Some years ago I co-edited a book about the different types of conditioning agents that are used by cosmetic chemists for hair and skin products. You can still get the book through (Conditioning Agents for Hair and Skin) and it still has a wealth of excellent information. There’s even a chapter by the famous dermatological researcher Peter Elias.
However, this is such basic information that I thought it would be helpful to have a general blog post on the topic of the book, conditioning agents for cosmetic products. Here are the 7 classes of conditioning agents used in cosmetic formulations.
The classic example of this type of ingredient is Petrolatum and in the book there is an entire chapter devoted to it. You’ll find the evidence that when it comes to skin moisturization, few (if any) ingredients are better than petrolatum. The way occlusives work is that the produce a film on the skin that substantially blocks the evaporation of water from the skin surface. This causes water to build up in the stratum corneum and subsequently makes skin feel moisturized. Dimethicone is another ingredient that can be considered an occlusive. Both dimethicone and petrolatum are used as hair conditioners but they work more like emollients/oils for that application.
Humectants are materials that can attract and bind water. The property is known as hygroscopicity. The humectants used in cosmetics are typically water soluble so they don’t work so well for products that are rinsed off. In skin, they have the ability to rehydrate so work well in lotions. In hair, they can attract moisture so they are good for leave-in styling products. Examples include glycerin, propylene glycol, and sorbitol.
Emollients & oils
These ingredients are designed to lubricate biological surfaces and in this way they condition. There are a wide variety of these types of ingredients including triglycerides, natural oils, lanolin, and synthetic esters. They work best in skin products to change the feel of an applied product however, they can also be used to increase flexibility in hair. The downside is that they can weigh hair down and make it feel greasy.
Proteins are a complicated polymer made up of a series of amino acid monomers. When present in living things, proteins can have complicated structures. But when used in cosmetic products, the proteins are typically denatured and have less complicated structures. At that point, it becomes more important what amino acids make up the protein than the specific sequence of the amino acids. Proteins conditions in the same way as humectants or as emollients depending on their water solubility and hygroscopicity. They are not a major conditioning agent and are used as secondary conditioners that support marketing stories.
Silicones are ultimately derived from sand and are some of the best ingredients for providing slickness and shine. There are a wide variety of silicones including things like dimethicone, cyclomethicone, and amodimethicone. They work in the same way that emollients work in that they are a non-polar molecule that stays near the surface. This property can have the drawback of weighing hair down or making a lotion feel too greasy.
Surfactants are versatile compounds in cosmetic products so it is no wonder that they find use as conditioning agents. They are not used so much in skin products as they can be irritating, but they are one of the primary ingredients in rinse-off hair conditioners. The thing that makes them so valuable is that they can be emulsified but will remain on the hair even during rinse-off due to electrostatic interactions between the positively charged cationic surfactants and the negatively charged damaged protein sites on the hair. Some common examples include Cetrimonium Chloride and Stearalkonium Chloride.
Polymers make up such a large class of conditioning agents that it is difficult to make general statements about them. The most common polymeric conditioning agents are used in hair care and are cationic polymers or Polyquaterniums. These can deposit on hair via electrostatic interactions or the dilution/deposition method where they become insoluble as the product gets more and more dilute. Other polymers like lauryl methyl gluceth-10 hydroxypropyl dimonium chloride are humectants.
As a cosmetic chemist, improving the condition of the biological surfaces that your products interact with will be a primary goal. Knowing the basic conditioning agents for hair and skin will get you a long way to achieving that goal. It will also take you a long time and a lot of experimentation to find just the right conditioning ingredient for your application.