Most of the hair care products we use (as well as skin care products, for that matter) are mixtures of components that don’t normally mix (are immiscible). These products are mixtures of various types of oils and water and other ingredients whose jobs range from making the oils compatible with the water to providing functions such as preserving the solution, thickening the mixture or adding color, scent or conditioning.
Oil and water don’t mix, right? We learn this as children when our parents show us how the oil floats on top of puddles on the ground and makes colorful patterns. The reason the two don’t mix is a fairly simple one; they are inherently different in their molecular makeup. Water is a polar molecule (meaning it has a side that is definitely more “negative” and one that is definitely more “positive”). In contrast, oils are non-polar, meaning they have no distinctive “positive” or “negative” portions.
This presents a unique challenge to the personal care product formulator. Her goal is to prepare a product that will be a predominantly aqueous (water-based) solution, but that contains all these different ingredients that add various benefits to her final product. This requires her to prepare mixtures of oils and water that are known as “emulsions”.
An emulsion is defined as a mixture of two normally unmixable liquids (e.g., oil and water) in which one is suspended or dispersed in the other (one exists as tiny particles within the other). Most typically, in shampoos and hair conditioners, the oil is the “dispersed phase,” meaning it exists as a suspension of fine droplets in the primarily aqueous solution. These droplets are not usually stable over time, and will eventually all come together creating a phase-separated solution (typically with the oil on top and the water layer on the bottom). For this reason and also to increase the performance of the product, formulators must use a variety of surfactants and “emulsion stabilizers,” in order to make these suspensions of oil-in-water last longer.
Most surfactants and emulsions stabilizers have in common the trait of having one distinct portion of the molecule that is polar and hydrophilic (water-loving) and one portion that is non-polar and hydrophobic (water-fearing). This allows them to interact with both the water phase and the dispersed phase. They anchor themselves along the outside of an oil droplet, and the non-polar portion can dissolve in the nonpolar or oily dispersed phase, while the polar portion is dissolved in the water. This acts as a sort of anchor mechanism to hold the molecule on the oil droplet. The outer polar portion of the droplet acts to prevent other droplets from getting too close due to electronic repulsion.
The dual nature of these surfactant molecules is also how detergency works, as many of these molecules form what is known as “micelles”. These micelles are clusters of molecules with an oily center or core made up of the nonpolar tail, surrounded by a shell formed by the polar portion of the molecule. These micelles can absorb oils from your skin, hair, or clothes, and hold them inside until they are rinsed out in the washing process.
Not only does the science of mixing oil and water have to be considered when making a hair care products, but also the formulator must make certain that her product is the right consistency (no one wants a runny conditioner that drips out of her hands before she can put it on her hair), the “right” color, smells pleasant, resists formation of molds or other bacterial growth. Other factors that may be pertinent are that the product provides sun protection (for color-treated hair especially), gives excellent conditioning benefits, smoothes frizz, adds body, offers a “pearlescent” appearance, gives hold to a hairstyle, etc.. The list of variables and desired effects of products are endless. As a result, there are literally thousands of ingredients approved for use in hair care products. They provide many different functions, and often perform multiple tasks in the same formula.
We've put together a list of the common choices formulators have for their products, and so you can refer to this list to find out what a particular ingredient may be doing in the hair care product you may have recently purchased or are considering purchasing. The list, a dynamic document will will amend/update as necessary, is by no means comprehensive, but it is a beginning. Soon, we will have a list of herbal and natural ingredients found in hair care products.