Surfactants are perhaps the most important of all cosmetic ingredients. Surfactants have so many useful applications that the cosmetic industry probably wouldn’t exist without them. In cosmetics, surfactants are used for cleansing, foaming, thickening, emulsifying, solubilizing, penetration enhancement, antimicrobial effects, and other special effects.
The key property of surfactant molecules that makes them useful cosmetic ingredients is that they are compatible with both water and oil. When put in a water solution, they naturally create structures with lipophilic portions aligning with lipids and hydrophilic portions aligning with water. The exact structure that is produced depends on the concentration of the surfactant solutions but the most important structure for cosmetics is micelles.
Detergency
One of the most common applications of surfactants in cosmetics is for cleansing formulations. When skin and hair get dirty there are really two types of dirt: solid particulates and oily deposits. The oily deposits come from natural sebum which is produced in the hair follicles. Solid particulates are just naturally picked up from the environment. They remain on hair and skin via Van der Waals forces.
Although rinsing the surface with water can remove some of the dirt, oily deposits will tend to adhere to the more lipophilic surfaces of hair and skin. Surfactants in detergent help get rid of these oily deposits. The lipophilic ends of the molecules are attracted to and align with the lipids on the surface of hair and skin. Meanwhile, the hydrophilic ends of the molecules align toward the surface of these deposits, thereby increasing the hydrophilicity. That allows the lipid deposits to lift off the surface of skin or hair where the rinse water washes them away.
Wetting
Surfactants are also wetting agents that reduce the contact angle between a solution put on a surface and the surface. This property allows surfactants to spread more easily on the surface and inject themselves between the oily deposit and the skin or hair surface. This lifts up the oil and allows it to be removed. Wetting also makes the product easier to spread and prevents it from balling up on the surface. This is useful in cosmetic creams and lotions.
Foam
Foam is an important characteristic of cleansing cosmetics. It is formed when air is dispersed in a continuous liquid medium. The air bubbles are surrounded by thin layers of liquid, and the surfactants help stabilize the bubbles that are formed, creating foam. It’s important to note that foam doesn’t really contribute much to the removal of dirt but consumers like it, so it’s very important for a cleansing product to foam.
Thickening
In a water/surfactant solution in which water is the major ingredient, surfactants align themselves in structures called micelles. These are tiny spherical structures in which the lipophilic tails orient inwards and the polar heads orient outwards toward the water. Micelles are important for the creation of emulsions and for thickening.
The thickness of a surfactant solution depends on how closely the micelles pack together. Since cleaning products are typically made from charged surfactants, the outer surfaces of the micelles have a specific charge density that causes them to repel other micelles. The more distance between the micelles, the thinner the solution. When the surface charge density is lowered by adding salt, for example— the particles pack together more closely, and the solution thickens. For this reason, salt is frequently added to adjust the viscosity of detergent systems.
Emulsification
Another major application of surfactants to cosmetics is in the creation of semi-stable mixtures of oil and water, or emulsions. Emulsions are the creams and lotions that deliver beneficial lipid materials to the surface of skin and hair. They can be simple oil-in-water or water-in-oil emulsions or more complex multiple emulsions. Each type has benefits that make it ideal for certain cosmetic applications. An entire article can be written about emulsions, but for our purposes, suffice it to say that nearly all creams and lotions are created using surfactants.
Types of surfactants
Surfactants can be classified according to the charge of their counter ion or whether they form ions in solution or not. There are anionic surfactants, which have a negatively charged ion. There are amphoteric surfactants, which are capable of both positive and negative charges depending on the pH conditions of the solution they are in. There are cationic surfactants, which are positively charged. And, finally, there are nonionic surfactants, which have no charge at all. All four of these surfactant types are used in cosmetics for different reasons.
Anionic
Anionic surfactants, the most common of which are the alkyl sulfates, are really the primary ingredient used in cleansing products. They are positively charged surfactant ions. Examples include sodium lauryl sulfate and ammonium lauryl sulfate (ALS).
Sometimes anionic surfactants are modified to make them less irritating. For example, ALS is commonly “ethoxylated” by reacting it with ethylene oxide to produce ammonium laureth sulfate. This additional chemical processing makes the final product significantly less irritating and slightly more water soluble.
Nowadays there is a tendency for companies to get away from using ingredients with the name “sulfate” in them, so other options are used. Other anionic surfactants include sulfosuccinates, alkyl benzene sulfanate, acyl methyl taurates, acyl sarcocinates, the isethionates, propyl peptide condensates, monoglyceride sulfates and fatty glycerol, ether sulfanates. These are all anionic surfactants that have been used in shampoos or body washes.
Anionics are used primarily as the main detergent in cosmetics because they are good at removing dirt and oil, they produce pleasing amounts of foam, and they are relatively inexpensive. Their primary drawback is that they can be irritating. This is why they are often blended with amphoteric surfactants.
Amphoteric
Amphoteric surfactants can have both a negative charge and a positive charge, depending on the pH. These materials are also referred to as zwitterionic materials, and they include ingredients such as cocamidopropyl betaine, cocoamphopropionate, and sodium lauraminopropionate. These three ingredients are probably the most commonly used amphoteric surfactants in cleansing products, particularly in shampoos.
Amphoterics are used because they have good detergency and are less irritating than the anionics. They also can help thicken a formula and have a positive effect on foam, as they make the bubbles smaller and feel creamier. The main drawback to using them is that they are significantly more expensive and, on their own, don’t really foam well enough to produce a good shampoo.
Nonionic
Nonionic surfactants are molecules that do not have a charge. When placed in a solution of water, the molecules do not dissociate as the previously mentioned surfactant molecules do. Salt also has no effect on whether these chargeless surfactants thicken or not. Some types include fatty alcohols and fatty alkanolamides, including lauramide diethanolamine (DEA) and cocamide DEA. Other nonionic surfactants found in cosmetics include amine oxides such as lauramine oxide or stearamine oxide.
There are a variety of reasons to use nonionics in cosmetics. They are good foam enhancers (when used with anionics) and can reduce irritation. They also can thicken systems and provide a conditioning effect. Additionally, they are very good for solubilizing fragrances and other natural oils in formulating. Finally, gentle cleansers such as baby shampoos are based on nonionics, the most common of which is PEG-80 sorbitan laurate. Nonionic surfactants are also the primary surfactants used to create emulsions.
The reason these surfactants aren’t used as the primary cleansing surfactant in most formulas is that they don’t foam nearly as well on their own and are significantly more expensive. Overall, nonionics do not work as well as anionics in shampoos.
Cationics
These are positively charged surfactant molecules. They are not used for cleansing formulas because they don’t clean, rinse, or foam as well, and they are more irritating—so they have a lot of drawbacks. They are also not compatible with anionics, so their positive benefits can’t be obtained from formulations that also contain an anionic surfactant.
Source:
AOCS. Perry Romanowski. April 2015. https://www.aocs.org/resource/an-introduction-to-cosmetic-technology/