Generally speaking, the “potency” of surfactants (in decreasing order) go something like this: anionic = cationic > amphoteric > nonionic. Anionic surfactants represent the most populated and varied group of surfactants. They are varied in the sense that they exhibit widely different degrees of potency. For example, sulfates and sulfonates are significantly more potent than isethionates. Newer amino-acid based surfactants such as the gluatames, sarcosinates, and glycinates are more gentle.
Anionic surfactants are used as emulsifiers in about 75% of all lotions. An anion is a negatively charged ion that is “surface active.” Each end of a soap molecule has a specific function; the hydrophilic “head” bonds with water, the hydrophobic “tail” bonds with dirt. The two ends work together to lift and carry dirt away.
These detergents usually consist of an alkali salt as soap, or ammonium salt of a strong acid. Can be irritating to the skin, depending on alkalinity. Whether or not it is irritating to the skin and/or eyes depends on the compound. Sodium laureth sulfate, for example, is very irritating, whereas triethanolamine (TEA) coco hydrolyzed animal protein is the least irritating.
Typical examples of anionic surfactants
alkyl benzene sulfonates
glycerol ether sulfonates
α-methyl ester sulfonates
fatty alcohol ether sulfates
glycerol ether sulfates
fatty acid ether sulfates
hydroxy mixed ether sulfates
monoglyceride (ether) sulfates
fatty acid amide (ether) sulfates
mono- and dialkyl sulfosuccinates
mono- and dialkyl sulfosuccinamates
ether carboxylic acids and salts thereof
fatty acid isethionates
fatty acid sarcosinates
fatty acid taurides
N-acylamino acids such as, for example, acyl lactylates, acyl tartrates, acyl glutamates and acyl aspartates
alkyl oligoglucoside sulfates
protein fatty acid condensates (particularly wheat-based vegetable products)
and alkyl (ether) phosphates
In cosmetics and skincare, the most commonly used anionic surfactants are: carboxylic acids, sulfates, and sulfonic acids. There are also acylated amino acids and acyl peptides, and phosphoric acid derivatives.
Acylated amino acids and Acyl peptides
The acyl sarcosinates are a special niche in cosmetics. They behave like soaps and the acylated amino acids generally are modest foamers and known to be mild. Downside: subject to microbial attack and preservation against spoilage is a problem in peptide-derived products.
Anionic surfactants like stearic acid are useful for creating stick products like deodorants and antiperspirants. The salt version (sodium stearate, which gels upon cooling) makes a typical bar of soap, and is used to harden soaps made with vegetable oil. The carboxylic acids are: alkanoic acids, ester & ether functional carboxylic acids.
These include the “harsher” surfactants: Sodium Laureth, Myreth, Lauryl Sulfate, Ammonium Lauryl and Laureth Sulfate, Sodium C14-16 Olefin Sulfonate, TEA Lauryl Sulfate, TEA-dodecylbenzenesulfonate, Sodium Alkylbenzene Sulfonate, Ammonium or Sodium Xylenesulfonate.
They are excellent foamers, cleansing agents, and are cheap, in comparison to others. The downside is that they can be irritating and drying and many people avoid them.
An Important Note on Sulfates
It’s important to remember though, that just because a cleanser has sulfates in it doesn’t mean it’ll be drying and irritating. Take note of the other ingredients in the product (like anti-irritants, emollients, moisturizers) and where on the ingredient list the sulfate surfactant is listed. The other thing to consider is the pH of the cleanser.
Here are some gentle, mild surfactants: Sodium Cocyl Isethionate, Sodium Lauryl Sulfoacetate, Sodium Socoyl (or lauryl/lauroyl) Sarcosinate, Ethyl PEG-15 Cocamine Sulfate, Dioctyl Sodium Sulfosuccinate, Sodium Lauryl Glucose Carboxylate, Methyl Cocoyl or Lauryl Taurate, Sodium Cocoyl Glycinate
Sulfonic acid surfactants
Generally more mild than sulfates. They include Taurates (derived from taurine & good foamers), Isethionates (derived from isethionic acid & known to be very mild), Alkylaryl Sulfonates, Olefin sulfonates, and Sulfosuccinates. These are more expensive to produce so they’re not used as often (and don’t provide enough of a benefit or reason to replace using sulfates). Sulfuric acid derivatives include: alkyl sulfates and alkyl ether sulfates.
The salts of alkylaryl sulfonates foam and cleanse well. They’re not usually used on their own and are blended with moisturizing ingredients, because they tend to leave the skin feeling stripped.
Sulfosuccinates are salts of strong acids and generally don’t produce stable foams but don’t interfere with the foaming ability of other surfactants they may be used with. Relatively speaking, they’re mild and can be used to reduce irritation of other surfactants.
Alkyl sulfates foam easily but requires stabilization (like with a foam booster). They’re considered to be somewhat irritating and have been claimed to strip the skin, so experimentation with the formula is necessary to combat this effect. They make great suspending agents, emulsifiers and solubilizers. In products meant to be left on (like moisturizers, for example) they should be used at concentrations of 0.5%-1%.