Abstract: Surfactants are considered to be among the most important cosmetic ingredients, as they are used for cleansing, thickening, stabilising, solubilising, emulsifying and foaming. However, surfactants also exhibit unwanted effects: skin irritation. The concept of responsible cleansing with respect to the skin, hair and the environment using surfactants with less irritation potential has taken a leading role in modern formulating.
Presern A, Kocevar Glavac N. Responsible cleansing. Cos ACTIVE J. 2022;1:1–7
FOR STARTERS: SURFACTANTS AND THE VERSATILITY OF THEIR USE
Surfactants or surface-active substances are used in many industrial sectors, primarily as a base for cleaning products. They are found, for example, in dishwashing gels, laundry detergents and car washes. Even when we focus on cosmetics, surfactants are considered to be among the most important cosmetic ingredients, as they are used for cleansing, thickening, stabilising, solubilising, emulsifying and also foaming alone (1–3).
STRUCTURE DEFINES PROPERTIES
Surfactants are amphiphilic or bipolar compounds, which means they contain both polar (hydrophilic) and non-polar (hydrophobic) groups. Due to this specific structure, surfactants are soluble in both polar and non-polar solvents. Or to put it simply, they are compatible with both water and oil, which accounts for the majority of their applications. They can be ‘absorbed’ at the interfaces between phases of different polarity and therefore lower the interfacial tension in systems of two non-miscible liquids, or lower the surface tension of water at the interface with the air (1–5).
In a solution, surfactants firstly self-organise at the interface. As their concentration in a solution increases, and all the places at the interface are already occupied, an increasing number of surfactant monomers are present in the solution. To minimise their movement and interactions and, most importantly, to reduce the surfactant contact surface with solvent molecules, the system tends to stabilise itself by combining monomers into aggregates called micelles (1–3).
The concentration at which this is spontaneously achieved is characteristic for each surfactant, and is referred to as critical micelle concentration. Surfactants in micelles have their hydrophobic tails aligning with non-polar solvent (lipids) and their hydrophilic heads aligning with a polar solvent (water) (1–3, 5). With regard to cleansing the skin or the scalp and hair, where we aim to remove sebum and environmental dirt, surfactants can solubilise in micelles, which are later washed away by water (2–5).
SURFACTANTS AND THE SKIN: REACTIONS
In addition to this desired effect of providing clean skin, cosmetologists and dermatologists have also focused their attention in recent years on the undeniable adverse skin effects of surfactants. The use of hair and body wash cosmetic products and household detergents is actually considered one of the primary causes of skin irritation. But that fact may not be so surprising if we simply consider how many times each day our skin is exposed to them. Surfactants can be adsorbed on the skin surface, interact with both proteins and lipids in the stratum corneum, and if they penetrate deeper, they may even damage the skin layers below (1, 3–5).
Surfactants primarily exhibit their unwanted effects in the outermost skin layer, the stratum corneum. They can bind to stratum corneum proteins, such as keratins, which can result in protein denaturation, leading to swelling of the stratum corneum and the accelerated washing away of proteins from the skin. Consequently, chemicals and pathogens from the environment may penetrate deeper into the skin more easily, causing immune responses, seen as itching and red patches. Swollen proteins also bind less water, so the skin becomes less moisturised and less flexible (1, 3).
Historically, the concentration of surfactant monomers in solutions was believed to be the greatest factor of surfactant-caused skin irritation. This was the result of a commonly recognised model of interactions between surfactants and stratum corneum constituents, known as a ‘surfactant monomer skin penetration model’, where only small-sized monomers were thought to be able to penetrate into this skin layer and interact with proteins. It was believed that after the critical micelle concentration of a surfactant in a solution was reached and the micelles were assembled, the severity of surfactant irritation was reduced (1, 5).
However, studies have shown that even the application of products in which the critical micelle concentration was exceeded caused skin irritation and that the negative effect increases with a higher surfactant concentration in a solution (even though the monomer concentration is constant above the critical micelle concentration). Because micelles are unstable aggregates, they are able to disintegrate into monomers following contact with the skin, after which small micelles are formed. All of this led to the conclusion that both monomers and micelles have the ability to cause protein denaturation. Later research also linked surfactants that form smaller micelles to an increased skin irritation effect (1, 5).
Another investigated mechanism of skin irritation caused by surfactants was their effect on lipids in the intercellular matrix of the stratum corneum. Again, both surfactants in monomer or micelle form can exhibit adverse effects. Surfactants at or above the micelle-forming concentration have the ability to solubilise stratum corneum lipids into micelles and wash them away with water, just as is desirable with sebum, sweat and dirt on the skin surface. This leads to stratum corneum (uneven) delipidation, resulting in the weakening of skin barrier function, seen in increased transepidermal water loss, excessive skin stiffness, dry skin, cracking and erythema (1, 3).
Some studies have also shown the ability of monomers to become embedded into the intercellular matrix of the stratum corneum, which increases permeability and enables monomers to penetrate even deeper into the skin, intensifying irritation. Embedded monomers also damage the enzymes responsible for production of intercellular matrix components (1, 5).
