This post will give you a very brief overview of what surfactants are, why they’re used, what products they’re used in and other important details regarding their skin friendliness and more. Enjoy!
What are surfactants?
Surfactants are, from a more technical standpoint, ‘surface active agents’ ( hence their name); ultimately they’re able to reduce the surface tension of a solution. Now stepping away from jargon, surfactants are ingredients that allow you to blend otherwise incompatible ingredients (e.g. fat-soluble and water-soluble), and to create a cleansing and foaming effect in your product. The reason for this comes down to their structure largely. Surfactants look a lot like sperm, they have a water-soluble (polar) head and a fat-soluble tail. This basic structure allows for a lot of its’ functions.
Now say you had a beaker of water with or without a bit of oil in it. With just a bit of surfactant in the beaker, you would end up with them clustering to the top of the solution, with their heads (water-loving) facing down and their tails up.
When you have enough surfactant in your solution, they begin to move downwards into the water phase to create micelles - a fundamental component of any surfactant-based product. Micelles form little balls, the tails all facing inwards, often trapping a bit of the once surface oil in between, as is the case in lotions. When you have enough micelles in the solution, water-based lotions/emulsions now become possible. Here’s where we arrive at the Critical Micelle Concentration; you’ve finally introduced enough surfactant to end up with enough micelles to create a stable emulsion product.
Below is what a micelle looks like, with the head of the surfactant facing outwards and the tail inwards, trapping oil in the ball.
Enough of these bad boys will create your stable and effective products, granted they are compatibly charged. For example, when you use negatively charged surfactants, you want to use only surfactants with that negative charge (or no charge) so that they’re able to sufficiently repel each other. In contrast, if you had negative and positive surfactants in a mixture together, since they wouldn’t repel each other in the same way, the micelles would tend to clump together, resulting in a not so functional end product. Instead, when you use only the negative surfactants, the micelles are able to stay dispersed throughout the formula, ultimately forming an emulsion, for example (emulsions/water-based lotions are really just a whole bunch of microscopic micelles containing oil, sitting in a water continuous phase).
There are a number of things that impact the way micelles act (e.g. HLB values, charge, surfactant type) - in the case of a surfactant with a cleansing ability, the picture below is a great diagram of how micelles help make that happen. In contrast, emulsifying surfactants don't have the same cleansing capacity and usually stick to your skin (e.g. lotions, creams, liquid makeup).
That’s a basic overview of what surfactants are, there are also many different types of surfactants out there, all with their own unique effects for your products. The first thing to look at for surfactants is their charge, which has a huge impact on how you should use them and which products they’ll be compatible with.
Classes of surfactants based on charge
Cationic: Positively charged surfactants, typically used for hair conditioner products. Note, these surfactants WILL NOT play well with anionic surfactants as mentioned above. Examples: quaternary ammonium compounds (e.g. cetrimonium chloride)
Anionic: Negatively charged surfactants, commonly used in emulsions and wash off products. These surfactants, because of their charge, are very good at cleansing the skin - if used alone, the products would be quite harsh to the skin. As a result, when these are used, they’re usually blended with an Amphoteric and Non-Ionic Surfactant for best results. Note, these surfactants WILL NOT play well with anionic surfactants as mentioned above. Examples: alkyl sulfates ( e.g. sodium lauryl sulfates), alkyl ether sulfates (Sodium Laureth Sulfates), sulfosuccinates (e.g. disodium lauryl sulfosuccinate), fatty acid isethionates (e.g. sodium cocoyl isethionate), fatty acid taurides (e.g. sodium methyl cocoyl taurate), acyl sarcosinates (e.g. sodium lauroryl sarcosinate) and carboxylate salts (soap ~natural option).
Amphoteric: Surfactants that can be either negatively or positively charged. Added with both cationic or anionic surfactants for more skin friendly products. Examples: alkyl betaines (e.g. cocamidopropyl betaine), alkyl sultaines (e.g. cocamidoproyl hydroxysultaine)
Non-Ionic: Uncharged surfactants, added to either cationic or anionic surfactants for more skin friendly products. Rarely used on their own in conventional cosmetic products, but, since these surfactant types have more ‘natural’ options, many indie beauty manufacturers will use them without charged surfactants. Examples: alkoxylates alcohols (e.g. oleth 5), ethoxylated oils/fatty acids/glycol esters/sorbitan (e.g. PEGs), alkyl polyglycosides (e.g. decyl glucoside)
I just have a few notes to add about the differently charged surfactants. Because of the fact that most readily available charged surfactants aren’t ‘natural’, all-natural formulas with surfactants are quite the challenge. For example, the amphoteric surfactant Cocamidopropyl betaine is naturally derived, but it also has some synthetic parts to it. This is all fine and well… until manufacturers are not truthful on their labels by calling their products all-natural, perpetuating a lot of undue misinformation. This ingredient can be found in a large percentage of ‘all-natural’ shampoos. Again, I’m not saying the ingredient is bad, it’s the marketing that bothers me. I find most ‘all-natural’ cleansing products on the market are either not all-natural, or are often not skin friendly. This isn’t always the case, but in my consultation work, overwhelmingly this is very common.
