What's the difference between soap and surfactants?

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What's the difference between soap and surfactants?

Definition  |  Cleansers  |  Surfactants  |  Benefits


Written by the QV science community.

Updated on June 30, 2023

Those of us with dry or sensitive skin are often advised to use a soap-free cleanser and avoid soap. But why? What exactly are soap-free alternatives, and how do they work if they don’t have soap in them?

Definition

What is soap?

Soaps have been around for thousands of years, traded by the Phoenicians as early as 600 B.C., and popular with the ancient Romans and Celts1. Chemically speaking, soaps are the salts of fatty acids, made by treating vegetable and animal fats with lye, also known as sodium or potassium hydroxide2. This process is called saponification. The resulting soap molecules have a head that’s attracted to polar molecules, like water, and a tail that’s attracted to non-polar molecules, like oils. What makes soap work is that it emulsifies oil and water. Normally, oil and water won’t mix, but with the help of soap the oil is broken into small soap-wrapped droplets that can be rinsed away with water.

Cleansers

How do I know if a cleanser is a 'soap-free cleanser'?

If a label doesn’t specifically state ‘soap-free’, or you just want to check the ingredient list yourself, how can you recognise a soap? Firstly, soaps have very poor solubility, so you’ll almost always find them in a solid bar form. Also, remember that soaps are anionic surfactants, so the first trick is to pick out the prefix (sodium or potassium) and the suffix (-ate). Soaps are named based on the oil or fat they’re made from, so olive oil, once it’s treated with sodium hydroxide, becomes sodium olivate8. Tallow becomes sodium tallowate. Coconut oil becomes sodium cocoate.

You get the idea. If you’re up for a challenge, try gathering up all the cleansers in your bathroom and pick out any with soap, or see if you can work out which other surfactants are hiding in there.

How a soap-free cleanser works

But soaps aren’t the only molecules that can perform this trick. In fact, they’re a small group within a much larger class of chemicals known as surfactants. The word ‘surfactant’ is short for ‘surface active agent’, which means a material that has both a polar and non-polar unit, and acts on the surface of two mediums2. Usually this means water and some type of oil, but it could also be water and air, or any other liquid/liquid or liquid/gas interface2.

Using a soap-free cleanser helps your skin to avoid dryness and irritation.

Surfactants

Defining the four groups of surfactants

Anionic surfactants3

The largest group, accounting for around 65%, they have negatively charged heads. Considered the high-foaming powerhouse surfactants, these include the soaps, the sulfates (e.g. sodium lauryl sulfate), as well as gentler alternatives like sodium lauroyl sarcosinate. If you see a surfactant beginning with ‘sodium’ and ending with the ‘-ate’ suffix, it’s safe to say it’s anionic.

Cationic surfactants4

Making up only around 3.5% of the surfactant market, these chemicals aren’t typically used for cleansing, but instead are known for their antimicrobial properties. Common cationic surfactants include cetrimonium bromide and benzalkonium chloride.

Non-ionic surfactants5

At around 28% of the market, non-ionic surfactants make up the second largest group. They don’t generally make good cleansers, being very low-foaming and mild, but find their way into many products as emulsifiers, ingredients that hold the oil and water parts together in a cream, the same way an egg helps hold together the ingredients in a cake. Some common non-ionic surfactants include glyceryl monostearate, PEG-100 stearate, and the ceteths, laureths, and ceteareths.

Amphoteric surfactants6

Another relatively small group (3.5%), these surfactants have large heads with both a positive and negative charge. They are very mild on the skin and eyes, and are often used as secondary surfactants, paired with a more powerful anionic surfactant. An amphoteric surfactant used in a lot of soap-free cleansers is cocamidopropyl betaine.

Benefits

What are the benefits of going soap-free?

Soap does a fantastic job at removing dirt and oils from our skin. In fact, it’s a little too good at it. As strong anionic surfactants, soaps remove a lot of the natural oils our skin produces to help keep the moisture in, which can be a problem for people already prone to dry skin. Milder anionic and amphoteric surfactants remove dirt without stripping away natural moisturisers, which means that using a soap-free cleanser helps your skin to avoid dryness and irritation7.

If you wanted to try a soap-free cleanser, give our QV Gentle Wash a go, in fact our entire QV range is designed to gently care and nourish your skin.

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References

1. Myers D, editor. An Overview of Surfactant Science and Technology. In: Surfactant Science and Technology, 3rd Edition. New Jersey: John Wiley & Sons, Ltd; 2006. page 1–28.

2. Gecol H. The Basic Theory. In: Farn RJ, editor. Chemistry and Technology of Surfactants. Oxford ; Ames, Iowa: Wiley-Blackwell; 2006. page 24–45.

3. Hibbs J. Anionic Surfactants. In: Farn RJ, editor. Chemistry and Technology of Surfactants. Oxford ; Ames, Iowa: Wiley-Blackwell; 2006. page 91–132.

4. Gadberry FJ. Cationics. In: Farn RJ, editor. Chemistry and Technology of Surfactants. Oxford ; Ames, Iowa: Wiley-Blackwell; 2006. page 153–66.

5. Hepworth P. Non-ionic Surfactants. In: Farn RJ, editor. Chemistry and Technology of Surfactants. Oxford ; Ames, Iowa: Wiley-Blackwell; 2006. page 133–52.

6. Otterson R. Amphoteric Surfactants. In: Farn RJ, editor. Chemistry and Technology of Surfactants. Oxford ; Ames, Iowa: Wiley-Blackwell; 2006. page 170–85.

7. Hannuksela A, Hannuksela M. Soaps and detergents in skin diseases. Clinics in Dermatology 1996;14(1):77–80.

8. Saffire Blue Inc. INCI Terms for Saponified Oils – Saffire Blue Inc. [Internet]. 2014 [cited 2018 Feb 12]; Available from: https://saffireblue.ca/resource-centre/inci-terms-for-saponified-oils/.