The skin barrier function is impaired due to the two above-described processes. Damaging agents are thus able to penetrate easily into deeper epidermal skin layers, where they can damage keratinocytes. Surfactants can impair keratinocytic cytoplasm and proteins, which can both lead to permanent cell damage or even cell death (1, 3).
The presence of surfactants can also cause the release of inflammatory mediators from cells, inducing dermal inflammatory reactions. The most common reaction is irritant contact dermatitis, where the inflammatory process is only present at the site of contact with the irritant, resulting in red skin, scaling, erythema and sensations of pain, discomfort and itching. Another inflammatory reaction is allergic contact dermatitis, where the immune response recognises a surfactant as an allergen. This results in an inflammatory response on the entire surface of the skin, causing bullae, redness, oedema and itching (1, 3).
UNDERSTANDING REACTIONS: SURFACTANT CHEMISTRY
The irritation effect of surfactants is closely linked to their chemical structure. If a surfactant dissolves in water due to the formation of hydrogen bonds with water molecules, it is classified as non-ionic. Non-ionic surfactants form weaker bonds and are therefore less capable of causing skin irritation. If a surfactant undergoes electrolytic dissociation in water, it is categorised as ionic. Ionic surfactants are further classified as anionic or cationic, and if a surfactant carries both a negative and positive charge, depending on the pH, it is called amphoteric. Due to strong electrostatic interactions with the skin’s proteins and lipids, ionic surfactants (the most well-known example being sodium lauryl sulphate) are primarily attributed to the aforementioned adverse interactions (1–3).
The second parameter important for affecting the barrier function is the length of alkyl chain. An increase in skin irritation, shown by measurements of transepidermal water loss, was detected when the length of alkyl chains is increased, but only up to C12 (again, the characteristic of sodium lauryl sulphate), and then decreased with C14 and C16 surfactants. This is due to an increase in the surfactant’s lipophilicity, which in turn decreases the affinity to stratum corneum proteins (1–3).
Thirdly, irritation potential also depends on the size of a surfactant’s polar part with a given alkyl chain length. Larger polar parts cause less damage and irritation due to a reduced negative charge density, thus producing weaker electrostatic interactions with proteins (1, 6).
IN THE SPOTLIGHT: HOW TO MINIMISE NEGATIVE EFFECTS
As explained, surfactants provide effective cleansing, but are also able to remove skin structural components and damage the barrier function, causing subclinical or clinical skin conditions. It is therefore of the utmost importance to select cleansing products with ingredients that respect our skin.
New generations of surfactants, which also possess better characteristics in terms of sustainability and biodegradation, are becoming increasingly available and used, e.g. alkyl (poly)glucosides and amino-acid based surfactants, such as alkyl glutamates, glycinates, alaninates and sarcosinates (6, 7). They also offer innovative formulating opportunities, such as so-called waterless and water-efficient cosmetics (8).
Nevertheless, keep in mind that the vast majority of cleansing products and household detergents on the market still contain sodium lauryl sulphate, which has been proven to be one of the most irritating surfactants. Due to its superior solubilisation power and strong effect on cell components, it is often used in biochemical gene research, as well, as a cell lysis reagent and in dermal studies as an irritation agent (3–5). Yet, we continue to apply massive amounts to our skin!
In order to formulate a cleansing product with less irritation potential, conscious formulators try to reduce the critical micelle concentration of the overall surfactant system, which leads to less monomers in the solution, and an increase in micelle size and improved stability. This could be achieved by selecting surfactants that have proven to cause less irritation (such as non-ionic surfactants or surfactants with larger polar heads), or by adding cosurfactant(s) to the system (1, 5).
Because the concept of the CosmEthically ACTIVE certificate focuses on evidence-based activity and skin compatibility, we do not permit the most irritating surfactants, such as sodium lauryl sulphate, ammonium lauryl sulphate and sodium coco sulphate. The overall composition of every CosmEthically ACTIVE certified cosmetic product must follow parameters of both the skin’s and hair’s natural physiology. We therefore attribute great importance to the pH value of formulations, as well. The appropriate pH level of the skin surface is crucial for an undisturbed skin barrier function, and the altering of that function disrupts the normal synthesis of enzymes responsible for the production of intercellular matrix components and their activity, and causes the dysbiosis of the skin’s microbiota. For this reason, we also do not permit the use of solid soaps obtained by saponification due to the high pH they leave after application to the skin during washing. To guarantee that CosmEthically ACTIVE certified products meet the highest level of environmental protection, surfactants obtained by alkoxylation, such as cocamidopropyl betaine and cocoamphoacetate, are also not permitted (9).
An increase in consumer awareness has spiked the demand for natural cosmetics and natural surfactants. Indeed, where there is demand, there is always supply: an increasing number of products available on the market contain natural surfactants with less potential to provoke adverse interactions with the skin’s components, which is of the utmost importance given the frequency of their use. New generations of surfactants of natural origin are both skin- and environment-friendly, and demonstrate excellent dermal compatibility and biodegradation.
Anja Presern, M. Pharm.
Modern CosmEthics, Velenje, Slovenia
Prof. Nina Kocevar Glavac, PhD., M. Pharm.
University of Ljubljana, Faculty of Pharmacy, Ljubljana, Slovenia
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