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surfactant definition

Article Summary

  • Explaining surfactants on a molecular level
  • The four types of surfactants: anionic, cationic, amphoteric, and non-ionic
  • The three types of cleansers: soap, synthetic, and lipid-free

Let’s learn about cleansers and surfactants!

Starting with the Basics

Cleansers work via a class of ingredients called surfactants (that stands for surface active agents).

Surfactants are long, two-headed molecules. One head is hydrophilic, meaning it’s drawn to water, and the other end is lipophilic, or drawn to oil. As you know, water and oil don’t play nice, but cleansing means we need them to so we can clean off grime and rinse it away. This two-headed molecule serves as a bridge between water and oil: one side binds to a water molecule, the other side bonds to an oil molecule, and presto change-o, water and oil are suddenly friends.

Sometimes, surfactants are used as detergents (bubbly, foaming, cleansing-focused ingredients), and sometimes they’re used as emulsifiers (non-foaming ingredients that make oil and water play nice within a formulation – for example, creams and lotions, or oil cleansers that rinse off cleanly with water).

Types of Surfactants

There are four types of surfactants: anioniccationicamphoteric, and non-ionic.

According to this study, “the order of surfactant potential for SC [stratum corneum] alteration and skin irritation is cationic=anionic>amphoteric>nonionic.” Translated, that means cationic and anionic are the most likely to irritate your skin and disrupt your moisture barrier; amphoteric is less likely; and non-ionic is the least likely. Of course, this varies widely surfactant-by-surfactant, but it’s a good general rule of thumb.


As you may remember from high school chem, an anion is a negatively-charged molecule (with more electrons than protons). Anionic surfactants have a strong negative charge, which makes them extremely effective cleansers, but they can also be harsh.
The most common anionic surfactant is soap. Others you might recognize are sodium lauryl sulfate (SLS) and sodium laureth sulfate (SLES). Speaking generally, anionic surfactants lather well and make lots of foam, and also have a higher incidence of irritation.
Fun fact: you know that tight feeling your skin can get after cleansing? There are three reasons for that:
  1. Your skin has been stripped of its moisturizing oils.
  2. Your cleanser is harsh enough that it’s disrupted your moisture barrier and there’s a sudden evaporation of water from within your skin.
  3. Surprisingly, the biggest factor is that the surfactant in your cleanser has been left behind and is irritating your skin – SLS is far and away the biggest culprit for this.
Boo, SLS.



The opposite of anionic, cationic surfactants have a positive charge. (For all of these surfactants, the charge is held on the hydrophilic side; the lipophilic side doesn’t have a charge.)
Cationic detergents are very effective but extremely harsh, so you see them more in household cleaners than facial cleansers. Common ingredients are benzalkonium chloride and cetrimonium bromide. Cationic surfactants do have antimicrobial properties, so you’ll sometimes see them in medical or prescription cleansers.
Cationic emulsifiers are much more common in beauty products. They’re a fundamental part of hair conditioners – they bond to your negatively-charged hair follicle, and they’re why your hair stays soft even after you rinse out the conditioner. They appear in skincare products, too – you might see them as anything with the suffix “-quat”.



Amphoteric surfactants have both positive and negative charge – their final charge depends on the pH they’re in.
These guys are milder and less irritating than the single-charge surfactant types above, so they’re popular in facial cleansers and gentle shampoos. The downside is that they foam less. Common amphoteric surfactants are cocoamidpropyl betaine and sodium cocoamphoacetate.



Some surfactants display no charge whatsoever, and we call those non-ionic. Similar to cationic, non-ionic detergents are often very harsh and we rarely see them in skincare.
Where we see them all over the place is as emulsifiers: the polysorbates, the sorbitans, the PEGs, and the laureth-[number]s, to name a few. The vast majority of emulsifiers are non-ionic, so we see these all the time.


Midpoint Summary

Let’s refresh. We’ve got four types of surfactants: anionic, cationic, amphoteric, and non-ionic. In terms of foaming detergents, we really only see anionic (more harsh) and amphoteric (less harsh) in facial cleansers; for emulsifiers, we see cationic and non-ionic all over the place.

Types of Cleansers

Now that we know what might be going into our cleansers, let’s take a look at the three major types of cleansers on the market.

Soap and Soap-Based Cleansers

Soap is one of the oldest personal care products in human history. It’s made by combining a plant or animal oil with a highly alkaline ingredient like lye (sodium hydroxide). That alkaline ingredient is crucial to the soap-making process, and it’s why cleansers tend to have high pHs – around 9.0-10.5. Since your skin is naturally slightly acidic, high pH products can disrupt your skin's healthy function and create a whole host of issues.
Soaps foam up well, as we’ve all experienced, and they strip off every bit of oil and grease they come in contact with. That’s good for dishes, but not as good for your delicate facial skin, which needs the protective, moisturizing oil it produces. Soaps can also strip away your beneficial flora and wreak general havoc on your skin.
Soaps have a lot of strikes against them. I award them zero points as a facial cleanser.


Synthetic Cleansers

Synthetic cleansers (also called synthetic detergents, or syndets) mix anionic and/or amphoteric surfactants with a number of other mild ingredients to create a gentler cleanser than soap can offer.
In general, these are formulated to have a low pH (between 5 and 7). Usually they don’t foam up as much as soap-based cleansers, but they’re much, much better for your skin. They leave more of your skin’s beneficial oil and flora in place, and penetrate your skin less to disrupt its natural structure. These types of cleansers are ideal for people with normal to oily skin.


Lipid-Free Liquid Cleansers

Also known as soap-free liquid cleansers, these products don’t foam at all. Our whole lives, we’re told that it’s the foam that gets us clean, so it can feel like these aren’t doing their job. Then again, we’re also told that oil is bad for our skin and that tight, squeaky-clean feeling is good, so sometimes common wisdom is exceptionally wrong.
Liquid cleansers rely on emulsifier-type surfactants rather than their detergent brethren. Remember from the beginning of the post that emulsifiers are those quiet, peaceful ingredients that allow oil and water to mix in things like lotions and creams. In this case, those emulsifiers are mixing the oil on your face with the water you’re cleansing with – same as detergents, except these don’t foam at all and are much, much gentler.
These are great for people with dry or sensitive skin. Dry skin underproduces oil, so needs to hang on to as much of it as possible, and liquid cleansers remove much less of that oil than their foaming compatriots. Liquid cleansers also have a lower risk of irritation and keep your moisture barrier in tip-top shape – perfect for sensitive-skin folks or people with rosacea.

So What Do I Do With This Information?

Cleansers are the hardest product to infer anything about from their ingredients lists. There are hundreds and hundreds of types of surfactants, and they all behave differently. Manufacturers rarely announce the pH of their products. Formulation is incredibly important in cleansers – even the right ingredients can destroy your moisture barrier if formulated poorly.