's lipid matrix by inserting themselves into it's structured layers, solubilising the natural oils, and disrupting the barrier function of the stratum corneum.
Surfactants affect the skin in four ways:
1/ Solubilisation and Removal
Surfactants lower surface tension and form structures called micelles that trap and wash away surface sebum, natural oils, and intercellular lipids such as free fatty acids, ceramides and cholesterol.
Free fatty acids appear particularly susceptible to surfactant-induced removal.
2/ Intercellular Insertion
Residual surfactant molecules that stay on or penetrate the skin can wedge themselves directly into the highly ordered lamellar lipid matrix of the stratum corneum.
Their interaction with our skin does not necessarily end when the cleanser is rinsed away.
3/ Disruption of Molecular Order
Interaction between surfactants and the lipid matrix can separate lipid chains and increases alkyl chain disorder, disrupting the tightly organised structure required for effective barrier function.
4/ Increased Permeability and Water Loss
Depletion and disorganisation of barrier lipids can increase transepidermal water loss (TEWL) and permeability, contributing to dryness, roughness and irritation while increasing the penetration of external substances.
These are known direct consequences of surfactant-induced barrier disruption. But the consequences can extend beyond damage to the physical barrier.
Skin lipids are not simply passive mortar
Ceramides, cholesterol and free fatty acids provide the physical architecture of the permeability barrier, but epidermal lipid biology extends beyond just structure.
The epidermis actively synthesises and metabolises these lipids, which, together with their metabolites, participate in signalling pathways involved in epidermal differentiation and barrier formation.
Cholesterol sulfate metabolism has an established role in regulating desquamation, which is the controlled shedding of corneocytes from the skin surface.
Free fatty acids contribute to acidification of the stratum corneum.
This matters.
Healthy skin is naturally acidic, typically pH 4.5 -5.5. This acidic environment regulates enzymes involved in lipid processing, barrier homeostasis, corneocyte cohesion and desquamation. It also contributes to antimicrobial defence and helps determine which microorganisms can successfully inhabit the skin surface.
The lipid barrier, skin pH and microbiome are not independent systems. They are interconnected components of skin homeostasis.
Removing skin lipids is therefore not simply removing oil from the surface.
We are interfering with a biologically active interface.
But mine is a “gentle” cleanser
Different surfactants behave differently when they come into contact with our skin.
Surfactant concentration, charge, micelle structure, pH and the composition of the cleansing system can all influence the degree of barrier disruption.
A “gentle” cleanser can therefore be less disruptive than a harsher cleanser or traditional soap.
But gentle is relative.
It describes one cleansing system in comparison with another. It does not mean that surfactants cease to interact with the skin.
This is not an argument against the appropriate use of surfactants. There are circumstances where their cleansing action is necessary, including handwashing for hygiene. The question is whether that same chemistry is necessary for routine facial cleansing.
Why expose an intact facial lipid barrier to surfactants when surfactants are not required for cleansing?
Warm water. Gentle physical action.
Vitis V recommends warm water and gentle physical action with a soft natural-fibre face cloth for cleaning.
Water removes water-soluble material. Gentle physical action helps lift surface debris without introducing surfactants whose function is to interact with and solubilise lipids.
The water should be warm, not hot. Hot water itself can impair barrier function, increasing transepidermal water loss and altering skin pH.
Warm enough to be comfortable. Not hot enough to challenge the barrier.
Where water-resistant sunscreen or oil-based makeup requires additional help, Vitis V recommends a small amount of castor oil to help lift the residue before removing it with warm water and a soft natural-fibre cloth.
Remove what needs to be removed from the surface. Preserve the structure.
The skin barrier is not a dead outer layer of our skin.
It is a highly organised, lipid-dependent biological interface that regulates permeability, water loss, desquamation, acidity and our relationship with the microorganisms living on its surface.
Its lipids are fundamental to its function.
Yet we have been taught to associate the tight, squeaky-clean sensation after cleansing with clean skin and then to reach for another product to relieve the dryness and tightness that follows. Mmmmmm.
Why are we encouraged to remove the very lipids our skin needs to maintain an intact, fully functioning barrier, only to apply more products in an attempt to replace what we have removed?
We should stop washing them down the sink!
References
Stratum corneum fatty acids: their critical role in preserving barrier integrity during cleansing
Cleansing formulations that respect skin barrier integrity
Skin Cleansing without or with Compromise: Soaps and Syndets.
The mechanisms by which lipids coordinately regulate the formation of the protein and lipid domains of the stratum corneum: Role of fatty acids, oxysterols, cholesterol sulfate and ceramides as signaling molecules.
The pH of the skin surface and its impact on the barrier function.