There are a crazy number of surfactants on the market. They can be grouped into four main categories:
This is how each group differs from the next:
Anionic surfactants, e.g. Sodium cocoyl isethionate (SCI), are negatively charged. They foam or lather best in hard / alkaline water or in neutral aqueous solutions. They produce a lot of foam and have very good cleaning power but they can also be quite irritating to skin if used without being combined with other milder surfactants. Most commercial detergents contain anionic surfactants.
Non-ionic surfactants, e.g. coco glucoside, decyl glucoside and lauryl glucoside are electrically neutral. Due to their mildness they are frequently used for products targeted towards babies. However, they don’t lather well. Non-ionic surfactants are not sensitive to the hardness of water.
Cationic surfactants, e.g. honeyquat and polyquats, are positively charged. They are commonly used in hair care products because they have conditioning and anti-static properties. Cationic surfactants can be added to conditioners.
Extra knowledge: due to their conditioning property cationic surfactants are also frequently found in detergents that claim to soften fabric.
Amphoteric surfactants, e.g. cocamidopropyl betaine, can be negatively or positively charged depending on the pH of the solution they are in. Amphoteric surfactants are great for reducing skin irritation, boosting foam and improving the conditioning capability of a shampoo. They produce less foam than anionic surfactants.
Besides being different in form, some are liquids, others are granular or needle-like and yet others are fine powders, they also vary in strength.
Which surfactant should you use?
A combination of two or more types of surfactant are usually used in one formulation because they have different properties. For instance, an anionic surfactant is an effective foam booster but it will tend to irritate skin. To counteract this an amphoteric surfactant is needed. To increase the conditioning property of a shampoo, a cationic surfactant is added. If you go to a shop and look at any shampoo bottle you will be able to confirm that there are always several surfactants in a shampoo.
Primary vs. Secondary vs. Co Surfactants
Primary surfactant can be used as the only surfactant in a shampoo. Secondary surfactants are added to boost a particular quality of the detergent, e.g. its conditioning properties or to create more lather or to reduce skin irritability. Co surfactants can be used on their own but benefit from being used in collaboration with other surfactants.
How about sulfates?
Sulfates are by far the most common surfactants used in shampoos. They are the strongest, i.e. most cleansing but also most irritating of all surfactants. As sulfates tend to dry out hair they are not recommended for products designed for drier kinky, curly or coily hair. They are okay for products targeting more greasy hair textures especially straight and wavy hair.
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You might also like:
Buffalo (Chemistry of Shampoo), ChemistryViews.org (Shampoo Science), The Science of Black Hair by Audrey Davis Sivasothy, SwiftCraftMonkey (Surfactants A Short Guide), (Intro to surfactants), GreenPeople (ALS vs. SLS)
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By Heather Katsonga-Woodward