An introduction to cosmetic technology
By Perry Romanowski
Surfactants and lipids are important ingredients in cosmetics. This article describes:
- their key properties and functions in cosmetics,
- the four types of surfactants used in cosmetics, and
- some common applications for surfactants and lipids in cosmetics.
Cosmetic science is not a subject typically covered in an undergraduate chemistry program, but it is the basis of an entire industry in which many chemistry graduates find themselves working. This review will introduce the basic technology and ingredients used by cosmetic formulators to create functional products.
The definitions for cosmetics differ slightly around the world, but they are basically any substance or mixture that is intended to be applied to the external parts of the human body or the teeth and mucous membranes for the purposes of improving the appearance. In the United States, cosmetics are regulated by the Food and Drug Administration, which defines cosmetics as “articles intended to be rubbed, poured, sprinkled, or sprayed on, introduced into, or otherwise applied to the human body . . . for cleansing, beautifying, promoting attractiveness, or altering the appearance” [Food, Drug and Cosmetic Act, Sec. 201(i)].
Ingredients in cosmetics
There are a number of ways to discuss the ingredients used to make cosmetics, but focusing on their primary purpose for being in the formula makes the most sense. This isn't perfect, since some ingredients have multiple functions, but it provides a good framework for the discussion. Basically, there are three reasons to use an ingredient in a formula: functionality, aesthetics, and marketing.
Functional ingredients provide the primary benefit to consumers who use the product. These are the ingredients that clean the skin, condition hair, and provide moisturization or even color. They are also the so-called active ingredients that make over-the-counter (OTC) drugs work.
The problem with many functional ingredients is that they do not feel nice or apply well on their own. They may even smell bad. Aesthetic ingredients are technologies that make formulas look and feel better; they improve the overall experience of using cosmetics.
Marketing ingredients are ingredients that are not expected to do anything in the formulation but are added specifically to help support the marketing story. Such ingredients are important, since they often provide the entire reason for purchasing a product, but they are not actually expected to have a significant impact on the formula’s performance. For example, consumers prefer to buy moisturizing products with aloe vera rather than petrolatum—even though the latter is the ingredient that actually provides the benefit. Marketing ingredients are not the focus of this review, but it is important to know how they are used.
The cosmetic ingredients that have the most impact on the way cosmetics work are the functional ingredients. These include:
- polymers, and
- active ingredients.
This review will focus on the first two categories, which include surfactants as well as lipids and oils.
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Surfactants and their applications in cosmetics
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.
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.
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 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.
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.
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 counterion 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 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 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 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.
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.
That being said, cationics are great for conditioning. They are substantive during use and are the primary ingredients for rinse-off hair conditioners.
Conditioning and moisturizing ingredients are materials that improve the feel or condition of whatever surface they are put on, including the skin or hair. They typically have an “oily” nature and , to be effective, they must also be substantive or left behind on the surface in some way. Materials that are easily washed away do not make good conditioning or moisturizing ingredients.
There are a wide range of different types of conditioning and moisturizing ingredients but the most common include: cationic surfactants, or quats; occlusive; emollients; and humectants.
Cationic surfactants used for conditioning are also known as quaternary ammonium compounds, quaternized compounds, or just simply “quats.” They contain at least one nitrogen atom bonded to four other hydrocarbon groups. Some common examples include stearalkonium chloride, dicetyldimonium chloride, and behentrimonium chloride.
Although cationic surfactants don't make good cleansing products, they are excellent conditioning ingredients, particularly for hair care products. The main reason is because they are substantive to the damaged, negatively charged protein sites on hair and skin. When a quat is put on hair or skin, the positive portion of the molecule is attracted to the negatively charged damaged site creating an electrostatic bond. While water rinses most things away, the cationic surfactant remains.
Since these are hydrocarbon molecules, the longer the hydrocarbon chain is, the more conditioning they will do. So, all things being equal, a material such as behentrimonium chloride, which is a C22 carbon chain, will be more conditioning than a shorter chain molecule such as cetrimonium chloride, which is a C16 carbon chain.
Perhaps the most effective skin moisturizing ingredients are occlusive agents. These are oily materials that can create a thin coating on the skin or hair. The most common types of occlusive agents used in cosmetics include petrolatum, mineral oil, and dimethicone.
When occlusive agents are put on the skin (or hair) they form a thin, continuous film on the surface. This film is flexible but feels slightly greasy to the touch, which is why only a small amount is used in formulations. The occlusive film is also resistant to water, which helps explain its moisturizing effect.
The body naturally loses water through skin. When atmospheric humidity is low, more moisture is lost and skin feels dryer. Occlusive agents create a film on the skin which slows the water loss. As water tries to leave the body, it hits this barrier and starts to accumulate in the outer layers of the skin (the epidermis). This extra moisture improves the way the skin looks and feels and also can reduce itching and redness. Occlusives such as petrolatum are so effective they can actually be used as OTC drugs for skin protection.
Emollients were some of the first ingredients used as cosmetics. They are not typically compatible with water and include ingredients such as oils, butters, waxes, and esters. Emollients are similar to occlusive agents, except that they tend to be lower-molecular-weight molecules and don't have the ability to form a continuous film to block water. But, emollients are important ingredients for improving the way the surface of the hair and skin feel, and they impart shine, which is the primary reason they are used.
When creating skin creams, lotions, and even hair products, emollients are used to modify the way formulations feel, how they rub into the skin, the ease at which they spread, and the length of time they remain “workable.”
Common examples of emollients include natural oils such as coconut oil, argan oil, almond oil, or olive oil. There are also a number of excellent esters that are emollients, including myristyl myristate, cetyl palmitate, and lauryl laurate. Each of these feels slightly different and has different abilities to absorb into the skin. In addition, many silicones make excellent emollients, as they provide great slickness and shine.
Humectants have been used as cosmetic compounds for as long as cosmetics have existed. They include natural materials such as honey, aloe, or glycerin, and are typically mild. The property that makes humectants useful is their ability to attract and hold water like a sponge. In fact, glycerin can hold as much as three times its weight in water. Such humectants are especially useful for surfaces that tend to dry out, such as skin or hair.
The most common humectants used in cosmetics include glycerin, propylene glycol, sorbitol, sodium pca, hyaluronic acid, and various hydrolyzed proteins.
Humectants have few significant negatives: They feel sticky and are easily rinsed away. This limits their use to cosmetic products that will be left on, making them great for skin lotions and leave-on hair conditioners but not so great for shampoos, body washes, or rinse-out hair conditioners.
There are numerous other technologies used to create cosmetic formulas, including colorants, preservatives, fragrances, polymers, and other active ingredients. You can find more information about the ingredients in cosmetics at http://chemistscorner.com.
Perry Romanowski is publisher of Cosmetic Chemists Corner, a website written for cosmetic chemists by cosmetic chemists. Romanowski has been formulating cosmetic products and inventing solutions to solve consumer problems since the early 1990’s. He has written and edited numerous articles and books, taught continuing education classes for industry scientists, and developed successful websites. More information on cosmetic ingredients can be found at http://chemistscorner.com.