Saponified oils (i.e. oils reacted with sodium/potassium hydroxide, e.g. liquid castile soap or cold-process soap, indicated by sodium/potassium -ate… e.g. sodium cocoate by INCI, or otherwise known as saponified coconut oil) are one of the few ‘natural’ options for anionic surfactants readily available, but there are a few problems when using these surfactant types when making products intended for your face or hair. These natural surfactants are only stable at very high pHs close to 8 or 9. In contrast, the pH of our skin and hair is closer to 5.2-5.4. Because products with these surfactant types cannot be pH adjusted (the product will lose stability at around pH 7), they can be quite unfriendly to the skin barrier and to your hair cuticle… e.g, in hair, a higher pH can cause the hair to swell and cuticle to open, making it very prone to damage. Many surfactants, including these, also react with metals in the water (that’s where chelating agents can come in). As a result, if you’re in an area with harder water, soap scum will become a problem. I see a lot of confusion for products especially among ‘shampoo bars’. A good shampoo bar will be pH adjusted to your scalp and use ingredients that don’t cause soap scum. Shampoo bars are not soaps, instead, they are syndet bars; they use a blend of dry surfactants to create a solid product. Instead of saponified oils, they’ll opt for different surfactants that allow for a skin friendly product. Most commonly, SLS will be used because it’s cheap and effective, but it can be a bit harsh depending on how it’s used. Shampoo bars don’t need this surfactant type, there are many other dry surfactant types available, although the manufacturer may need to go directly to ingredient manufacturers. There’s a trend in my own community for soap shampoo bars by some of the local makers…. These aren’t really going to be a nice shampoo bar… I love the idea of eliminating packaging waste by creating solid products… but this trend might cause consumers to think all shampoo bars cause residue and make their hair feel gross…. At the end of the day, there are many ways you can create a great 'natural' surfactant-based product, you just need to be a good formulator with access to some of the more innovative ingredients that you often can only find directly through ingredient manufacturers (rather than suppliers). Note, these challenges are one of the reasons why I personally love natural formulating :). That's my two cents, let's move on!
Classes of surfactants based on function
You can also classify surfactants based on how they act in formula or the products they’re capable of producing.
Surfactants: All of these classes are surfactants, but in the cosmetic industry, ‘surfactants’ are surfactants capable of detergency or foam. Surfactants are used in cleansing products, like soap, cleansers, shampoos, etc.
Emulsifiers: These surfactants will react in a way to create an emulsion, e.g. a cream or a lotion. Emulsifiers are found in lotions and creams, conditioners, liquid to solid makeup, and cream cleansers.
Solubilizers: Ingredients like essential oils don’t mix with water-soluble ingredients naturally. For example, they would just sit at the surface of a product undiluted, which is not a good thing for a product. Solubilizers make it possible for them to solubilize throughout their formula, ultimately diluting them for a safe end-product.
Note: Surfactants are used in varying percentages for different product types, for example, at around 10% total in face wash and upwards to 20% in a shower gel. Different areas of our body are, for example, more sensitive than the others. A good formulator will tailor the different products they make based on these differences. For the best end-product, you typically want a blend of different surfactants. For example, when you use anionic surfactants, you'll usually also use a bit of amphoteric and non-ionic to give you a nice, skin friendly end product.
For more information on surfactants, listen to my podcast on surfactant science with Perry Romanowski, internationally renowned cosmetic chemist and founder at Chemists Corner.
I think that’s a good basic review of surfactants. There’s so much more to dive into, but I think the information I presented will give you a good starting point to start to really understand the different surfactants out there, why they’re used, and how they impact products. That’s a wrap! Questions, queries, conundrums or concerns? Leave em’ below or on any of our social media channels!
Carli, B (2017). Apply surface science to formulate basic skin care. Diploma of Personal Care Formulation Text Book, Institute of Personal Care Science.